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. Further Further- . . ", "' , .: .. ' 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 weil well as on the estimateestlmate of the total mass of the universe. • lS
2. NGCNGC 3109: Observations and TABlETABLE 1:1: Stellar photometry in dwarf irregular galaxies DataData Reduction Object OistanceDistance Ref. Material No. of stars NGCNGC 3109 = DDO 236 (Fig.(Fig. 1) is a modulus used measured magellanicmagellanic spiral locatedlocated at the peripheryperiphery of the Local Group: itsits disdis- NGeNGC 6822 23.5 mag 1 phot. - tancetance modulus, based on Cepheids, is 2 CCDeeo 3475 leIC 1613 24.5 3 phot. 318 (m-M)B(m-M), = 26 according toto Sandage and - 4 CCDeeo 2224 Carlson (1988;(1988; hereafter referred to as WlMWLM 25.0 5 phot. 68 SC). While comparable toto the Small 6 CCDeeo 2250 MagellanicMagellanic Cloud from thethe point of view lGS3LGS 3 25.0 7 CCDeeo 66 of luminosity (MB(MB = -16),-16), thethe size of GR8GR 8 25.7 8 CCDeeo 84 NGCNGC 3109 makes itit one of thethe largestlargest 9 CCDeeo 142 magellanicmagellanic systems known so far (D = UKS 2323 26.0 10 CCDeeo 204 1414 kpc,kpc, corresponding toto 09O? 5 on the NGeNGC 310931 09 26.0 11 phot. - sky).sky). Previous studies of its stellar concon- 12 phot. 83 Pegasus 26.1 13 phot. 54 tenttent are by Demers et al. (1985)(1985) and by Pegasus 14 CCDeeo - SC; both of themthem are based on photophoto- Sextans A 26.2 15 phot. 70 graphiegraphic material. 16 CCDeeo 652 Our observations were made during 17 CCDeeo 2279 threethree different runs runs at ESO, La Silla, inin Sextans B 26.2 18 phot. 77 MarchMarch and May 1988, and inin March 19 CCDeeo 1273 1989,1989, with thethe CCD cameras of the DanDan- leoLeo A 27.1 20 phot. 92 ishish 1.5-m1.5-m and thethe ESO/MPIESOIMPI 2.2-m teletele- 21 phot. - 27.3 phot. scopes.scopes. We collected BandB and V images of Sculptor 27.3 22 phot. 33 HolHo l 27.5 23 CCDeeo 279 1111 fieldsfields of thethe galaxy, distributed inin Ho 11II 27.5 23 CCDeeo 468 suchsuch a way as toto cover a totaltotal area of Ho IXlX 27.5 24 CCDeeo 367 ~-21'21' x 2'. The six central fields were 000187DDO 187 28.8 25 CCDeeo 77 also imaged through an Ha filter (and in a contiguous band). Deep exposures and fair seeing conditions (FWHM ;== References 0':9-10"-1!2), ':2), allowed toto measure stars (1) Kayser, S.E., 1967, Astron. J.,J, 72,134;72, 134; (2) Hoessel, J.G., and Anderson, N., 1986, Astrophys. J.J Suppl. down toto magnitude V = 24 using DAODAO- SeroSer. 