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The Stellar Content of the Sagittarius Dwarf Galaxy ⋆

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The Stellar Content of the Sagittarius Dwarf Galaxy ⋆ AND Your thesaurus codes are: View metadata, citation and similar papers at core.ac.uk ASTROPHYSICSbrought to you by CORE (03.20.4; 08.08.1; 08.12.3; 11.09.5; 11.19.5) 15.3.1997 provided by CERN Document Server The stellar content of the Sagittarius Dwarf Galaxy ? G. Marconi1, R. Buonanno1, M. Castellani1,2, G. Iannicola1, P. Molaro3,L.Pasquini4,L.Pulone1 1 Osservatorio Astronomico di Roma Via dell’Osservatorio 2, Monte Porzio Catone, 00040 (Roma). Italy [marconi,buonanno,mkast,iannicola,pulone]@coma.mporzio.astro.it 2 Universita’ ”La Sapienza”, Istituto Astronomico Via Lancisi 29, 00161 Roma 3 Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131 Trieste 4 ESO, La Silla Received September; accepted Abstract. We present V,I deep CCD photometry for Sgr is similar in mass to the Fornax dwarf, which contains three fields of the dwarf galaxy in Sagittarius (Sgr), lo- a planetary nebula as well. The other, less massive, dwarfs cated at l=5.6o, b=-14.1o. One of the fields is centered on of the Local Group seem not to contain any star in such the globular cluster NGC 6715 (M54), which lies in one of a short evolutionary phase. the dense clumps of the Sgr galaxy. Comparing the CMD Although the CM diagram of IGI is heavily contam- of Sgr with those of globular clusters which are believed inated by the rich population of foreground disk and to be kinematically associated with the dwarf galaxy (Da bulge stars, nonetheless it clearly shows the presence of Costa & Armandroff, 1995), we conclude that the stel- an intermediate-old dominant population. lar population of Sgr presents a spread in metallicity of -0.71≤[Fe/H]≤-1.58, and that the dominant population Mateo et al. 1995a (hereafter MUSKKK), presented ('10 Gyr old) is extremely similar to the star content of the CCD photometry of a field in Sagittarius, which the associated globular cluster Terzan 7. The estimated reaches I'22.3. After having statistically removed the field distance to Sgr is d'24.55 Kpc. stars from the CM diagram, they concluded that Sgr is dominated by a intermediate-old population which is Key words: techniques: photometry - stars: HR- younger than that of the bulk of galactic globular clusters. diagrams - stars: luminosity function - galaxies: irregular They also found evidence of a feature in the CMD which - galaxies: stellar content could be interpreted as a weak component of intermediate age or, alternatively, as a population of blue stragglers. Finally, from the color of the red giant branch they esti- mated that the metallicity of the dominant population is about [Fe/H]'-1.1. Mateo et al. (1995b) extensively dis- 1. Introduction cussed the properties of a handful of RR Lyrae variables Ibata, Gilmore and Irwin (1994, 1995; hereafter both in- in the field of Sgr. dicated with IGI) identified a new dwarf galaxy of the Lo- Sarajedini & Layden (1995, hereafter SL) presented cal Group, located in the constellation of Sagittarius. The the V,I CCD color magnitude diagram for two fields. The dwarf, detected as a moving group of stars with mean ra- first field is centered near the globular cluster M54 and dial velocity significatively different from that of the stars covers both the cluster and the central region of Sgr, while in the bulge, is located at a distance of about 25 Kpc from the second field is centered 12 arcmin north of the cluster. the Sun, and subtends an angle of '10 degrees on the sky. From the analysis of the CMD they found that the bulk The new galaxy is comparable in size and luminosity to of Sgr is formed by a relatively metal-rich population of the dwarf spheroidal Fornax, which is the largest galaxy [Fe/H]'-0.52, and that a component of lower metallicity known in the Local Group. The recent discovery of two ([Fe/H]'-1.3) could also be present. planetary nebulae (Zijlstra & Walsh, 1996) confirms that Sgr is the nearest galaxy to the Milky Way which has Send offprint requests to:G.Marconi been discovered until now, and numerical simulations con- ? Based on observations collected at the European Southern firm that Sgr is presently