2,380 research outputs found
The ACS Virgo Cluster Survey. XIV. Analysis of Color-Magnitude Relations in Globular Cluster Systems
We examine the correlation between globular cluster (GC) color and magnitude
using HST/ACS imaging for a sample of 79 early-type galaxies (-21.7<M_B<-15.2
mag) with accurate SBF distances from the ACS Virgo Cluster Survey. Using the
KMM mixture modeling algorithm, we find a highly significant correlation,
d(g-z)/dz = -0.037 +- 0.004, between color and magnitude for the subpopulation
of blue GCs in the co-added GC color-magnitude diagram of the three brightest
Virgo galaxies (M49, M87 and M60): brighter GCs are redder than their fainter
counterparts. For the single GC systems of M87 and M60, we find similar
correlations; M49 does not appear to show a significant trend. There is no
correlation between (g-z) and M_z for GCs of the red subpopulation. The
correlation d(g-z)/dg for the blue subpopulation is much weaker than d(g-z)/dz.
Using Monte Carlo simulations, we attribute this to the fact that the blue
subpopulation in M_g extends to higher luminosities than the red subpopulation,
which biases the KMM fits. The correlation between color and M_z thus is a real
effect. This conclusion is supported by biweight fits to the same color
distributions. We identify two environmental dependencies of the
color-magnitude relation: (1) the slope decreases in significance with
decreasing galaxy luminosity; and (2) the slope is stronger for GCs at smaller
galactocentric distances. We examine several mechanisms that might give rise to
the observed color-magnitude relation: (1) presence of contaminators; (2)
accretion of GCs from low-mass galaxies; (3) stochastic effects; (4) capture of
field stars by individual GCs; and (5) GC self-enrichment. We conclude that
self-enrichment and field-star capture, or a combination of these processes,
offer the most promising means of explaining our observations.Comment: 15 pages, 12 figures, accepted for publication in the Astrophysical
Journal. Uses emulateapj.cl
The ACS Virgo Cluster Survey III. Chandra and HST Observations of Low-Mass X-Ray Binaries and Globular Clusters in M87
The ACIS instrument on board the Chandra X-ray Observatory has been used to
carry out the first systematic study of low-mass X-ray binaries (LMXBs) in M87.
We identify 174 X-ray point-sources, of which ~150 are likely LMXBs. This LMXB
catalog is combined with deep F475W and F850LP images taken with ACS on HST to
examine the connection between LMXBs and globular clusters in M87. Of the 1688
globular clusters in our catalog, f_X = 3.6 +- 0.5% contain a LMXB and we find
that the metal-rich clusters are 3 +- 1 times more likely to harbor a LMXB than
their metal-poor counterparts. In agreement with previous findings for other
galaxies, we find that brighter, more metal-rich clusters are more likely to
contain a LMXB. For the first time, however, we are able to demonstrate that
the probability, p_X, that a given cluster will contain a LMXB depends
sensitively on the dynamical properties of the host cluster. Specifically, we
use the HST images to measure the half-light radius, concentration index and
central density, \rho_0, for each globular, and define a parameter, \Gamma,
which is related to the tidal capture and binary-neutron star exchange rate.
