We present the first deep color-magnitude diagram of the putative central region (0N5 ; 0N5) of the Canis Major stellar overdensity (l; b) ¼ (240; À8) found recently by Martin and coworkers, which has been proposed as the remnant of a dwarf satellite accreted onto the Milky Way on a near-equatorial orbit. We find a narrow (in apparent magnitude) main sequence extending 6 mag below the turnoff to our limiting magnitude of B $ 24:5 mag. This main sequence has very high contrast (>3) with respect to the thin/thick disk/halo background; its narrowness at brighter magnitudes clearly implies the presence of a distinct and possibly still bound stellar system. We derived the line-of-sight size (r 1/2 ) of this system based on the B-band width of the lower main sequence, obtaining 0:94 AE 0:18 (random) AE 0:18 (systematic) kpc. That size matches a model prediction for the main body of the parent galaxy of the Monoceros tidal stream. The high-density contrast and limited spatial extent in the radial direction are very hard to reconcile with the alternative explanation put forward to explain the Canis Major stellar overdensity: a flared or warped Galactic disk viewed in projection, as found in the recent work of Momany and coworkers. We also derived a central surface brightness of V ;0 ¼ 23:3 AE 0:1 mag arcsec À2 and an absolute magnitude of M V ¼ À14:5 AE 0:1 mag. These values place the Canis Major object in the category of dwarf galaxy, considering the L V -size and M V À V planes for such objects. However, like the Sagittarius dwarf, it is an outlier in the ½Fe/ H À M V plane in the sense that it is too metal-rich for its estimated absolute magnitude. This suggests that the main mechanism driving its recent and current star formation history (possibly tidal stripping) is different from that of isolated dwarfs.

We present the first detailed analysis of resolved stellar populations in the dwarf galaxies NGC 185 and NGC 205 based on archival V-and I-band WFPC2 pointings. For NGC 185 we deduce from the brightest main-sequence and blue-loop stars that star formation was probably still active about 4 ; 10 8 yr ago and have three key abundancerelated results: (1) We identify ancient stars with ½Fe/H P À1:5 dex by a well-defined horizontal branch ( HB).(2) We find a prominent clump/bump-like feature along the red giant branch/faint asymptotic giant branch (RGB/ faint AGB) with the same mean V-band magnitude as in the HB, within uncertainties [i.e., ÁV ( bump À HB) ¼ 0]; from a comparison with theory, the implication is that ancient stars have ½Fe /H $À1:5 dex, with a higher abundance level for intermediate-age stars. (3) From color information we infer that the median ½Fe /H > À1:11 AE 0:08 dex for ancient stars [assuming E(B À V ) ¼ 0:18 mag]. For NGC 205 we record a new distance modulus, (m À M ) 0 ¼ 24:76 AE 0:1 mag, taking E(B À V ) ¼ 0:11 mag, based on the RGB tip magnitude method in the I band. We find that stars were probably still forming less than 3 ; 10 8 yr ago in NGC 205, which is compatible with star formation triggered by an interaction with M31. There are three key abundance-related results for NGC 205: (1) The RGB/faint AGB is significantly skewed to redder values than those for a control field in the outskirts of M31; this probably results from a relatively narrow metallicity and/or age range for a significant fraction of the dwarf's stars. (2) From a comparison with models, the most metal-rich RGB stars reach ½Fe /H k À 0:7 dex (k0.2 Z ). (3) For ancient stars we infer from color information that the median ½Fe /H > À1:06 AE 0:04 dex [for E(B À V ) ¼ 0:11 mag]. We briefly compare the stellar populations of NGC 205, NGC 185, and NGC 147. Finally, we study several V-and R-band structural properties of the nuclear star cluster in NGC 205 for the first time; the apparent V-and R-band effective radii indicate a blue excess in the cluster's outer region. In terms of size, the cluster is like a typical Galactic globular cluster or a nuclear cluster in a nearby late-type spiral galaxy, but it is quite bright (10 6 L , R ), unlike an ancient globular cluster, and by matching with models, the blue color hints that its stellar population is young, up to a few times 10 8 yr old.

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We have measured the absolute proper motion of the candidate Canis Major dwarf galaxy (CMa) at $(l,b) = (240\arcdeg, -8\arcdeg)$. Likely main-sequence stars in CMa have been selected from a region in the color-magnitude diagram that has very little contamination from known Milky Way components. We obtain $\mu_{l} cos b = -1.47 \pm 0.37$ and $\mu_{b} = -1.07 \pm 0.38$ mas yr$^{-1}$, on the ICRS system via Hipparcos stars. Together with the radial velocity of 109 km s$^{-1}$, and the assumed distance of 8 kpc, these results imply a space motion of $(\Pi, \Theta, W) = (-5 \pm 12, 188 \pm 10, -49 \pm 15)$ km s$^{-1}$. While CMa has in-plane rotation similar to the mean of thick disk stars, it shows significant $(3\sigma)$ motion perpendicular to the disk, and differs even more $(7\sigma)$ from that expected for the Galactic warp. The $W$ velocity lends support to the argument that the CMa overdensity is part of a satellite galaxy remnant.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter

We present the first Hubble Space Telescope (HST ) WFPC2 V and I photometry for the Sculptor Group galaxy NGC 300 in four fields ranging from the center to the outer edge. We have made the first measurement of the star formation histories in two disk fields: the oldest stars were born at similar epochs and formation activity increased but at different mean rates. The main disk stellar population is predominantly old, consisting of red giant branch (RGB) and asymptotic giant branch (AGB) stars, based on a synthetic color-magnitude diagram analysis. The metallicity Z is found to have been less than 0.006 (or 0.33 Z ), with no evidence for significant change in mean Z over time in both disk fields. In the central region, we find a dearth of bright stars with respect to the two disk fields that cannot be explained by observational effects. Taken at face value, this finding would agree with the Davidge report of suppressed star formation there during the past 10 9 yr with respect to his disk fields at larger radii; but the possibility remains that significant central extinction affects our finding. We have also determined the first distance modulus estimate based on the tip of the red giant branch method. On the Cepheid distance scale of Ferrarese et al., we find ðm-M Þ 0 ¼ 26:56 AE 0:07ðAE0:13Þ mag and a similar value from the Cepheid-independent empirical method of Lee, Freedman, & Madore, both in good agreement with the Cepheid distance determined by Freedman et al. A discrepancy between this value and the theoretical calibration of the red giant branch tip magnitude method remains. Finally, we report a newly detected young (up to about 10 Myr) stellar association of about average size ($140 pc) in one of the disk fields.