60,60,507;507: (3) Sandage, A.,A, and Katem, B., 1976, Astron. J.,J, 81,743;81, 743; (4) Freedman, W.W.L., L., 1988, Astron. J.,J, 96, 1248; (5) Sandage, A., and Carlson, G., 1985, Astron. J., 90, 1464; (6) Ferraro, F. R., Fusi Pecci, F., Tosi, PHOT.PHOT. InstrumentalInstrumental magnitudes were 96, 1248; (5) Sandage, A, and Carlson, G., 1985, Astron. J, 90, 1464; (6) Ferraro, F. R., Fusi Pecci, F., Tosi, M., and Buonanno, R., 1989, ESO preprint; (7) Christian, C., and Tully, R.B.,R. B., 1983, Astron. J.,J, 88, 934; calibratedcalibrated by a large set of standard (8) Hoessel, J.J.G.,G., and Danielson, G. E., 1983, Astrophys. J.,J, 271, 65; (9) Aparicio, A.,A, et al., 1988, Astron. starsstars (Landolt(Landolt 19831983a,a, b). The zero point Astrophys. Suppl. Ser., 74, 375:375; (10)(lo) Capaccioli, M., Ortolani, S., and Piotto, G., 1987, inin Proceedings of01 the errors of our photometry are estimated ESO Workshop on "Stellar Evolution and Dynamics in the Outer Halo of01 thethe Galaxy, ed. M. Azzopardi and F. Matteucci, p. 281 ; (1 1) Demers, S., et al., 1985, Astron. J., 90,1967; (12) Sandage, A., and Carlson, G., 1988, toto be ~-0.030.03 mag in V and 0.05 mag in Matteucci, p. 281; (11) Demers, S., et al. , 1985, Astron. J, 90, 1967; (12) Sandage, A, and Carlson, G., 1988, Astron. J,J., 96, 1599; (13) Sandage, A,,A, 1986, Astron. J.,J, 91, 496; (14) Hoessel, J.G., and Mould, J.J.R., R., 1982, (B-V).(B-V). Note that thethe comparison of Astrophys. J,J., 254, 38:38; (15) Sandage, A.,A, and Carlson, G., 1982, Astrophys. J., 258, 439; (16)(16) Hoessel, J.G., ~-200200 stars in common with SC has et al., 1983, Astrophys. J,J., 274, 577; (1(17) 7) Aparicio, A,,A, et al., 1987, Astron. Astrophys. Suppl. Ser., 71, 297; revealedrevealed thethe presence of a systematic (18)(1 8) Sandage, A,A,, and Carlson, G., 1985, Astron. J.,J, 90, 1011019;9; (1(19) 9) Tosi, M., et al., 1989, The Messenger, 57, 57; (20) Sandage, A., 1986, Astron. J., 91, 496; (21) Demers, S., et al., 1984, Astron. J., 89, 1160; difference in thethe zero point, our photomphotom- 57; (20) Sandage, A, 1986, Astron. J, 91, 496; (21) Demers, S., et al., 1984, Astron. J, 89, 1160; (22) Lequeux, J., and West, R.R.M.,M., 1981, Astron. Astrophys., 103, 319; (23) Hoessel, J.G.,J. G., and Danielson, etry being 0.3 mag brighter thanthan SC's; inin G.E., 1984, Astrophys. J,J., 286, 159; (24) Hopp, U., 1987, preprint. (25) Aparicio, A.,A, et al., 1988, Astron. other words, our scale implies a dis- Astrophys. Suppt. Suppl. Ser., 74, 367. 38 (Fig. 4). They are weilwell confined to a ~-300300 pc thick stripe, with a maximum Fl density a few arcminutes to the west of the optical centre. We have drawn the isophotes of these regions, and superimposed the blue star distribution. In general, the brightest blue stars and the main stellar associations are found i~ " 21
040 10 0 20 10 20 o 60 200 0 Acknowledgements 20 10 0 0 c() 20 o 0 0 0 20 20 40 We are indebtedindebted toto thankthank G. Bertelli, 40 0 6 10 2010 0 0 C. Chiosi, and E. Nasi, forfor providing us 0 0 40 20 o 20 00 00 6 0 with the isochronesisochrones for for thethe massive 10 o 10 0 10 o 20 10 stars inin advance of publication. We also 20 thankthank S. Ortolani for the Danish obserobser- vations.
