arXiv subjects

William E. Harris

Publications and source records attributed to William E. Harris.

At least 109 records · Page 6Linked to original sources

arXiv2007

Wide-Field Survey of Globular Clusters in M31. II. Kinematics of the Globular Cluster System

We present a kinematic analysis of the globular cluster(GC) system in M31. Using the photometric and spectroscopic database of 504 GCs, we have investigated the kinematics of the M31 GC system. We find that the all GC system shows strong rotation, with rotation amplitude of v_rot~190km/s, and that a weak rotation persists even for the outermost samples at |Y|>5kpc. The rotation-corrected velocity dispersion for the GC system is estimated to be sigma_{p,r}~130km/s, and it increases from sigma_{p,r}~120km/s at |Y|<1kpc to sigma_{p,r}~150km/s at |Y|>5kpc. These results are very similar to those for the metal-poor GCs. This shows that there is a dynamically hot halo in M31 that is rotating but primarily pressure-supported. We have identified 50 "friendless" GCs, and they appear to rotate around the major axis of M31. For the subsamples of metal-poor and metal-rich GCs, we have found that the metal-rich GCs are more centrally concentrated than the metal-poor GCs, and both subsamples show strong rotation. For the subsamples of bright and faint GCs, it is found that the rotation for the faint GCs is stronger than that for the bright GCs. We have identified 56 GCs and GC candidates with X-ray detection. It is found that the majority of X-ray emitting GCs follow the disk rotation, and that the redder, more metal-rich, and brighter GCs are more likely to be detected as X-ray emitting GCs. We have derived a rotation curve of M31 using the GCs at |Y|<0.6kpc. We have estimated the dynamical mass of M31 using `Projected Mass Estimator(PME)' and `Tracer Mass Estimator(TME)'. We finally discuss the implication of these results and compare the kinematics of GCs with that of planetary nebulae in M31.

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arXiv2007

Wide-Field Survey of Globular Clusters in M31. I. A Catalog of New Clusters

We present the result of a wide-field survey of globular clusters (GCs) in M31 covering a 3deg x 3deg field c. We have searched for GCs on CCD images taken with Washington CMT1 filters at the KPNO 0.9 m telescope using steps: (1) inspection of morphological parameters given by the SExtractor package such as stellarity, full maximum, and ellipticity; (2) consulting the spectral types and radial velocities obtained from spectra takena spectrograph at the WIYN 3.5 m telescope; and (3) visual inspection of the images of each object. We have and GC candidates, of which 605 are newly found GCs and GC candidates and 559 are previously known GCs. Amoects there are 113 genuine GCs, 258 probable GCs, and 234 possible GCs, according to our classification critee known objects there are 383 genuine GCs, 109 probable GCs, and 67 possible GCs. In total there are 496 genprobable GCs and 301 possible GCs. Most of these newly found GCs have T1 magnitudes of 17.5 - 19.5 mag, [17.9 < V < 19.9 mag assuming (C-T1) ~ 1.5], and (C-T1) colors in the range 1 - 2.

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arXiv2007

The Kinematics and Dynamics of the Globular Clusters and the Planetary Nebulae of NGC 5128

A new kinematic and dynamic study of the halo of the giant elliptical galaxy, NGC 5128, is presented. From a spectroscopically confirmed sample of 340 globular clusters and 780 planetary nebulae, the rotation amplitude, rotation axis, velocity dispersion, and the total dynamical mass are determined for the halo of NGC 5128. The globular cluster kinematics were searched for both radial dependence and metallicity dependence by subdividing the globular cluster sample into 158 metal-rich ([Fe/H] > -1.0) and 178 metal-poor ([Fe/H] < -1.0) globular clusters. Our results show the kinematics of the metal-rich and metal-poor subpopulations are quite similar. The kinematics are compared to the planetary nebula population where differences are apparent in the outer regions of the halo. The total mass of NGC 5128 is found using the Tracer Mass estimator (Evans et al. 2003), to determine the mass supported by internal random motions, and the spherical component of the Jeans equation to determine the mass supported by rotation. We find a total mass of (1.0+/-0.2) x 10^(12) Msun from the planetary nebulae data out to a projected radius of 90 kpc and (1.3+/-0.5) x 10^(12) Msun from the globular clusters out to a projected radius of 50 kpc. Lastly, we present a new and homogeneous catalog of known globular clusters in NGC 5128. This catalog combines all previous definitive cluster identifications from radial velocity studies and HST imaging studies, as well as 80 new globular clusters from a study of M.A. Beasley et al. (2007, in preparation).

