671 research outputs found
The Birthplace of Low-Mass X-ray Binaries: Field Versus Globular Cluster Populations
Recent Chandra studies of low-mass X-ray binaries (LMXBs) within early-type
galaxies have found that LMXBs are commonly located within globular clusters of
the galaxies. However, whether all LMXBs are formed within globular clusters
has remained an open question. If all LMXBs formed within globular clusters,
the summed X-ray luminosity of the LMXBs in a galaxy should be directly
proportional to the number of globular clusters in the galaxy regardless of
where the LMXBs currently reside. We have compared these two quantities over
the same angular area for a sample of 12 elliptical and S0 galaxies observed
with Chandra and found that the correlation between the two quantities is
weaker than expected if all LMXBs formed within globular clusters. This
indicates that a significant number of the LMXBs were formed in the field, and
naturally accounts for the spread in field-to-cluster fractions of LMXBs from
galaxy to galaxy. We also find that the "pollution" of globular cluster LMXBs
into the field has been minimal within elliptical galaxies, but there is
evidence that roughly half of the LMXBs originally in the globular clusters of
S0 galaxies in our sample have escaped into the field. This is due to higher
globular cluster disruption rates in S0s resulting from stronger gravitational
shocks caused by the passage of globular clusters through the disks of S0
galaxies that are absent in elliptical galaxies.Comment: To appear in ApJ, 1 October 2005, v631 2 issue, 9 pages, 3 figures,
typos and a few minor issues correcte
The Spin of M87 as measured from the Rotation of its Globular Clusters
We revisit the kinematical data for 204 globular clusters in the halo of M87.
Beyond 3 r_eff along the major axis of the galaxy light, these globular
clusters exhibit substantial rotation (~ 300 +/- 70 km/s) that translates into
an equally substantial spin (lambda ~ 0.18). The present appearance of M87 is
most likely the product of a single major merger, since this event is best able
to account for so sizable a spin. A rotation this large makes improbable any
significant accretion of material after this merger, since that would have
diluted the rotation signature. We see weak evidence for a difference between
the kinematics of the metal-poor and metal-rich population, in the sense that
the metal-poor globular clusters appear to dominate the rotation. If, as we
suspect, the last major merger event of M87 was mainly dissipationless and did
not trigger the formation of a large number of globular clusters, the kinematic
difference between the two could reflect their orbital properties in the
progenitor galaxies; these differences would be compatible with these
progenitors having formed in dissipational mergers. However, to put strong
kinematic constraints on the origin of the globular clusters themselves is
difficult, given the complex history of the galaxy and its last dominant merger
event.Comment: 20 pages (AAS two column style, including 1 table and 7 figures)
accepted in the AJ (November issue), also available at
http://www.ucolick.org/~mkissler
Globular cluster systems II: On the formation of old globular clusters and their sites of formation
We studied the metal-poor globular cluster (GC) populations of a large
variety of galaxies (47 galaxies spanning about 10mag in absolute brightness)
and compared their mean [Fe/H] with the properties of the host galaxies. The
mean [Fe/H] of the systems lie in the -1.65<[Fe/H]<-1.20 range (74% of the
population). Using only GC systems with more than 6 objects detected, 85% of
the population lie within -1.65<[Fe/H]<-1.20. The relation between the mean
[Fe/H] of the metal-poor GC systems and the Mv of their host galaxies presents
a very low slope which includes zero. An analysis of the correlation of the
mean [Fe/H] with other galaxy properties also leads to the conclusion that no
strong correlation exists. The lack of correlation suggests a formation of all
metal-poor GC in similar gas fragments. A weak correlation might exist between
mean [Fe/H] of the metal-poor GC and host galaxy metallicity. This would imply
that some fragments in which metal-poor GC formed were already embedded in the
larger dark matter halo of the final galaxy (as oppose to being independent
satellites that were accreted later). Our result suggests a homogeneous
formation of metal-poor GC in all galaxies, in typical fragments of masses
around 10^9-10^10 solar masses with very similar metallicities, compatible with
hierarchical formation scenarios for galaxies. We compared the mean [Fe/H] of
the metal-poor GC populations with the typical metallicities of high-z objects.