in phase of being disrupted by Observatory, La Silla, Chile. the Galaxy (Kathryn, Spergel & Hernquist 1995). This idea is supported by Mateo et al. (1996) and by δ2000 30 28 23 (field 1) and the other centered at h o Alcock et al. (1997), who found that Sgr is considerably α2000=18 59’44”, δ=-30 59’45” (field 2). more extended than previously believed, suggesting that The three observed fields are sketched in figure 1 this dwarf has probably suffered at least one - and perhaps (a,b,c). Several Landolt (1992) stars (from 7 to 15), were many - strong tidal encounters with the Milky Way during observed for calibration throughout each night. past perigalacticon passages. The details of the observations, with the exposure time Another piece of evidence of the strong interaction be- and seeing conditions, are listed in Table 1. tween Sgr and the Milky Way is that two of the clusters (Ter 7 and Arp2) which have radial velocities similar to that of Sgr, and then suspected to be cinematically as- Table 1. Journal of observations sociated with Sgr (Da Costa and Armandroff, 1995), are anomalously young compared to the bulk of galactic glob- ulars (Buonanno et al. 1994, 1995a,b). Recently Fahlman et al. (1996; hereinafter FMRTS) Date F ilter Exp.time Seeing F ield T el. detected the upper end of the main sequence in the back- 6 aug. 95 V 240 s 0.7000 Sgr 1 NTT ground field around the globular cluster M55, which is 6 aug. 95 I 300 s 0.7400 Sgr 1 NTT projected in the sky about 40 arcmin east of Sgr. Having 7aug.95 V 20s 1.1200 Sgr 1 NTT interpreted this feature as the turn-off of Sgr, they used 7aug.95 I 30s 0.9500 Sgr 1 NTT the Vandenberg & Bell (1985) isochrones to give an esti- 6 aug. 95 V 240 s 0.8900 Sgr 2 NTT mate of the age of the galaxy, and concluded that Sgr is 6 aug. 95 I 300 s 0.6700 Sgr 2 NTT between 10 and 13 Gyr old, although this conclusion is 7 aug. 95 I 300 s 0.6500 Sgr 2 NTT admittedly based on an handful of stars. 7aug.95 V 20s 0.8000 Sgr 2 NTT 00 Finally, the complex history of Sgr has been confirmed 7aug.95 I 30s 0.86 Sgr 2 NTT 7sep.94 B 60s 0.8500 M54 2.2 by Ng & Schulthesis (1996) who detected a carbon-star 00 which, if its membership to Sgr will be confirmed, hints at 7 sep. 94 B 1200 s 0.93 M54 2.2 7 sep. 94 B 600 s 0.9800 M54 2.2 the existence of a stellar population about 4 Gyr younger 7 sep. 94 B 300 s 0.9500 M54 2.2 than the dominant population. 7 sep. 94 V 300 s 0.8900 M54 2.2 In this paper we present deep V,I CCD photometry 7 sep. 94 V 300 s 0.9200 M54 2.2 of Sgr, in two separate fields, far from those of SL and 7 sep. 94 V 240 s 0.9500 M54 2.2 MUSKKK. Our aim is twofold: first, to explore whether 7 sep. 94 V 240 s 0.9500 M54 2.2 the results above summarized depend on the specific re- 7 sep. 94 V 120 s 0.9200 M54 2.2 gion of Sgr examined (considering the extension of Sgr this 7 sep. 94 I 300 s 0.8800 M54 2.2 aspect is particularly important) and, second, to investi- 8 sep. 94 I 180 s 0.8200 M54 2.2 00 gate the nature of the Sgr population. 8 sep. 94 B 300 s 0.90 M54 2.2 00 Deep V,I photometry of the field near M54, already 8 sep. 94 B 300 s 0.99 M54 2.2 8 sep. 94 B 480 s 0.9500 M54 2.2 studied by SL, is also presented. 8 sep. 94 I 300 s 0.8900 M54 2.2 In section 2, we describe the observations and the re- 8 sep. 94 I 180 s 0.9100 M54 2.2 duction procedures, while the resulting C-M diagrams are 8 sep. 94 I 180 s 0.9300 M54 2.2 presented and discussed in section 3. The stellar content 8 sep. 94 I 180 s 0.8200 M54 2.2 of Sgr is studied and compared to that of associated glob- 8 sep. 94 I 180 s 0.8500 M54 2.2 ular clusters in section 4. The conclusions are drawn in 8 sep. 94 I 180 s 0.8500 M54 2.2 section 5. 8sep.94 I 60s 0.8600 M54 2.2 8sep.94 I 60s 0.8800 M54 2.2 8sep.94 I 60s 0.9300 M54 2.2 2. Observations and reductions 2.1. Observations Observations of a field which includes a portion of Sgr and of M54 (α =18h55’03”, δ =-30o28’42”) were first ob- 2000 2000 2.2. Reduction Procedures tained using EFOSC2 at the ESO/MPI 2.2 m at La Silla during September 1994. After having applied the basic procedures of pre-reduction In August 1995 additional observations of Sgr were (bias and dark current subtraction, trimming, flatfield obtained with the 3.5 m ESO-NTT.
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