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arXiv2006

Structural Parameters for Globular Clusters in NGC 5128. II: HST/ACS Imaging and New Clusters

We report the first results from an imaging program with the ACS camera on HST designed to measure the structural characteristics of a wide range of globular clusters in NGC 5128, the nearest giant elliptical galaxy. From 12 ACS/WFC fields, we have measured a total of 62 previously known globular clusters and have discovered 69 new high-probability cluster candidates not found in any previous work. We present magnitudes and color indices for all of these, along with rough measurements of their effective diameters and ellipticities. The luminosity distribution of this nearly-uncontaminated sample of clusters matches well with the normal GCLF for giant elliptical galaxies, and the cluster scale size and ellipticity distributions are similar to those in the Milky Way system. The indication from this survey is that many hundreds of individual clusters remain to be found with carefully designed search techniques in the future. A very rough estimate of the total cluster population from our data suggests N_GC = 1500 in NGC 5128, over all magnitudes and within a projected radius R = 25' from the galaxy center.

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arXiv2005

Two Populations of Young Massive Star Clusters in Arp 220

We present new optical observations of young massive star clusters in Arp 220, the nearest ultraluminous infrared galaxy, taken in UBVI with the Hubble Space Telescope ACS/HRC camera. We find a total of 206 probable clusters whose spatial distribution is centrally concentrated toward the nucleus of Arp 220. We use model star cluster tracks to determine ages, luminosities, and masses for 14 clusters with complete UBVI indices or previously published near-infrared data. We estimate rough masses for 24 additional clusters with I < 24 mag from BVI indices alone. The clusters with useful ages fall into two distinct groups: a ``young'' population (< 10 Myr) and an intermediate-age population (~300 Myr). There are many clusters with masses clearly above 10^6 Msun and possibly even above 10^7 Msun in the most extreme instances. These masses are high enough that the clusters being formed in the Arp 220 starburst can be considered as genuine young globular clusters. In addition, this study allows us to extend the observed correlation between global star formation rate and maximum cluster luminosity by more than an order of magnitude in star formation rate.

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arXiv2005

Radial Velocities of Newly Discovered Globular Clusters in NGC 5128

We present radial velocity measurements for 74 globular clusters (GCs) in the nearby giant elliptical NGC 5128, of which 31 are newly discovered clusters. All the GC candidates were taken from the list of possible new clusters given in the Harris, Harris, & Geisler (2004) photometric survey. In addition to the newly confirmed clusters, we identified 24 definite foreground stars and 31 probable background galaxies. From a combined list of 299 known GCs in NGC 5128 with measured radial velocities and metallicity-sensitive (C - T_1) photometric indices, we construct a new metallicity distribution function (MDF) for the cluster system. The MDF shows an approximately bimodal form, with centroids at [Fe/H] = -1.46 and -0.53, and with nearly equal numbers of metal-poor and metal-rich clusters in the two modes. However, there are many intermediate-color clusters in the distribution, and the fainter clusters tend to have a higher proportion of red clusters. These features of the MDF may indicate a widespread age range within the cluster system as well as an intrinsically broad metallicity spread.

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arXiv2004

The Metallicity Distribution in the Outer Halo of M33

We present the results of deep I and V-band photometry of the halo stars near the southeast minor axis of the Local Group spiral galaxy M33. An (I,V-I) color-magnitude diagram distinctly reveals the red giant branch stars at the bright end of the color-magnitude diagram. A luminosity function in the I-band which utilizes a background field to remove the contaminating effects of Galactic foreground stars and distant, unresolved galaxies reveals the presence of the tip of the red giant branch at (m-M)_{I}=20.70. Assuming an absolute magnitude of the tip of the red giant branch of M_{I,TRGB}=-4.1 and a foreground reddening of E(V-I)=0.054, the distance modulus of M33 is determined to be (m-M)=24.72 (880 kpc). The metallicity distribution function derived by interpolating between evolutionary tracks of red giant branch models is dominated by a relatively metal poor stellar population, with a mean metallicity of [m/H]=-0.94 or [Fe/H]=-1.24. We fit a leaky-box chemical enrichment model to the halo data, which shows that the halo is well represented by a single-component model with an effective yield of y_{eff}=0.0024.