Our preferred form for p_X is then p_X \propto \Gamma \rho_0^{-0.42\pm0.11}
(Z/Z_{\odot})^{0.33\pm0.1}. We argue that if the form of p_X is determined by
dynamical processes, then the observed metallicity dependence is a consequence
of an increased number of neutron stars per unit mass in metal-rich globular
clusters. Finally, we find no compelling evidence for a break in the luminosity
distribution of resolved X-ray point sources. Instead, the LMXB luminosity
function is well described by a power law with an upper cutoff at L_X ~ 10^39
erg/s. (abridged)Comment: 23 pages, 21 figures. Accepted for publication in ApJ. Also available
at http://www.physics.rutgers.edu/~pcote/acs/publications.htm
Rapid Heterotrophic Ossification with Cryopreserved Poly(ethylene glycol-) Microencapsulated BMP2-Expressing MSCs
Autologous bone grafting is the most effective treatment for long-bone nonunions, but it poses considerable risks to donors, necessitating the development of alternative therapeutics. Poly(ethylene glycol) (PEG) microencapsulation and BMP2 transgene delivery are being developed together to induce rapid bone formation. However, methods to make these treatments available for clinical applications are presently lacking. In this study we used mesenchymal stem cells (MSCs) due to their ease of harvest, replication potential, and immunomodulatory capabilities. MSCs were from sheep and pig due to their appeal as large animal models for bone nonunion. We demonstrated that cryopreservation of these microencapsulated MSCs did not affect their cell viability, adenoviral BMP2 production, or ability to initiate bone formation. Additionally, microspheres showed no appreciable damage from cryopreservation when examined with light and electron microscopy. These results validate the use of cryopreservation in preserving the viability and functionality of PEG-encapsulated BMP2-transduced MSCs
Applications of Direct Injection Soft Chemical Ionisation-Mass Spectrometry for the Detection of Pre-blast Smokeless Powder Organic Additives
Analysis of smokeless powders is of interest from forensics and security perspectives. This article reports the detection of smokeless powder organic additives (in their pre-detonation condition), namely the stabiliser diphenylamine and its derivatives 2-nitrodiphenylamine and 4-nitrodiphenylamine, and the additives (used both as stabilisers and plasticisers) methyl centralite and ethyl centralite, by means of swab sampling followed by thermal desorption and direct injection soft chemical ionisation-mass spectrometry. Investigations on the product ions resulting from the reactions of the reagent ions H3O+ and O2+ with additives as a function of reduced electric field are reported. The method was comprehensively evaluated in terms of linearity, sensitivity and precision. For H3O+, the limits of detection (LoD) are in the range of 41-88 pg of additive, for which the accuracy varied between 1.5 and 3.2%, precision varied between 3.7 and 7.3% and linearity showed R20.9991. For O2+, LoD are in the range of 72 to 1.4 ng, with an accuracy of between 2.8 and 4.9% and a precision between 4.5 and 8.6% and R20.9914. The validated methodology was applied to the analysis of commercial pre-blast gun powders from different manufacturers.(VLID)4826148Accepted versio
The ACS Virgo Cluster Survey XI. The Nature of Diffuse Star Clusters in Early-Type Galaxies
We use HST/ACS imaging of 100 early-type galaxies in the ACS Virgo Cluster
Survey to investigate the nature of diffuse star clusters (DSCs). Compared to
globular clusters (GCs), these star clusters have moderately low luminosities
(M_V > -8) and a broad distribution of sizes (3 < r_h < 30 pc), but they are
principally characterized by their low mean surface brightnesses which can be
more than three magnitudes fainter than a typical GC (mu_g > 20 mag arcsec^-2).
The median colors of diffuse star cluster systems are red, 1.1 < g-z < 1.6,
which is redder than metal-rich GCs and often as red as the galaxy itself. Most
DSC systems thus have mean ages older than 5 Gyr or else have super-solar
metallicities. We find that 12 galaxies in our sample contain a significant
excess of diffuse star cluster candidates -- nine are lenticulars (S0s), and
five visibly contain dust. We also find a substantial population of DSCs in the
halo of the giant elliptical M49, associated with the companion galaxy VCC
1199. Most DSC systems appear to be both aligned with the galaxy light and
associated with galactic disks, but at the same time many lenticulars do not
host substantial DSC populations. Diffuse star clusters in our sample share
similar characteristics to those identified in other nearby lenticular, spiral,
and dwarf galaxies, and we suggest that DSCs preferentially form, survive, and
coevolve with galactic disks. Their properties are broadly consistent with
