VII. NGC 3109 a Galaxy Without a Stellar Halo
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A&A 410, 795–801 (2003) Astronomy DOI: 10.1051/0004-6361:20031286 & c ESO 2003 Astrophysics Carbon star survey in the Local Group? VII. NGC 3109 a galaxy without a stellar halo S. Demers1;??, P. Battinelli2, and B. Letarte1;??? 1 D´epartement de Physique, Universit´edeMontr´eal, CP 6128, Succursale Centre-Ville, Montr´eal, Qu´ebec H3C 3J7, Canada e-mail: [email protected] 2 INAF, Osservatorio Astronomico di Roma Viale del Parco Mellini 84, 00136 Roma, Italia e-mail: [email protected] Received 4 June 2003 / Accepted 19 August 2003 Abstract. We present a CFH12K wide field survey of the carbon star population in and around NGC 3109. Carbon stars, the brightest members of the intermediate-age population, were found nearly exclusively in and near the disk of NGC 3109, ruling out the existence of an extensive intermediate-age halo like the one found in NGC 6822. Over 400 carbon stars identified have M = 4:71, confirming the nearly universality of mean magnitude of C star populations in Local Group galaxies. Star h I i − counts over the field reveal that NGC 3109 is a truncated disk shaped galaxy without an extensive stellar halo. The minor axis star counts reach the foreground density between 40 and 50, a distance that can be explained by an inclined disk rather than a spheroidal halo. We calculate a global C/M ratio of 1:75 0:20, a value expected for such a metal poor galaxy. ± Key words. galaxies: individual: NGC 3109 – galaxy stellar content – galaxy structure 1. Introduction stars at large radii in IC 1613. Do all Magellanic type galax- ies have an intermediate-age population in their halo? Is there Halos of galaxies may result from gravitational collapse something special about NGC 6822? (Eggen et al. 1962), early in their life, or from the accretion of small fragments (Searle & Zinn 1978). The halo build- To try to answer these questions we follow two approaches: ing by accretion should be a continuous process leading to 1) We are investigating the kinematical properties of C stars in halo populations of different ages and metallicities. Recently, NGC 6822 from radial velocity follow-up observations already dwarf galaxies have been found to have halos of old stars acquired; 2) We look at other dwarf galaxies similar or dis- (Lee 1993; Minniti & Zijlstra 1997; Minniti et al. 1999); or similar to NGC 6822 to search for intermediate-age halos. In stars older than 1.5 Gyr (Aparicio & Tikhonov 2000; Aparicio this regard, the natural comparison to NGC 6822 is NGC 3109, et al. 2000). In this regard, our recent CFH12K observations a galaxy of the same morphological type. In both galaxies, of NGC 6822 (Letarte et al. 2002) represent a major break- de Blok & Walter (2000) and Barnes & de Blok (2001) have through. We detected numerous carbon stars (C stars) in the recently mapped extensive HI envelopes. halo of this Magellanic-type galaxy implying the presence of NGC 3109, a Magellanic dwarf on the outskirts of the Local an intermediate-age population extending at least three times Group, is a spectacular object seen through a large telescope. the optical radius. Albert et al. (2000) have also identified C With a diameter of nearly 300, NGC 3109 is one of the largest and brightest galaxy of the southern hemisphere. Well resolved Send offprint requests to: S. Demers, into stars, it has been classified Sm IV by Sandage & Tammann e-mail: [email protected] (1981). A prime focus CTIO 4 m photo of this galaxy can be ? The complete Table 2 is only available in electronic form at the found in Demers et al. (1985). This galaxy, first believed to be CDS via anonymous ftp to cdsarc.u-strasbg.fr irregular because of the lack of an obvious nucleus, is actually a (130.79.128.5) or via disk galaxy and could possibly be a small spiral (Demers et al. http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/795 1985). ?? Visiting observers, Canada-France-Hawaii Telescope, operated by the National Research Council of Canada, the Centre National de la During the last decade several photometric investigations of Recherche Scientifique de France, and the University of Hawaii. small areas in NGC 3109 have been published. We now have ??? Current address: Kapteyn Astronomical Institute, Posbus 800, a fairly good estimate of the distance of this galaxy: Musella Groningen, 9700 AV, The Netherlands. et al. (1997) derive (m M) = 25:67 0:16 from 24 Cepheids − 0 ± Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20031286 796 S. Demers et al.: NGC 3109. VII. Table 1. Summary of the observations. 