A comparison of the near-infrared spectral features of early-type galaxies in the Coma Cluster, the Virgo cluster and the field

Earlier researchers derived the relative distance between the Coma and Virgo clusters from color-magnitude relations of the early-type galaxies in each cluster. They found that the derived distance was color-dependent and concluded that the galaxies of similar luminosity in the two clusters differ in their red stellar populations. More recently, the color-dependence of the Coma-Virgo distance modulus has been called into question. However, because these two clusters differ so dramatically in their morphologies and kinematics, it is plausible that the star formation histories of the member galaxies also differed. If the conclusions of earlier researchers are indeed correct, then some signature of the resulting stellar population differences should appear in the near-infrared and/or infrared light of the respective galaxies. We have collected near-infrared spectra of 17 Virgo and 10 Coma early-type galaxies; this sample spans about four magnitudes in luminosity in each cluster. Seven field E/S0 galaxies have been observed for comparison. Pseudo-equivalent widths have been measured for all of the field galaxies, all but one of the Virgo members, and five of the Coma galaxies. The features examined are sensitive to the temperature, metallicity, and surface gravity of the reddest stars. A preliminary analysis of these spectral features has been performed, and, with a few notable exceptions, the measured pseudo-equivalent widths agree well with previously published values

Faint Stars in the Ursa Minor Dwarf Spheroidal Galaxy: Implications for the Low-Mass Stellar Initial Mass Function at High Redshift

The stellar initial mass function at high redshift is an important defining property of the first stellar systems to form and may also play a role in various dark matter problems. We here determine the faint stellar luminosity function in an apparently dark-matter-dominated external galaxy in which the stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a system with a particularly simple stellar population - all of the stars being old and metal-poor - similar to that of a classical halo globular cluster. A direct comparison of the faint luminosity functions of the UMi Sph and of similar metallicity, old globular clusters is equivalent to a comparison of the initial mass functions and is presented here, based on deep HST WFPC2 and STIS imaging data. We find that these luminosity functions are indistinguishable, down to a luminosity corresponding to 0.3 solar masses. Our results show that the low-mass stellar IMF for stars that formed at very high redshift is apparently invariant across environments as diverse as those of an extremely low-surface-brightness, dark-matter-dominated dwarf galaxy and a dark-matter-free, high-density globular cluster within the Milky Way.Comment: Accepted by New Astronomy. 64 pages, including 9 embedded postscript tables and 20 embedded postscript figures, plus 14 separate jpeg figures. Postscript versions of the jpeg figures and a complete version of the paper with all figures embedded can be found at http://tarkus.pha.jhu.edu/~mlh

Giants in old open clusters - Temperatures, luminosities, and abundances from infrared photometry

We present infrared observations for more than 100 red giants and clump stars in eight old open clusters. We have assembled the best available optical photometry for these stars and determined a consistent set of integrated physical parameters (reddening, distance modulus, abundance, and age) for the clusters. From color-color and color-magnitude diagrams that make use of both the infrared and optical data, we are able to identify quite a few stars as probable field stars rather than cluster members. However, because of a general scarcity of bright stars, it is often difficult to distinguish between cluster members on the asymptotic giant branch and field interlopers. In a ( U-V)_0, ( V-K)_0 plot, stars from the most metal-poor open clusters tend to lie between the relations defined by field and globular cluster giants. On the other hand, nearly all of the open cluster stars lie near the field giant line in a (H-K)_0 plot. The mean CO strengths of the giants in each open cluster show a range consistent with the optically determined range in [Fe/H], but the correlation between these two quantities is weak, probably because of the small total range of each and the significant uncertainties in [Fe/H]. The results for the open clusters, though, are consistent with the relation between CO and [Fe/H] established for globular clusters and considerably strengthen that relation near the solar metallicity end. For these eight open clusters, there is a modest linear correlation between [Fe/H] and age which shows a gradient in metallicity of about —0.1 dex per Gyr and gives [Fe/H] ~ -0.6 for τ = 5 Gyr, the age of the sun. The relation is quite similar in slope and zero point to that exhibited by clusters in the Large Magellanic Cloud. If the open cluster data are adjusted for the galactic radial metallicity gradient, the age-metallicity relation becomes intermediate to those of the Large Magellanic Cloud and the solar neighborhood. This could be an indication that the old open clusters are representative of the stellar population of the galactic thick disk. The temperatures of the cluster giant branches determined from infrared observations are tightly correlated with the optically derived values for age and [Fe/H]. Also, there is general agreement between the location of the cluster giant branches in an H-R diagram determined from infrared photometry and the predictions of the Revised Yale Isochrones. Specific differences that exist between the theoretical and semiempirical parameters can at least partially be attributed to uncertainties in the (primarily) optical data and/or the presence of convective overshootin

The Frequency of Barred Spiral Galaxies in the Near-IR

We have determined the fraction of barred galaxies in the H-band for a statistically well-defined sample of 186 spirals drawn from the Ohio State University Bright Spiral Galaxy survey. We find 56% of our sample to be strongly barred at H, while another 16% is weakly barred. Only 27% of our sample is unbarred in the near-infrared. The RC3 and the Carnegie Atlas of Galaxies both classify only about 30% of our sample as strongly barred. Thus strong bars are nearly twice as prevalent in the near-infrared as in the optical. The frequency of genuine optically hidden bars is significant, but lower than many claims in the literature: 40% of the galaxies in our sample that are classified as unbarred in the RC3 show evidence for a bar in the H-band, while for the Carnegie Atlas this fraction is 66%. Our data reveal no significant trend in bar fraction as a function of morphology in either the optical or H-band. Optical surveys of high redshift galaxies may be strongly biased against finding bars, as bars are increasingly difficult to detect at bluer rest wavelengths.Comment: LaTeX with AASTeX style file, 23 pages with 6 figures. Accepted for publication in The Astronomical Journal (Feb. 2000