1982ApJS...49...53H © 1982.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. The AstrophysicalJournalSupplementSeries,49:53-88,1982May National ScienceFoundation. Astronomy, Inc.,undercontract with theNationalScienceFoun- operated byAssociatedUniversities, Inc.,undercontractwiththe dation; andtheNationalRadioAstronomy Observatory,whichis operated bytheAssociationof UniversitiesforResearchin brightness, implyingarecentepochofenhancedstar with anomalousUBVcolors,disturbedopticalmorphol- many ofthegalaxieswithunusualcolorsareinteracting ogies, emissionlines,andregionsofhighsurface galaxies thatshowevidenceforunusualstellarpopula- systems, butthoseisolatedsystemswithunusualcolors formation. LarsonandTinsley(1978)haveshownthat tions arenotunderstood.Thesegalaxiesincludeones The star-forminghistoriesofnoninteractingirregular Visiting Student,KittPeakNational Observatory,whichis © American Astronomical Society •Provided by theNASA Astrophysics DataSystem 3 + This factmayshowthatthesegalaxiesarelessevolvedthantypicalspiralsormixingfroma reservoir ofunprocessedmaterialistakingplace.Webrieflycomparethesepropertieswithpredic- The nitrogen-to-sulfurratioremainsfairlyconstant,risingsteeplyonlyforhighoxygenabundance. mean). NocorrelationisfoundbetweentheSFRandglobalgasorabundanceparameters,implying nevertheless, thereisalargerangeinSFR,anditcanbeveryhigh(upto10timesthelocalgalactic current rate.Thiscorrelation,coupledwiththelowmetallicities,suggeststhatstar-forming Subject headings:galaxies:photometry—stellarcontentnebulae:Hnregions activity decreaseswithincreasingSFRanddecreasingtimescaleforexhaustingthegasat N/S, andtheradiodatagiveinformationongasmasseskinematics. primarily from4500-7500Á,and21cmHiobservations.Fromthefilterphotometrywedetermine star-induced starformationmodel,andthegasinfallmodel. activity movesaroundthegalaxyandthatoverallSFRislikelytovarysubstantiallywithtime. that localprocessesdominate.Thefractionalinvolvementofthegalaxyincurrentstar-forming irregular galaxiesandafewcomparisonobjects.Programwerechosenprimarilyonthebasis tions fromthreeglobalmodelsofgalaxianevolution:theclosedsystemmodel,stochastic The dataconsistof40"aperturephotometrythroughintermediate-bandfilters,25" formation rate(SFR)isestimatedfromthenumberofionizingphotonscoupledwithaMillerand applied tothelargeaperturespectrainorderseparateHßemissionfromabsorption,andastar a Mgindexandreddening-freecolorparameterQ.Aniterativepopulationsynthesistechniqueis spectrophotometry intheregion3500-5900À,ofindividualHnregions of theirbluecolorsandthusmaybeexpectedtorepresentsystemswithhighratesstarformation. Scalo initialmassfunction.LinefluxesareusedtodeterminetheabundanceratiosO/Hand None oftheprogramgalaxieshavespiralarms,andmanylowrotationalvelocities; We presentopticalandradioobservationsofglobalpropertiesforasamplenoninteracting I. INTRODUCTION radio sources:21cmradiation—stars:formation GLOBAL PROPERTIESOFIRREGULARGALAXIES Department ofAstronomy,UniversityIllinoisatUrbana-Champaign 1 Received 1981June8;acceptedOctober12 D. A.HunterandJ.S.Gallagher Steward Observatory,UniversityofArizona D. Rautenkranz ABSTRACT AND low rotationvelocities,andnonehavespiralarms,sothe lar mediumbytheevolving massivestarsthroughHn by meansoftheenergydumped backintotheinterstel- ing thestarformationratesandrelationshipofthese would liketouncoverthelarge-scaleprocessesgovern- not dominanthere.Nevertheless,thereareirregular present afurtherpuzzle.We wouldliketoknowwhat efficient, atleastthecurrentepoch;therefore,one galactic shockdensitywavemodelofstarformationis still arenotunderstood.Manyofthesegalaxieshave cess, andsystemswithglobally extendedstarformation most relevant.However,this isaninherentlylocalpro- region expansion,stellarwinds, andsupemovaemaybe systems inwhichthestarformationisseentobehighly stochastic processesinwhichstarformationcontinues systems tothelessactivelowluminositydwarfs.The 1982ApJS...49...53H 54 Nevertheless, alldataarepresentedherewiththehope individual Hnregions,andi21cmobservations. photometry, smallaperturespectrophotometryof mediate-band filterphotometry,largeaperturespectro- radio observations.Thedatapresentedhereconcernthe on globalandlocalscalesthroughvariousoptical explored thestellarandgaseouscontentsmetallicity paper presentinglocalpropertiesoftheseirregulars lined in§III,theglobalpropertiesarepresentedand in §II,analysisofthespectrophotometricdataisout- global propertiesofthesesystemsandconsistinter- terms ofthehistorystarformationandrelation made in§V,andVIsummarizesourresults.The meant thatnotallgalaxiesarepresentindatasets. Limits totelescopetimeand/orweather,however,have the starformationtoothersystemicproperties,wehave (Hunter 1981)iscurrentlybeingprepared. discussed in§IV,acomparisonwithvariousmodelsis possible bursts,andtheformparameterdependence parameters controlthestarformation,whethersto- ies wouldcompriseanoddmixture ofunrelatedobjects. high starformationrate(see BalickandHeckman1981). NGC 972,2146,and NGC 7625,andthepeculiar parison (seeFig.1foracolor-colorplot).Alsoincluded progressed, somesystemswithunknowncolorsand influenced byrecentencounters.Astheinvestigation hence, areselectedtobethoseinahighlyactivestar ies arequiteblue(£/—i?<—0.3;seeTable1)and, that gapswillbefilledinthefuture.Thedataaregiven underlying galaxy,thehistoryofSFR,including low luminosityspiralNGC 3310whichcurrentlyhasa NGC 5474,7292,thedustyspiral-likepeculiars morphologies andemissionlineswereaddedforcom- primarily byanomalousUBVcolors.Mostofthegalax- intermediate luminosityirregularschosenforshowing of theinitialmassfunction(IMF)are. dances, andstellarcontent,whatthenatureof formation rates(SFR),efficiency,distribution,abun- chastic processescanaccountfortheobservedstar our samplehavenocloseneighbors,andtheirstar H iiregionswhicharemostlikelytheresultofepisodic formation phaseoftheirevolution.Wehave,however, evidence ofunusualevolutionaryhistories,asindicated gram wereselectedfromapoolofnoninteracting, However, thishasturnedout nottobethecase.Video for comparisonwerethelessactiveirregularsDDO168, some withmorenormalspiral-likecolorsbutdisturbed formation ratesarethereforeunlikelytohavebeen star formationeventsingas-dominatedgalaxies(Searle, avoided thelowluminosity,extremelyblue,intergalactic One mightsupposeatfirst that suchasampleofgalax- Sargent, andBagnuolo1973).Mostofthegalaxiesin In ordertodevelopapictureofthesegalaxiesin The galaxiesobservedinouropticalandradiopro- © American Astronomical Society •Provided by theNASA Astrophysics DataSystem II. OBSERVATIONSANDDATAREDUCTION HUNTER, GALLAGHER,ANDRAUTENKRANZ -1 columns areasfollows:column(1)—galaxyidentifica- —morphological typefromdeVaucouleursand tion; column(2)—alternateidentifications;(3) Freeman (1972)ordeVaucouleurs,and photometry ofHiiregions.Columns(8)-(9)—total Prairie Observatory(PO)photometry;largeaperture “Y” indicatesthatdatawereobtainedforgalaxy: Corwin (1976;hereafterRC2).Incolumns(4)-(7)a reddening-corrected UBVcolorsfromdeVaucouleurs, Reticon (1RS)spectrophotometry;21cmHi,or Burstein andHeiles(1978);column(11)—distanceto galaxy inMpc.TheHubbleconstantischosentobe (1977û); column(10)—colorexcessesusedtocorrect de Vaucouleurs,andCorwin(1981),theRC2,orHuchra Steward Observatory90inch(2.3m)Reticonspectro- (Table 5).Column(12)—themajoraxisinarcminutes from therecessionvelocitiesofRood(1980)orourdata Korteweg andTammann(1979),wheneverpossible,or have calculatedtheirfactorforcorrectingtheoptical on Holmberg’s(1958)system.IfanoriginalHolmberg 50 kmsMpc.DistancesaretakenfromKraan- lar instructure,butwiththeexceptionsnotedabove,no ingly homogeneousmorphologicalclass.Theyareirregu- diameter wasnotavailable,oneconstructedfrom camera images(Hunter1981)throughb,y,/,andHa UGC) ortheisophotaldiameterDfromRC2using one galaxystandsoutasbeingdistinctfromtherest. filters showthattheprogramgalaxiesformasurpris- respectively, thereisnomeasurablecorrection.Column dimension forgalacticextinction,butexceptNGC the bluemajoraxisfromNilson(1973;hereafter brightness, brightknots,etc. labeled “?colors”werechosenonthebasisofmorphol- umn (14)—commentsonthenatureofeachirregular (13)—the ratioofminortomajorHolmbergaxis;col- the relationshipsgivenbyFisherandTully(1980).We UBV colorsforgalacticreddening,determinedfrom chosen forthissample.“BlueIrr”identifiesthosethat galaxy whichmayhelpclarifythequestionofwhyitwas ogy alone,includingirregularstructure,highsurface form theprimarythrustofthisinvestigation.Those with theUniversityofIllinoisPrairieObservatory1m 1569 andIC334,forwhichthisfactoris21.12, photometer withathermoelectricallycooledRCA 25 used, butthevfilterwasshiftedtoacentralwavelength telescope atOakland,Illinois,usingasingle-channel was usedforallobservations. The 38,52,and74filterson Wood’s(1969)photometric of 4200Àwithawidth150 ÀinordertoavoidHÔ. curred. A40"aperture,centered onthegalaxybyeye, C31034A photomultiplier.Thestandardubyfilterswere system werekindlyloaned to usbyS.M.Faberand traced toensurethatnomajor deteriorationhadoc- The programgalaxiesarelistedinTable1,andthe Intermediate-band filterphotometrywasobtained a) Photometry 00 LO i-D a
TABLE 1
© American Astronomical Society Program Galaxies ■5 iS O a; oS O B Ö ë w XJ £S (Xi OQ oq g op : ^ oc o I o' , ,i : : _l 'S ag SS22o§2£sss T3 c-.ÆX) \ s!$ ÖÖÖÖÖÖÖÖÖÖÖÖÖÖÖ^ÖÖ uouo'^'^uotr>'0(Nr^''OON*r)ON(N'^J'Ot^-x 1 OO (N—'cncN(N—uomcN cniN^^cNrn —^‘OcncnONOrocnr^uoTtONvo oof-Hr- (Nioc -£1.4 x Qo Mg = 0.4( j )0 - (52 - )0 - 0.073, (2) •e8,° «.¡E 1.8 where (52— y) has been transformed to the Faber sys- E sb tem. This index differs slightly from Faber’s Mg index * ^30 * 22 VE since the 45 filter she used is narrower than the b filter. •Sb Here we have constructed the index so that stars of spectral class G and hotter have an index of approxi- 2.6 JJL 01 0.2 0.3 0.4 0.5 0.6 0.7 0.8 mately zero. Column (10) presents a reddening-free in- (b-y)0 dex g, where Fig. 1.—Color-color distribution of galaxies observed at Prairie Observatory and with the 1RS at Kitt Peak. Q=-lJ(b-y) + (v-y)-, (3) and, finally, column (11) lists the V magnitude corrected The galaxies were observed differentially with respect for reddening determined from y. The second line for to a nearby star (less than Io away). Not more than 10 each galaxy contains the estimated uncertainty in the minutes were allowed between star and 4 minutes be- colors (see Appendix A). The ratio of the color excess to tween sky observations. A typical observation sequence E(B — V) is taken to be 0.7 for (b — y), 1.19 for (t> — y), would be star-sky-galaxy-sky-galaxy-sky, ending with a 1.12 for (w — b), 0.24 for (52— y), -1.1 for (74-y), 1.6 star-sky. Good signal-to-noise with narrow-band ultra- for (38 —j), and 3.1 for V. Figure 1 displays (u — b) violet filters is difficult to achieve at sea level in a 0 versus (b — y) for the objects in Table 2, as well as for reasonable amount of time, so filter 38 was eliminated 0 a selection of galaxies not included in this photometric part way through the observing program. The typical list which were observed with the 1RS (Table 4). observing time per filter was 10-20 s on the galaxy and Two galaxies were also observed in 1980 June on the 5-10 s on the star. Additional observations were some- KPNO No. 4 16 inch (41 cm) telescope using a dry ice times made through the u and 52 filters. Unless standard cooled GaAs-C31034 photometer through BVI filters stars were observed on a particular night, mean extinc- and a 55" aperture. These data are presented in Table 3. tion and transformation coefficients were used for the The quantity E(B — V) = 0 for both galaxies. differential colors. The statistical analysis of the raw data is given in Appendix A. The galaxy comparison stars, as well as library stars b) Large Aperture Spectrophotometry for population synthesis described in § IIIû, were Multichannel spectrophotometric observations were calibrated on separate nights. Typically 12 standard obtained with the cooled dual beam intensified Reticon Crawford and Barnes’s (1970) and six Faber (1973û) scanner (1RS) mounted on the white spectrograph of the standards were observed over a wide range in air mass No. 1 36 inch (19 cm) telescope at KPNO. Grating No. and were chosen to cover a variety of spectral types, 26 with 600 lines mm-1 provided a useful wavelength although A stars were avoided since the presence of the coverage of 3500 to 5800 Á. Object and sky observations HS feature affects the transformation in v. An iterative were made through two apertures of diameter 25" sep- least-squares routine borrowed from E. C. Olson was arated by 1'; the spectral resolution is ~15 A. Except used to determine the transformation and extinction for DDO 168, which was acquired by a blind offset, coefficients simultaneously. Transformations followed galaxies were normally centered visually and guided the equations of Crawford and Barnes. Second-order with the gold automatic offset guider. When contamina- color terms were not included in the extinction. A subset tion of the second beam by the galaxy was negligible, of Wood’s (1969) stars was measured at Prairie Observa- object-sky subtraction was effected by switching be- tory, and a linear transformation was found between tween the two apertures. Where the contribution of the Wood’s and Faber’s (52— y) and (74— j) colors: galaxy to the sky beam was not negligible, we beam switched between galaxy and a suitably chosen blank (52-y)F= —0.035 +1.198(52— ^)w sky north or south of the object. Each night, observa- 0) tions were also made of a quartz lamp, He-Ne-Ar lamp, (74— /)F = 0.091 + 1.091(74- j)w. bias or dark, and typically four standard stars. The observations of the galaxies and comparison ob- The 1RS was still in a developmental stage at the time jects (including globular clusters) are presented in Table these observations were made and, as a consequence, we © American Astronomical Society • Provided by the NASA Astrophysics Data System 1982ApJS...49...53H NGC972 . NGC 2968 NGC 2146 NGC 1569 NGC 1012 NGC 7089 NGC 7078 NGC 6779 NGC 6205 NGC 5904 NGC 5272 NGC 5962 NGC 4254 NGC 4449 NGC 3912 NGC 3310 NGC 3077 NGC 3034 NGC 7331 NGC 4111 NGC 205. NGC 7625 NGC 3738 Mrk 31 Mrk 32 Mrk 35.... Mrk 33.... a The spectraltypesofthecomparison galaxiesare,inorder,Ep,Sb,E,SO,Sc,andSb. © American Astronomical Society •Provided by theNASA Astrophysics DataSystem Object N(b-y)(e-y)(u~b)(52-y)(74-jOo(38-MgQV 0 (1) (2)(3)(4)(5)(6)(7)(8)(9)(10)(11) 0.368 0.016 0.006 0.024 0.009 0.631 0.018 0.507 0.509 0.005 0.031 0.493 0.232 0.025 0.035 0.227 0.009 0.231 0.007 0.366 0.008 0.633 0.011 0.820 0.020 0.711 0.015 0.191 0.028 0.546 0.430 0.016 0.401 0.031 0.403 0.419 0.012 0.478 0.005 0.488 0.008 0.613 0.007 0.517 0.003 0.554 0.007 0.021 0.216 0.244 0.016 0.012 0.420 0.367 0.005 0.975 0.027 0.889 0.028 0.958 0.023 0.006 0.012 0.034 0.431 0.029 0.013 0.006 0.010 0.010 0.014 0.606 0.031 0.590 0.035 0.963 0.047 0.572 0.014 0.546 0.028 0.823 0.012 0.019 0.021 0.042 0.861 0.944 0.013 0.008 0.882 0.027 0.711 0.014 0.008 1.043 1.055 1.513 1.143 1.184 1.416 1.364 1.529 1.212 1.828 1.173 1.064 1.367 0.064 0.006 0.024 0.015 0.011 0.022 0.028 0.980 0.826 0.026 0.051 0.059 0.022 0.617 0.042 0.007 0.034 0.009 2.014 0.026 0.013 0.009 0.016 0.020 0.950 0.007 2.020 0.068 0.063 2.053 0.019 0.051 0.899 0.023 0.011 2.560 2.230 a 1.424 1.522 1.402 1.810 1.489 1.480 1.401 1.517 1.532 1.568 1.957 1.359 1.288 1.968 1.559 1.009 1.476 Comparison Galaxies Irregular Galaxies Globular Clusters Photometry TABLE 2 0.029 0.031 0.108 0.008 0.153 0.017 0.132 0.037 0.183 0.081 0.006 0.152 0.007 0.153 0.011 0.304 0.007 0.237 0.223 0.005 0.193 0.146 0.031 0.019 0.059 0.021 0.161 0.012 0.252 0.014 0.394 0.034 0.300 0.033 0.126 0.026 0.128 0.041 0.290 0.007 0.294 0.006 0.306 0.005 0.410 0.016 0.166 0.055 0.022 0.049 0.096 0.021 0.015 0.041 -0.210 -0.593 -0.094 +0.026 +0.046 -0.414 +0.027 + 0.014 -0.309 + 0.025 -0.425 + 0.022 -0.874 +0.040 -0.841 +0.021 +0.017 -0.392 -0.658 +0.005 -0.303 -0.264 +0.036 -0.345 + 0.007 -0.451 +0.009 -0.394 +0.008 -0.585 -0.543 +0.009 +0.007 -0.561 +0.007 -0.643 +0.017 -0.357 +0.051 -0.341 +0.058 -0.642 -0.271 +0.009 +0.011 +0.004 -0.616 -0.339 +0.027 +0.008 0.058 0.039 0.119 0.060 0.012 0.975 0.022 0.013 0.013 0.057 0.070 0.997 0.007 0.009 0.055 0.067 2.191 1.242 1.296 1.462 1.074 1.220 1.914 1.838 1.129 1.178 + 0.032 -0.090 — 0,039 + 0.021 + 0.013 -0.072 -0.139 + 0.035 -0.123 -0.054 +0.041 -0.144 -0.107 +0.050 -0.021 -0.050 + 0.034 +0.020 -0.088 +0.014 -0.089 + 0.033 +0.028 +0.008 -0.045 -0.095 +0.030 +0.008 -0.034 -0.031 +0.011 + 0.010 -0.092 + 0.006 -0.122 + 0.007 -0.172 -0.262 +0.016 -0.009 -0.072 +0.022 -0.069 +0.016 -0.055 +0.018 +0.039 + 0.013 + 0.008 -0.125 + 0.008 + 0.005 -0.092 + 0.056 0.055 0.075 0.186 0.152 0.031 0.201 0.018 0.136 0.433 0.047 0.033 0.107 0.235 0.040 0.244 0.226 0.195 0.248 0.021 0.027 0.157 0.063 0.012 0.207 0.272 0.047 0.231 0.021 0.231 0.015 0.225 0.019 0.441 0.033 0.281 0.015 0.319 0.013 0.374 0.015 0.485 0.010 0.587 0.019 0.257 0.239 0.061 0.297 0.030 0.022 0.245 0.038 0.060 0.045 10.812 12.375 11.862 11.120 13.170 11.651 11.772 12.832 11.785 13.431 12.697 13.282 12.817 11.720 10.894 12.434 10.768 12.893 12.258 0.145 0.020 0.003 0.040 0.013 0.005 0.088 0.019 0.003 0.009 0.012 0.012 0.018 0.079 0.038 0.029 0.017 0.038 0.036 0.032 0.058 0.037 0.040 0.012 7.851 9.999 9.046 9.196 0.026 0.033 0.021 9.571 8.836 8.471 8.546 58 HUNTER, GALLAGHER, AND RAUTENKRANZ Vol. 49 TABLE 3 comparison observations were used to determine the B VI Photometry instrumental dispersion relation. The standard stars (primarily from the list of Oke Galaxy V B— V /— V 1974) provided the basis for transforming observed to absolute fluxes; in order to extrapolate to fluxes outside NGC 4449... 11.33 0.30 -0.69 NGC 4214... 11.93 0.40 -0.87 the atmosphere, mean atmospheric extinction coeffi- cients were used. The different scans were summed, corrected for extinction, and calibrated, and the two beams were summed. If persistence was a problem, the had to correct for several technical problems. First, and sky observation was subtracted from the object after most severe, was the changing alignment of the beam on calibration. the Reticon arrays. To deal with this problem, quartz The program galaxies and one comparison irregular observations were taken at the beginning and end of the dwarf, DDO 168, which was chosen for its low star- night and after every object observation (including forming activity, are listed in Table 4. Column (1) gives standard stars). Second, the bias current varied; conse- the galaxy identification, column (2) the total integra- quently dark scans had to be taken at least every 20 tion time, column (3) the mode of observation (S = stellar minutes or whenever an odd/even pattern began to or N = nebular) columns (4)-(6) the synthesized ubvy appear in the data. Third, the image tube flashed. We colors from the 1RS spectra, corrected for galactic red- compensated for this by employing a software patch dening, and columns (7)-(8) the nearest neighboring that removed any scans that passed a predetermined galaxy with roughly the same radial velocity, found in threshold. Fourth, the image tube exhibited significant the UGC, RC2, or Fisher and Tully (1975, 1980). Given variations of sensitivity with wavelength, but quartz are the projected distance, d, in kpc and the absolute normalizations removed them with no apparent residu- value of the difference in the velocities. As can be seen, als. Fifth, some galaxies were sufficiently bright to cause most of the galaxies are over 100 kpc from any neigh- phosphor persistence. In such cases the nebular mode bor. Observations of pairs of interacting galaxies show was used with the west aperture centered on the galaxy, that only in the more extreme colhsions, e.g., inter- and sky observations in the west aperture, which were penetrating mass distributions, is the star formation contaminated by persistence from the object, were dis- appreciably disturbed in either galaxy (cf. Larson and carded and replaced by sky observations in the east Tinsley 1978). Hence, we do not expect the current aperture. SFRs in the program irregulars to be affected by inter- Data from each night were reduced separately. First, actions. The spectra, corrected for galactic reddening the quartz observations were summed, fitted with a and continuum emission, are plotted in Figure 2 as polynomial, and normalized (divided by the fit) in order logF(X) in order to display better the weaker features. to eliminate the broad-band color of the lamp and leave Also observed for comparison were two red irregulars, only the gain variations in adjacent pixels. The He-Ne-Ar NGC 2968 and 5363. These spectra contain no emission TABLE 4 Objects Observed with the 1RS Nearest Neighbor t d V -1 Object (min) Mode {b-y)0 (v-y)0 {u-b) (kpc) (km s ) (1) (2) (3) (4) (5) (6) (7) (8) NGC 1569 16+16 N +0.20 +0.39 +0.62 577 40 NGC 1800 28 + 24 S, N +0.20 +0.57 + 1.02 70 10 NGC 2146 16 N +0.62 + 1.29 + 2.13 122 670 NGC 2366 16+16 S -0.09 -0.14 -0.17 224 33 Haro 22 ... 80 s +0.23 + 0.53 + 0.82 629 123 NGC 3274 32 N +0.32 + 0.69 + 1.00 622 39 NGC 3310 8 N +0.25 +0.53 + 0.67 962 6 Mrk35 .... 12 S +0.28 + 0.57 + 0.78 939 6 NGC 3510 56 s +0.30 + 0.71 + 1.20 173 1 NGC 3738 32 s +0.17 +0.43 + 0.88 500 11 NGC 4214 12 N +0.20 +0.46 + 0.52 17 6 NGC 4449 24 N + 0.23 + 0.52 + 0.96 57 1 NGC 5253 12 N +0.08 +0.28 +0.41 213 97 DDO 168 . 80 S +0.26 + 0.52 + 1.12 26 33 © American Astronomical Society • Provided by the NASA Astrophysics Data System 1982ApJS...49...53H interval foreachchannelinthebandpass.Alinear product oftheflux,filterresponse,andwavelength in CrawfordandBarnes(1970)JohnsonMorgan No. 1,1982 in Table9.ThePrairiephotometrywasobtainedwitha lengths greaterthan3800Àandtothe(usuallysmaller) ing totheobject’s1RSHßradialvelocityforwave- were thenrecomputedwiththefollowingcorrections (1951). Thecolorswerecalculatedbysummingthe lines andareshowninFigure3.Thespectrumof Although aquasi-photonstatistical uncertaintycouldbe were foundbyfittingthefeatureswithGaussians,using which hadtobecorrectedforunderlyingabsorption, idea ofhowgoodthe1RSsynthesizedcolorsare.We parison forobjectsincommoncangiveanapproximate intrinsic differencesareexpected.Nevertheless,acom- larger aperturethanthatusedwiththe1RS,sosome lines replacedbyacubicfittothesurroundingcon- Lindman andHauck1973;JohnsonMcNamara and standardcolorsforobjectsobservedatPrairieOb- transformation wasfoundbetweenthese“raw”colors NGC 5363ishighlycontaminatedbythesuperposed was fittedwithpolynomials:typicallyonebluewardof graph. Colorswerealsocalculatedwiththeemission rected forreddeningusingafittotheWhitford(1958) mation coefficientswereapplied.(3)Colorscor- servatory andelsewhere(CrawfordBarnes1970; ubvy, 52,UBVfiltersbyusingafittothefilterresponses in thisgalaxy,buttheemissionregionmustnotbe The appropriatecontinuumwassubtractedfromthe nature andmoreconsistentfromobjecttoobject. (1969) code.First,thecontinuumofentirespectrum of lessthan0.04magwasmadetothefinalcolors constructed. Asmallwavelength-dependentcorrection (b —y),0.05for(vand0.1(w—y). find thatthereisanaveragedeviationof0.04magfor [O il]X3727redshiftfortheblueend.(2)Thetransfor- applied: (1)Thebandpassendswereredshiftedaccord- Gaussians. feature, andwhatwasleft fittedwithoneormore this waythechoiceofcontinuumwouldbeglobalin 3750 Á,oneredwardof4000À,andinbetween.In an interactivegraphicsprogrambasedonBevington’s system usedonthe1RStoOkeandShield(1970) emission subtracted. extensive. star. Emissionlineshavebeendetected(Caldwell1980) simple orasstraightforward asforphotometricdata. system usedbyO’Connell. transform theHayesandLatham(1975)absoluteflux tinuum andthecontributionduetocontinuum 1969; EggenandGreenstein1965,1967).Thecolors The 1RSubvycolorsareincludedinTable4andUBV Colors weresynthesizedfromthe1RSspectrafor The fluxesoftheemissionlinesotherthanhydrogen, Estimating theuncertainty in Reticondataisnotas O’Connell (1973)colorsrelativeto5050Awerealso © American Astronomical Society •Provided by theNASA Astrophysics DataSystem GLOBAL PROPERTIESOFIRREGULARGALAXIES introduces moreuncertainty,asforexampleinthechoice instrumental factors,suchasachangingalignment,and values. (2)Intheconstructionofbestlinearleast- in thesynthesisofcolors.Inordertogetsomeidea data processingfactors,suchastheuseofmeanextinc- constructed, theactuallimitstodataaresetby credibility ofthedatawehavelookedattheircon- of thecontinuumthesehighlycompositesystemsor (3) Fivestarswererepeated2to7timesduringtherun, (1) Theabsolutefluxesoftheobservedstandardstarsat tion coefficients.Furthermore,eachstepofdataanalysis Three galaxieswererepeatedonceeach(oneatanair 4200 À,0.04at5300and0.10forthe5050Áflux.(4) the starsof~0.04magubvycolorsand0.15forV. there isameanscatter(averagepositivedifference)for squares transformationsfrom1RStostandardcolors 5050 Ádeviateanaverageof0.1magfromtheaccepted sistency inavarietyofwayswiththefollowingresults: The ratiosofHyandH8toHßdependonunderlying mass of3),andtheaveragestandarddeviationfor from nighttowas0.04mag.ForO’Connellcolors and theaveragestandarddeviationoftheirubvycolors uncertainty willincreaseastheflux,i.e.,signal-to-noise, we againfindtheaveragetobeoffby12%.Thus, ratio of[Oin]5007/4959tobe~2.88fromatomic conclude thatouremissionlinefluxesareprobablygood at 5300,and0.12forthe5050Áflux.(5)Weexpect the observeddeviationwas0.07at3570Á,0.05 decreases. and theobservedratiosdividedbyexpectedratios, absorption andreddening.Whencorrectionsaremade an averageof3.00witha12%standarddeviation.(6) theory (Nussbaumer1979).Theprogramgalaxiesgive O’Connell colorswas0.09at3570Á,0.104200,0.07 to ~10%forthestrongerlines.Forweakerlines beam Reticonsystem(Hege,Cromwell,andWoolf1979). runs from1978Novemberto1980JuneontheSteward regions intheprogramgalaxieswasobtainedfive The systemwasusedbothinabluesensitivemodewith were recordedbytwoparallelReticonarraysof936 Varo imageintensifierchain.Inbothcasesthespectra Observatory 2.3mtelescopeusingtheintensifiedtwo first stagedetectorandinaredsensitivemodewith an RCAmagneticallyfocusedimageintensifierasthe blue sensitivesystemfora wavelength coveragesimilar diodes each.Entranceapertureswereusuallytwocir- cular holes~20"apart.Foramoredetaileddescription made withthemorestable red sensitivesystemwitha to thatobtainedwiththe1RS. Theremainingrunswere see Williamsetal.