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arXiv2004

Photometry and the Metallicity Distribution of the Outer Halo of M31. II. The 30 Kpc Field

We present the results of a wide-field (V,I) photometric study of the red-giant branch (RGB) stars in the outer halo of M31, in a field located 30 to 35 kpc from the center of the galaxy along the southeast minor axis. At this remote location, we find that RGB stars belonging to M31 are sparsely but definitely present, after statistical subtraction of field contamination. We derive the metallicity distribution (MDF) for the halo stars using interpolation within a standard (I,V-I) grid of RGB evolutionary tracks. The halo MDF is quite broad but dominated by a moderately high-metallicity population peaking at [m/H] ~ -0.5, strikingly different from the [m/H] ~ -1.3 level which characterizes the outer halo of the Milky Way. However,the shape and peak metallicity for this region are entirely similar to those found in other studies for the inner regions of the M31 halo, particularly our previous study of a 20-kpc region (Durrell, Harris, & Pritchet 2001) employing similar data. In summary, we find no evidence for a metallicity gradient or systematic change in the MDF out to quite large distances in the M31 halo: it appears to be a homogeneous and moderately metal-rich subsystem of the galaxy at all locations. The star counts in the 30-kpc field are also consistent with the r^1/4 law that fits the interior regions of the M31 spheroid surface brightness profile. The metal-rich MDF and the r^1/4 spheroid suggests M31 more strongly resembles a giant elliptical galaxy than other, Milky-Way-like, spirals.

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arXiv2003

A Two-Phase Chemical Enrichment Model for the Milky Way Globular Cluster System

Many globular cluster systems have a distinct bimodal metallicity distribution function (MDF) which has strikingly similar features in many large galaxies of all types. By using the Milky Way cluster system as a typical example, we show that bimodal MDFs can be very well matched with a double ``accreting-box'' chemical enrichment model in which both the halo (metal-poor) and bulge (metal-rich) clusters form during an early phase of gas inflow simultaneously with star formation. However, differences in effective yield between the two phases are not enough by themselves to reproduce the observed MDF shape: gas infall is required for both phases, and either the initial gas or the infalling gas must have very different metallicities in the two separate phases.

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arXiv2002

Origin of the metallicity distribution of the NGC 5128 stellar halo

Recent {\it Hubble Space Telescope} photometry in the nearby elliptical galaxy NGC 5128 shows that its halo field star population is dominated by moderately metal-rich stars, with a peak at [m/H] $\simeq$ -0.4 and with a very small fraction of metal-poor ([m/H] $<$ -1.0) stars. In order to investigate the physical processes which may have produced this metallicity distribution function (MDF), we consider a model in which NGC 5128 is formed by merging of two major spiral galaxies. We find that the halo of an elliptical formed this way is predominantly populated by moderately metal-rich stars with [m/H] $\sim$ -0.4 which were initially within the outer parts of the two merging discs and were tidally stripped during the merger. To match the NGC 5128 data, we find that the progenitor spiral discs must have rather steep metallicity gradients similar to the one defined by the Milky Way open clusters, as well as sparse metal-poor haloes (5% or less of the disc mass). Very few stars from the central bulges of the spiral galaxies end up in the halo, so the results are not sensitive to the relative sizes (bulge-to-disc ratios) or metallicities of the initial bulges. Finally, we discuss the effects on the globular cluster system (GCS). The emergent elliptical will end up with metal-poor halo clusters from the original spiral haloes, but with moderately metal-rich halo stars from the progenitor discs, thus creating a mean offset between the MDFs of the halo stars and the GCS.

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arXiv2002

Simultaneous Modelling of the Stellar Halo and Globular Cluster System of NGC 5128

An important test for models of galaxy formation lies in the metallicity distribution functions (MDFs) of spheroid stars and their globular clusters (GCs).We have compared the MDFs obtained from spectroscopy of the GCs and the star-by-star photometry of red giants in the nearby elliptical NGC 5128, with the predictions of a semi-analytic galaxy formation model. We have selected model ellipticals comparable in luminosity and environment to NGC 5128, and derived their MDFs. A direct comparison between models and data shows that the MDFs are qualitatively similar, both have stellar components which are predominantly metal-rich (0.8Z), with a small fraction of metal-poor stars extending down to 0.002Z. The model MDFs show only small variations, whether they are brightest cluster galaxies or low luminosity group ellipticals. Our comparison also reveals that these model MDFs harbour a greater fraction of stars above solar metallicity than the observations, producing generally more metal-rich (by 0.1 dex) MDFs. One possibility is that the outer-bulge observations are missing some of the highest metallicity stars. We find good agreement between the model and observed GC MDFs, provided that the metal-poor GC formation is halted early in the model. Under this proviso, both the models and data are bimodal with peaks at 0.1Z and Z, and cover similar metallicity ranges. This broad agreement for the stars and GCs suggests that the bulk of the stellar population in NGC 5128 may have been built up in a hierarchical fashion, involving both quiescent and merger-induced star formation. The existence of age structure amongst the metal-rich GCs needs to be tested against high-quality data for this galaxy.

