762 research outputs found
Large-scale study of the NGC 1399 globular cluster system in Fornax
We present a Washington C and Kron-Cousins R photometric study of the
globular cluster system of NGC 1399, the central galaxy of the Fornax cluster.
A large areal coverage of 1 square degree around NGC 1399 is achieved with
three adjoining fields of the MOSAIC II Imager at the CTIO 4-m telescope.
Working on such a large field, we can perform the first indicative
determination of the total size of the NGC 1399 globular cluster system. The
estimated angular extent, measured from the NGC 1399 centre and up to a
limiting radius where the areal density of blue globular clusters falls to 30
per cent of the background level, is 45 +/- 5 arcmin, which corresponds to 220
- 275 kpc at the Fornax distance. The bimodal colour distribution of this
globular cluster system, as well as the different radial distribution of blue
and red clusters, up to these large distances from the parent galaxy, are
confirmed. The azimuthal globular cluster distribution exhibits asymmetries
that might be understood in terms of tidal stripping of globulars from NGC
1387, a nearby galaxy. The good agreement between the areal density profile of
blue clusters and a projected dark-matter NFW density profile is emphasized.Comment: 9 pages, 9 figures. Accepted for publication in A&
Globular cluster systems in low-luminosity early-type galaxies near the Fornax Cluster centre
We present a photometric study of the globular cluster systems of the Fornax
cluster galaxies NGC 1374, NGC 1379, and NGC 1387. The data consists of images
from the wide-field MOSAIC Imager of the CTIO 4-m telescope, obtained with
Washington C and Kron-Cousins R filters. The images cover a field of 36 x 36
arcmin, corresponding to 200 x 200 kpc at the Fornax distance. Two of the
galaxies, NGC 1374 and NGC 1379, are low-luminosity ellipticals while NGC 1387
is a low-luminosity lenticular. Their cluster systems are still embedded in the
cluster system of NGC 1399. Therefore the use of a large field is crucial and
some differences to previous work can be explained by this. The colour
distributions of all globular cluster systems are bimodal. NGC 1387 presents a
particularly distinct separation between red and blue clusters and an
overproportionally large population of red clusters. The radial distribution is
different for blue and red clusters, red clusters being more concentrated
towards the respective galaxies. The different colour and radial distributions
point to the existence of two globular cluster subpopulations in these
galaxies. Specific frequencies are in the range S_N= 1.4-2.4, smaller than the
typical values for elliptical galaxies. These galaxies might have suffered
tidal stripping of blue globular clusters by NGC 1399.Comment: 14 pages, 12 figures. Accepted for publication in MNRA
A Variable Black Hole X-Ray Source in a NGC 1399 Globular Cluster
We have discovered an accreting black hole (BH) in a spectroscopically
confirmed globular cluster (GC) in NGC 1399 through monitoring of its X-ray
activity. The source, with a peak luminosity of L_x=2x10^39 ergs/s, reveals an
order of magnitude change in the count rate within ~10 ks in a Chandra
observation. The BH resides in a metal-rich [Fe/H]~0.2 globular cluster. After
RZ2109 in NGC 4472 this is only the second black-hole X-ray source in a GC
confirmed via rapid X-ray variability. Unlike RZ2109, the X-ray spectrum of
this BH source did not change during the period of rapid variability. In
addition to the short-term variability the source also exhibits long-term
variability. After being bright for at least a decade since 1993 within a span
of 2 years it became progressively fainter, and eventually undetectable, or
marginally detectable, in deep Chandra and XMM observations. The source also
became harder as it faded. The characteristics of the long term variability in
itself provide sufficient evidence to identify the source as a BH. The long
term decline in the luminosity of this object was likely not recognized in
previous studies because the rapid variability within the bright epoch
suppressed the average luminosity in that integration. The hardening of the
spectrum accompanying the fading would also make this black hole source
indistinguishable from an accreting neutron star in some epochs. Therefore some
low mass X-ray binaries identified as NS accretors in snapshot studies of
nearby galaxies may also be BHs. Thus the discovery of the second confirmed BH
in an extragalactic GC through rapid variability at the very least suggests
that accreting BHs in GCs are not exceedingly rare occurences.Comment: 8 pages, 2 figs. Accepted for publication in Ap
The globular cluster system of NGC 1316. II - The extraordinary object SH2
SH2 has been described as an isolated HII-region, located about 6.5 arcmin
south of the nucleus of NGC 1316 (Fornax A), a merger remnant in the the
outskirts of the Fornax cluster of galaxies. We give a first, preliminary
description of the stellar content and environment of this remarkable object.
We used photometric data in the Washington system and HST photometry from the
Hubble Legacy Archive for a morphological description and preliminary aperture
photometry. Low-resolution spectroscopy provides radial velocities of the
brightest star cluster in SH2 and a nearby intermediate-age cluster. SH2 is not
a normal HII-region, ionized by very young stars. It contains a multitude of
star clusters with ages of approximately 0.1 Gyr. A ring-like morphology is
striking. SH2 seems to be connected to an intermediate-age massive globular
cluster with a similar radial velocity, which itself is the main object of a
group of fainter clusters. Metallicity estimates from emission lines remain
ambiguous. The present data do not yet allow firm conclusions about the nature
or origin of SH2. It might be a dwarf galaxy that has experienced a burst of
extremely clustered star formation. We may witness how globular clusters are
donated to a parent galaxy.Comment: 5 pages, to appear in A&A, format slightly different from the printed
versio
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