(1978).Onerunwasmadewiththe coverage of4000to7500À andaresolutionof~5Á. Alignment wasalsoaproblem withtheStewardRe- Small aperturespectrophotometryofindividualHn c) SmallApertureSpectrophotometry 59 CTi CO CTi OO LO '"O a Log F (erg/cm2/sec/A) -12 x © American Astronomical Society •Provided by theNASA Astrophysics DataSystem 60 HUNTER,GALLAGHER,ANDRAUTENKRANZVol.49 X5577 nightskyemissionline.Datareductionwasdone ing wasappliedtothequartz observationspriorto residuals. Thequalityofthedataimprovedsteadily of thenightskysubtractioncanbeseenfrom[Oi] dures weresimilartothoseforthe1RSobservations. used tochoosetheHnregions,andobservingproce- determining thelampcoefficients inordertoremove except thatawide(~50-75 channel)Gaussiansmooth- at Stewardinamannersimilar tothatforthe1RSdata Entrance aperturesof2'.'5to5"wereused.Thequality of veryhighquality.TheTVacquisitionsystemwas from oneruntothenextsothat1980Junedataare did notmoveandsodividedoutwithnoapparent the opticaloilcouplingimageintensifierstages,butit small scalestructure. sometimes noisy.Foronerunabubblewaspresentin the spectrumincludingH¿>andHy;Kßregionwas ticon, resultinginvaryingamountslostontheendsof Observatory intensifiedReticonsystem.NotethatthesetwospectrahavelinearFscalesinunitsofergsscmÀ. x Fig. 2.—1RSopticalspectra,correctedforreddeningandcontinuumemission,samplesofHnregionspectratakenwiththeSteward of theaperturecanintroduce significantvariationsin regions. Iftheentireregion isnotincluded,placement use ofsmallaperturesonusually inhomogeneousHu man 1980;Talent1980)where thereisoverlap.However, uncertainties inthesedatacanonlybeverycrudely 2.81 and13%;forD,3.036%.Ourlarge estimated. Takingtheratioof[Om]5007/4959,wefind only forgalacticreddening,areshowninFigure2,and there isageneral,oftenoverlooked, problemwiththe agree withthoseofmostotherauthors(Peimbertand small aperturespectrophotometricresultsalsogenerally standard deviationis17%;forB,3.33and35%;C, Smith, andWeedman1974;Alloin,Bergeron,Pelat Spinrad 1970;WelchForrester1973;Ossmer, that forrunA(see§Illfr)theaverageis3.11and the dataarepresentedbelowin(Table8)§III6.The 1978; Lequeuxetal.Kennicutt,Balick,andHeck- Two examplesofspectrafrom1980June,corrected CTi co CTi 00 LO '"O a o Log F (erg/cm2/sec /A ) x No. 1,1982GLOBALPROPERTIESOFIRREGULARGALAXIES61 NRAO. Thecooleddualchannel21cmfrontendand results. observed fluxratiosasdifferentareasaresampled (cf. Hawley1978;KwitterandAller1981).Thus,cau- used withabandwidthof10MHzandanaverage tion mustbeexercisedininterpretingandgeneralizing receivers. off position,orcomparing thesignalsintwo Reality ofweaklinefeatures wasoftentestedbyshifting system temperatureof~55K.Thetelescopewasoper- the centerofbandpass, changing theIF,moving tion onblankskyfollowing thegalaxyobservation. ated inthetotalpowerobservingmodewithanobserva- 384 channelautocorrelationspectrallinereceiverwere 1980 Marchwiththe300foot(91m)telescopeat © American Astronomical Society •Provided by theNASA Astrophysics DataSystem Observations at21cmweremadeduring10daysin d) HiRadioObservations Fig. 2.—Continued were firedbeforeandafterthesourcesignal. were obtainedonastripchartrecorder.Asourcewas was obtained.Thenpositions10',onebeamwidthfrom point calibrationsourcesfrom—10°to65°declination give Jy/Kasafunctionof declination foreachchannel calibrated beforeslidingthetworeceiverstogether.For observations weresummed,averaged,smoothed,and discarded. Eachdaythetwochanneldeflectionsoffive observed. Scanswithobviouslybadbaselineswere quadratic functionindeclination. (see FisherandTully1975 for adetaileddescriptionof and aratioofnoise-to-signal deflectionswasusedto Baselines werefittedwithalinearfunction.Multiple allowed todriftthroughthebeam,andnoisetubes the problem).Thesecalibration curveswerefittedwitha the calibration,5daysofmeasurementswereaveraged, the centerofgalaxy,usuallynorthorsouth,were several minuteseachdayuntiladequatesignal-to-noise The galaxieswereobservedoveranumberofdaysfor Reduction ofthedatawasdoneattelescope. CTi CO CTi 00 LO '"O a Log F\ (erg/cm2/sec/Â ) 62 HUNTER,GALLAGHER,ANDRAUTENKRANZVol.49 (1979) listandsomedwarfsfromFisherTully of smallwell-calibratedsourcesinAllenandShostak’s Fisher andTully(1980),provided22measurementsfor (1975) wereobserved.These,plussomeoverlapwith minus otherauthors’datadividedbytheseandthe number ofobjects)was24.Ourresultsforotherindivid- age percentdifferenceinemission-lineareas(thesedata estimating theoveralluncertaintyindata.Theaver- (Bottinelli etal.1973;PetersonandShostak1974;Dean ual galaxiesalsoagreefairlywellwithotherauthors Table 5presentstheresults.Thecolumnexplanations and Davies1975;Allsopp1979;Reakes1980;Gordon follow. Unlessotherwisespecified, calculationswere and Gottesman1981). done usingrelationsgivenin FisherandTully(1980). © American Astronomical Society •Provided by theNASA Astrophysics DataSystem In additiontothegalaxiesinthisprogram,anumber Figure 4showsmostoftheemissionprofiles,and Column (3).—Totalintegration time,t,inminutes. Column (1).—Galaxyidentification. Column (2).—Beamposition. Fig. 2.—Continued -1 -1 -1 62 5 _1 l km s. line at20%ofmaximumintensityinkms,minus Jy kms.Thiswasobtainedintwoways:usingthe using aroutineavailableon-lineatNRAO.Bothmeth- corrected fortheSun’smotionaccordingtoV= of theantennaresponse,taken fromFisherandTully for correctingtheobservedfluxGaussianshape ods gavethesameresults. Illinois interactivegraphicssystemtofitGaussians,and (BSC) (F),assumingthegas isopticallythin. units of10MMpc“.This comesfrom2.36X10 (1975). 5 kmstocorrectforvelocityresolution. + 0sin/cosb,where©istakentobe250kms~. Ge 0 Column (4).—Systemicheliocentricvelocity,K,in Column (5).—Systemicgalactocentricvelocity,V, Column (7).—JF20,thefullwidthofemission Column (6).—F,theareaunderemissionlinein Column (8).—Thebeamshapecorrectionfactor,BSC, Column (9).—TheH1mass perdistancesquaredin 0 G CTi co CTi OO LO '"O K a 2 _ 1 8 Log Fx (erg/cm /sec/Ä) A/E(B-V) =A.\. No. 1,1982GLOBALPROPERTIESOFIRREGULARGALAXIES63 blue luminosityinM/L,wherethehas 4.72. been determinedusingasolarabsolutemagnitudeof included hereandusedincomputing belowbecause use theirequation(8)withAt)^11kmsandaguess must beviewedasadhoc. Theinclinationangleis The intrinsicshapeandflattening ofirregularsystems RC2 andcorrectedforgalacticreddeningusing determination, afterGordonandGottesman(1981).We at AiS(rms)foreachspectrum.ThequantityUstedis an intrinsicflatteningof0.2 (FisherandTully1980). are notwellknown,andso thevalueof0.2usedhere b 10 M.Thisisthesumofalldetections. Q 0 © American Astronomical Society •Provided by theNASA Astrophysics DataSystem Column (13).—Mj/T#,totalHimassdividedby Column(12).—B, thebluemagnitudetakenfrom Column (11).—Mj,thetotalHimassinunitsof Column (10).—Anestimateofthequalitymass Column (14).—Inclinationanglez,foundbyassuming H T h ! inpercent. Fig. 2.—Continued 9 we expectthesampleofgalaxiestocoverarangein is takentobethecosmicabundanceratio0.25/0.73 in unitsof10M,foundfromtheWq- ing Misneeded,butwhattheactualvaluesaremust mass dividedbyindicativemass.TheHetoHigas orientations relativetous.Somecorrectionincomput- (Allen 1973),givingatotalgasmassof1.34Af(Hi). await afurtherunderstandingofirregulargalaxies. important toolinthestudy oftheintegratedlight composite systemsinthat it providesameansforde- termining thefractionoflight contributedbyeachof the mainstellargroupscomprising thesystem.This 02 t Column (15).—M,theindicativeordynamicalmass Column (16).—M/M,hydrogenplusheliumgas Spectral synthesisofspectrophotometric dataisan t gasz III. ANALYSISOFSPECTROPHOTOMETRICDATA a) PopulationSynthesis 64 HUNTER, GALLAGHER, AND RAUTENKRANZ Vol. 49 since the synthesis techniques cannot tell you when a necessary stellar component is missing from the library, and the results will be misleading (Peck 1980). Most libraries currently in the literature lack hot luminous stars. The problem with completeness, coupled with uncertainties in the data, means that metallicity infor- mation is hard to extract. Current libraries do not contain the range in metallicities necessary to fit popula- tions with nonsolar metallicities, and, even if they did, fitting the metallicity indicators requires greater signal- to-noise than required for fitting the continuum. Also necessary are a long baseline in wavelength, a suffi- ciently small grid so that there are no large gaps in color from one stellar group to the next, and observed stars which are average for their stellar type, as indicated by the dereddened UBV colors. Plans have been made for constructing a good high resolution stellar library for use with an improved weighted least-squares synthesis technique developed by Peck (1981). In the mean time we have put together a stellar library covering the colors of O’Connell (1973), ultra- Fig. 3.—1RS spectra, corrected for reddening, of red irregular violet to infrared colors from Wu et al. (1980), UBVRI systems. from Johnson (1968), and ubvy,52,14,3$. The nbvy pho- tometry was taken from Crawford and Barnes (1970), technique has been used in a variety of forms by a Wood (1969), and that done by D.A.H. at Prairie Ob- number of authors in the study of nuclei of galaxies servatory. The 38,52,74 data came from Faber (1973a), (e.g., Tumrose 1976; Pritchet 1977), M82 (O’Connell Wood (with the transformation given by eq. [1]), and 1970), ellipticals (O’Connell 1976; Faber 1972), and Prairie Observatory. These colors were dereddened by other systems (e.g., binary stars; Beavers, Cook, and comparing the observed UBV colors from Blanco etal. Pick 1978). The stellar mix of a galaxy determined in (1968) to the spectral type average given by Johnson this way can be used to explore its evolutionary history (1968). The groups used were a subset of O’Connell’s through the luminosity function, ratios of different stel- and are given in Appendix B along with the lar age groups, the presence and size of a young compo- ubvy, 52,14,3$ group colors which are not collected nent, etc. and is useful in calculating parameters such as elsewhere in the literature. the mass-to-light ratio. Furthermore, and perhaps most Because the O’Connell data set is normalized to importantly, population synthesis is a means for correct- 5050 A while all other colors use V (or y, which is taken ing observed nebular hydrogen emission line strengths to be essentially the same), both sets could not be used for underlying stellar absorption which is essential if as input colors for spectral synthesis at the same time. such information as the star formation rate, excitation For the 1RS data we chose to construct models with the parameter 5007/H/?, abundances, and internal redden- O’Connell colors and then by renormalizing (described ing are to be extracted from data on composite systems. in Appendix B) predict the other colors as a check. As a Using unconstrained linear programming modeling de- further check, models were run using ubvy colors from veloped by Peck (1980), we have successfully applied the Prairie and the 1RS spectra. population synthesis to our large aperture 1RS spectra The synthesis procedure used here for each spectrum of irregular galaxies. We have adopted the attitude that was an iteration on the Hß emission flux or, equiva- imposing astrophysical constraints on the fits unduly lently, the electron temperature. A first guess at the prejudices the results, especially for systems such as correction for the underlying absorption was based on irregular galaxies which may have had unusual stellar the equivalent width of the observed HS absorp- populations; e.g., metal-poor young stellar components tion feature in the spectrum of the galaxy. Only for in which evolved massive stars are important optical NGC 2366 and NGC 5253 was [O m] X4363 reliably light contributors. In fact, it is just such anomalies in the detected, so electron temperatures for the remaining population structure that we are trying to understand. galaxies were determined using Pagel etal.’s (1979, Fig. Hence, we have computed only unconstrained models 8) relation between ([O n] 3727+ [0 m] 4959 + here. 5007)/H/l and Te. These parameters were used in the A crucial part of population synthesis is the stellar synthesis program to correct the colors for continuum library. Completeness in stellar types is very important emission (Osterbrock 1974) before finding a stellar fit to © American Astronomical Society • Provided by the NASA Astrophysics Data System 1982ApJS...49...53H ubvy colors.Hereweseethat therecanbetrade-offs O’Connell colorswerecompared withthosefittedto color ofthecompositespectrum. (3)Modelsfittedto emission linesandarenotlinear inthesensethatadding generally bluer.TheJohnsonbroad-bandcolorsarenot with anaveragecontributionof4%at5050Á,andthe the colorsoftwostarsis not quitethesameas suited tospectralsynthesisingeneralsincetheyinclude (t> —y),and0.08for(wwiththe1RScolorsbeing