473 research outputs found
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
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
Tip of the red giant branch distances to the dwarf galaxies dw1335-29 and dw1340-30 in the Centaurus group
The abundance and spatial distribution of dwarf galaxies are excellent
empirical benchmarks to test models of structure formation on small scales. The
nearby Centaurus group, with its two subgroups centered on CenA and M83, stands
out as an important alternative to the Local Group for scrutinizing
cosmological model predictions in a group of galaxies context. We have obtained
deep optical images of three recently discovered M83 satellite galaxy
candidates with the FORS2 instrument mounted on the Very Large Telescope. We
aim to confirm their group membership and study their stellar population. Deep
VI-band photometry is used to resolve the brightest stars in our targets.
Artificial star tests are performed to estimate the completeness and
uncertainties of the photometry. The color-magnitude diagrams reveal the red
giant branch (RGB) stars allowing to use the Sobel edge detection method to
measure the magnitude of the RGB tip and thus derive distances and group
membership for our targets. The mean metallicity of the dwarf galaxies are
further determined by fitting BASTI model isochrones to the mean RGB locus. We
confirm the two candidates, dw1335-29 and dw1340-30, to be dwarf satellites of
the M83 subgroup, with estimated distances of 5.03 +- 0.24 Mpc and 5.06 +- 0.24
Mpc, respectively. Their respective mean metallicities of = -1.79 +-
0.4 and = -2.27 +- 0.4 are consistent with the metallicity-luminosity
relation for dwarf galaxies. The third candidate, dw1325-33, could not be
resolved into stars due to insufficiently deep images, implying its distance
must be larger than 5.3 Mpc. Using the two newly derived distances we assess
the spatial distribution of the galaxies in the M83 subgroup and discuss a
potential plane-ofsatellites around M83.Comment: 10 pages, 9 figures, accepted for publication in A&
An imaging and spectroscopic study of the planetary nebulae in NGC 5128 (Centaurus A): Planetary nebulae catalogues
Imaging and spectroscopic observations of planetary nebulae (PNe) in the
nearest large elliptical galaxy NGC 5128 (Centaurus A), were obtained to find
more PNe and measure their radial velocities. NTT imaging was obtained in 15
fields in NGC 5128 over an area of about 1 square degree with EMMI using [O
III] and off-band filters. Newly detected sources, combined with literature
PNe, were used as input for VLT FLAMES multi-fibre spectroscopy in MEDUSA mode.
Spectra of the 4600-5100A region were analysed and velocities measured based on
emission lines of [O III]4959,5007A and often H-beta. The chief results are
catalogues of 1118 PN candidates and 1267 spectroscopically confirmed PNe in
NGC 5128. The catalogue of PN candidates contains 1060 PNe discovered with EMMI
imaging and 58 from literature surveys. The spectroscopic PN catalogue has
FLAMES radial velocity and emission line measurements for 1135 PNe, of which
486 are new. Another 132 PN radial velocities are available from the
literature. For 629 PNe observed with FLAMES, H-beta was measured in addition
to [O III]. Nine targets show double-lined or more complex profiles, and their
possible origin is discussed. FLAMES spectra of 48 globular clusters were also
targetted: 11 had emission lines detected (two with multiple components), but
only 3 are PNe likely to belong to the host globular. The total of 1267
confirmed PNe in NGC 5128 with radial velocity measurements (1135 with small
velocity errors) is the largest collection of individual kinematic probes in an
early-type galaxy. This PN dataset, as well as the catalogue of PN candidates,
are valuable resources for detailed investigation of the stellar population of
NGC 5128. [Abridged]Comment: 19 pages, 11 figures. Accepted for publication in Astronomy and
Astrophysics. Tables 7 - 11 available in electronic form at CDS. Replaced
with a few typos fixe
The Planetary Nebula Luminosity Function: Pieces of the Puzzle
Extragalactic surveys in the emission line of [O III] 5007 have provided us
with the absolute line strengths of large, homogeneous sets of planetary
nebulae. These data have been used to address a host of problems, from the
measurement of the extragalactic distance scale, to the study of stellar
populations. I review our current understanding of the [O III] planetary nebula
luminosity function (PNLF), and discuss some of the physical processes that
effect its structure. I also describe the features of the H-alpha PNLF, a
function that, upon first glance, looks similar to the [O III] PNLF, but which
includes a very different set of objects. Finally, I discuss recent
measurements of alpha, the number of PNe found in a stellar population,
normalized to that population's bolometric luminosity. I show that, contrary to
expectations, the values of alpha found in actively star-forming spirals is
essentially the same as those measured in late-type elliptical and lenticular
systems. I discuss how this result sheds light on the physics of the planetary
nebula phenomenon.Comment: 7 pages, including 7 figures; presentation at the workshop on the
Legacies of the Macquarie/AAO/Strasbourg H-alpha Planetary Nebula project,
accepted for publication in PAS
A faint extended cluster in the outskirts of NGC 5128: evidence of a low mass accretion
We report the discovery of an extended globular cluster in a halo field in
Centaurus A (NGC 5128), situated \sim 38\kpc from the centre of that galaxy,
imaged with the Advanced Camera for Surveys on board the Hubble Space
Telescope. At the distance of 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.Comment: 9 pages, MNRAS, in pres
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