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Getting to Know Our New Neighbor (1Dec94) (from NOAO HIGHLIGHTS!, NOAO Newsletter No. 40, 1 December 1994) This year, Ibata, Gilmore, and Irwin (1994) discovered that the Milky Way has a neighbor that previously escaped our notice. Our "new" neighbor is a dwarf spheroidal galaxy in the constellation of Sagittarius, and is only 25 kiloparsecs from the Sun on the opposite side of the galactic bulge. To make up for our previous sins of omission, Ata Sarajedini (KPNO) and Andy Layden (CTIO) decided to take a good look at the stellar population of the field around and including the globular cluster M54, a system apparently in the line of sight to the Sgr dwarf. What they found was that M54 and Sgr may be different parts of a common system, an intriguing clue to the formation and evolution of both globular clusters and dwarf spheroidals. The clumpy distribution of stars in Sgr suggests that it may be in the process of being tidally disrupted and accreted by the Milky Way galaxy. This, along with its relative proximity, make Sgr an excellent candidate for detailed studies, which can potentially reveal much about the processes whereby small galaxies are absorbed by larger ones and the role such processes play in galaxy formation and evolution. M54 is apparently close to the densest clump in Sgr, so Sarajedini and Layden decided that V and I images of M54 (obtained with the CTIO 0.9-m) would be valuable in determining its metallicity, distance, and relationship to Sgr. A first glance at the images, in fact, just shows M54, a typical bright globular cluster, with no hint whatsoever of another population belonging to a dwarf galaxy (Figure 1). However, the color data identifies a population of extremely red stars (Figure 2). In general, extremely red stars such as these are very rare in a relatively metal deficient object like M54, implying the existence of another population. [Figure not included] The real picture of what's going on is provided by color-magnitude diagrams (CMD) of fields on and off the M54 cluster. The upper panel in Figure 3 shows the CMD of the on-cluster frames with the red stars indicated as crosses, with the lower panel serving as interpretive guide. The CMD of the off-cluster region is shown in Figure 4, but includes the extremely red giant stars from Figure 3 as a guide because the observed off- cluster field lies in a low density portion of Sgr and therefore contains few of these stars. The nearly vertical red giant branch (RGB) and blue horizontal branch (HB) in Figure 3 appear to belong to M54 because they do not appear in Figure 4. In contrast, the metal-rich horizontal branch can be seen in both figures, as can a weakly-populated red giant branch (Metal- Rich RGB). Stars are not present at these locations in CMDs constructed by Mateo et al. (1994) at a "control" position on the opposite side of the galactic bulge and thus are likely to be stars associated with Sgr. Lastly, the vertical scattering of stars at 0.8 < V - I < 1.3 are the foreground bulge stars that have masked the Sgr galaxy for so long. [Figures not included] One would be inclined to attribute the Intermediate Metallicity HB to M54 because it is not apparent in Figure 4. However, the radial distribution of these HB stars is not concentrated toward the cluster center, as are the M54 RGB and Blue HB stars. Furthermore, the stars are about 0.15 mag fainter than expected from the horizontal branch fiducials of other clusters, when fit to the Blue HB and RR Lyrae stars of M54. Finally, detailed luminosity functions of the cluster frames, along with luminosity functions from the on- Sgr and control CMDs of Mateo et al. (1994), suggest that the Red HB is present in all of the on-Sgr fields, but is not in the control fields. The red HB is thus likely to belong to Sgr, not to M54. This interpretation is further supported by the "sequence" of stars between the M54 and Sgr Metal-Rich RGBs in Figure 3, which may be the red giants associated with this Red HB population. Their positions suggest a metal abundance intermediate between that of M54 and the more prominent metal-rich Sgr population. Sarajedini and Layden argue from Figure 3, as well as a more detailed analysis, that the three horizontal branch populations (one associated with M54 and two with Sgr) all lie at the same distance of ~25 kiloparsecs, to within the accuracy of the RR Lyrae distance scale. This suggests that M54 is in fact a part of the Sgr dwarf galaxy. The cluster and galaxy also have very similar velocities (131 vs 140 km/s), and M54 lies in the highest- density region of Sgr. These further strengthen the interpretation that M54 formed in, and is a member of, the Sgr dwarf galaxy. The association of M54 with Sgr leads to intriguing conclusions. Firstly, rescaling existing integrated luminosity estimates of M54 for its new (greater) distance indicates that it is nearly as luminous as Omega Centauri, the most luminous globular cluster in the Milky Way. M54 is then two orders of magnitude brighter than the brightest cluster in Fornax, which is the only other Galactic dwarf spheroidal satellite possessing globulars. Secondly, the high metal abundance of the Sgr field population implies a high integrated luminosity for Sgr, assuming it follows the well-established absolute magnitude-metallicity relation for dwarf galaxies. In contrast, star counts suggest a much lower absolute magnitude. Perhaps much of this galaxy has already been "stripped off" in its interaction with the Milky Way. The foreground confusion of the galactic bulge could easily mask tidal tails extending from the main body of Sgr. Sgr is clearly a surprising galaxy. Its anomalously luminous globular cluster and its high metallicity, along with its position as the "smoking gun" of galactic formation-by-accretion, make it an extremely attractive target for further study.
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