If we add the constraint that GC need a high column density of gas to form,
DLAs are the most likely sites for the formation of metal-poor GC populations.Comment: accepted for publication in AJ, scheduled for the May 2001 issu
Towards an Understanding of the Globular Cluster Over--abundance around the Central Giant Elliptical NGC 1399
We investigate the kinematics of a combined sample of 74 globular clusters
around NGC 1399. Their high velocity dispersion, increasing with radius,
supports their association with the gravitational potential of the galaxy
cluster rather than with that of NGC 1399 itself. We find no evidence for
rotation in the full sample, although some indication for rotation in the outer
regions. The data do not allow us to detect differences between the kinematics
of the blue and red sub-populations of globular clusters.
A comparison between the globular cluster systems of NGC 1399 and those of
NGC 1404 and NGC 1380 indicates that the globular clusters in all three
galaxies are likely to have formed via similar mechanisms and at similar
epochs. The only property which distinguishes the NGC 1399 globular cluster
system from these others is that it is ten times more abundant. We summarize
the evidence for associating these excess globulars with the galaxy cluster
rather than with NGC 1399 itself, and suggest that the over-abundance can be
explained by tidal stripping, at an early epoch, of neighboring galaxies and
subsequent accumulation of globulars in the gravitational potential of the
galaxy cluster.Comment: AJ accepted (March issue), 27 pages (6 figures included), AAS style,
two columns. Also available at http://www.eso.org/~mkissle
The central region of the Fornax cluster -- II. Spectroscopy and radial velocities of member and background galaxies
Radial velocities of 94 galaxies brighter than about V_tot = 20 mag in the
direction of the central Fornax cluster have been measured. Except for 8 Fornax
members, all galaxies lie in the background. Among the 8 members, there are 5
nucleated dwarf ellipticals that are already listed in the FCC (Ferguson 1989,
AJ 98, 367). Two of the 3 ``new'' members are very compact and have surface
brightnesses comparable to globular clusters, however their luminosities are in
the range of dwarf elliptical nuclei. The measured line indices (especially
Mg2, H beta, and iron) of the brighter of the compact objects suggest a solar
metallicity, whereas the fainter compact object as well as the dE,Ns have line
indices that are similar to those of old metal-poor globular clusters (GCs).
However, with these data it is not possible to clearly classify the compact
objects either as very bright globular clusters, isolated nuclei of dE,Ns, or
even compact ellipticals. A background galaxy cluster at z = 0.11 has been
found just behind the center of the Fornax cluster. This explains the excess
population of galaxies reported in Paper I. The brightest galaxy of the
background cluster lies only 1.1 arcmin south of NGC 1399 and is comparable in
absolute luminosity with the central Fornax galaxy itself.Comment: 12 pages, LaTeX2e, uses aa.cls, including 9 PostScript figures;
accepted for publication in A&AS, also available at
http://www.astro.puc.cl/~mhilker/publication.htm
The Star Cluster Population in the Tidal Tails of NGC 6872
We present a photometric analysis of the rich star cluster population in the
tidal tails of NGC 6872. We find star clusters with ages between 1 - 100 Myr
distributed in the tidal tails, while the tails themselves have an age of less
than 150 Myr. Most of the young massive ()
clusters are found in the outer regions of the galactic disk or the tidal
tails. The mass distribution of the cluster population can be well described by
power-law of the form , where , in very good agreement with other young cluster populations found in a
variety of different environments. We estimate the star formation rate for
three separate regions of the galaxy, and find that the eastern tail is forming
stars at times the rate of the western tail and times the
rate of the main body of the galaxy. By comparing our observations with
published N-body models of the fate of material in tidal tails in a galaxy
cluster potential, we see that many of these young clusters will be lost into
the intergalactic medium. We speculate that this mechanism may also be at work
in larger galaxy clusters such as Fornax, and suggest that the so-called
ultra-compact dwarf galaxies could be the most massive star clusters that have
formed in the tidal tails of an ancient galactic merger.Comment: 12 pages, 10 figures, accepted A&
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