arXiv subjects

W. E. Harris

Publications and source records attributed to W. E. Harris.

At least 37 records · Page 2Linked to original sources

arXiv2006

The Faint End of the Luminosity Function in the Core of the Coma Cluster

We present optical measurements of the faint end of the luminosity function in the core of the Coma cluster. Dwarf galaxies are detected down to a limiting magnitude of R approx. 25.75 in images taken with the Hubble Space Telescope. This represents the faintest determination of the Coma luminosity function to date. With the assumption that errors due to cosmic variance are small, evidence is found for a steep faint end slope with alpha < -2. Such a value is expected in theories in which reionization and other feedback processes are dependent on density.

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arXiv2005

Structural Parameters from Ground-based Observations of Newly Discovered Globular Clusters in NGC 5128

We have investigated a number of globular cluster candidates from a recent wide-field study by Harris et al. (2004a) of the giant elliptical galaxy NGC 5128. We used the Magellan I telescope + MagIC camera under excellent seeing (0.3"-0.6") and obtained very high resolution images for a sample of 44 candidates. Our images allow us to study the light profiles of the likely clusters. This is the first ground-based study of structural parameters for globular clusters outside the Local Group. We compare the psf-deconvolved profiles with King models and derive structural parameters, ellipticities and surface brightnesses. In general, our clusters are similar in size, ellipticity, core radius and central surface brightness to their counterparts in other galaxies, in particular those in NGC 5128 observed with HST by Harris et al. (2002). However, they extend to higher ellipticities and larger half-light radii than their Galactic counterparts. Combining our results with those of Harris et al. fills in the gaps previously existing in $r_h - M_V$ parameter space and indicates that any substantial difference between presumed distinct cluster types in this diagram, including for example the Faint Fuzzies of Larsen & Brodie (2000) and the `extended, luminous' M31 clusters of Huxor et al. (2005) is now removed and that clusters form a continuum in this diagram. Indeed, this continuum now extends to the realm of the Ultra Compact Dwarfs. We have carried out additional analysis to quantify the contamination by background galaxies. This shows that, although galaxies cannot be easily told apart from clusters in some of the structural diagrams, the combination of excellent image quality and Washington photometry should limit the contamination to roughly 10% of the population of cluster candidates.

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arXiv2005

Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High-Luminosity Clusters

We present new (B,I) photometry for the globular cluster systems in eight Brightest Cluster Galaxies (BCGs), obtained with the ACS/WFC camera on the Hubble Space Telescope. In the very rich cluster systems that reside within these giant galaxies, we find that all have strongly bimodal color distributions All the BCGs show population gradients, with much higher relative numbers of red clusters within 5 kpc of their centers, consistent with their having formed at later times than the blue, metal-poor population. A striking new feature of the color distributions emerging from our data is that for the brightest clusters (M_I < -10.5) the color distribution becomes broad and less obviously bimodal. we suggest that it may be a characteristic of many BCGs. Furthermore, the blue (metal-poor) clusters become progressively redder with increasing luminosity, following a mass/metallicity scaling relation Z ~ M^0.55. We argue that these GCS characteristics are consistent with a hierarchical-merging formation picture in which the metal-poor clusters formed in protogalactic clouds or dense starburst complexes with gas masses in the range 10^7 - 10^10 M_Sun, but where the more massive clusters on average formed in bigger clouds with deeper potential wells where more pre-enrichment could occur.

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arXiv2005

Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch

Using the HST Wide Field Camera of the Advanced Camera for Surveys (ACS), we have obtained deep (V,I) photometry of an outer-halo field in NGC 5128, to a limiting magnitude of I~29. Our photometry directly reveals the core helium-burning stellar population (the ``red clump'' or horizontal branch) in a giant E/S0 galaxy for the first time. The color-magnitude diagram displays a very wide red giant branch (RGB), an asymptotic giant branch (AGB) bump, and the red clump; no noticeable population of blue HB stars is present, confirming previous suggestions that old, very metal-poor population is not ubiquitous in the halo of this galaxy. From the upper RGB we derive the metallicity distribution, which we find to be very broad and moderately metal-rich, with average [M/H]=-0.64 and dispersion 0.49 dex. The MDF is virtually identical to that found in other halo fields observed previously with the HST, but with an enhanced metal-rich population which was partially missed in the previous surveys due to V-band incompleteness for these very red stars. Combining the metallicity sensitive colors of the RGB stars with the metallicity and age sensitive features of the AGB bump and the red clump, we infer the average age of the halo stars to be 8^{+3}_{-3.5} Gy. As part of our study, we present an empirical calibration of the ACS F606W and F814W filters to the standard V and I bands, achieved with ground-based observations of the same field made from the EMMI camera of the New Technology Telescope of the ESO La Silla Observatory.

