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The Stellar Content of the Dwarf Galaxy NGC 3109 1 2 1 F

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The Stellar Content of the Dwarf Galaxy NGC 3109 1 2 1 F sen and West, 1980, Whitmore et al., have massive pragenitors. Since the 1987); in Figure 1 the ring is visible as a number of spectroscopically and photo­ Latest News disk-like feature almost oriented north­ metrically well-sampled supernovae Ib According to M. M. Phillips, CTIO, south and SN 1990 I is located close to is rather smalI, the follow-up of SN spectra of SN 19901 show that it its southern edge. Although it cannot be 1990 I is very interesting, even if it is of type Ib (IAUC 5032; June 14, exluded that the alignment of the SN should turn out not to be of type Ia. 1990). 1990 I with the polar ring is a projection The favourable location of SN 1990 I effect, it is rnore probable that the SN in the sky will allow La Silla observers to References was really formed in the ring. follow it during the next 5 months or so. The possibility that SN 1990 I is of Branch, D. et al., 1983: Ap. J. 270, 123. type la and is associated with the ring, Danziger, I.J., Lucy, L.B., Gouiffes, C., Bouchet, P., 1990: Supernovae S.E. makes this object particularly interest­ Acknowledgements Woosley ed. Springer. ing, because the ring has quite blue GaskeIl, C. M. et al., 1986: Ap. J. Left. 306, colours, is knotty and very rich in H I1 Special thanks and congratulations L77. regions indicating significant recent star go to O. Pizarro, who discovered SN Harksen, R.P. et al., 1987: Ap. J. 317, 355. formation (Laustsen and West, 1980); on 1990 I in his long-term survey of Laustsen, S., West, R. M., 1980: J. Astroph. the other hand, SN type la are typical of Schmidt plates. I also appreciate the Astr. 1, 177. galaxies in which no young stellar popu­ help of M. Bahamondes, J. Miranda and Mayerott, R. E., 1980: Ap. J. 239, 257. lation is present and it is generally J. Borquez in obtaining some of the Pasquini, L., Jarvis, B., Leibundgut, B., 1990: agreed that Ia supernovae are associ­ early observations, as weil as several lAU Circular 5003. Pizarro, 0., Miranda, J., Pasquini, L., ated with a low mass, old population visiting astronomers, who kindly spent Leibundgut, B., 1990: lAU Circular 5003. (Woosley et al., 1986). part of their time observing SN 1990 I: V. Whitmore, B.C., McElroy, D. B., Schweizer, The location of SN 19901 would not Burwitz, D. Pollacco and J. P. Sivan. Fi­ F., 1987: Ap. J. 314, 439. be unusual if, instead, we were dealing nally, I am grateful to B. Leibundgut and Woosley, S. E., Taam, R. E., Weaver, T.A., with a type Ib, which are thought to E. Oliva for helpful discussions. 1986: Ap. J. 301, 601. The Stellar Content of the Dwarf Galaxy NGC 3109 1 2 1 F. BRESOUN , M. CAPACCIOU , and G. PIOTTo ,3 10ipartimento diAstronomia, Universita di Padova, Italy; 20sservatorio Astronomico, Padova, Italy; 3European Southern Observatory 1. Introduction spects. First, the high mass-to-light content of the universe. Observations Despite their modest appearance, ratios measured for the smallest sys­ suggest that the ratio of hidden-to­ dwarf irregular galaxies (DIGs) form an tems make DIGs some of the best can­ luminous matter increases when going interesting class of objects in many re- didates for the study of the dark matter fram irregular systems such as Sextans • ... .. • • • • . .~ .. • •• ".., .•. Figure 1: The SBm galaxy NGC 3109 trom an ESO 3.6-m 60-min exposure on Kodak II/a-J + GG 385. 36 16 I~~~~I~~~~I~~~~sewolrsservoirs of gas supports thethe bellefbelief thatthat - - dwarf Irregularsirregulars are young and st111still rela-rela­ - 9 fields 9 fields tlvelytively unevolved galaxlesgalaxies wlthwith regard toto - 8250 stars - star formatlon.formation. The question of whether 18 - - - - star formatlonformation ISis a contlnuouscontinuous process - or rather a sequence of slnglesingle bursts, 1sis - - st111still controvers~al.controversial. The burst model 1sis 20 - - usually preferred Inin lnterpretlnginterpreting the - ,. - properties of the blue compact galaxies, - , whllewhile more normal DIGSDIGs are thought to 3 .. k - - .....'-a ;"- . , - experience a contlnuouscontinuous and constant . .*, -. Y. - 2, >,-?%s,* . 22 - . actlvltyactivity (Hunter and Gallagher, 1985). 22 - ;$p%-%3e:* ,,. ,:, :. b, .. - - . ~,2,fi~~p%$st:.+~.s>IJ::,>$+,,:, :-/ q - More than a dozen DIGSDIGs have alreadyal ready .*$P>$< ~$A$>$$~$,:&~&!$L;~;$$ h5 .+ - been ldentlf~edidentified Inin the nelghbourhoodneighbourhood of - -,&:, yi L$&:$4s-c .2:f>;-$d v+, vI; ,>' : - - , .