94 research outputs found
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.Comment: 10 pages, 7 figures, 4 tables - accepted in Astronomical Journa
NGC 5128: The Giant Beneath
I review what we have learned about the old stellar population of NGC 5128,
the only large E galaxy close enough that we can currently observe individual
stars as faint as the horizontal branch. Although its galaxy type is still a
matter of debate for some, the uncertainties over distance are now largely
resolved; comparison of five stellar distance indicators gives d=3.8 +- 0.1
Mpc. The globular cluster system, which was once perplexingly invisible, is now
known to be predominantly old with a substantial metal-rich component. The GCS
total population and luminosity function are normal and the clusters follow the
same fundamental plane relation as those in the Milky Way and M31. Finally, the
halo out to at least ~7r_{eff} is dominated by metal-rich stars which are also
predominantly old, with age and metallicity tantalizingly similar to the
majority of globular clusters.Comment: PASA in pres
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.Comment: 13 pages, 9 figures and a table, accepted by MNRA
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. It may, in addition, have relatively few
stars more metal-rich than [m/H] = -0.3$, although interpretation of the
high-metallicity end of the MDF is limited by photometric completeness that
affects the detection of the reddest, most metal-rich stars. NGC 3377 appears
to have an enrichment history intermediate between those of normal dwarf
ellipticals and the much larger giants. As yet, we find no clear evidence that
the halo of NGC 3377 contains a significant population of ``young'' (< 3 Gy)
stars.Comment: 40 pages, 17 figure
The Distance to NGC 5128 (Centaurus A)
In this paper we review the various high precision methods that are now
available to determine the distance to NGC 5128. These methods include:
Cepheids, TRGB (tip of the red giant branch), PNLF (planetary nebula luminosity
function), SBF (surface brightness fluctuations) and Long Period Variable (LPV)
Mira stars. From an evaluation of these methods and their uncertainties, we
derive a best-estimate distance of 3.8 +- 0.1 Mpc to NGC 5128 and find that
this mean is now well supported by the current data. We also discuss the role
of NGC 5128 more generally for the extragalactic distance scale as a testbed
for the most direct possible comparison among these key methods.Comment: in press PASA; minor text change
On the Formation of Galaxy Halos: Comparing NGC 5128 and the Local Group Members
The metallicity distribution function (MDF) for the old red-giant stars in
the halo of NGC 5128, the nearest giant elliptical galaxy, is virtually
identical with the MDF for the old-disk stars in the LMC and also strongly
resembles the halo MDF in M31. These galaxies all have high mean halo
metallicities ( ~ -0.4$) 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.Comment: 10 pages, LaTeX, plus 3 figures in separate postscript files;
Astronomical Journal, in press for December 200
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.Comment: AASTex, 33 preprint pages including 9 Figures. Accepted for
publication in Astronomical Journal, volume 132 (2006
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