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 $Delta$[M/H]/$Delta$ 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.
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)
We report the discovery of an extended globular cluster in a halo field in Centaurus A (NGC 5128), situated $sim 38kpc$ from the centre of that galaxy, imaged with the Advanced Camera for Surveys on board the Hubble Space Telescope. At the distance o
f the galaxy, the half-light radius of the cluster is r_h ~ 17pc, placing it among the largest globular clusters known. The faint absolute magnitude of the star cluster, M_(V,o)=-5.2, and its large size render this object somewhat different from the population of extended globular clusters previously reported, making it the first firm detection in the outskirts of a giant galaxy of an analogue of the faint, diffuse globular clusters present in the outer halo of the Milky Way. The colour-magnitude diagram of the cluster, covering approximately the brightest four magnitudes of the red giant branch, is consistent with an ancient, i.e., older than ~8 Gyr, intermediate-metallicity, i.e., [M/H] ~-1.0 dex, stellar population. We also report the detection of a second, even fainter cluster candidate which would have r_h ~ 9pc, and M_(V,o)=-3.4 if it is at the distance of NGC 5128. The properties of the extended globular cluster and the diffuse stellar populations in its close vicinity suggest that they are part of a low mass accretion in the outer regions of NGC 5128.
We use deep images taken with the Advanced Camera for Surveys on board the Hubble Space Telescope of the disk galaxy NGC 891, to search for globular cluster candidates. This galaxy has long been considered to be a close analog in size and structure t
o the Milky Way and is nearly edge-on, facilitating studies of its halo population. These extraplanar ACS images, originally intended to study the halo field-star populations, reach deep enough to reveal even the faintest globular clusters that would be similar to those in the Milky Way. From the three pointings we have identified a total of 43 candidates after culling by object morphology, magnitude, and colour. We present (V,I) photometry for all of these, along with measurements of their effective radius and ellipticity. The 16 highest-rank candidates within the whole sample are found to fall in very much the same regions of parameter space occupied by the classic Milky Way globular clusters. Our provisional conclusion from this survey is that the total globular cluster population in NGC 891 as a whole may be almost as large as that of the Milky Way.