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arXiv2005

Extragalactic Globular Clusters in the Near-Infrared: V. IC 4051 and NGC 3311

We present the results of combined optical and near-infrared photometry for the globular cluster systems of the giant ellipticals IC 4051 and NGC 3311. We use the reduced age-metallicity degeneracy in (V-I) vs.(V-H) color-color diagrams to derive the cumulative age distribution within the red sub-population of globular clusters and to search for age sub-populations. The age distribution is then compared to the one determined for simulated globular cluster systems in order to set constraints on the relative age and size of these globular cluster sub-populations. In both galaxies we find a significant fraction of globular clusters with ages between 2- 5 Gyr. We also investigate the metallicity distribution in both systems. Small number statistics prevent us from making any definite statements concerning NGC 3311, but we find that the derived metallicity distribution of the IC 4051 clusters strongly depends on the assumed age distribution. Based on our most likely result that finds a large number of young/intermediate age clusters (~2 Gyr) within the selected globular cluster sample, we find metallicity peaks at $\sim$ -0.2 for the old clusters and +0.8 for the young clusters. Only few very metal poor clusters are found. However, the metallicity distribution within the young/intermediate globular cluster population is significantly affected by our choice of the applied Single Stellar Population model. The mean metallicity of the second generation of globular clusters changes from the above mentioned and extremely high +0.8 dex to +0.2 dex. Note that the model dependency becomes less severe with an increasing age of the cluster population.

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arXiv2003

X-ray Point Sources in the Sombrero Galaxy: Supersoft sources, the globular cluster/LMXB connection, and an overview

We report on the population of point sources discovered during an 18.5 ksec Chandra ACIS-S observation of the Sombrero Galaxy. We present the luminosity function, the spectra of the 6 brightest sources, consider correlations with globular clusters (GCs) and with planetary nebulae (PNe), and study the galaxy's population of SSSs. We detected 122 sources, 22 of them are identified as luminous supersoft X-ray sources (SSSs). There is an over density of SSSs within 1.5 kpc of the nucleus, which is itself the brightest X-ray source. SSSs are also found in the disk and halo, with 1 SSS in a globular cluster (GC). Several sources in Sombrero's halo are good candidates for SSS models in which the accretor is a nuclear-burning white dwarf. In total, 32 X-ray sources are associated with GCs. The majority of sources with luminosity > 1e38 erg/s are in GCs. These results for M104, an Sa galaxy, are similar to what has been found for elliptical galaxies and for the late-type spiral M31. We find that those optically bright GCs with X-ray sources house only the brightest X-ray sources. We find that, in common with other galaxies, there appears to be a positive connection between young (metal-rich) GCs and X-ray sources, but that the brightest X-ray sources are equally likely to be in metal-poor GCs. We propose a model which can explain the trends seen in the data sets from the Sombrero and other galaxies. Thermal-time scale mass transfer can occur in some of the the younger clusters in which the turn-off mass is slightly greater than $0.8 M_\odot$; multiplicity may play a role in some of the most massive clusters; accretion from giant stars may be the dominant mechanism in some older, less massive and less centrally concentrated clusters.

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arXiv2002

Photospheric radius expansion X-ray bursts as standard candles

We examined the maximum bolometric peak luminosities during type I X-ray bursts from the persistent or transient luminous X-ray sources in globular clusters. We show that for about two thirds of the sources the maximum peak luminosities during photospheric radius expansion X-ray bursts extend to a critical value of (3.79+/-0.15)x10^{38} erg/s, assuming the total X-ray burst emission is entirely due to black-body radiation and the recorded maximum luminosity is the actual peak luminosity. This empirical critical luminosity is consistent with the Eddington luminosity limit for hydrogen poor material. Since the critical luminosity is more or less always reached during photospheric radius expansion X-ray bursts (except for one source), such bursts may be regarded as empirical standard candles. However, because significant deviations do occur, our standard candle is only accurate to within 15%. We re-evaluated the distances to the twelve globular clusters in which the X-ray bursters reside.

