arXiv subjects

G. L. H. Harris

Publications and source records attributed to G. L. H. Harris.

15 recordsLinked to original sources

arXiv2024

The turbulent life of NGC 4696 as told by its globular cluster system

In this work we perform the photometric analysis of the globular cluster system (GCS) of the giant elliptical NGC4696, which is the brightest member of Centaurus, a rich and dynamically young galaxy cluster. We obtained deep Magellan 6.5 m/MegaCam (g', r', i') photometry, with which we identified a sample of 3818 stellar clusters around NGC4696 that were analyzed in the context of possible interactions and its assembly history. After carefully modeling and subtracting the galaxy light, we used selection criteria based on the shape, colors, and magnitudes to identify GC candidates. We find a number of features that indicate a disturbed GCS that points toward a complex evolution with other neighboring members of Centaurus. Formally, two subpopulations could be found at (g'-i')_0 = 0.763 $\pm$ 0.004 and (g'-i')_0=1.012 $\pm$ 0.004. Moreover, the color distribution does not show the presence of a significant blue tilt, but it presents a trend with the radius, where at small galactocentric distances a unimodal distribution is preferable to a bimodal one, suggesting the presence of an intermediate GC population. Besides the color distribution, the metallicity distribution also shows a bimodal trend, with peaks at [Fe/H]=-1.363 $\pm$ 0.010 and [Fe/H]=-0.488 $\pm$ 0.012. The radial density profiles show different slopes for the blue and red populations and the azimuthal distributions are well fitted by an asymmetrical sinusoidal function, with peaks projecting toward two nearby galaxies, NGC4696B and NGC4709, indicating past interactions among these three galaxies. Finally, we derived a GC specific frequency of S_N=6.8 $\pm$ 0.9, in good agreement with the values obtained for other giant ellipticals and with previously estimated S_N of NGC4696. All these results point toward a complex GCS, strongly influenced by the interaction history of NGC4696 with the other galaxies of the Centaurus cluster.

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arXiv2021

The outermost stellar halo of NGC 5128 (Centaurus A): Radial structure

The extended stellar halos of galaxies contain important clues for investigating their assembly history and evolution. We investigate the resolved stellar content and the extended halo of NGC 5128 as a function of galactocentric distance. We used HST images to resolve individual red giant branch (RGB) stars in 28 independent pointings. Star counts from deep VI color-magnitude diagrams reaching at least 1.5 mag below the tip of the RGB are used to derive the surface density distribution of the halo. The contamination by Milky Way stars is assessed with a new control field, with models, and by combining optical and near-IR photometry. We present a new calibration of the WFC3 F606W+F814W photometry to the ground-based VI photometric system. The photometry shows that the stellar halo of NGC 5128 is dominated by old RGB stars that are present in all fields. The V-band surface brightness changes from 23 to 32 mag/arcsec$^2$ between 8.3 kpc from the galaxy center to our outermost halo fields located 140 kpc away from the center along the major axis and 92 kpc along the minor axis. Within ~30 kpc, we also find evidence for a 2-3 Gyr old population traced by bright asymptotic giant branch stars. This population contributes only up to 10% in total stellar mass if it is 2 Gyr old, but a larger fraction of 30-40% is required if its age is 3 Gyr. The stellar surface density profile is well fit by a r$^{1/4}$ curve or a power-law $\sim r^{-3.1}$ over the full radial range, with no obvious break in the slope, but with large field-to-field scatter. The ellipticity measured from integrated-light photometry in the inner parts, $e=(b/a)=0.77$, flattens to $e=0.54 \pm 0.02$ beyond 30 kpc. Considering the flattening of the outer halo, the projection of the elliptical isophote on the semimajor axis for our most distant field reaches nearly 30 effective radii. [abridged]

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arXiv2014

Tracing the outer halo in a giant elliptical to 25 Reff

We have used the ACS and WFC3 cameras on board HST to resolve stars in the halo of the nearest giant elliptical (gE) galaxy NGC 5128 out to a projected distance of 140 kpc (25 effective radii, Reff) along the major axis and 90 kpc (16 Reff) along the minor axis. This dataset provides an unprecedented radial coverage of the stellar halo properties in any gE galaxy. Color-magnitude diagrams clearly reveal the presence of the red giant branch stars belonging to the halo of NGC 5128, even in our most distant fields. The star counts demonstrate increasing flattening of the outer halo, which is elongated along the major axis of the galaxy. The V-I colors of the red giants enable us to measure the metallicity distribution in each field and so map the gradient out to ~16 Reff from the galaxy center along the major axis. A median metallicity is obtained even for the outermost fields along both axes. We observe a smooth transition from a metal-rich ([M/H]~0.0) inner galaxy to lower metallicity in the outer halo, with the metallicity gradient slope along the major axis of $Δ$[M/H]/$Δ$ R=-0.0054 $\pm$ 0.0006 dex/kpc. In the outer halo, beyond ~10 Reff, the number density profile follows a power law, but also significant field-to-field metallicity and star count variations are detected. The metal-rich component dominates in all observed fields, and the median metallicity is [M/H]>-1 dex in all fields.