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arXiv2001

Globular Cluster Systems in Four BCGs: A262, A3560, A3565 and A3742

We have used deep I-band (F814W) images from the HST archive to study the globular cluster systems around the brightest cluster galaxies (BCGs) in Abell 262, 3560, 3565 and 3742. Three of these BCGs have inner dust lanes and peculiar structural features which indicate past histories of low-level interaction and accretion. The deep I-band WFPC2 images have photometric limits which, for all four galaxies, reach near or just beyond the GCLF turnover point. Their specific frequencies are 8.24 \pm 1.65, 4.66 \pm 0.93, 2.58 \pm 0.52 and 2.62 \pm 0.52 respectively, all within a factor of two of the normal range for giant ellipticals. We obtain new estimates of the GCLF turnover magnitudes, which are shown to be consistent with an adopted Hubble constant of H_0 ~ 70 km s^{-1} Mpc^{-1} on the ``Hubble diagram'' of GCLF turnover apparent magnitude versus redshift, on a distance scale where the fundamental GCLF calibrator E galaxies (M87 and others) in Virgo are at d=16 Mpc.

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arXiv2001

Washington Photometry of New Globular Clusters in M31

We present a progress report on Washington photometry of several hundred new globular cluster (GC) candidates in M31 which were recently found from our new CCD survey of GCs. Washington CMT1 filters we used are very efficient to survey extragalactic GCs and to estimate the metallicity of GCs. Preliminary color-magnitude diagrams and color-color diagrams of the new GC candidates and known GCs in M31 are obtained.

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arXiv2001

Photometry and the Metallicity Distribution of the Outer Halo of M31

We have conducted a wide-field CCD-mosaic study of the resolved red-giant branch (RGB) stars of M31, in a field located 20 kpc from the nucleus along the SE minor axis. In our (I, V-I) color-magnitude diagram, RGB stars in the top three magnitudes of the M31 halo are strongly present. Photometry of a more distant control field to subtract field contamination is used to derive the `cleaned' luminosity function and metallicity distribution function (MDF) of the M31 halo field. From the color distribution of the foreground Milky Way halo stars, we find a reddening E(V-I)= 0.10 +/- 0.02 for this field, and from the luminosity of the RGB tip, we determine a distance modulus (m-M)_o = 24.47 +/- 0.12 (= 783 +/- 43 kpc). The MDF is derived from interpolation within an extensive new grid of RGB models (Vandenberg et al. 2000). The MDF is dominated by a moderately high-metallicity population ([m/H]~ -0.5) found previously in more interior M31 halo/bulge fields, and is much more metal-rich than the [m/H]~ -1.5 level in the Milky Way halo. A significant (~30% - 40%, depending on AGB star contribution) metal-poor population is also present. To first order, the shape of the MDF resembles that predicted by a simple, single-component model of chemical evolution starting from primordial gas with an effective yield y=0.0055. It strongly resembles the MDF recently found for the outer halo of the giant elliptical NGC 5128 (Harris et al. 2000), though NGC 5128 has an even lower fraction of low-metallicity stars. Intriguingly, in both NGC 5128 and M31, the metallicity distribution of the globular clusters in M31 does not match the halo stars; the clusters are far more heavily weighted to metal-poor objects. We suggest similarities in the formation and early evolution of massive, spheroidal stellar systems.

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arXiv1998

Ages for Globular Clusters in the Outer Galactic Halo: The Second Parameter Clusters Palomar~3, Palomar~4 and Eridanus

We have used the WFPC2 camera on the Hubble Space Telescope to obtain photometry of the outer-halo globular clusters Palomar 3, Palomar 4, and Eridanus. These three are classic examples of the "second parameter" anomaly because of their red horizontal-branch morphologies in combination with their low-to-intermediate metallicities. Our color-magnitude diagrams in (V, V-I) reach $V_{lim} \simeq 27.0$, clearly delineating the subgiant and turnoff regions and about three magnitudes of the unevolved main sequences. The slopes and dereddened colors of the giant branches are consistent with published [Fe/H] estimates that rank the clusters (Pal 3, Eridanus, Pal 4) in order of increasing metallicity, with all three falling near or between the abundance values of the classic nearby halo clusters M3 and M5. Differential fits of their color-magnitude diagrams are made to each other and to M3 and M5 for relative age determinations. We find that the three outer-halo cluster CMDs differ from the nearby clusters in a way that is consistent with their being younger by $\sim 1.5 - 2 $Gyr, if we have correctly estimated the clusters' chemical-abundance ratios. Conversely, the inferred age difference could be smaller ($\ltsim 1 $Gyr) if either [Fe/H] or [$α$/Fe] for the outer-halo clusters is significantly lower than we have assumed. Possible age spreads of order 1$ $Gyr among both the nearby and outer-halo clusters may also be present.