for (U-V),0.12(B-F),0.06(¿>y),0.18 bulk ofthelightwasascribedtoFKIIIgroups. We foundthatthemodelsdemandedan07Vgroup (2) Thepredictedcolorsfromthemodelsarecompared to 1RScolors.Wefindanaveragedeviationof0.15mag als fromthefit:(1)Wehavealsocomputed'modelsfor the globularclustersandellipticalgalaxyNGC3379. sistency ofthemodelsotherthanlookingatresidu- colors ineachmodel.Rarelydotheerrorsexceed10%. (calculated flux/observedflux-1)X100,forthedifferent literal manner.Thesesolutionsarethoseinwhichthe residuals areminimized.Othersolutions,whichthe present andshouldnotbeinterprettedinanoverly tions, buttheyhavenotbeenexploitedhere(seePeck comers ofapolyhedralconvexset,arealsovalidsolu- luminosity groupsarenotwelldetermined.Theresults are anindicationoftheroughstellartemperaturemix nants sothattherelativenumbersofstarsintwo percent contributionbyastellargrouptothetotalflux presented inFigure5.Theresultsaretheformof stellar groups,ourdatalackgoodluminositydiscrimi- at 5050Á.AlthoughweusedbothluminosityVandIII 1980). InFigure6areshownthepercentresiduals, into accountthattheemissionfeatureisbroaderthan parameters, fromtheobservedcolorat4861Á,taking which thecorrectionwentashigh0.55.Thesynthesis ble forwavelengthsredderthan3800Áandtypically hydrogen absorptionandemissioncomponentsmeant color andcontinuumemission,whichdependsonboth Hß. AnewguessfortheHßemissionfluxandelectron model thengivesapredictedstellarcolorcenteredon under 0.1magforthosebluer,exceptNGC2366 cases theproperdecrementisrecoveredeventhough ors inpassbandsnotusedthefit.Theseparationof again withthenewguesses.Convergencewasalways No. 1,1982 several ofthelinesdonotappearabovecontinuum. corrected forunderlyingabsorption,andpredictedcol- consist ofthestellarcomponentsgalaxyfittedto achieved inthreecomputerrunsorless.Theresults the colorpassband,andfittingprocedureisbegun temperature isfoundbysubtractingthepredictedstellar that theBalmerdecrementcouldbecomputed.Insome tion ofthecontinuumemissiontocolorswasnegligi- the emission-freespectrum,hydrogenemissionfluxes all colorsnotcontainingemissionlines.Thecontribu- We haveseveralwaystocheckthegeneralcon- The optimalsolutionstothespectralsynthesisare © American Astronomical Society •Provided by theNASA Astrophysics DataSystem GLOBAL PROPERTIESOFIRREGULARGALAXIES -3 3 + 2-3 A usedindeterminingthe branchingratiosweretaken plying theoldemissivityby2.121for[Sn]and0.933 portant forthesedensities. The transitionprobabilities (1976) TableIfor[On].Thisadjustmentmeansmulti- ments aremadefornewcollisionstrengthsgivenin lation ofBrocklehurst’s(1971)TableV(a).TheK Nq inthebranchingratio bwhichisrarelyim- [O ii]. Pradhan’s (1978)Table1for[Sn]andin (1970) Table11(e),andassuggestedbyTalent(1981), for densitiesgreaterthan1000cm(Kaler1978). would bedistortedandthederivedabundancesincor- A6716/À6731 indicatesA^^IO(Osterbrock1974). from Nussbaumer’s(1979) Table3for0and (3869 À)weignoredthecollisional de-excitationterm emissivity functionistabulatedinSaraphandSeaton of equation(4)inDufour’spaper.Thearecombina- rect (seeAlloin,Bergeron,andPelat1978).ForHei the advisedcorrectionsfor[Sn]areignoredandadjust- sional excitationwasignoredsinceitisonlyapplicable Peimbert (1971)wereusedhere.Acorrectionforcolli- (5876 A),theequationsofPeimbertandTorres- regions inMagellanicirregularsshowthatthisisa tion coefficientsweretakenfromTalent’s(1980)formu- shock heatingwereaproblem,theemissionspectrum Fortunately theresultsarenotverysensitivetoN.If among Hnregions,formostregionsthe[Sn] data (Table8)indicatethatthereissomevariation reasonable choicefortheaverage;althoughourown of shockheatinginthegas.Talent’s(1980)dataonHn 1RS datafollowedtheformalismoutlinedbyDufour (1975) withtheassumptionsofameansquaretempera- ture fluctuationt=0,500cm,andtheabsence kl is takenintoaccount,wefindthat,ontheaverage, luminosity classesofagiventemperaturegroup.Ifthis between adjacenttemperaturegroupsand internal extinction(seeTable6).Thisisverifiedempiri- percent contributionsoftheindividualstellartempera- with themodel,asisillustratedbycaseofdusty most asmalleffectonthecolors,duetolowaverage cally bythegenerallygoodfitsacrossBalmerdis- galaxies, weexpectthatinternalextinctionhashadat (see Tumrose1976).Inmostoftheprogramirregular kl galaxy NGC2146inthepresentdata. However, extremeextinctioncancauseseriousproblems strong featureeveninamoderatelyreddenedsystem. continuity, whichwouldappearasananomalously sitive toeffectsdueinternalinterstellarextinction ture groupsareroughlygoodto—15%. ekJ kl Hß e + + In calculating0(5007,4959Á)andNe Abundance determinationsofionizedgasfromthe For 0(3727Á),notethatafactorofNwasleftout Population synthesismodelsarealsopotentiallysen- e b) Abundances 65 1982ApJS...49...53H + + + + + + 66 Ne, Ne/H=ICF(Ne/H ), wheretheionization usual way(Dufour1975).For O,0wascalculated for À4959andA.5007separately andthenaveraged.For Garstang’s (1968)Table3forNe.Themeancollision 0 andPradhan’s(1974) Table 4forNe. taken fromEissnerandSeaton’s (1974)Table4for strengths (2(k,l)),ory(k,l)inmanypapers,were N3510-Center by2. The elementalabundances weredeterminedinthe Fig. 4.—HI21cmprofilesinJyvs.heliocentricvelocity.Note: thefluxscaleofN1012-Centershouldbemultipliedby10and © American Astronomical Society •Provided by theNASA Astrophysics DataSystem HUNTER, GALLAGHER,ANDRAUTENKRANZ + + + is within200Aoftheedge oftheReticonarrayandis is notconsideredveryreliable.BecausetheusualICF rather uncertain.However, wenotethat,exceptfor correction factor(IGF),0/0,isusedeventhoughit the fluxdeterminationof5876 isnotwelldetermined(it for Heisconsideredevenless trustworthyandbecause DDO 168andNGC3510,the valuesofHe/Hhein adjacent toNaD),aconversion ofHetowillbe Vol. 49 1982ApJS...49...53H + + + No. 1,1982GLOBALPROPERTIESOFIRREGULARGALAXIES67 here He/Hmaybeafair approximationtoHe/H. when neutralheliumis present inlargeamounts by 0/,whichrangesfrom 0.2to0.9,asisfound no correlationofHe/Hwithionization,asindicated value forHe/Hof0.1(Pageletal1979).Alsothereis (Peimbert andTorres-Peimbert 1977).Thissuggeststhat a bandbetween0.05and0.1,justbelowthecanonical derived abundances. Theitemspresented are asfollows: In Table6wegivethe1RS emission-linedataand © American Astronomical Society •Provided by theNASA Astrophysics DataSystem Fig. 4.—Continued rows (l)-(ll)presenttheobservedandreddening-cor- ratio ofHÔandHytoHß, allcorrectedforunderlying emission observedabovethe continuumdividedbythat not seenabovethecontinuum. Row(12)givestheHß rected emissionfluxesrelativetoHß.HyandHôare corrected forunderlyingabsorption, rows(13)-(14)the the observedfluxes,butYLßisvaluecorrectedfor stellar absorption, row(15)thefluxofHß inunitsof stellar absorption;adashindicates thoselinesthatwere |- 0 a TABLES H i Observations and Results American Astronomical Society •Provided bythe NASAAstrophysics DataSystem 13 - H ^ O sJT »<0 « o % -ZQ ^ I ^ X x o »5 w © X *® ffi 2o - ^ r I I I oq ^ CÔ OO o 0 o Os’ io^OsÖ — «qcqp 2 «• o ö a> Oo»o ö ?a o — : ■ oo*otctvcovor^'*o • ^r-cNoovo^'o^o^'^t .oor-r^cNcoovTi-voco‘oco • CN«o—cor'"uSvoov ; ovoo—<5enO^vq(N vorn Oco —Ov^iOOOvvo ON —^-cqoopoovqCN|Ti- 'ÖOQOOOVO —CNO'OOv OOvmcNvomoooo —OOv Tí; «o • ö—' : TABLE 5 — Continued © American Astronomical Society •Provided by theNASA Astrophysics DataSystem 13 - 01 o: Q- ^ Ie S C n r 60 v N1800 40 20 V V III 0 J ^ I I I 1 L i r i r 40 m V N1569 20 III III 0 J ^ \ L J L 0 B A F G K Stellar Spectral Type Fig. 5.—Percent contribution of stellar groups to the total galaxian light at 5050 À from the population synthesis models fitted to the 1RS data. The symbols V and III refer to luminosity classes. © American Astronomical Society • Provided by the NASA Astrophysics Data System K 00LO CTi GLOBAL PROPERTIES OF IRREGULAR GALAXIES 71 '"O a CTiCO Fig. 6.—Percent residuals of the spectral synthesis fits presented in Fig. 5 10“13 ergs s-1 cm-2, row (16) the electron temperature, them. However, valuable information can be extracted + Te, rows (17)-(20) ionic abundances relative to H , row from the density sensitive ratio [S n] 6731/6717, the (21) the ICF for Ne+ + and rows (22)-(23) the elemental abundance sensitive ratio [N n]/[S n], and the excita- abundances of Ne and O, relative to H. tion parameter 5007/Hß in cases where absorption is In Table 7 are given the results for the blue Steward not a problem. data in the same format as Table 6. However, the The Steward red Reticon data are presented in Table quality of these data is the lowest of all the Reticon data 8 where the columns are as follows: and should be interpreted cautiously. Column (1).—Galaxy identification. The red Steward Reticon data could not be analyzed Column (2).—Region observed in the galaxy. in the same detail as the 1RS data since [O n] X3121 and Column (3).—Data set: A = 1978 December, B = 1979 [O ni] X4363 are lacking. Also the correction of hydro- March, C = 1979 October, D = 1980 June, E-1978 gen lines for underlying absorption using population November. The apertures used were, respectively, 8"X synthesis would not be accurate since there are only 11 50", 275, 375, 5", and 375. O’Connell colors in this region, and Ha is not one of Column (4).— t, total integration time in minutes. © American Astronomical Society • Provided by the NASA Astrophysics Data System 1982ApJS...49...53H 4 5 2 13a a Object (23) 10O/H (22) 107Ne/H (21) ICF(Ne) (20) 10Ne^ (19) 10?0++ (18) 10?0+ (17) 1Ö.He (16) T(°K) (15) 10-F(Hß) (14) Hy/Hß (12) Hß (11) He (10) (9) (6) (4) (3) (2) (13) (8) (7) (5) (1) F(Hß)inunitsoferg/sec/cm : measurementuncertain © American Astronomical Society •Provided by theNASA Astrophysics DataSystem He+[Ne III]3970 He I4472 Hy 4340 Hô 4102 H8+He I3889 [O III]5007 [Ne III]3869 [O III]4363 [O III]4959 [O II]3727 I 5876 /Hß 4 4+ 4+ á+ l4a 11600 (20) 10O/H73.5: (18) ICF(Ne) (17) 1004.73: (16) 10068.8 (15) 10Ne (14) T(K)4300. (8) [Oin]49590.22: (7) [Oin]4363 (6) Ha4340— (5) HS101— (4) He+[Nein]3970... (3) H8+Hei3889 (13) 10F(H/?)2.66 (12) Ha/H/30.45 (11) H i/Hß 0.23 (10) H0/0.27 (9) [Ohi]50070.20: (2) [Nein]3869 (1) [On]37270.62 C obs 5.69 67.0 7.13 1.99 1569 180021462366Har2232743310Mk3535103738421444495253D168 1.39 1.78 1.3 1.82 1.37 .28: .06: a1 2 .82 .3: .43 .30 .41 .25 .10 F(Hß) inunitsofergsscm . 4.53 7800 7.17 3.16 5.58 1.20 1.05 1.79 Object nuc 2 .66 .34 .29 .10 .44 xx ... 2.14: Reticon AEmission-LineDataandAbundances 6900 8.86 5.9 7.35 2.10 1.3: 1.51 .03: .995 .21: .92 .56 .67 .65 1RS EmissionDataandAbundances 14600 6.76 8.12 2.18 1.1 1 .15 19.7 .99 .45 .22 .03 .45 .23 .23 .44 .08 .10 .13 .78 .70 .19 .77 .1 972 2.2 5.07 3.35 9200 3.05 3.03 2.65 1.66 1.39 .961 1 .1 .47 .61 .16 .32 .04 .35 .98 .02 .07 TABLEÓ TABLE? 72 3.07 5.13 2.9 9900 4.02 2.69 1.75 1.11 1 .5 .24 .59 .44 .87 .50 .94 10900. 6400.10700. bib 2nuc 1569 21462814 31.7 3.167.51 0.38 10.20.83 5.2 ...3.7 3.95 2.24 0.43 0.180.42 0.21 0.250.18 0.89 0.830.90 0.35 —0.42 0.07 0.14 —0.18 0.17: ...0.20 0.08 ...0.15 0.58 ...0.30 5.71 0.564.70 1.76 11.42.08 1.3 ...1.7 1.38 1.201.25 1.96 0.18:1.43 1.35 2.132.64 2.35 3.64 2.42 9100 3.3 3.76 3.49 1.07 10.6 14.8 1 .2 .49 .65 .19 .27 .15 .79 .01 .25 .29 3.62 9600 2.71 2.1 2.83 1.46 1 .40 5.62 1.17 1.31 11.5 .49 .83 .16 .30 .27 .09 .13 .3 4.5 5.19 2.13 3.04 2.36 42.0 9800 5.21 .343 2.3 .80 .58 .17 .25 .61 .49 .68 5.96 3.0 2.33 6.13 9100 2.43 3.50 3.05 3.63 1.17 1.8 .56 .01 .26 .44 .09 .86 .39 3.45 8800 2.67 2.6 2.22 6.27 3.62 2.83 1.40 10.7 .01: .44 .93 .09 .32 .20 .25 .91 .13 .10 .9 2.06 4.05 2.62 8600 3.3 2.82 8.74 7.46 3.35 1.23 .53 .60 .12 .06 .24 .81 .14 .9 10300 4.66 3.02 9.43 45.5 2.22 2.49 1.7 1.66 1.31 .02: .91 .43 .96 .13 .20 .37 .16 .36 .04 .13 .12 .5 6.65 6500 8.15 22.0 1.50 1.92 .39: .40: .204 .93 .48 .20 .59 .33 1982ApJS...49...53H NGC 972 NGC 1569. NGC 1012. NGC 2146 NGC 3077. NGC 2814. 4214 A1004+10. 4670 4449 Har 22 3952 3738 NGC 3274. Mrk 33 NGC 5474fromnucleus(Hodge 1966): Hiiis63"Sand4"W.NGC7292fromSN1964(Zwicky 1965): NWis28"Nand3"E. blb2, blbl.SEis8"Eoftail20"toSE, blblgal is20"toEofblbl,blb2filblb2,fil NGC 1012:SWisbrightregionjusttoofdustlane.1569 (Hodge1974,Fig.2):threeblobsfromNWtoSEareNW, 7800 7625 7292 5474 5253 NGC 7625fromnucleus(Lynds1974): NWis5"Nand3"W.NGC7800frombrightstartoNE(Hunter 1981):NEis11"Sand (Hunter 1981):Hncenis55"N and 79"W,HnUpis80"N86"W.NGC3738fromthenuc (Hunter1981):NWis11"W measurement wasmade.NGC972,measuredfromthecenter(Lynds 1974):NWis23"Nand20"W,NE16"10"W. 6" W,SWis55"Sand21"W. and 5.5"N.NGC3952fromstar to N(Hunter1981):Eis9"Sand11"E,W13"4"W.NGC 4214fromnucleus(Hodge and 5"E,NWis11"N10"W,SE21"S15"SEgal is20"toEofSE.A1004+10fromthebrightstarSE 31 mmSand24WfrombrightstartoNofgalaxyinHodge’s Fig.1.NGC2146fromthenucleus(Lynds1974):cenis0.5"S 19" N,Hii2is31"Eand91"N. NGC4670fromnucleus(Hunter1981):Eis6"E.5253:brspot isHIIregionatcenter. 1966): Scenis4"and16"E, 24"Sand20"E,cenis8"NW.NGC4449fromnuc(Hodge 1966):Hn1is8"Eand C b a Notes.