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arXiv2010

How old are the stars in the halo of NGC 5128 (Centaurus A)?

NGC 5128 (Centaurus A) is, at the distance of just 3.8 Mpc, the nearest easily observable giant elliptical galaxy. Therefore it is the best target to investigate the early star formation history of an elliptical galaxy. Our aims are to establish when the oldest stars formed in NGC 5128, and whether this galaxy formed stars over a long period. We compare simulated colour-magnitude diagrams with the deep ACS/HST photometry. We find that that the observed colour-magnitude diagram can be reproduced satisfactorily only by simulations that have the bulk of the stars with ages in excess of ~10 Gyr, and that the alpha-enhanced models fit the data much better than the solar scaled ones. Data are not consistent with extended star formation over more than 3-4 Gyr. Two burst models, with 70-80% of the stars formed 12+/-1 Gyr ago and with 20-30% younger contribution with 2-4 Gyr old stars provide the best agreement with the data. The old component spans the whole metallicity range of the models (Z=0.0001-0.04), while for the young component the best fitting models indicate higher minimum metallicity (~1/10 - 1/4 Z_sun). The bulk of the halo stars in NGC5128 must have formed at redshift z>=2 and the chemical enrichment was very fast, reaching solar or even twice-solar metallicity already for the ~11-12 Gyr old population. The minor young component, adding ~20-30% of the stars to the halo, and contributing less than 10% of the mass, may have resulted from a later star formation event ~2-4 Gyr ago. (abridged)

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arXiv2007

HST Photometry for the Halo Stars in the Leo Elliptical NGC 3377

We have used the ACS camera on HST to obtain (V,I) photometry for 57,000 red-giant stars in the halo of the Leo elliptical NGC 3377. We use this sample of stars to derive the metallicity distribution function (MDF) for its halo field stars, and comment on its chemical evolution history compared with both larger and smaller E galaxies. Our ACS/WFC field spans a radial range extending from 4 to 18 kpc projected distance from the center of NGC 3377 and thus covers a significant portion of this galaxy's halo. We find that the MDF is broad, reaching a peak at [m/H] ~ -0.6$, but containing virtually no stars more metal-poor than log [m/H] = -1.5$. It may, in addition, have relatively few stars more metal-rich than [m/H] = -0.3$, although interpretation of the high-metallicity end of the MDF is limited by photometric completeness that affects the detection of the reddest, most metal-rich stars. NGC 3377 appears to have an enrichment history intermediate between those of normal dwarf ellipticals and the much larger giants. As yet, we find no clear evidence that the halo of NGC 3377 contains a significant population of ``young'' (< 3 Gy) stars.

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arXiv2007

The Leo Elliptical NGC 3379: A Metal-Poor Halo Emerges

We have used the ACS camera on HST to obtain (V,I) photometry for 5300 red-giant stars in the halo of the dominant Leo-group member NGC 3379, a galaxy usually regarded as a classic normal giant elliptical. We use this sample of stars to derive the metallicity distribution function (MDF) for its outer-halo field stars 33 kpc from the galaxy center. The MDF is distinctly unlike all the other E galaxies for which we have similar data (including the Local Group dwarf ellipticals, the intermediate-luminosity NGC 3377, and the giant NGC 5128). First, the MDF for the NGC 3379 outer halo is broad and flat, with many stars at every interval in [m/H]. Second, we see a metallicity gradient across our ACS field such that in its outermost region the blue, low-metallicity stars ([m/H] < -0.7) are beginning to dominate and the higher-metallicity stars are rapidly diminishing. Our target field is centered at a projected distance about equal to 12 R_e, twice as far out in units of effective radius as in any of the other galaxies that we have surveyed. If NGC 3379 is indeed representative of large E/S0 galaxies, we predict that such galaxies in general will reveal diffuse low-metallicity subpopulations, but that photometry at radii r ~ 10 - 15 R_e will be necessary to see the faint low-metallicity component clearly.

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arXiv2007

Structural parameters for globular clusters in M31 and generalizations for the fundamental plane

The structures of globular clusters (GCs) reflect their dynamical states and past histories. High-resolution imaging allows the exploration of morphologies of clusters in other galaxies. Surface brightness profiles from new Hubble Space Telescope observations of 34 globular clusters in M31 are presented, together with fits of several different structural models to each cluster. M31 clusters appear to be adequately fit by standard King models, and do not obviously require alternate descriptions with relatively stronger halos, such as are needed to fit many GCs in other nearby galaxies. The derived structural parameters are combined with corrected versions of those measured in an earlier survey to construct a comprehensive catalog of structural and dynamical parameters for M31 GCs with a sample size similar to that for the Milky Way. Clusters in M31, the Milky Way, Magellanic Clouds, Fornax dwarf spheroidal and NGC 5128 define a very tight fundamental plane with identical slopes. The combined evidence for these widely different galaxies strongly reinforces the view that old globular clusters have near-universal structural properties regardless of host environment.

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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

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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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

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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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

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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