those of merged star cluster complexes, and we note that despite being 3 - 5
magnitudes brighter than DSCs, ultra-compact dwarfs have similar surface
brightnesses. The closest Galactic analogs to the DSCs are the old open
clusters. We suggest that if a diffuse star cluster population did exist in the
disk of the Milky Way, it would be very difficult to find. (Abridged)Comment: 16 pages, 18 figures, 2 tables. Accepted for publication in the
Astrophysical Journal. Version with high resolution images and Figures 9, 10,
14 in color is available at
http://www.physics.rutgers.edu/~pcote/acs/publications.html#acsvcs1
The ACS Virgo Cluster Survey IX: The Color Distributions of Globular Cluster Systems in Early-Type Galaxies
We present the color distributions of globular cluster (GC) systems for 100
Virgo cluster early-type galaxies observed in the ACS Virgo Cluster Survey. The
color distributions of individual GC systems are consistent with continuous
trends across galaxy luminosity, color, and stellar mass. On average, almost
all galaxies possess a component of metal-poor GCs, with the average fraction
of metal-rich GCs ranging from 15 to 60%. The colors of both subpopulations
correlate with host galaxy luminosity and color, with the red GCs having a
steeper slope. To convert color to metallicity, we also introduce a preliminary
(g-z)-[Fe/H] relation calibrated to Galactic, M49 and M87 GCs. This relation is
nonlinear with a steeper slope for [Fe/H] < -0.8. As a result, the
metallicities of the metal-poor and metal-rich GCs vary similarly with respect
to galaxy luminosity and stellar mass, with relations of [Fe/H]_MP ~ L^0.16 ~
M_star^0.17 and [Fe/H]_MR ~ L^0.26 ~ M_star^0.22, respectively. Although these
relations are shallower than the mass-metallicity relation predicted by wind
models and observed for dwarf galaxies, they are very similar to the
mass-metallicity relation for star forming galaxies in the same mass range. The
offset between the two GC populations varies slowly (~ M_star^0.05) and is
approximately 1 dex across three orders of magnitude in mass, suggesting a
nearly universal amount of enrichment between the formation of the two
populations of GCs. We also find that although the metal-rich GCs show a larger
dispersion in color, it is the *metal-poor GCs* that have an equal or larger
dispersion in metallicity. Like the color-magnitude relation, these relations
derived from globular clusters present stringent constraints on the formation
and evolution of early-type galaxies. (Abridged)Comment: 25 pages, 16 figures, 7 tables. Accepted for publication in the
Astrophysical Journal. Version with Figures 1,5-9,11-16 in color is available
at http://www.physics.rutgers.edu/~pcote/acs/acsvcs_9.pd
The ACS Virgo Cluster Survey. XII. The Luminosity Function of Globular Clusters in Early Type Galaxies
We analyze the luminosity function of the globular clusters (GCs) belonging
to the early-type galaxies observed in the ACS Virgo Cluster Survey. We have
obtained estimates for a Gaussian representation of the GC luminosity function
(GCLF) for 89 galaxies. We have also fit the GCLFs with an "evolved Schechter
function", which is meant to reflect the preferential depletion of low-mass
GCs, primarily by evaporation due to two-body relaxation, from an initial
Schechter mass function similar to that of young massive clusters. We find a
significant trend of the GCLF dispersion with galaxy luminosity, in the sense
that smaller galaxies have narrower GCLFs. We show that this narrowing of the
GCLF in a Gaussian description is driven by a steepening of the GC mass
function above the turnover mass, as one moves to smaller host galaxies. We
argue that this behavior at the high-mass end of the GC mass function is most
likely a consequence of systematic variations of the initial cluster mass
function. The GCLF turnover mass M_TO is roughly constant, at ~ 2.2 x 10^5
M_sun in bright galaxies, but it decreases slightly in dwarfs with M_B >~ -18.
We show that part of the variation could arise from the shorter dynamical
friction timescales in smaller galaxies. We probe the variation of the GCLF to
projected galactocentric radii of 20-35 kpc in the Virgo giants M49 and M87,
finding that M_TO is essentially constant over these spatial scales. Our fits
of evolved Schechter functions imply average dynamical mass losses (Delta) over
a Hubble time that fall in the range 2 x 10^5 <~ (Delta/M_sun) < 10^6 per GC.
We agree with previous suggestions that if the full GCLF is to be understood in
more detail GCLF models will have to include self-consistent treatments of
dynamical evolution inside time-dependent galaxy potentials. (Abridged)Comment: 46 pages, 20 figures, 6 tables. Accepted for publication in ApJS.
Also available at
http://www1.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/community/ACSVCS/publications.htm
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