Showing papers by "William E. Harris published in 2000"

The Halo Stars in NGC 5128. II. An Outer Halo Field and a New Metallicity Distribution

Abstract: We present new Hubble Space Telescope WFPC2 (V, I) photometry for field stars in NGC 5128 at a projected distance of 8 kpc from the galaxy center, which probes a mixture of its inner halo and outer bulge. The color-magnitude diagram shows an old red giant branch that is even broader in color than our two previously studied outer halo fields (at 21 and 31 kpc), with significant numbers of stars extending to solar metallicity and higher. The peak frequency of the metallicity distribution function (MDF) is at [m/H] -0.4, with even fewer metal-poor stars than in the outer halo fields. If we use the 21 and 31 kpc fields to define template "halo" MDFs and subtract these from the 8 kpc field, the residual "bulge" population has a mean [m/H] -0.2, similar to the bulges of other large spiral and elliptical galaxies. We find that the main features of the halo MDF can be reproduced by a simple chemical evolution model in which early star formation goes on simultaneously with an initial stage of rapid infall of very metal-poor gas, after which the infall dies away exponentially. Finally, by comparison with the MDFs for the NGC 5128 globular clusters, we find that in all the halo fields we have studied there is a clear decrease of specific frequency SN (number of clusters per unit halo light) with increasing metallicity. At the lowest-metallicity range ([Fe/H] -1 it has dropped to 1.5. This trend may indicate that globular cluster formation efficiency is a strong function of the metallicity of the protocluster gas. However, we suggest an alternate possibility, which is that globular clusters form preferentially sooner than field stars. If most of the cluster formation within a host giant molecular cloud takes place sooner than most of the distributed field-star formation and if the earliest most metal-poor star-forming clouds are prematurely disrupted by their own first bursts of star formation, then they would leave relatively few field stars with a high-SN population. The high specific frequency at low metallicity may therefore be related to the comparably large SN values found in the most metal-poor dwarf elliptical galaxies.

The Globular Cluster Systems in the Coma Ellipticals. I. The Luminosity Function in NGC 4874 and Implications for Hubble’s Constant*

Abstract: We have used deep Hubble Space Telescope WFPC2 images in V (F606W) and I (F814W) to measure the luminosity distribution of the globular clusters in NGC 4874, the central cD galaxy of the Coma cluster. We —nd the ii turnover ˇˇ point of the globular cluster luminosity function (GCLF) to lie at V \ 27.88 ^ 0.12, while the overall GCLF shape matches the standard Gaussian-like form with disper- sion We use the GCLF as a standard candle by matching the turnover points in NGC p V \ 1.49 ^ 0.12. 4874 and another Coma elliptical, IC 4051, with those of the giant ellipticals in the Virgo cluster (M87 and —ve others). The result is *(m ( M) (ComaVirgo) \ 4.06 ^ 0.11 mag, which converts to a Coma distance d \ 102 Mpc if the Virgo distance modulus is

The Globular Cluster Systems in the Coma Ellipticals. II. Metallicity Distribution and Radial Structure in NGC 4874 and Implications for Galaxy Formation

Abstract: Deep Hubble Space Telescope WFPC2 images in V and I are used to investigate the globular cluster system (GCS) in NGC 4874, the central cD galaxy of the Coma cluster. Although the luminosity function of the clusters displays its normal Gaussian-like shape and turnover level, other features of the system are surprising. We find the GCS to be (a) spatially extended, with core radius rc ~ 22 kpc, (b) entirely metal poor (a narrow, unimodal metallicity distribution with [Fe/H] ~ -1.5), and (c) modestly populated for a cD-type galaxy, with specific frequency SN = 3.7 ± 0.5. Model interpretations suggest to us that as much as half of this galaxy might have accreted from low-mass satellites, but no single one of the three classic modes of galaxy formation (accretion, disk mergers, in situ formation) can supply a fully satisfactory model for the formation of NGC 4874. Even when they are used in combination, strong challenges to these models remain. We suggest that the principal anomaly in this GCS is essentially the complete lack of metal-rich clusters. If these were present in normal (M87-like) numbers in addition to the metal-poor ones that are already there, then the GCS in total would more closely resemble what we see in many other giant E galaxies. This supergiant galaxy appears to have avoided forming globular clusters during the main metal-rich stage of star formation that built the bulk of the galaxy.

The Globular Cluster Systems in the Coma Ellipticals. III. The Unique Case of IC 4051

Abstract: IC 4051 is a giant E galaxy on the outskirts of the Coma Cluster core. Using archival HST WFPC2 data, we derive the metallicity distribution, luminosity function, and spatial structure of its globular cluster system (GCS). The metallicity distribution derived from the V-I colors has a mean Fe/H -0.3, a near-complete lack of metal-poor clusters, and only a small metallicity gradient with radius. We tentatively suggest that the GCS has two roughly equal metallicity subcomponents, one centered at [Fe/H] ~ 0.0 and the second at [Fe/H] ~ -1.0, although their identification is blurred by the photometric uncertainties. The luminosity distribution (globular cluster luminosity function) has the standard Gaussian-like form observed in all other giant E galaxies, with a peak (turnover) at V0 = 27.8, consistent with a Coma distance of 100 Mpc. The radial profiles of both the GCS and the halo light show an unusually steep falloff, which may indicate that the halo of this galaxy has been tidally truncated. Lastly, the specific frequency of the GCS is remarkably large: we find SN = 11 ? 2, at a level that rivals M87 and most others in the central cD-type category, even though IC 4051 is not a cD or brightest cluster elliptical. This galaxy exhibits a combination of GCS characteristics found nowhere else. A formation model consistent with most of the observations would be that this galaxy was subjected to removal of a large fraction of its protogalactic gas shortly after its main phase of globular cluster formation, probably by its first passage through the Coma core. Since then, no significant additions due to accretions or mergers have taken place, in strong contrast to the central Coma galaxy NGC 4874.

The Globular Cluster Systems in the Coma Ellipticals. III: The Unique Case of IC 4051

Abstract: Using archival \hst WFPC2 data, we derive the metallicity distribution, luminosity function, and spatial structure of the globular cluster system around IC 4051, a giant E galaxy on the outskirts of the Coma cluster core. The metallicity distribution derived from the (V-I) colors has a mean [Fe/H] = -0.3, a near-complete lack of metal-poor clusters, and only a small metallicity gradient with radius; it may, however, have two roughly equal metallicity subcomponents, centered at [Fe/H] ~ 0.0 and -1.0. The luminosity distribution (GCLF) has the Gaussian-like form observed in all other giant E galaxies, with a peak (turnover) at V = 27.8, consistent with a Coma distance of 100 Mpc. The radial profiles of both the GCS and the halo light show an unusually steep falloff which may indicate that the halo of this galaxy has been tidally truncated. Lastly, the specific frequency of the GCS is remarkably large: we find S_N = 11 +- 2, resembling the central cD-type galaxies even though IC 4051 is not a cD or brightest cluster elliptical. A formation model consistent with most of the observations would be that this galaxy was subjected to removal of a large fraction of its protogalactic gas shortly after its main phase of globular cluster formation, probably by its first passage through the Coma core. Since then, no significant additions due to accretions or mergers have taken place.