\",.W:F;.; - the Galaxy, and so close that thelrtheir stellar SQ -:<,pi, L,: , 'i , .,- " - - 24 - ." content ISis qultequite wellweil resolved. The avall-avail­ # - - - - abllltyability of software packages allowlngallowing - - photometry ~nin crowded f~elds,fields, such as - - DAOPHOT or ROMAFOT, has permitted 26 /11111111111111,,113 the constructlonconstruction of C-M d~agramsdiagrams for a -1 1 2 - 1 0o 1 2 certalncertain number of these objects (B(B-V)-V) (Table 1). Comb~nedCombined wlthwith stellar evolu-evolu­ FlgureFigure 22: C-M dlagramdiagram of the stars prolect~ngprojecting on the 21' x 3' reglonregion centred on NGC 3109, not tion models, these measurements have corrected for f~eldfield star contam~nat~oncontamination. shed some llghtlight on the characterlstlcscharacteristics of the stellar populatlonspopulations of the nearest galaxles,galaxies, showlngshowing s~rnllarltlessimilarities Inin the~rtheir content of massive stars, as wellweil as Inin the shape of the Initialln~t~alMass Funct~on.Function. A and LGS 3 towards the smaller dwarf processes. Such processes can be We have recently undertaken a sys-sys­ spheroldalsspheroidals Ursa MlnorMinor and Draco (cf. spectacular Inin dwarf Irregulars,irregulars, espe-espe­ tematlctematic programme for the study of the TrimbleTrlmble 1987). ciallyclally inIn the blue compact galaxies.galax~es.The stellar populations inIn some nearby DIGsDIGS Second, sinceslnce DIGsDIGS are generally be­be- intenseIntense activityactlvlty relatively to the small through multicolourmultlcolour CCD photometry. lievedlleved to be simpleslmple and rather unevolved sizes of these galaxiesgalaxles accounts for theirthew MaterialMaterlal has alalreadyready been collected for objects, they are excellent laboratorieslaborator~es blue colours and for the presence of UKS 2323, IC 1613, and NGC 3109, and to study the early phases of the chemi­cheml- several associationsassoclatlons of young stars; the a firstflrst account of the results obtainedobtalned for cal evolution of galaxies.galaxles. Spec­Spec- latter are often embedded inIn large UKS 2323 has been givenglven by CapaccioliCapaccloll trophotometrictrophotometrlc investigationslnvestlgatlons have re­re- clouds of ionizedlonlzed hydrogen, and give et al. (1987). Here we present some new vealed that the metal content is1s general­general- these galaxiesgalaxles theirthelr clumpy and knotty results concerning a classical photo­photo- ly low, ranging from approximately a few appearance. The availabilityavallablllty of great re- metriemetrlc target, NGC 3109.31 09. hundredths to half of the solar value. The low metallicitymetalllc~tyis1s of consequence inIn cosmology, sinceslnce it~tfavours the estimateestlmate of the primordialpr~mordlalabundance of Helium,Hehum, a figureflgure of paramount importancelmportance inIn testingtestlng the BigBlg Bang theory. Further­Further- . ", "' , .: .. ' s . more, DIGsDIGS are probably the most com­corn- . ":~'..? :,. : . ' y ., .. :' . , • " .." , ..-',.;2.,, .zj: ..: . , . mon galaxiesgalaxles inIn the universe; thisthls too *.. ': . .. ; .. a. Lr++o. :,:.,:* . - I. can be of cosmologicalcosmolog~calinterestInterest inIn that it~t .. e . * .. .. .. * bears on the theoriestheorles of galaxy forma­forma- ..: . tiontlon as weilwell as on the estimateestlmate of the total mass of the universe. • lS<V<191R'V<19 -2«B-V)<.42<(B V)<4 Third,Thlrd, DIGsDIGS play sosomeme role inIn the . 19<V<2019<\1<20 problem of the cosmic distance scale: problem of the cosmlc dlstance scale: 20<V<21ZO<V<Z1 CepheidCepheld variablesvar~ableshave been identified~dent~f~ed 21<V<2221<v<22 E inIn thethe nearest galaxies,galaxles, and used toto AN estimateestlmate theirthelr distancedlstance moduli.modull. These , ..*. measurements have ledled toto thethe extension .*." . .,*.:-. -.. ....:. .. ",.. "0.: -, of thethe faintfa~ntend of thethe relationrelatlon between .: . a.. ., . ... ..-:: ., .':. ..: *. ..I*, .a' luminositylumlnoslty of the brightest blue and redred . ..:. .. .: . : ..a..... -.. .:; c* . -. supergiantssuperglants and absolute magnitudemagn~tudeof • ,',. -. : • :. thethe parent galaxy (Humphreys,(Humphreys, 1983). Such a calibrationcallbratlon isIS of great importancelmportance forfor thethe determinationdetermlnatlon of distancesd~stancestoto .lS<V<19ls<v<19 1.3«B-V)<31 3<(~-~)<3 more distantdlstant galaxiesgalaxles inIn whwhlchich CepheidsCephelds • 19<V<2019<v<20 can no longerlonger be seen. .20<V<21.20<v<21 3' Finally,Flnally, given theirthew structural simplicitys~mpl~c~ty . 21<V<222 1<v<22 M (e.(e.g.g. lacklack of spiralsplral arms), DIGsDIGS are better FigureF~gure3:3 Upper panel: projected distributiond~str~butlonof thethe brightestbrlghtest blue stars of NGC 3109 places thanthan thethe more complex and containedcontalned inln thethe diagramd~agramof FigureFlgure 2;2, thethe centralcentral bar and thethe twotwo eastern spiralsplral arms are largerlarger spirals,splrals, toto study star formationformatlon discernible.d~scern~bleLower panel: map of thethe brightestbrlghtest redred stars.stars 37 tance modulus 0.3 mag smaller than given by SC. The mean internal error, as
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