—“Nuc” referstonucleus.RoughpositionsoftheH n regions.Referencesrefertotheimageonwhich C ^correctedforunderlyinghydrogenabsorption;N=notobviously present;Y=absorptionpresent. ? =notabletodetermine;xHß/Haratiowentthewrongway. r inminutes,numbersparenthesisrefertoseparateskyobservations. a © American Astronomical Society •Provided by theNASA Astrophysics DataSystem Object RegDrModeE(B-Vf (1) (2)(3)(4)(5)(6) .. HilcenB SE TAIL blblgal H iiUpB blb2fil SEgal C Brspot D H n2B Hill S cenD blbl blb2 C blb2 C blb2 blb2 NW NW NW NW C NW D NW NW D H ilD nuc nuc A SW NE nuc A nuc B nuc B nuc B nuc B nuc B cen B SW NE cen C cen D SE SE C W B fil E B E S D A A C c C C C C C C E C C C C C C B B 20 30 30 20(20) 10(10) N(W) 10(5) N(E) 10(5) N(W) 10 10(10) 10(5) N 10(5) N(W) 10(10) N(E) 10(10) N 10(10) N(E) 10(10) N(W) 10(5) N 10(10) N 15(8) N 10 10 10 10 15 10 10 10 10+20 10 10 5 5 5 4 5(5) N 2.5 5 5 5 5 5 5 5 5 5 H ilRegionObservationsandLineRatios N N N s S S S s S S S S S S S S S S S S s s s s s S S S s S s 0.14 0.52 0 0.16 0.22 0.13 0 0.11 0.73 0.33 0.07 0.14 0.46 0.05 0.82 0.11 0.19 2.06 0.31 0.81 0.06 0.08 1.03 1.29 x ? 0 ? ? ? ? x ? X ? ? ? ? ? ? ? ? ? ? ? TABLE 8 N N N N N N N N N 73 N Y Y Y N N Y N N N N N N N N N Y C C N N N N N N N N N N N Y Y Y Y C C [O in] 0.54: 0.88: 0.20: 2.99 2.91 2.75 5007 2.81 2.22 2.59 2.60 3.12 2.77: 3.24 3.22 2.11 0.972.860.39 2.01: 3.26 3.07: 4.38 2.11 0.68: 2.85 2.82 2.89 2.84 2.85 2.58 3.30 2.96 2.23 2.94 2.94 3.29 3.30 3.00 2.69: 3.01 3.13 3.13 3.05 1.14 0.79 2.870.42 0.25: 4.64 2.27 0.56 3.47 9.98; 7.52 5.71 8.07 3.77 4.12: 4.70 0.78: 4.62 2.24 2.42 3.90 3.97 3.95 5.69 3.48 3.53 3.04 1.83: 1.18 1.45 1.86 1.84 2.87 2.87 2.84 2.83 2.85 2.85 2.84 9.90: 2.87 2.88 0.49 2.90 2.87 2.85 2.86 2.87 2.84 2.86 5.24: 2.85 2.86: 2.85 2.85 2.69 1.64: 0.12 1.28 0.098 0.030 0.19 0.39 0.54 0.13 0.093 0.109: 0.050 0.11 0.12 0.110 0.14 0.089 0.17 0.25 0.19 0.069 0.15 0.094 0.094 0.081 0.12 0.093 0.12 0.43 0.095 0.87 0.81 0.87 0.87 0.73: 0.88 0.80 0.52: 5.32: 0.67 0.86: 0.85 0.72 0.95 0.84 0.67 0.86 0.74 0.93 0.89 0.50: 0.72 0.709 0.691 0.698 0.92 0.75 0.81 0.64 1.02 1.00 1.22 1.78: 1.55 1.30 1.30: 1.52 0.13 0.039 0.23 0.24 0.31 0.22 0.15: 0.26 0.047 0.55 0.22 0.18 0.24: 0.16 0.20 0.096 0.23 0.07: 0.15 0.42 0.20 0.22 0.31 0.14 0.29 0.20 0.094 0.081 0.18 0.40 90. 0.55 0.77 0.80 2.10 2.29 2.27 3.55 2.29 0.65 0.62 0.94 0.79 0.68 0.77 0.53 2.27 0.90 0.55 0.80 0.68 2.42 2.46 3.54 1.01 1.39 1.03 1.08 1.86 1.28: 1.39 1.29 1.30 1.49 1.63 1.40 1.09 21.3: 11.3 11.1 13.2 13.9 12.9: 11.5: 4.8: 4.3 6.1 2.9: 7.8 5.9 5.7 4.3 4.4 7.8 7.7: 8.03: 4.1 4.1 6.6 4.8 6.5 7.3 9.0 6.0 3.7 5.7 7.2 5.4 8.6 3.1 5.3 3.2 3.9 74 HUNTER, GALLAGHER, AND RAUTENKRANZ Vol. 49 Column (5).—Mode of observing: for S = stellar the We assume that the Hß emission flux represents the telescope was wobbled, for N = nebular a sky observa- ionization due to stars more massive than 15 MQ. To tion was made separately. estimate the number of ionizing photons emitted per Column (6).—E(B — V) due to reddening internal to star, we used Osterbrock’s (1974) Table 2.3 to find that each galaxy determined from the observed YLß/YLa approximately ratio. This could only be done for spectra not obviously underscored with absorption. The emission fluxes were log m = 0.171 »log (A^r,3) +0.495. (6) not corrected for internal reddening if a number is not 3 present here, but all fluxes in the table were corrected After solving for NeNpr{ as a function of mass, we can for galactic reddening. calculate an IMF-weighted average which is 105 67, cor- Column (7).—Appearance of absorption in the spec- responding to a mass of 29 MQ. This value is used with trum. If yes, then flux ratios with Hß and Ha are the appropriate aB, which is a function of temperature, omitted. from Osterbrock’s Table 2.1 and with his equation (2.19) Column (8).—The ratio [O m] 5007/4959. This ratio to find the average number of ionizing photons per star is included as an indicator of the quahty of the data. for each galaxy. The total number of ionizing photons Column (9).—The ratio [O m] 5007/Hß. emitted from the galaxy can be calculated from the flux Column (10).—Ha/H/L of Hß, knowing the distance and using Osterbrock’s Column (11).—[N ll] 6584/Ha. Tables 2.1 and 4.4 to scale the Hß photons to total Column (12).—[S n] 6731/6717. ionizing photons. The star formation rate, R, for masses Column (13).—[S n] 6731+6717/Ha. greater than 15 Me, is then the number of stars divided Column (14).—\3 [N n] 6584/([S n] 6731+6717). by the average lifetime of the H n regions. However, we + + Column (15).—N /S . do not know the lifetimes of the H n regions, and so Two methods involving the interactive graphics here we have used the longer time of 6X106 yr, corre- routine for fitting emission lines with Gaussians were sponding to the main-sequence phase of a 25 Mö star used to deconvolve the lines [N n] 6548-Ha-[N n]6584: (Lamb, Iben, and Howard 1976). To go now to the total 1 (1) the Unes were fitted simultaneously with three SFR \p in Mq yr“ , we must go back to the assumed Gaussians whose widths were required to be equal, or IMF. For stellar lifetimes rm less than the age of the (2) the Ha line was fitted with one Gaussian and galaxy, the number of stars of mass m is given by subtracted from the data; the remainder to the red was fitted with another Gaussian, and X6548 was taken to be n(w) = $(m)^Tm. (7) a Gaussian of the same shape but one-third maximum intensity; both [N n] lines were subtracted from the Dividing both sides of this equation by rw, integrating region, what was left was fitted as Ha, and the proce- from 15 to 50 M0, taking ÿ out of the integral, and dure iterated. This latter procedure was suggested by setting Smith (1975), but unfortunately our routine sometimes diverged, and the [N n] fits would become unduly „ rso n(m) , /rA R= -^-t-dm, (8) broad. Hence, both methods were applied to each spec- •'15 Tm trum, and the most reasonable in terms of expected instrumental widths and flux ratios of 6584/6548 was we obtain the total SFR: taken as the final result. c) Star Formation Rates A very interesting parameter is the rate of star forma- tion in these galaxies. We have estimated this from the number of massive stars found from the total number of This can then be divided by the area covered in the 25" ionizing photons and an initial mass function (IMF) aperture to obtain the SFR per unit area, which is from Miller and Scalo (1979) given by independent of distance. We can also estimate the number of massive stars 0.354 m“14 O.Km^l from the fractional contribution of the 07 V group to V 0.354 m“2 5 Km<10 > (4) light in the population synthesis models. Using an abso- lute visual magnitude of —5.3 for an 07 V star (Mihalis ^2.231 m“33 10 © American Astronomical Society • Provided by the NASA Astrophysics Data System 1982ApJS...49...53H /M 2 (18) -12 -12 7 2 25*’ Region: (3) (1) (KMfl/yr) Total: (6) (5) (4) (2) ^' (9) (7) (10) (8) (15) logM+Oíq) (13) Mu/Lj>(Mfl/Ln) (12) logHß/HI (ID (14) (16) |1 (27) lo^L^Lg) (2G Íotali (25) M^/L*(M^/Lj.) (°) (4'J^W (19) loÍJ$)._ "'exit (17) A(D-B) No. 1,1982 (23) p (22) logM*(M) (24) a(kpc) (21) T aer atr NGC 1569agreesreasonablywellwiththatfoundby include itatall.Weshallusethemoreconservative mentioned earher,whichwouldexplainwhysomemod- cause thestellarlibrarythatwasusedisdeficientinhot 0 els predicttoomuchofthe07groupandothersdonot stars andbecauseofthetrade-offsbetweengroups, are expectedtobeuncertainforspectralsynthesisbe- Israel (1980)fromradioobservations. other parametersexplainedbelow.Parametersarein throughout therestofthispaper.Theresultfor SFRs foundfromthenumberofionizingphotons which hasbeenscaleddown to25"bydividingeither log ofthenumberionizingphotonsspc;(4) log oftheSFRperunitarea,\p,inMyrpc;(3) two sets:thoseforthe25"regionobservedwith1RS, exceptions: (i)thesizeof HiextentinNGC1569is optical Holmbergareafrom Table1.Therearesome (8) theabsoluteVmagnitude, M;(9)(25"),the rected forreddening;(7)SurfacebrightnessatV,S , lines, correctedforreddening;(6)1RS(U—B),cor- rected forreddening;(5)1RS(B—V)withemission and thoseapplicabletothegalaxyasawhole. observed Himassinunits of10MfromTable3 given byK+6.73whichisthemagnitudeof1arcsec; the beamsizeifitisknown to extendbeyond10'8orthe V magnitudesynthesizedfromthe1RSspectra,cor- a0 yH I y 0 H Ipeakingfactorusedis2.0ratherthan2.5 -1 In Table9arepresentedtheseresults,alongwith For the25"region:(1)totalSFR,\p,inMyr;(2) ö Parameter 1 © American Astronomical Society •Provided by theNASA Astrophysics DataSystem 2 V MgiOO Mg) “V 7 iög^C^/yj/pc^ logN (secpc OhiCMo/pc) Sy (U-B) log T(yr) (B-V) 0 0 -17 .10 -7.35 -6.01 47.16 25 .8 11.26 9.23 7.79 17.99 9.02 -.54 8.05 1569 180021462366Har2232743310Mk3535103738421444495253DDO168 7.55 4.5 .249 .11 .43 .125 .01 .02 .28 .21 .31 .046 .45 .08 .66 -16 .70 -8.49 -7.75 45.59 20.42 13.70 8.88 8.35 9.33 15.4 GLOBAL PROPERTIESOFIRREGULARGALAXIES -.33 6.70 9.10 2.55 6.3 1.7 .030 .58 .09 .05 .10 .132 .47 .45 .18 .31 .18 -9.70 10.26 11.08 -18.29 a -8.04 45.33 -.25 8.41 9.78 20.17 13.44 1.11 3.6 6.71 9.09 4.78 3.0 .052 .010 .19 .07 .42 .18 .022 38 .42 .92 -8.74 -12.90 -1.00 46.64 -6.47 46.5 8.49 21.55 14.82 5.07 2.66 8.16 -.60 8.26 7.26 9.2 .048 .01 .047 .19 .37 .26 .88 .005 .31 .66 .99 1RS DerivedParameters -7.63 9.87 -16.91 -7 .94 45.32 8.74 22.03 8.33 15.31 9.79 .085 -.52 6.67 9.00 7.79 6.5 .103 .45 .77 .20 .23 .12 .58 2.0 .25 .33 TABLE 9 -8.95 -15.80 -7 .83 45.38 4.36 8.21 20.84 8.61 14.11 3.94 8.78 .04 -.39 8.59 7.04 5.9 .016 .036 .016 .28 .97 .78 .077 .17 .97 .41 .46 2 log ofthetimescalerinyearsneededtoexhaust width and~2.0forthosemoreextended.Thus,this peaking oftheintensityHidistributionat Tully (1975)andGallagher,Hunter,Knapp(1981), propriately correctedforadifferentchoiceofdistances. mass; (12)logoftheHß flux dividedbyHiflux,a mass, HeplusHi,istaken tobe1.34timestheHi parameter isaroughestimateoftheHimassin NGC 1569(Reakes1980)and4214(Allsopp respectively. Acrudecorrectionisalsomadeforthe Heidmann (1971)forNGC5253andfromvanWoerden, (14) Massinstarsdividedby L.Thisisestimatedfrom unitless number;(13)Himass dividedbyBluminosity current gascontentattheSFR,where region studiedwiththe1RS.(10)TheaverageHimass circular withadiameterof8';(iii)themassesfor Bosma, andMebold(1975)forNGC4449,ap- H iaretakenfromBottinelli,Gougnenheim,and from (9)dividedbytheareaofaperture.(11)The density inthe25"regionMpc~;thisismass for Hidistributionslessthanorequaltothebeam center ofthegalaxy.Fromintensitycontoursfor DDO 168andNGC1800aretakenfromFisher For NGC5253theHidistributionisassumedtobe taken fromReakes(1980);(ii)themassesandsizesof the 1RSUBVcolorsandLarson andTinsley’s(1978) found froman1RSBmagnitude withoutemissionlines; 1979), wedeterminedthatthiscorrectionfactoris2.5 y 0 10.05 -19.70 -6.76 a 46.51 9.56 5.14 11.90 9.85 18.63 1.03 .01 -.59 8.09 7.59 2.59 .881 .010 .58 .06 .69 .45 23 .28 -8.48 -17.75 -7.15 46.08 9.23 8.66 20.53 13.80 9.68 16.9 .11 -.57 8.50 7.16 8.12 8.9 1.7 .344 .15 .45 .41 .27 .56 .21 .26 .19 .05 -15.91 37.46 -9.01 -8.35 44.87 8.26 21.42 8.81 8.97 14.69 2.74 10.05 -.28 5.59 7.53 .009 .043 .05 .97 .09 .72 .97 .64 16 .43 .36 .22 -15.25 -9.10 -7.44 45.82 8.51 20.14 8.69 7.41 2.07 13.42 .01 -.48 7.80 7.89 5.5 .012 .31 .035 .13 .022 .016 .25 .91 .97 .45 .19 .07 -9 .05 -15.59 -6.92 46.36 9.27 7.63 13.07 9.71 1.26 15.3 19.80 .05 -.76 7.48 8.23 1.7 a .041 .17 .46 .31 .007 .38 17 .19 .14 .52 -8.36 -16.33 -7 .08 46.21 2.74 7.48 9.41 13.33 9.58 19.06 a .004 -.42 8.07 7.65 2.08 9.4 .028 .23 .12 .039 .11 .092 .23 .65 .97 .29 .05 -8.66 -17 .08 -6.20 47.00 9.22 7.86 6.61 12.11 9.39 18.84 .008 -.75 -.07 8.47 8.0 7.14 .350 .11 .52 .064 .66 .13 .23 .003 .97 .31 .36 -11.76 23.98 .00025 -8.75 -9.35 22.69 44.64 15.97 5.65 1.18 10.26 -.25 7.89 8.11 5.56 2.89 5.8 .11 .006 .30 .60 .70 .01 .91 .58 .34 .25 75 76 HUNTER, GALLAGHER, AND RAUTENKRANZ Vol. 49 Tables 1 and 2 in which they present UBV colors and M/Lv at various ages for model galaxies with various star formation histories. The model used was the one most closely matching both colors. The observed colors have been corrected for the shift to the blue due to low metalhcity according to Larson and Tinsley’s Figure 7. We have added 0.1 mag to B — V and JJ—B for blue galaxies and 0.1 and 0.2 mag, respectively, for red galaxies. (15) Log of mass in stars, M*, in M0 found from M*/Lv in. (14) and the 1RS Lv, using an absolute magnitude for the Sun of 4.72; (16) gas fraction parame- ter (i — Mg2LS/(Mgas + Mstars); note that our method for finding ¡i differs from the more usual approach in which Fig. 7.