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arXiv2002

Structural Parameters for Globular Clusters in NGC 5128

We present new imaging measurements of 27 individual globular clusters in the halo of the nearby elliptical galaxy NGC 5128, obtained with the Hubble Space Telescope STIS and WFPC2 cameras. Combining these with similar data for selected inner-halo clusters from Holland et al. 1999 (AAp, 348, 418), we now have a total sample of 43 NGC 5128 globular clusters with measured structural properties. Classic King-model profiles match the clusters extremely well, and their core- and half-light radii, central surface brightness, and central concentration fall in very much the same range as do the clusters in the Milky Way and M31. We find half a dozen bright clusters which show tentative evidence for light that extends beyond the nominal tidal radius, or possibly ones in which anisotropic velocity distributions are important. We also confirm previous indications that NGC 5128 contains relatively more clusters with large (> 0.2) ellipticity than does the Milky Way. Finally, calculations of the cluster binding energies E_b as defined by McLaughlin 2000 (ApJ, 539,618) show that the NGC 5128 clusters occupy the same extremely narrow region of the parametric ``fundamental plane'' as do their Milky Way counterparts. Our data are thus strongly consistent with the claim that the globular clusters in both NGC 5128 and the Milky Way are fundamentally the same type of object: old star clusters with similar mass-to-light ratios and King-model structures.

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arXiv2002

The Halo Stars in NGC 5128. III: An Inner-Halo Field and the Metallicity Distribution

We present new HST/WFPC2 (V,I) photometry for the red-giant stars in NGC 5128 at a projected distance of 8 kpc from the galaxy center, which probe a mixture of its inner halo and outer bulge. The color-magnitude diagram shows an old red-giant branch which 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. 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. A comparison with the MDF for the NGC 5128 globular clusters indicates that there is a clear decrease of specific frequency $S_N$ (number of clusters per unit halo light) with increasing metallicity, from S_N ~ 4-8 at [Fe/H] < -1.6 down to S_N = 1.5 at [Fe/H] > -1. This trend may indicate that globular cluster formation efficiency is a strong function of the metallicity of the protocluster gas.

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arXiv2001

Globular Cluster Mass and Metallicity Distributions: Reconstructing the Events During Halo Formation

Globular clusters in most large galaxies are a mixture of metal-poor and metal-rich (bimodal), but the halo stars are almost entirely metal-rich. This and other lines of evidence argue that the metal-poor globular clusters formed within widely distributed 10^8-10^9 Solar masses gas clouds (supergiant GMCs) during an early burst in which most of the gas was ejected or unused till later rounds of star formation. New simulations of the growth of pre-galactic potential wells in the early universe now indicate that the initial power-law form of the globular cluster mass distribution (dN/dM ~ M^-1.8) is a miniature replica of the mass distribution of the SGMCs themselves, which grow hierarchically in the CDM potential wells of large protogalaxies.

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arXiv2001

On the Formation of Galaxy Halos: Comparing NGC 5128 and the Local Group Members

The metallicity distribution function (MDF) for the old red-giant stars in the halo of NGC 5128, the nearest giant elliptical galaxy, is virtually identical with the MDF for the old-disk stars in the LMC and also strongly resembles the halo MDF in M31. These galaxies all have high mean halo metallicities ( ~ -0.4$) with very small proportions of low-metallicity stars. These observations reinforce the view that metal-rich halos are quite normal for large galaxies of all types. Such systems are unlikely to have built up by accretion of pre-existing, gas-free small satellite galaxies, unless these satellites had an extremely shallow mass distribution (d log N / d log M > -1). We suggest that the halo of NGC 5128 is more likely to have assembled from hierarchical merging of gas-rich lumps in which the bulk of star formation took place during or after the merger stage.

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arXiv2000

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

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.

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arXiv1999

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

Deep HST/WFPC2 (V,I) photometry is used to investigate the globular cluster system (GCS) in NGC 4874, the central cD galaxy of the Coma cluster. The luminosity function of the clusters displays its normal Gaussian-like shape and turnover level. Other features of the system are surprising: the GCS is (a) spatially extended, with core radius r_c = 22 kpc, (b) entirely metal-poor (a narrow, unimodal metallicity distribution with mean [Fe/H] = -1.5), and (c) modestly populated, with specific frequency S_N = 3.7 +- 0.5. We suggest on the basis of some simple models 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 formation picture. Even when they are used in combination, strong challenges to these models remain. 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.

