2,205 research outputs found
Star Clusters in Virgo and Fornax Dwarf Irregular Galaxies
We present the results of a search for clusters in dwarf irregular galaxies
in the Virgo and Fornax Cluster using HST WFPC2 snapshot data. The galaxy
sample includes 28 galaxies, 11 of which are confirmed members of the Virgo and
Fornax clusters. In the 11 confirmed members, we detect 237 cluster candidates
and determine their V magnitudes, V-I colors and core radii. After statistical
subtraction of background galaxies and foreground stars, most of the cluster
candidates have V-I colors of -0.2 and 1.4, V magnitudes lying between 20 and
25th magnitude and core radii between 0 and 6 pc. Using H-alpha observations,
we find that 26% of the blue cluster candidates are most likely HII regions.
The rest of the cluster candidates are most likely massive (>10^4 Msol) young
and old clusters. A comparison between the red cluster candidates in our sample
and the Milky Way globular clusters shows that they have similar luminosity
distributions, but that the red cluster candidates typically have larger core
radii. Assuming that the red cluster candidates are in fact globular clusters,
we derive specific frequencies (S_N) ranging from ~0-9 for the galaxies.
Although the values are uncertain, seven of the galaxies appear to have
specific frequencies greater than 2. These values are more typical of
ellipticals and nucleated dwarf ellipticals than they are of spirals or Local
Group dwarf irregulars.Comment: 46 pages, 14 figures, 3 tables, accepted by AJ. Higher quality PS
version of entire paper available at
http://www.astro.washington.edu/seth/dirr_gcs.htm
Investigating evidence for different black hole accretion modes since redshift z~1
Chandra data in the COSMOS, AEGIS-XD and 4Ms CDFS are combined with
optical/near-IR photometry to determine the rest-frame U-V vs V-J colours of
X-ray AGN hosts at mean redshifts 0.40 and 0.85. This combination of colours
(UVJ) provides an efficient means of separating quiescent from star-forming,
including dust reddened, galaxies. Morphological information emphasises
differences between AGN split by their UVJ colours. AGN in quiescent galaxies
are dominated by spheroids, while star-forming hosts are split between bulges
and disks. The UVJ diagram of AGN hosts is then used to set limits on the
accretion density associated with evolved and star-forming systems. Most of the
black hole growth since z~1 is associated with star-forming hosts.
Nevertheless, ~15-20% of the X-ray luminosity density since z~1, is taking
place in the quiescent region of the UVJ diagram. For the z~0.40 subsample,
there is tentative evidence (2sigma significance), that AGN split by their UVJ
colours differ in Eddington ratio. AGN in star-forming hosts dominate at high
Eddington ratios, while AGN in quiescent hosts become increasingly important as
a fraction of the total population toward low Eddington ratios. At higher
redshift, z~0.8, such differences are significant at the 2sigma level only at
Eddington ratios >1e-3. These findings are consistent with scenarios in which
diverse accretion modes are responsible for the build-up of SMBHs at the
centres of galaxies. We compare our results with the GALFORM semi-analytic
model, which postulates two black hole fuelling modes, the first linked to
star-formation and the second occuring in passive galaxies. GALFORM predicts a
larger fraction of black hole growth in quiescent galaxies at z<1, compared to
the data. Relaxing the strong assumption of the model that passive AGN hosts
have zero star-formation rate could reconcile this disagreement.Comment: MNRAS accepte
A Strong-Lens Survey in AEGIS: the influence of large scale structure
We report on the results of a visual search for galaxy-scale strong
gravitational lenses over 650 arcmin^2 of HST/ACS imaging in the DEEP2-EGS
field. In addition to a previously-known Einstein Cross (the "Cross," HST
J141735+52264, with z_lens=0.8106 and a published z_source=3.40), we identify
two new strong galaxy-galaxy lenses with multiple extended arcs. The first, HST
J141820+52361 (the ``Dewdrop''; z_lens=0.5798, lenses two distinct extended
sources into two pairs of arcs z_source=0.while), 9818 the second, HST
J141833+52435 (the ``Anchor''; z_lens=0.4625), produces a single pair of arcs
(source redshift not yet known). All three definite lenses are fit well by
simple singular isothermal ellipsoid models including external shear. Using the
three-dimensional line-of-sight (LOS) information on galaxies from the DEEP2
data, we calculate the convergence and shear contributions, assuming singular
isothermal sphere halos truncated at 200 h^-1 kpc. These are also compared
against three-dimensional local-density estimates. We find that even strong