16 Filter Exp: times Average seeing (00) Mean airmass R 8 400 s 0:79 1:534 × I 6 360 s 0:69 1:540 CN 7 × 1167 s 0:70 1:571 TiO 7 × 1167 s 0:76 1:629 18 × while Minniti et al. (1999) obtain (m M) = 25:62 0:1 from − 0 ± the I magnitude of the tip of the red giant branch (TRGB). 20 More recently, M´endez et al. (2002) re-determined distances to out-lying members of the Local Group using the TRGB method. For NGC 3109 they quote (m M) = 25:52 0:06. We − 0 ± adopt, for our investigation, the weighted mean of these three 22 estimates (m M)0 = 25:56 0:05. The colour excess and the amount of− Galactic extinction± toward NGC 3109 are small. Schlegel et al. (1998) obtain E(B V) = 0:04, from the COBE and IRAS maps at 100 µm. This corresponds− to E(R I) = 0:03 − 24 and AI = 0:054 using the reddening ratios of Rieke & Lebofsky (1985). This low reddening is consistent with the colour of the 012 ridge of bright stars, seen in Fig. 1. Fig. 1. Colour–magnitude diagram of the whole CFH12K field. Stars The photometric study of a 9 9 region, centered on 0 × 0 with colour errors <0.10 are plotted. We acquire the first 1.5 mag of NGC 3109, by Minniti et al. (1999) revealed that this galaxy the giant branch, sufficient to identify the C star population. possesses a Population II halo extending to 4.50 (1.8 kpc) above and below the plane of the galaxy. These∼ authors deter- mined, from the position of the red giant branch on the CMD, were done by fitting model point-spread functions (PSFs) using that the metallicity of that population is [Fe=H] = 1:8 0:2. DAOPHOT-II/ALLSTAR series of programs (Stetson 1987, They also noted the presence of red asymptotic giant− branch± 1994). Instrumental magnitudes are calibrated using equa- stars (AGB), presumably C stars, in the main body of the tions provided by the CFHT QSO team. The equations for R galaxy. This metallicity estimate roughly confirms the abun- and I magnitudes are: dance of 1:6 0:2 determined in the same way by Lee (1993). R = 26:190 + mr 0:09(Xr 1) + 0:0094(R I); M´endez et− al. (2002)± obtained [Fe=H] = 1:69 0:06 also from − − − − ± the colour of the red giant branch. We adopt, for our discussion, I = 26:185 + mi 0:04(Xi 1) 0:0511(R I); the weighted mean value of [Fe=H] = 1:7. − − − − − where Xr and Xi are the airmasses of the R and I exposures. For 2. Observations the instrumental magnitudes r and i, obtained with DAOPHOT, and for the given exposure times, mr and mi correspond to: The results presented here are based on observations obtained, m = (r 25:0) + 2:5 log(400); in Service Queue observing mode in March 2002, with the r − CFH12K camera installed at the prime focus of the 3.66 m m = (i 25:0) + 2:5 log(360): Canada-France-Hawaii Telescope. The camera consists in a i − 12k 8k pixel mosaic covering a field of 420 280, each pixel × × By subtracting the above R and I equations and using the corresponding to 0.206 arcsec. Images were obtained through appropriate airmasses we obtain: Mould I and R filters and narrowband CN and TiO filters, cen- tered at 808.6 nm and 768.9 nm, respectively. A summary of (R I) = 0:099 + 1:064(r i); the acquired data is presented in Table 1. The total exposure − − time on NGC 3109 was 6 hours, under excellent seeing. I = i + 7:5542 0:0511(R I): − − The data distributed by the CFHT have been detrended. This means that the images have already been corrected with 3. Results the master darks, biases, and flats. Fringes have been removed on I exposures under 60 s and large scale structures such as the The colour–magnitude diagram (CMD) of the whole CFH12K “Skyring” effect have been removed when relevant. This pre- field is presented in Fig. 1. 38 600 stars, with (R I) colours analysis produces 12 CCD images, of a given mosaic, with the with photometric errors less than 0.1 mag. are plotted.− Since same zero point and magnitude scale. our observations were acquired to survey the C star population, We carefully combine images taken with the same fil- they reach only 1.5 mag below the tip of the red giant branch. ter, making sure that the final FWHM was similar to the av- This limit is quite sufficient to detect all the C stars that can be erage FWHM given in Table 1. The photometric reductions resolved.