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arXiv1999

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

We have used deep HST/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 find the ``turnover'' 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 dispersion SIGMA_V = 1.49 +/- 0.12. We use the GCLF as a standard candle by matching the turnover points in NGC 4874 and another Coma elliptical, IC 4051, with those of the giant ellipticals in the Virgo cluster (M87 and five others). The result is (m-M)_(Coma - Virgo) = 4.06 +/- 0.11 magnitudes, which converts to a Coma distance d = 102 Mpc if the Virgo distance modulus is (m-M)_0 = 30.99 +/- 0.04. The Hubble constant which emerges from our GCLF measurement is then H0 = (69 +/- 9) km/s/Mpc. We confirm this H0 value with a novel presentation of the ``Hubble diagram'' for GCLFs in giant E galaxies. Measurements of additional GCLFs in the Coma ellipticals, as well as calibrating galaxies in Virgo and Fornax, have excellent potential to refine this result in the near future.

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arXiv1998

The Metallicity Distribution in the Halo Stars of NGC 5128: Implications for Galaxy Formation

The Hubble Space Telescope has been used to obtain WFPC2 (V,I) photometry for a sample of 10,000 red giant stars in the outer halo of the giant elliptical NGC 5128. Comparison with the fiducial RGBs of Milky Way globular clusters and model isochrones demonstrates that this outer-halo population is completely dominated by old stars, with an extremely broad metallicity range extending from the most metal-poor Galactic globulars at [Fe/H] = -2 up to Solar abundance. By interpolation within the fiducial lines we derive the metallicity distribution function (MDF) for the halo, which turns out to have a distinct two-component structure: about 2/3 of the stars belong to a sharply peaked metal-richer component ([Fe/H] = -0.3, sigma = 0.22 dex), while the remaining one-third belong to a more extended metal-poor distribution sloping gradually down to low metallicity. The MDF can be remarkably well matched by a simple closed-box model of chemical enrichment, where the first component starts with an initial abundance Z_0 =0 and the second component with Z_0 = 0.18 Z_Sun. A basic "in situ" formation picture with two distinct epochs of star formation fits the observations better than other models involving major contributions from accretions or mergers.

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arXiv1998

A Color-Magnitude Diagram for a Globular Cluster In the Giant Elliptical Galaxy NGC 5128

The Hubble Space Telescope has been used to obtain WFPC2 (V,I) photometry for a large sample of stars in the outer halo of the giant elliptical NGC 5128 (d = 4 Mpc). The globular cluster N5128-C44, at the center of the Planetary Camera field, is well enough resolved to permit the construction of a color-magnitude diagram (CMD) for it which covers the brightest two magnitudes of the giant branch. The CMD is consistent with that of a normal old, moderately low-metallicity ([Fe/H] = -1.30 globular cluster, distinctly more metal-poor than most of the field halo stars at the same projected location (which average [Fe/H] ~ -0.5). This is the most distant globular cluster in which direct color-magnitude photometry has been achieved to date, and the first one belonging to a giant E galaxy.

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arXiv1998

The Hubble Constant from Observations of the Brightest Red Giant Stars in a Virgo-Cluster Galaxy

The Virgo and Fornax clusters of galaxies play central roles in determining the Hubble constant H_0. A powerful and direct way of establishing distances for elliptical galaxies is to use the luminosities of the brightest red-giant stars (the TRGB luminosity, at M_I = -4.2). Here we report the direct observation of the TRGB stars in a dwarf elliptical galaxy in the Virgo cluster. We find its distance to be 15.7 +- 1.5 Megaparsecs, from which we estimate a Hubble constant of H_0 = 77 +- 8 km/s/Mpc. Under the assumption of a low-density Universe with the simplest cosmology, the age of the Universe is no more than 12-13 billion years.

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arXiv1998

M87, Globular Clusters, and Galactic Winds: Issues in Giant Galaxy Formation

New VRI photometry is presented for the globular clusters in the innermost 140'' of the M87 halo. The results are used to discuss several issues concerning the formation and evolution of globular cluster systems in supergiant ellipticals like M87. (1) we find no significant change in the globular cluster luminosity function (GCLF) with galactocentric radius, for cluster masses M < 10^5 solar masses, indicating that the main effects of dynamical evolution may be only on lower-mass clusters. (2) Within the core radius (1') of the globular cluster system, the metallicity distribution is uniform, but at larger radii the mean metallicity declines steadily as Z ~ r^-0.9. (3) The various options for explaining the existence of high specific frequency galaxies like M87 are evaluated, and scaling laws for the GCSs in these galaxies are given. Interpretations involving secondary evolution (formation of many globular clusters during mergers, intergalactic globular clusters, etc.) are unlikely to be the primary explanation for high-S_N galaxies. (4) We suggest that central-supergiant E galaxies may have formed in an exceptionally turbulent or high-density environment in which an early, powerful galactic wind drove out a high fraction of the protogalactic gas, thus artificially boosting the specific

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