1,522 research outputs found
Globular cluster formation efficiencies from black-hole X-ray binary feedback
We investigate a scenario in which feedback from black-hole X-ray binaries
(BHXBs) sometimes begins inside young star clusters before strong supernova
feedback. Those BHXBs could reduce the gas fraction inside embedded young
clusters whilst maintaining virial equilibrium, which may help globular
clusters (GCs) to stay bound when supernova-driven gas ejection subsequently
occurs. Adopting a simple toy model with parameters guided by BHXB population
models, we produce GC formation efficiencies consistent with
empirically-inferred values. The metallicity dependence of BHXB formation could
naturally explain why GC formation efficiency is higher at lower metallicity.
For reasonable assumptions about that metallicity dependence, our toy model can
produce a GC metallicity bimodality in some galaxies without a bimodality in
the field-star metallicity distribution.Comment: Accepted to ApJ Letters on 19th July. 6 pages. The definitive version
is available from: http://iopscience.iop.org/2041-8205/809/1/L16
Chandra Detection of Intra-cluster X-ray sources in Virgo
We present a survey of X-ray point sources in the nearest and dynamically
young galaxy cluster, Virgo, using archival Chandra observations that sample
the vicinity of 80 early-type member galaxies. The X-ray source populations at
the outskirt of these galaxies are of particular interest. We detect a total of
1046 point sources (excluding galactic nuclei) out to a projected
galactocentric radius of 40 kpc and down to a limiting 0.5-8 keV
luminosity of . Based on the cumulative
spatial and flux distributions of these sources, we statistically identify
120 excess sources that are not associated with the main stellar content
of the individual galaxies, nor with the cosmic X-ray background. This excess
is significant at a 3.5 level, when Poisson error and cosmic variance
are taken into account. On the other hand, no significant excess sources are
found at the outskirt of a control sample of field galaxies, suggesting that at
least some fraction of the excess sources around the Virgo galaxies are truly
intra-cluster X-ray sources. Assisted with ground-based and HST optical imaging
of Virgo, we discuss the origins of these intra-cluster X-ray sources, in terms
of supernova-kicked low-mass X-ray binaries (LMXBs), globular clusters, LMXBs
associated with the diffuse intra-cluster light, stripped nucleated dwarf
galaxies and free-floating massive black holes.Comment: 29 pages, 8 figures. Accepted for publication in ApJ. Comments
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The ACS Fornax Cluster Survey. III. Globular Cluster Specific Frequencies of Early-Type Galaxies
The globular cluster (GC) specific frequency (), defined as the number
of GCs per unit galactic luminosity, represents the efficiency of GC formation
(and survival) compared to field stars. Despite the naive expectation that star
cluster formation should scale directly with star formation, this efficiency
varies widely across galaxies. To explore this variation we measure the z-band
GC specific frequency () for 43 early-type galaxies (ETGs) from the
Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) Fornax Cluster
Survey. Combined with the homogenous measurements of in 100 ETGs from
the HST/ACS Virgo Cluster Survey from Peng et al. (2008), we investigate the
dependence of on mass and environment over a range of galaxy
properties. We find that behaves similarly in the two galaxy
clusters, despite the clusters' order-of-magnitude difference in mass density.
The is low in intermediate-mass ETGs (), and increases
with galaxy luminosity. It is elevated at low masses, on average, but with a
large scatter driven by galaxies in dense environments. The densest
environments with the strongest tidal forces appear to strip the GC systems of
low-mass galaxies. However, in low-mass galaxies that are not in strong tidal
fields, denser environments correlate with enhanced GC formation efficiencies.
Normalizing by inferred halo masses, the GC mass fraction,
, is constant for ETGs with stellar masses
, in agreement with previous
studies. The lack of correlation between the fraction of GCs and the nuclear
light implies only a weak link between the infall of GCs and the formation of
nuclei.Comment: 16 pages, 7 figures, 6 tables; accepted by Ap
The ACS Virgo Cluster Survey. XVII. The Spatial Alignment of Globular Cluster Systems With Early-Type Host Galaxies
We study the azimuthal distribution of globular clusters (GCs) in early-type
galaxies and compare them to their host galaxies using data from the ACS Virgo
Cluster Survey. We find that in host galaxies with visible elongation (epsilon
> 0.2) and intermediate to high luminosities (M_z<-19), the GCs are
preferentially aligned along the major axis of the stellar light. The red
(metal-rich) GC subpopulations show strong alignment with the major axis of the
host galaxy, which supports the notion that these GCs are associated with
metal-rich field stars. The metal-rich GCs in lenticular galaxies show signs of
being more strongly associated with disks rather than bulges. Surprisingly, we
find that the blue (metal-poor) GCs can also show the same correlation. If the
metal-poor GCs are part of the early formation of the halo and built up through
mergers, then our results support a picture where halo formation and merging
occur anisotropically, and where the present day major axis is an indicator of
the preferred merging axis.Comment: 8 pages, 7 figures, accepted for publication in The Astrophysical
Journa
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