—log of the total SFR per blue luminosity vs. (U—B) Mgas + Mstars is assumed to be equal to the dynamical color for the 25" region observed with the 1RS. Vertical axis mass. should be shifted down by 0.29. Solid line is an eye estimate of a Total galaxy parameters: (17) A (U— B), RC2 minus best fit to the data. Dashed curve represents the trajectory an old the 1RS (U—B), is a measure of the fraction of the population would follow if more and more young stars were added galaxy involved in the current star-forming activity. to it. Unfilled circle identifies NGC 2366 here and in Figs. 8-10, 15, and 18-20. Since the galaxy was centered in the aperture by eye, the regions of highest surface brightness, and hence activity, were preferentially selected. (18) Total extrapolated -1 star formation rate, ^extr, in Mö yr ; (19) log of been estimated by many authors from star counts the galaxy-wide SFR per unit area, (^fl)extr, in -1 -2 (Ostriker, Richstone, and Thuan 1974; Tinsley 1980), Mo yr pc extrapolated from the 25" region using total Lyc photons (Smith, Biermann and Mezger 1978; (U—B), Lb, and the relation between the total SFR Mezger 1978), age-metallicity relationships and model- normalized to blue luminosity and (U—B) color found ing (Twarog 1980), and the surface density of stellar for the 25" region. The areas are found from the blue mass (Tinsley 1980, 1981). The average of the SFRs optical dimensions in the UGC or D25 in the RC2. In -9 -1 -2 found in ~ 5 X10 M0 yr pc which is comparable Figure 7 we have also plotted a curve which illustrates to the least active irregulars in this program. The SFRs the effect of adding a young component to an old of massive stars in the Magellanic Clouds (MC) by population. We have used Larson and Tinsley’s (1978) comparison have been calculated from star counts of models from their Tables 1 and 2: the old population is supergiants by Dennefeld and Tammann (1980). When represented by a decreasing SFR model with a. B — V of 7 combined with an IMF as for the rates in Table 9, this 0.95, and the young component by a 10 yr old star -1 gives 0.268 M0 yr in the LMC and 0.099 in the SMC. formation model with a B — V oí —0.20. Die curve is For diameters of 10.7 and 6.4 kpc for the LMC and constructed by connecting the tips of the vectors de- -9 -1 -2 SMC the SFRs become 3X10 MG yr pc for termined for a particular ratio of total numbers of old both, again on the low end. However, in making this stars to young. Increasing this ratio rotates the vector comparison it is important to keep in mind that our clockwise and shortens it, as is shown for models with sample has been intentionally biased toward high SFR various SFRs in Larson and Tinsley’s Figure 2. (20) The galaxies. ratio of extrapolated to 25" SFRs; (21) total mass in We might expect that the SFR would correlate with stars divided by visual luminosity in M^/Lv, de- color; the higher the SFR, the bluer the colors due to the termined as for (14), but here using total UBV colors; dominating young population. A plot of SFR against (22) log of the total mass in stars, M*, found from (21) the 1RS (U—B) and (B — V), with and without emis- and VT = BT — (B — V)T from the RC2; (23) total gas sion lines, does show this relationship. However, there is fraction, fi, where the Mgas includes all mass detected in sen average scatter in color of ~ 0.08 mag if NGC 2146 Table 5; (24) Holmberg major axis diameter in kpc; (25) is ignored. We expect some scatter due to uncertainties total Mh ! divided by blue luminosity in MQ/LQ; (26) from emission flux determinations, passband synthesis, total mass in gas and stars divided by the indicative and calibration errors and because the colors average mass, M¿, found from JT2o in Table 5; (27) log of the over a longer time scale than the emission flux. The total blue luminosity, LB, from BT in the RC2. observed scatter is about twice the expected observa- tional uncertainty. Figure 7, a plot of \p/LB versus IV. PROPERTIES (U—B) for the 1RS data, shows a tighter correlation than found for the SFR alone. Since both axes are a) The SFR normalized to blue luminosity, this is really a plot of The most striking feature in Table 9 is the wide range, SFR against U light and indicates that U, which in- a factor of 103, in SFR. The SFR for the Galaxy has cludes the region below the Balmer discontinuity where © American Astronomical Society • Provided by the NASA Astrophysics Data System K 00LO CTi No. 1, 1982 GLOBAL PROPERTIES OF IRREGULAR GALAXIES 77 istics prevail. An important task for the future will be to |-D a measure these parameters for normal galaxies. CO CTi b) The Fractional Involvement in Star Formation The parameter A(i/ —^), which is the galaxy-wide (U~ B) found in the RC2 catalog minus the (I/ — B) in the 1RS region, is in some sense a measure of the fractional involvement of the galaxy in the current star formation activity. A h(U—B) of zero means that the region sampled with the 1RS is typical of the galaxy as a whole. In Figure 9 this fractional involvement is plotted against SFR and r, the time scale to exhaust the gas at the current rate. The correlation apparent in the graph cannot be due to emission lines contaminating U and B since any emission contribution present in the 1RS colors would also be present in the total colors. The bottommost point is NGC 2146, a dusty galaxy sitting in a very extensive cloud of H i (Fisher and Tully 1976). A galaxy undergoing a true global burst of star forma- tion would occupy the region of the graphs near A(£/ — B) of zero, i.e., the region observed would be typical of the galaxy as a whole, and have both low r and high Fig. 8.—log of SFR per unit area plotted against (a) the log of the hydrogen mass per blue luminosity (the vertical axis should be SFR. Such a galaxy is NGC 1569, which may be an shifted down by 0.29), (b) the natural log of the inverse of the gas example of a true global burst. Other data (Hunter fraction (gas mass per total mass), and (c) the log of the surface 1981; de Vaucouleurs, de Vaucouleurs, and Pence 1974) H I density. All parameters refer to the 25" region observed with show that this galaxy even has the appearance of a the 1RS. burst, with H n regions distributed in a ring around a central stellar core, rather like the flames in a brushfire which are propagating outward. hot stars dominate, is much more sensitive to the current One can see that in most galaxies the more localized SFR than B. That the colors alone are not adequate is the star-forming region, the higher the SFR and, conse- also due to the effects of the underlying older popula- quently, the shorter the time that will be needed to tion. Increasing the SFR against a given background of exhaust the gas. This suggests that as the gas in a stars will cause the total color to become bluer, but a particular region becomes exhausted, the star formation given SFR against an increasingly younger background activity decreases in that region and other unused or will do the same. Thus, the colors are a folding together replenished areas take over as the center of activity of the current SFR with the entire evolutionary history of the region, including the history of the SFR, IMF, enrichment in metals, and subsequent mixing in the interstellar medium (ISM). Furthermore, the path to any given point on the UBV color-color diagram is not unique (Huchra 19776). With what other properties might the SFR correlate? One would expect a correspondence between gas density or gas fraction (see, e.g., Madore, van den Bergh, and Rogstad 1974). The higher the mean volume density in the interstellar medium, the higher the SFR should be. Likewise, the more gas a galaxy has, the more material it has for forming stars and the fewer stars it has formed in the past. In Figure Sa-c are plotted aH , (if scale heights are similar, then surface density is a reasonable indicator of volume density), ln(l//i), and MH j/Lg against SFR. The H i masses are total masses scaled to Fig. 9.—Difference in (U—B) color between the total galaxy the 1RS region. The lack of any relationship indicates and the 25" region observed with the 1RS plotted against the logs that global gas characteristics are not dominant in the of ( a) the time scale to exhaust the gas at the current rate and (b) star formation process and suggests that local character- the SFR per unit area. The SFR refer to the 25" region only. © American Astronomical Society • Provided by the NASA Astrophysics Data System 78 HUNTER, GALLAGHER, AND RAUTENKRANZ Vol. 49 (Gerola and Seiden 1978). Hence, the region of activity moves around the galaxy giving it the appearance of a stellar “mudpot.” The random bubbling in a geological mudpot is caused by escaping steam in active thermal regions, while in irregular galaxies it is caused by the escaping energy due to the percolation of star formation (Gerola, Seiden, and Schulman 1980). That the star-for- ming regions do shift in irregular galaxies has been shown for the LMC by Payne-Gaposchkin (1974) and for IC 1613 and NGC 6822 by Hodge (1980). However, an important question is to what extent is the “bub- bling” in galaxies actually random, or is it propagating because of the influence of neighboring bubbles? Why does the SFR increase as the fractional involve- ment decreases? We would expect the SFR per unit area of a given star-forming region to decrease as more and more inactive surroundings (which are not contributing Fig. 10.—Abundance of oxygen relative to hydrogen vs. log of to the FLß flux) were included in the sampling. In this SFR per unit area for the 1RS observations. context h(U— B) corresponds to a filling factor in the observed region. The maximum SFR per unit area would increasing SFR. This suggests that the evolved massive be measured for a filling factor of 1. A correlation of stars associated with the current star-forming activity SFR per unit area with Ä(U— B) would be expected if are dominating the cool stellar population. all the galaxies had the same total SFR but different filling factors. If there is also a characteristic spatial c) The Metallicity scale of the star-forming regions, we would expect Figure 10 shows a plot of O/H versus SFR. The h(U— B) to increase with decreasing linear dimension, values for solar, Orion, and the LMC and SMC are which is not the case for these data. Perhaps Figure 9 is taken from Kaler (1980) and Smith (1975). Most of the a reflection of differences in local characteristics. If a galaxies have a metallicity typical of the LMC, regard- particular galaxy, for example, has a very clumpy inter- less of the SFR, although a few of the less active galaxies stellar medium, those clumps could be very efficient at have solar-like metallicities. The fact that the least active turning themselves into stars, but if the clumps are irregular, DDO 168, has such a high O/H could mean somewhat dispersed, it might be more difficult for the that we are catching it in a postactive phase since its on-going star formation to induce star formation in current SFR is too low to provide such an enrichment in other clumps and hence spread the activity quickly to a a stochastic star-forming process if it represents the larger region. Again, this SFR was determined from the mean of the past activity. massive stars, so that the current star formation ob- The low metal content of the gas in combination with served does not remain local as a result of a lack of the high SFR which means stars should be rapidly sources of energy input. Information on local character- enriching the ISM indicates one of the following: (1) the istics of the irregulars may shed light on this problem. current star-forming activity is unusual in the history of A look at the spectra in Figure 2 shows that, except the galaxy, i.e., a burst; (2) the region of activity moves for NGC 2366 which is a low luminosity galaxy around the galaxy like bubbles in a “mudpot” so that dominated by one very big H u region, the irregular any one spot is not allowed to build up metals; (3) the galaxies have absorption features of cool stars, such as gas is being continually diluted by infall of unprocessed Ca il H and K, regardless of the SFR and the fractional material; or (4) the IMF is unusually skewed toward involvement in the star formation. This could be due to luminous high mass stars (cf. Huchra 19776) so that, the massive supergiants undergoing core He burning although the enrichment rate is high and the star-form- (Brunish and Truran 1981), but an older underlying ing activity is quite visible, gas is not locked up in low stellar population cannot be ruled out. For example, in mass stars, leaving a large pool of unprocessed material the LMC Payne-Gaposchkin (1974) finds that an old in which to continually dilute the returned metals. The component is indeed present. This could mean that metallicity of the gas is therefore an important con- galaxies actively producing stars have done so in those straint on evolutionary modeling. regions before. However, video camera images (Hunter We can learn something about the metallicity of the 1981) through b, y, and /, which are free of emission stars from the photometric data in Table 2. Figures 11 lines, and through Ha show that the same regions stand to 13 show plots of the Mg index against (t> —y)0, out in all of these passbands. Furthermore, the surface (6—74)0, and Q, the reddening-free parameter defined brightness at V (without emission lines) increases with in § lia. A number of comparison objects are plotted © American Astronomical Society • Provided by the NASA Astrophysics Data System No. 1,1982 GLOBAL PROPERTIES OF IRREGULAR GALAXIES 79 x Irregular Galaxies • Normal Galaxies o Globular Clusters x XD * xM82 x . o x2146 -01 x x972 2968 x -0.2- J I ^ L J L -0.30.