References Capaccioli, M., Ortolani, S., and Piotto, G., 1' G., 1987, in Proceedings of thethe ESO WorkWork- shop on "Stellar Evolution and Dynamics in the Outer Halo of the Galaxy", eds. M. Azzopardi and F. Matteucci, p. 281. Demers, S., Kunkel, W.W.E.,E., and Irwin, M.J., 1985, Astron. J. 90,1967.90, 1967. Figure 7: Map of the star-forming regions seleetedselected in the six eentralcentral fjelds; fields; the radi! radii of the de Vaucouleurs, G., and Freeman, K. K.C.,C., 6 eire/escircles are proportional to the size of the regions. Ages (in 10lo6yr) of the youngest stars found in 1972, Vistas Astron. 14, 163. the C-M diagrams are indieated.indicated. Humphreys, R.R.M.,M., 1983, Astrophys. J. 269, 335. Hunter, D.A,D.A., and Gallagher, J.S., 1985, As- isis accurate enough to get an estimate of NGC 3109 have ages of the order of trophys. J. Suppl. SeroSer. 58, 533. the agageses of the single regions. We have ~- 6 x·1x 10O6 6 years, with masses ~- 30 MG);Ma; Landolt, AU.,A. U., 1983a, Astron. J. 88, 439. used models calculated for a solar inin other words, this galaxy isis still active Landolt, AU.,A. U., 1983b,1983 b, Astron. J. 88, 853. metaliicitymetallicity and adopted the internal abab- in forming stars. Moreover, since the Piotto, G., King, I.I.R.,R., Capaccioli, M., Ortolani, sorption values given by SC. A larger part of the diagrams corresponding to S., and Djorgovski, S., 1990, Astrophys. J. 350,662.350, 662. extinction correction had to be adopted agageses of 20-100 million years appears Sandage, A.,A, and Carlson, G., 1988, Astron. for star-forming regions belonging to the rather uniformly populated by stars, we J. 96, 1599. central parts of the galaxy. An example may conclude that, in this time interval, Stetson, P.ß.,P. B., 1987, P.A.S.P.P.A. S. P. 99,191.99, 191. of the procedure is shown in Figure 6. stars have formed in an almost regular Trimble, V., 1987, Ann. Rev. Astron. Astro-Astro We find that the youngest stars of manner (see the plot in Fig. 7, where phys. 25, 67.
Probing the Hidden Secrets of Seyfert Nuclei I. AAPPENZELLER PPENZEL LER and S. WAGNER, Landessternwarte Heidelberg-Konigstuhl,Heidelberg-Königstuhl, F. R. Germany
The nuclei of active galaxies are clear-clear rates up to about one earth mass per became increasingly evident that in ly among the most spectacular and vio-vio second, such black holes give rise to many AGNs the central engines are lent places that can be found in our huge rotating magnetic fields and elec-elec not directly visible but hidden behind present universe. Most extreme are the tric current systems which can explain opaque dusty matter concentrations bright Quasars, where we observe a to-to the astonishing properties of these sys-sys along the line of sight. Even in the case tal energy output equivalent to a large tems. of nearby Seyfert galaxies (the nearest galaxy cluster from galactic core regions However, in spite of a large research known examples of AGNs), direct opti-opti comparable in size to our solar system. effort during the past decades we are cal radiation from the centre of the ac-ac In addition to optical and radio radiation still lacking a reliable observational con-con tive nuclei seems to be observed only in we often observe intense X-ray and firmation of the basic physical models of exceptional cases. Moreover, when di-di even energetic Gamma radiation as wellweil the AGNs and our knowledge of the rect radiation is detected, it is often mix-mix as collimated streams of matter moving detailed physical processes occurring in ed with light of the normal stellar galac-galac at velocities close to the velocity of light. AGNs is still highly incomplete. One tic core and with emission from circum-circum Most currrent theories assume that reason for the slow progress of our nuclear normal HI1H 11 regions ionized by the enormous radiation power of active understanding of the AGNs is the great stars. galactic nuclei (AGNs) isis produced by distance of most active galaxies which Fortunately, modern observing tech-tech massive rotating black holes residing in makes it impossible toto resolve thethe active niques provide various methods toto over-over thethe dynamical centres of thesethese galaxies. nuclei by direct imagingimaging techniques.techniques. come some of thethe observational difficul-difficul Swallowing surrounding material at Furthermore, during thethe past decade itit tiesties mentioned above. High-resolution 40