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arXiv1997

Dwarf Galaxies in the Coma Cluster. II. Photometry and Analysis

We study the dwarf galaxy population in the central ~700 arcmin^2 of the Coma cluster, the majority of which are early-type dwarf elliptical (dE) galaxies. Analysis of the statistically-decontaminated dE galaxy sequence in the color-magnitude diagram reveals a highly significant trend of color with magnitude (Δ(B-R)/ΔR = -0.056\pm0.002 mag), in the sense that fainter dEs are bluer and thus presumably more metal-poor. The mean color of the faintest dEs in our sample is (B-R)~1.15 mag, consistent with a color measurement of the diffuse intracluster light in the Coma core. This intracluster light could then have originated from the tidal disruption of faint dEs in the cluster core. The total galaxy luminosity function (LF) is well modeled as the sum of a log-normal distribution for the giant galaxies, and a Schechter function for the dE galaxies with a faint-end slope α= -1.41\pm0.05. This value of αis consistent with those measured for the Virgo and Fornax clusters. The spatial distribution of the faint dE galaxies (19.0 < R \le 22.5 mag) has R_c = 22.15 arcmin (~0.46h^{-1} Mpc), significantly larger than the R_c = 13.71 arcmin (~0.29h^{-1} Mpc) found for the cluster giants and the brighter dEs (R \le 19.0 mag), consistent with tidal disruption of the faint dEs. Finally, we find that most dEs belong to the general Coma cluster potential rather than as satellites of individual giant galaxies: An analysis of the number counts around 10 cluster giants reveals that they each have on average 4\pm 1 dE companions within a projected radius of 13.9h^{-1} kpc. (Abridged)

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arXiv1997

Dwarf Galaxies in the Coma Cluster. I. Detection, Measurement and Classification Techniques

Deep B- and R-band CCD images of the central ~700 arcmin^2 of the Coma cluster core have been used to measure the dwarf-galaxy population in Coma. In this paper, we describe a newly developed code for automated detection, photometry and classification of faint objects of arbitrary shape and size on digital images. Intensity-weighted moments are used to compute the positions, radial structures, ellipticities, and integrated magnitudes of detected objects. We demonstrate that Kron-type 2r_1 aperture aperture magnitudes and surface brightnesses are well suited to faint-galaxy photometry of the type described here. Discrimination between starlike and extended (galaxy) objects is performed interactively through parameter-space culling in several possible parameters, including the radial moments, surface brightness, and integrated color versus magnitude. Our code is tested and characterized with artificial CCD images of star and galaxy fields; it is demonstrated to be accurate, robust and versatile. Using these analysis techniques, we detect a large population of dE galaxies in the Coma cluster core. These dEs stand out as a tight sequence in the R, (B-R) color-magnitude diagram.

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arXiv1997

Photometry and Spectroscopy of Coma Dwarf Ellipticals

We have used deep R- and B-band CCD images (with ~1.2 arcsec seeing) of the central ~700 arcmin^2 of the Coma cluster core (and an associated ~270 arcmin^2 control field) to study the early-type dwarf elliptical (dE) galaxy population. Analysis of the color-restricted control-field-corrected color-magnitude diagram (CMD) shows that the dE galaxy sequence is linear over the range 15.5 <= R <= 22.5 mag, with a slope Delta (B-R)/Delta R = -0.056+/-0.002. The faintest dEs in this sample have a color (B-R) = 1.15 mag, which is within the color range measured for the diffuse intracluster light. Analysis of the completeness-corrected background-subtracted radial number density profiles reveals that the faint dEs are deficient in the cluster core, consistent with their destruction in this dense environment. To determine ages and abundances for a subsample of these Coma dEs, we used the Keck I telescope and the Low-Resolution Imaging Spectrograph (LRIS) in ~1.0 arcsec seeing. We obtained 2x1800 sec exposures of four fields, with ~22 objects per field. Our wavelength coverage is 3800 < λ< 6200 Å, with 8 Åresolution and S/N~50 for the dEs. Preliminary analysis of an LRIS field located ~3.8 arcmin west of NGC 4874 reveals that only four of the 17 target objects have Coma velocities, compared to 9-10 expected from analysis of the control-field CMD.

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