Red Giants in the Halo of the S0 Galaxy NGC 3115: A Distance and a Bimodal Metallicity Distribution

Using the Hubble Space Telescope, we resolve the red giant branch in the halo of the S0 galaxy NGC 3115. We measure magnitudes and $(V-I)$ colours for stars down to 1.5 magnitudes below the tip of the red giant branch. From the brightest stars we estimate a distance modulus $(m-M)_0=30.21 \pm 0.30$, corresponding to a distance of $11.0 \pm 1.5$ Mpc. This is in excellent agreement with the value $(m-M)_0=30.17 \pm 0.13$ determined from the planetary nebula luminosity function. Our results rule out the shorter distance modulus $(m-M)_0=29.65$ determined from surface brightness fluctuations. A histogram of $(V-I)$ colours shows a clear bimodality, indicating the presence of two distinct halo populations of roughly equal size. One has [Fe/H]$\sim -0.7$ and one has [Fe/H]$\sim -1.3$. This is the most distant galaxy in which a Population II halo has been resolved, and it is the first time a colour bimodality has been observed among the halo stars of any early-type galaxy.Comment: 15 pages, 4 figures. MNRAS, in pres

HST Star Counts at High Galactic Latitudes

We use star counts from 13 deep HST fields imaged with the {\it Wide Field Camera - 2} in order to constrain the amount of dark matter in the Galaxy that can be in the form of low-mass main sequence stars or white-dwarfs. Based on the number of red stars found in our fields, we exclude the possibility that more than 25\% of the massive dark halo is made up of M dwarfs or subdwarfs; fairly massive ($M \sim 0.2 M_\odot$) and yet extremely faint ($M_I$ \gtsima $14.5$) stellar candidates would have to be invoked in order to make the observed number of stars compatible with that predicted by a stellar dark halo. White dwarfs must also be intrinsically very faint ($M_I$ \gtsima $14$) in order to be consistent with the observed number of faint stars in the HST fields. We also rule out an increasing or flat stellar luminosity function beyond $M_V \sim 13$. The inferred slope of the disk luminosity function is intermediary between local, volume-limited surveys and ground-based photometric ones. Finally, the magnitude counts are well fitted by existing models for the structure of the Galaxy, with only small changes in the fiducial values of the model parameters. The colour distribution, however, is not well reproduced by the models. It is unclear at present if this reflects inadequacies of the available models or uncertainties in the colour-magnitude diagrams for low metallicity stars and in the photometric calibration.Comment: 18 pages plus 6 figure

HST Observations of the Field Star Population in the Large Magellanic Cloud

We present $V$ and $I$ photometry, obtained with the Hubble Space Telescope, for $\sim 15,800$ stars in a field in the inner disk of the Large Magellanic Cloud. We confirm previous results indicating that an intense star formation event, probably corresponding to the formation of the LMC disk, occurred a few times $10^9$ years ago. We find a small but real difference between our field and one further out in the disk observed by Gallagher et al (1996): either star formation in the inner disk commenced slightly earlier, or the stars are slightly more metal rich. We also find evidence for a later burst, around 1 Gyr ago, which may correspond to the formation of the LMC bar. About 5% of the stars in our field are substantially older than either burst, and are probably members of an old disk or halo population with age $\sim 12$ Gyr.Comment: 18 pages, 7 figures: only 3 available ellectronically - complete copies by request from [email protected]

The Ages and Abundances of the M87 Globular Clusters

A subset of 150 globular clusters in M87 has been selected on the basis of S/N ratio for abundance and age determinations from the sample of Paper I. Indices measuring the strength of the strongest spectral features were determined for the M87 GCs and from new data for twelve galactic GCs. Combining the new and existing data for the galactic GCs and comparing the $(U-R)$ colors and the line indices gives qualitative indications for the ages and abundances of the GCs. Quantitative results are obtained by applying the Worthey (1994) models for the integrated light of stellar systems of a single age, calibrated by observations of galactic GCs, to deduce abundances and ages for the objects in our sample. We find that the M87 GCs span a wide range in metallicity, from very metal poor to somewhat above solar metallicity. The mean [Fe/H] of -0.95 dex is higher than that of the galactic GC system, and there is a metal rich tail that reaches to higher [Fe/H] than one finds among the galactic GCs. The mean metallicity of the M87 GC system is about a factor of four lower than that of the M87 stellar halo at a fixed projected radius $R$. The metallicity inferred from the X-ray studies is similar to that of the M87 stellar halo, not to that of GCs. We infer the relative abundances of Na, Mg, and Fe in the M87 GCs from the strength of their spectral features. The behavior of these elements between the metal rich and metal poor M87 GCs is similar to that shown by the galactic GCs and by halo stars in the Galaxy. The pattern of chemical evolution in these disparate old stellar systems is indistinguishable. We obtain a median age for the M87 GC system of 13 Gyr, similar to that of the galactic GCs, with a small dispersion about this value.Comment: 56 pages with included postscript figures; added derived M87 GC metallicities to Table 2, a statistical analysis of possible bimodality, an appendix on the metallicity calibration of U-R and the Washington system, and other smaller changes. Accepted for publication in ApJ. (See paper for complete version of the Abstract.

The Surface Brightness Fluctuations and Globular Cluster Populations of M87 and its Companions

Using the surface brightness fluctuations in HST WFPC-2 images, we determine that M87, NGC 4486B, and NGC 4478 are all at a distance of ~16 Mpc, while NGC 4476 lies in the background at ~21 Mpc. We also examine the globular clusters of M87 using archived HST fields. We detect the bimodal color distribution, and find that the amplitude of the red peak relative to the blue peak is greatest near the center. This feature is in good agreement with the merger model of elliptical galaxy formation, where some of the clusters originated in progenitor galaxies while other formed during mergers.Comment: 5 pages, 2 figure