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 (v-y)0 Fig. 11.—Mg index plotted against (t? —^)0 for objects ob- Fig. 12.—Mg index plotted against 74)0 for the same served at Prairie Observatory. objects as in Fig. 11. also. One can see, first of all, that the Mg index does not have a large dynamic range; M 31 differs from globular clusters by at most 0.25 mag. Second, the graphs of (v~y)o (^-74)0 are very similar. Mg versus (u — b)0 appears essentially the same as (v — y)0i but the uncertainties in the ultraviolet observations are much greater than for (t> — y)0. Increasing the baseline alone only increases the problem of reddening. The dustier galaxies are labeled in these figures. In order to get around the reddening problem we have used the param- eter Q. In going from Figure 12 to Figure 13, we see that the dusty galaxies move to the left. In Figure 13 the irregular galaxies he in the region occupied by the globular clusters. However, we should correct the Mg index first for contamination by the younger stars, par- ticularly giants and supergiants. O’Connell’s (1973) Fig- ure 5 of Mg versus a red passband color for the stars in Fig. 13.—Mg index vs. ß, a reddening-free parameter. Notice that the dusty galaxies identified in Figs. 11 and 12 have shifted to his library shows that the evolved hot stars will not the left here. See text for an explanation of the vector. affect Mg very much. We have used the population synthesis models for galaxies observed with the 1RS to dances of these two elements can serve as clues to the determine the Mg and Q indices for the full model and evolutionary history of a galaxy (similar to N/O; for the portion of the model cooler than F. The vector Edmunds and Pagel 1978). Suppose, for example, that drawn in Figure 13 is the average difference between the one follows the relative abundances of N and S as a indices predicted by only cool stars and those predicted stellar population with a normal IMF ages. At the by the full model. The length of the vector is highly beginning the massive stars which evolve faster would uncertain, but the effect is to move a point along the dominate the enrichment process, and N and S would observed sequence toward higher metallicity. What we increase together. Later, however, the lower mass stars can say from all of this is that the stars in these would begin making their contribution, and N, whether irregulars are neither extremely metal poor nor rich. primary or secondary, would rise more quickly than S because there are so many more low mass- stars d) [Nii]/[Sii] (Dearborn, Tinsley, and Schram 1978; Iben and Truran The element S is produced only in massive stars, 1978; Becker and Iben 1980). while N is produced in all stars. The massive stars have In Figure 14 are plotted [N 11] against [S 11] for H 11 the maximum S/N production ratio. For example, a regions in different galaxies. If other authors’ (Talent 25 Mq star produces S and N in the ratio 35:5 (Weaver, 1980; French 1980; Lequeux etal. 1979; Kinman and Zimmerman, and Woosley 1978). As a result, the abun- Davidson 1981) data for irregulars are also plotted in © American Astronomical Society • Provided by the NASA Astrophysics Data System 1982ApJS...49...53H + 80 who hasshownthatforplanetarynebulaethereisa The ratioN/SisapproximatelyproportionaltoN/S perature. However,Kaleralsoshowsacorrelationof correlate withotherevolutionaryindicators,suchas (Peimbert 1979),andsotheratio[Nn]/[Sn]should Figure 14,thepointsfallinregionalreadyoccupied. Jensen etalhaveindeedfoundacorrelationbetween metallicity relationshipissuggestedbyKaler(1978), galaxies observedatStewardObservatory. (6717-31 À),normalizedtoHßforHIIregionsinirregular correlation between5007/Hßandthecentralstartem- O/H forthe1RSdataandobtainexpectedcorrela- O/H, oralternatively[Oni]/HjS(Jensen,Strom,and tion (seealsoPageletal1979).Thatthismightnotbea Strom 1976).InFigure15weplot5007/Hßagainst [N n]/[Sii]and[Oin]/Hß.Figure16showsourdata temperature with3869/Hß,whichwedonotseehere. irregulars andcompactgalaxies observed byLequeuxetal.(1979),theMagellanicCloudHn regions observedbyDufour(1975),Hii regions inMagellanicirregularsobserved byTalent(1980),andtheemissiongalaxiesofFrench(1980). Pointsinparenthesisareuncertain. Fig. 14.—Nitrogen(6548-84A)emissionfluxvs.sulfur o Fig. 16.—Ratioofnitrogento sulfur emissionfluxagainst5007/HßfortheHnregionsobserved atStewardObservatory,blue © American Astronomical Society •Provided by theNASA Astrophysics DataSystem if) Op? 18 L- HUNTER, GALLAGHER,ANDRAUTENKRANZ 3.0 3.4 2.6 06 14 ID (x) (x) 1*" □ X □ x X •* Pi*n, A* h Qlxo x 4 56 5007/Hß T + a Dufour(1975) o Lequeuxetal.(1979) □ French(1980) x ThisPaper • Talent(1980) to the25"regionobservedwith1RS. the oxygenabundancerelativetohydrogen.Bothparametersrefer plot oftheaverageN/Sforagivengalaxyagainst lagher andHunter1981). A discussionofN/Sasanevolutionaryparameterand dent andcanvarybyuptofactorsof4.Figure17isa only forlow5007/Hß(highO/H).However,[O in]/Hß lessthan~3,theionizationcorrectionfactors comparison withdatawillbepresentedelsewhere(Gal- O/H forthoseobjectscommontobothTables6and8. to change[Nn]/[Sn]intoN/Sarequitemodeldepen- [N n]/[Sii]remainsroughlyconstant,turningupward as wellthatofotherauthors,andwefind Tully 1976)andNGC4449(vanWoerden,Bosma, moderately strongdetections.NGC2146(Fisherand optical centerofthegalaxiesandhalfthesewere mapped weredetectedatpositionscentered10'fromthe and kinematics.First,weseethat~60%ofthegalaxies something abouttheglobalpropertiesofHimass T Fig. 15.—Theexcitationparameter,5007/H/?,plottedagainst From theHi21cmprofilesinFigure4wecanlearn e) ExtentandKinematicsoftheHydrogenGas T T 10 Vol. 49 1982ApJS...49...53H -1 region observedwiththe1RS. entire galaxy(circledpointsarerotating galaxies)and(b)the25" against thenaturallogofinverse ofthegasfractionfor(a) Rouble peakstypicalofarotatingdisk,andtherestare I Ofthegalaxiesdetected,one-thirdhavesinglepeak pjoundaries. praction havehydrogenextendingbeyondtheiroptical The othersarelessspectacular,but,nevertheless,afair irogen extendedto6and10timestheopticaldiameter. fMebold 1974)arewell-knownextremeexamplesofhy- !¡1979) areincludedalso.Errorbarsthestandarddeviationsin in between.Therotationvelocitiesarealsoratherlow: group. Emission-linestudieshaveshownthatthestellar over halfhaveafullwidthat20%intensityoflessthan Igainst the1RSoxygenabundance.DatafromLequeuxetal. emission profiles,alittleoverone-thirdhavetheusual 2 150 kms.Allofthesingle-peakedprofilesareinthis ¡¡he averages. i >10- aiitrogen tosulfurfortheHnregionsineachgalaxyplotted r £ 2- ?-+° 1 f« 91- Fig. 18.—1RSoxygenabundance relativetohydrogenplotted c’ |o. 1,1982 i-3h Fig. 17.—Averageoftheratioionicabundances © American Astronomical Society •Provided by theNASA Astrophysics DataSystem 4 - 0 - 1- 1 - 11- 12- 13- 14- Ql 1 2345 6 789 o° (b) (a) i 1r J x_ £ x X ? 4 4 10 0/H" 25 100/H" 25 GLOBAL PROPERTIESOFIRREGULARGALAXIES o Lequeuxetal.(1979) x ThisPaper a Lequeuxetal.(1979)_ x ThisPaper "T -1 -1 figure remainsthesame.One canseeinFigure18athat colors fromHuebra1977a, RC2,andGordon objects inthemannerdescribed in§IIIc(someUBV In (l//i),andincludedarethedatafromLequeuxetal the pointstranslateup—1 in In,butthenatureof Gottesman 1981).Iftheirvalues oftotalmassareused, (1979) forfourirregularandbluecompactgalaxiesplus the LMCandSMC.We have computed¡xfortheir predictions oftheclosedsystemmodel,galaxiesasa group indicatethatsuchamodelisnotentirelycorrect. metallicity ofanyonegalaxymaybeconsistentwiththe galaxies inthisprogramshowsnocorrelation.Whilethe into stars,themetallicitywillriselogarithmically.Aplot of the1RSln(l/ja)againstO/H(Fig.186)for by Lequeuxetal.(1979).Thispredictsthatasthegas locked upinstars,isempiricallydeterminedtobe0.004 fraction decreases,i.e.,moreandgasisconverted element yieldfactordefinedasejectedmassto metallicity ofthegasZ=y\n(1/jit),wherey,heavy chemical evolutionmodel(SearleandSargent1972).In cycling approximationprovidesthesimplestgalaxian would beconsistentwiththedisturbedopticalstructure of thisgalaxy. Nevertheless, fromthepropertiesofhydrogengas this modelthegasisturnedintostarssuchthat alone aninteractioncannotberuledoutandindeed but noneisapparent.Thenearestgalaxyinprojected DDO 39whichisataprojecteddistanceof1.3Mpc. 670 kms.Thenearestgalaxyinvelocityspaceis width at20%intensityis760kms,andtheedgesare distance isNGC2146A,butthevelocitydifference very broadprofilewiththreecomponents.Thetotal likely tobeofthisorigin.However,NGC2146hasa somewhat sloped.Thisissuggestiveofaninteraction, be evidenceforongoingtidalinteractionsbetweengal- presence oflargevelocitywingsonthefineprofilescan g 7625 areseentohaveverysmallwingswhichnot axies. InoursampleNGC1012,3310,and der Kruit1976),NGC4214(Allsopp1979),and motions: NGC3310(WalkerandChincarini1967;van 5253 (WelchandWallerstein1969). (Allsopp 1979).Othersshowpecuharitiesornoncircular NGC 1800(Gallagher,Hunter,andKnapp1981), NGC 2366(GoadandRoberts1981;Cheriguene1974), turn over:NGC1569(Reakes1980)and4214 2146, NGC4214(Cheriguene1974),andothers(Hunter systems inirregularsalsohaveverylittlerotation: 1981). Plotted inFigure18aisO/Hagainstthegalaxy-wide The closedsystemmodelwiththeinstantaneousre- Gallagher, Knapp,andFaber(1981)suggestthatthe A fewoftherotationcurveswhichshowalso V. COMPARISONWITHMODELS a) TheClosedSystemModel 81 1982ApJS...49...53H be thateventhesecombineddatasufferfromthesame lack ofsufficientbaselineasdidLequeuxetal.’salone. 82 yM^/M. UsingZ=O/HfromTable6,M*=^ of whatmassestouseincomputing/i;i.e.,isthe galactic Hnregions.Furthermore,thereisthequestion for thecombineddatathereisnocorrelation.Itcould uncertainty associatedwiththedeterminationof really behaveasaclosedsystem?Thereisanintrinsic However, Peimbert(1979)alsofindsnocorrelationfor percolation process;i.e.,theprobabilityofanactivecell have usedparametersapplicabletothe1RSregion,but widely betweenburstandquiescentlevels.However,the high SFRstate,buttheinstantaneousfluctuates primarily ontheratioofgalaxysizeto irregular galaxieswithvaryingpropertieswhichdepend igniting aneighborislessthan1.Thephysicalmecha- galaxies withlowrotationalvelocitiesandnoobvious involved inthemixingofmetals.Wemightexpect if thegalaxyisdrawinggasfromohtsideofactive dynamical andobservedmassesiswellknown.Inaddi- true massofthegalaxyandoverwhatregiondoesit whole galaxybecausethere aresofewcells.Larger rotation (Gerola,Seiden,andSchulman1980)produce massive starsthroughexpansionofHnregions,winds, nism forthisistheenergydumpedbackintoISMby from onecelltoaneighboringviastochastic for explainingthepropagationofstarformationin (SSPSF) ofGerolaandSeiden(1978)isveryattractive region, thenclearlyfimustincludetheentireregion form ofmoleculargas.IncomputingfiinTable9we total massofanygalaxy,andthediscrepancybetween metallicity, andlowerfractional gasmass.Whilesome Supernovae explosions,etc.Modelswithnodifferential spiral arms.Inthismodelstarformationpropagates tion, wedonotknowhowmuchmattermaybeinthe depletion bythe starformationprocess,other cellshave regions ofthegalaxyarealways exhaustedbecauseof dwarf witharelativelysmoothlydistributedoldpopula- star-forming cells.Asmallergalaxyspendslesstimeina galaxies withdouble-peakedHiprofilesarecircled.One systems wouldexhibitamore constantandhighermean average SFRislow.Mostoftenitwouldappearasa we findthattheenrichmenttimescaleiscomparableto can seeatrendtowardseparationofthetwogroups. that havenoappreciablerotation.InFigure18«those that rotatinggalaxieswouldmixdifferentlythanthose SFR, morestructurein the activeregions,higher tion, butwhenonecellrandomlyturnson,itignitesthe and totalMfromTable9,y=0.002foroxygen, ating theequationforZabovetoobtain= current SFR(Table9)fortheirregulars. gasextr the timescalertoexhaustcurrentgascontentat gas g The stochasticself-propagatingstarformationmodel An interestingtimescalecanbederivedbydifferenti- ce) American Astronomical Society •Provided by theNASA Astrophysics DataSystem b) PercolationTheory HUNTER, GALLAGHER,ANDRAUTENKRANZ galaxy-wide gasfraction,(b)the1RSoxygenabundance,and(c) lar structures,butcoveringarangeinSFR,canbetest had timetoreplenish,andsostarformationisalways from therelationinFig.7. the logofSFRperunitareaextrapolatedtoentiregalaxy high fortheirsize.Alsoitis possiblethatwepreferen- in §IIIc.Noclearcorrelation isseenforanyofthese is plottedagainsttheSFR, O/H,andtotal/ITGas volvement inOBstarproduction(Fig.9),andthelack will fluctuatewidelyforthesmallergalaxies. be thebestparameterssincetheydependonSFR moves around.Therefore,acomparisonoftheevolu- parameters. Somescatteris expected, butnotethatthere of thistheory.Theabundanceandgasfractionshould going onsomewhereinthegalaxy,andactivity Figure 19a-c,wheretheblueopticalmajoraxisinkpc not asfavorable.Predictedsizecorrelationsaretestedin (Figs. 8,10)tendtobequalitativelyconsistentwiththe of correlationsSFRwithglobalgalaxianproperties fractions forobjectsfromother authorsarecomputedas the correlationofSFRwithdecreasingfractionalin- ters, suchasMj/Ly,colors,andinstantaneousSFR, averaged overthelifetimeofgalaxy.Otherparame- tionary statusofgalaxieshavingsimilarsizesandirregu- are somegalaxieswithgas fractions thataremuchtoo SSPSF theory.However,quantitativecomparisonsare h 2 Fig. 19.—Blueopticalmajoraxisplottedagainst(a)the The largerangeinSFRamongirregulars(Table9), ^0.4 u_ o?-9 CO -2-10 I Q- P cn £5 O 4 X io CM b c 0.6 0.8 02 8 7 3 2 1 0 10203040 Blue OpticalMajorAxis(kpc) Vol. 49 No. 1,1982 GLOBAL PROPERTIES OF IRREGULAR GALAXIES 83 tially select galaxies near the star formation peak in a burst or oscillation (Seiden, Schulman, and Feitzinger x This Paper 1981). o Lequeux et al. (1979) A Kinman and Although no quantitative evidence here bears the Davidson (1981) SSPSF model out, these data are global in nature, and o □ French (1981) the percolation theory is inherently a phenomenon 4 s dominated by local conditions. That star-induced star formation is a viable process is known from observa- 2 - □ tions of OB associations in our own Galaxy (e.g., Lada, D a&< Blitz, and Elmegreen 1978; Herbst and Assousa 1978) 0 .xÊE where we can clearly see evidence of causal relation- 9 10 11 log (Mgas+M*)T ships. Perhaps in irregulars, which have no global mech- anism to stimulate star formation like the density waves Fig. 20.—O/H vs. the log of the total mass of a galaxy of spirals, the star formation can propagate readily within a cell but not between cells. Then the large scale pattern of star formation would be purely stochastic. A I ^ I study is currently underway to investigate local char- x This Paper □ Lequeux et al. (1979) acteristics of these irregular galaxies (Hunter 1981), and A Kinman and perhaps that will prove a better test of the applicability ^ Davidson (1981) of the percolation process. A x □ c) Gas Infall □ Gas infall from a halo of unprocessed gas has some- □ B times been invoked (see references in Chiosi 1980) as a A A □ means for supplying fuel for star formation and at the same time for diluting its by-products. Chiosi (1981) has A suggested that a galaxy itself may act as its own pool of 10 11 12 hydrogen and gas infall from the inactive outer optical log M¡ (Mo) regions (not the invisible halo which could extend much Fig. 21.—log O/H plotted against the log of the dynamical further out) toward the center may keep the center alive. mass of a galaxy. This model predicts that initially a galaxy is a loose “puddle” of gas with a low SFR per unit gas mass. As gas becomes locked up in stars, the outlying gas falls toward the center. The galaxy becomes denser, and the objects which is certainly arbitrary. These two graphs do efficiency of star formation increases. Finally a point is show that any correlation between metallicity and reached where the exhaustion of the gas pool becomes galaxian mass established for low mass galaxies does not important, and the SFR efficiency again decreases. Un- extend to the more massive systems. This is what one til that point the metallicity of the system has been kept might expect, since smaller galaxies probably cannot roughly low and constant, but with the loss of the support sustained star formation (Gerola, Seiden, and dilution factor the metal content would increase rapidly Schulman 1980). Figure \9b is another statement of this. as the already formed stars evolved. While none of the properties of the program irregulars The bright star-forming regions of these irregulars are are inconsistent with the infall model, none uniquely somewhat centrally concentrated with respect to the point to it. Further observations of stellar and gas extent of the blue continuum (Hunter 1981). The clump- density distributions and gas motions are necessary. ing of the galaxies around a metallicity like that of the LMC regardless of SFR, as in Figure 10, and the low VI. SUMMARY N/S ratio are also consistent with this picture, although We have presented optical and radio data on global the details depend on the infall rates. Inconsistent with properties of noninteracting irregular galaxies and have this model would be an increase in metallicity with total found the following: mass of a galaxy as proposed by Lequeux etal. (\919) 1. These galaxies are generally blue, but not ex- and Kinman and Davidson (1981). Plotted in Figure 20 tremely so, and thus are different from the extragalactic is O/H against total mass (M* calculated as in § IIIc). H ii regions (Searle and Sargent 1972) in which the If O/H is plotted instead against dynamical mass, as in young OB stellar population is overwhelmingly domi- Figure 21, only the addition of the data of Kinman and nant. Davidson at low mass suggests a trend, but these authors 2. Many of the galaxies do not show the classical assumed an inchnation angle of 60° for all of their two-peaked H121 cm profile, and the rotation velocities © American Astronomical Society • Provided by the NASA Astrophysics Data System 1982ApJS...49...53H 3 84 low ascomparedtomostspirals. inferred fromthewidthsofprofilesarerelatively very high(upto~10timesthemeanvalueforsolar neighborhood) overanextensiveregion(>1kpc).Thus, rotating galaxiescanbequiteeffective. even withoutspiralarmsortheaidofanexternal rather thanglobal,characteristicsarewhatgovernthe gas orabundanceparameters.Thismayimplythatlocal, stimulons fromcollisions,thestarformationinslowly probably becausethecolorsfoldtogethercurrent star formationprocessinthesegalaxies. region. star formationactivitywiththepasthistoryinthat have beenasactivetheyarenowforverylong,or prise onlyasmallfractionoftheirregulargalaxies the MagellanicClouds.CoupledwithhighSFRs, Of thesepossibilitieswefavortimevariationsofthe this saysthattheobservedstar-formingregionscannot (de Vaucouleurs,deandButa1981;Fisher they donotformaclosedsystem,ortheIMFisunusual. and Tully1980). SFR sinceblue,highsurfacebrightnessgalaxiescom- hour anglesforeachfilterobservation ofthegalaxy,comparisonstar,andsky.We analyzethisdatausingweighted in theobservedregion.Thesefeaturesareprobablydue averaging andpropagation of uncertainties.Therawdataisfirstcorrectedforextinction, dividedbytheintegration current star-formingactivitydecreaseswithdecreasing plicate effortstodeducethedetailedstellarpopulation indicate thatthegalaxies,orregionsofcurrentstar-for- nomical mudpot. rate. Thissuggeststhatthestarformationactivitymoves by Hunter(1981). brightness andB~VcolordescribedbyGallagher, from integratedspectra(e.g.,thiseffectmayberespon- stars inthespectraregardlessofstrengthSFR ming activity,arelessevolvedthantypicallocationsin creasing SFR.Thisisnotunderstoodatpresent. around thegalaxy,givingitappearanceofanastro- time scalenecessarytoexhaustthegasatcurrent optical colorsofhighstarformationrateregionsisgiven Hunter, andKnapp1981).Afurtherdiscussionofthe to evolvedmassivestars,whichwillconsiderablycom- sible forthelackofcorrelationbetweenbluesurface 4. ThereisnocorrelationbetweentheSFRandglobal 3. ThereisalargerangeintheSFR,anditcanbe 5. ThecorrelationofSFRwithcolorsisratherloose, 6. Themetallicitiesaregenerallylow,comparableto The rawphotometricdataobtained atPrairieObservatoryareintheformoftotal counts,integrationtimes,and 7. Thereareabsorptionfeaturescharacteristicofcool 8. Thefractionalinvolvementofthegalaxyin 9. Thefractionalinvolvementalsodecreaseswithin- 10. Fortheirregulars,lowN/SandO/Hratios © American Astronomical Society •Provided by theNASA Astrophysics DataSystem STATISTICAL ANALYSIS OFFILTERPHOTOMETRICDATA HUNTER, GALLAGHER,ANDRAUTENKRANZ APPENDIX A but mayalsobeduetodifficultiesinestimatingthej processed gasistakingplace. ¡i betweenrapidlyandslowlyrotatinggalaxies. portant comesfromanapparentseparationofeffectivej reservoir. Ahintthatthislattereffectisindeedim-j reflection ofuncertaintiesinthestellarandgasmassesj degree towhichmetalsaremixedintotheoverallgasJ simple, closedsystemmodel.Thismay,inpart,bea spiral disksorthatmixingfromareservoirofun-j dictiohs fromthreeglobalmodelsofgalaxianevolution:i formation model,andthegasinfallmodel.Theobserva-i Jor asummerresearchassistantshiptoD.A.H.,com- the closedsystemmodel,stochasticstar-inducedstari nature. Furtherworkonthisproblemisobviouslyneces- for star-inducedstarformationbetweencells,the| the percolatingstarformationmodelmaynotbecorrectj tions seemtoagreebestwiththegasinfallmodel.Whilej formation processinirregularsmaywellbestochastic| National RadioAstronomyObservatoryforgenerousj National Observatory,Stewardandthej sary. j with theobservations.WewouldparticularlyliketoI assistants oftheseinstitutionsfortheirinvaluablehelpj amounts ofobservingtimeandthestaffsj reducing the1RSdata,KeithHegeforhelpwith! his thesis.WealsothankJimKaler,RichardKron,and thank JeanetteBarnesforheraidinobtainingandj Jim Truranforusefuldiscussionsandcommentsonan Greenbank, EdOlsonandSandraFaberforuseoftheir; the StewardReticon,AllanWirthforassistanceati filters, andDavidTalentforaprepublicationcopyof participants, andespeciallySandraFaberPhil was completedandwouldalsoliketothankmanyofthe early draftofthispaper.D.A.H.andJ.S.G.aregrateful viewing thispaper.Thisworkwassupportedinpartby a stimulatingenvironmentinwhichpartofthiswork to the1981AspenAstrophysicsWorkshopforproviding to thankananonymousrefereeforconstructivelyre- Seiden, fortheircommentsonthisproject.Wealsowish happy tothanktheUniversityofIllinoisResearchBoard puter funds,andanimproveddrivesystemforthe Foundation totheUniversityofIllinois,andweare grant NSFAST79-09977fromtheNationalScience Prairie Observatorytelescope. 11. Thechemicalabundancesarenotwellfittedbyaj We havebrieflycomparedthesepropertieswithpre-i We wishtothanktheUniversityofIllinois,KittPeak Vol.49 î 1982ApJS...49...53H integration time,containingobservationswehave galaxy’s sky,star,andstar’ssky—thecountsforagiven filteraregroupedbyintegrationtime.Soforeacht,kth magnitudes areformedimmediately,andthesandwiches arethenaveragedasdescribedabove. where 7}isthetransformationcoefficientandZzero-point constantforthey'thcolor. where nisthenumberofsandwiches.Finallymagnitudesareformedsothat k -1 calculated by y where yreferstotheythcolor.Thecolorsfromallofthesandwichesareaveragedtogether,anda“real”uncertainty is where ireferstotheithfilterandonly2isanexponent.Thencolors,Lj,fornmthfiltersareformed: For eachsandwich,thegalaxyisratioedtoaveragestar: The twobracketingstarsegmentsineachsandwichareaveragedtoprovideasimpletimeaverage: sequence. Foreachsegment,then,aweighted-meancountrateanditsassociatedweightarecomputed: is thoughtofasconsistingaseries“sandwiches”composedgalaxysegmentbetweentwostarsegmentsthe where tistheintegrationtimeins.Thesequenceofgalaxyandcomparisonstarobservations,star-galaxy-star-galaxy, has aweightw,whichisproportionaltotheintegrationtime,sothat No. 1,1982GLOBALPROPERTIESOFIRREGULARGALAXIES85 time, andsky-subtractedtoprovideaseriesofcountssinthedifferentfilters.Forgivenfiltereachobservation,C, We alsocomputeanuncertaintyaduetophotonstatistics asfollows.Foreachofthefour“objects”—galaxy, One exceptiontotheaboveprocessisincomputingV.In thiscaseinsteadofformingaratio,foreachsandwichthe p © American Astronomical Society •Provided by theNASA Astrophysics DataSystem m =T{-2.5logL)+Z,a7/1.086^,( JrA7 C* L,= — _ Ri C*,i +C*; c* C= l/w= 1/1+1/<(sky),(Al) 2>vC KW n 2 —n k ’ k w* 1 r rr WR‘ («-1)2^ m 1/2 w 1 *,1 » .J- + K W„R w Rj}2 *,2 1/2 4* (A9) (A8) (A6) (A5) (A3) (A4) (A2) 1982ApJS...49...53H 2 2 2 Then forthegalaxyandstaranuncertaintycanbecomputed: 86 HUNTER,GALLAGHER,ANDRAUTENKRANZ The uncertaintyintheratioofgalaxytostarfor/thfilteris where For colors, and magnitudes, colors. , deviation arecomputed.Therealandphotonuncertaintiespropagatedasusualinformingthemagnitude thenj Each observationisassignedaweightasabove,thentheweightedaverageofallcountsforgivenfilterandstandard j represents themiddleofgroup. colors ofthecomparisonstarareaddedtodifferentialgalaxywithafinaluncertaintywhichis=o +a/. j adding sandwiches)andthatforthetrueuncertaintyinamean(l/a=S1/a,).Thehighestischosen.Finally, theI mean. Theuncertaintyofthismeanisfoundintwoways:thatforaveragingaseriesmeasurementstogether (asfor| effective wavelengthofafilterdependsonthetemperature ofthestar,eachstellargroupwasassignedatemperature, there aresmallergapsbetweenO’Connellcolorsinthat wavelengthregionthanthereareintheofV.Since A'=5050 À.Thegalaxycolor atwavelengthXinO’Connell’ssystemis to determineS/A),thefluxperunitfrequencyofith stellarcomponentattheO’Connellnormalizingwavelength and itseffectiveBwavelengthwasdeterminedfromAllen (1973).Alinearinterpolationisdonebetweencolorsinorder stellar groupstothetotalgalaxianlight,foundwithrespect to5050ÁwasaccomplishedthroughtheBmagnitudesince g To averageseveralnightstogether,thehighestoftwocomputeduncertaintiesischosenandusedinfiguring thej For objects,suchasstars,whichwerenotobservedindifferentialmode,thestatisticalanalysisismuchsimpler, j The renormalizationnecessarytopredictcolorswithrespect toKfromactivitylevels,thefractionalcontributionsof A sectionofthestellarlibraryusedforpopulationsynthesis ispresentedinTable10.Thespectraltypelisted © American Astronomical Society •Provided by theNASA Astrophysics DataSystem D, 1 u POPULATION SYNTHESIS W) _{