lenses in demonstrably underdense local environments may be considerably
affected by LOS contributions, which in turn, may be underestimates of the
effect of large scale structure.Comment: ApJ Letters, submitted. Part of the AEGIS ApJL Special Issue. 4
Figures, 1 Table. For a version with full-resolution figures, please see
http://www.slac.stanford.edu/~pjm/HAGGLeS/astroph/legs.pd
The Effects of an AGN on Host Galaxy Colour and Morphology Measurements
We assess the effects of simulated active galactic nuclei (AGNs) on the
colour and morphology measurements of their host galaxies. To test the
morphology measurements, we select a sample of galaxies not known to host AGNs
and add a series of point sources scaled to represent specified fractions of
the observed V band light detected from the resulting systems; we then compare
morphology measurements of the simulated systems to measurements of the
original galaxies. AGN contributions >20 per cent bias most of the morphology
measurements tested, though the extent of the apparent bias depends on the
morphological characteristics of the original galaxies. We test colour
measurements by adding to non-AGN galaxy spectra a quasar spectrum scaled to
contribute specified fractions of the rest-frame B band light detected from the
resulting systems. A quasar fraction of 5 per cent can move the NUV-r colour of
an elliptical galaxy from the UV-optical red sequence to the green valley, and
20 per cent can move it into the blue cloud. Combining the colour and
morphology results, we find that a galaxy/AGN system with an AGN contribution
>20 per cent may appear bluer and more bulge-dominated than the underlying
galaxy. We conclude that (1) bulge-dominated, E/S0/Sa, and early-type
morphology classifications are accurate for red AGN host galaxies and may be
accurate for blue host galaxies, unless the AGN manifests itself as a
well-defined point source; and (2) although highly unobscured AGNs, such as the
quasar used for our experiments, can significantly bias the measured colours of
AGN host galaxies, it is possible to identify such systems by examining optical
images of the hosts for the presence of a point source and/or measuring the
level of nuclear obscuration.Comment: 18 pages, 19 figures, 1 table. Accepted for publication in MNRA
The Colors of Dwarf Elliptical Galaxy Globular Cluster Systems, Nuclei and Stellar Halos
We present the results of a Hubble Space Telescope WFPC2 F555W and F814W
survey of 69 dwarf elliptical galaxies (dEs) in the Virgo and Fornax Clusters
and Leo Group. The colors of the dE globular clusters, nuclei, and
underlying field star populations are used to trace the dE star-formation
histories. We find that the dE globular cluster candidates are as blue as the
metal-poor globular clusters of the Milky Way. The observed correlation of the
dE globular cluster systems' color with the luminosity of the host dE is
strong evidence that the globular clusters were formed within the the halos of
dEs and do not have a pre-galactic origin. Assuming the majority of dE clusters
are old, the mean globular cluster color- host galaxy luminosity correlation
implies a cluster metallicity galaxy luminosity relation of , which is significantly shallower than the field star
metallicity - host galaxy luminosity relationship observed in Local Group
dwarfs (). The dE stellar envelopes are
magnitudes redder in than their globular clusters and nuclei. This color
offset implies separate star-formation episodes within the dEs for the clusters
and field stars, while the very blue colors of two dE nuclei trace a third
star-formation event in those dEs less than a Gyr ago.Comment: 39 pages, including 5 tables and 10 figures; accepted by the
Astrophysical Journa
Ischaemic preconditioning of the liver before transplantation
Purpose: Assessment of the effect of a short ischaemic time prior to liver transplantation on the liver graft. Methods: White X Landrace pigs (N=10) were subjected to liver transplantation. Before being removed from the donor animal, the livers were randomised into two groups: group 1 - pre-procurement ischaemia (15 minutes' temporary arrest of portal venous and hepatic arterial inflow to the liver, followed by reperfusion of these vessels for a period of 15 minutes); group 2 - no prior inflow occlusion (control group). In group 1 a spleno-jugular bypass was established to prevent venous congestion, portal venous hypertension, intestinal oedema and bacterial translocation. The livers were perfused with Eurocollins solution (4oC), after which they were stored on ice for a period of 3 hours' cold ischaemic time. Hepatocellular injury was assessed according to liver cell function tests (aspartate aminotransferase, AST), biochemical indicators of reperfusion injury (malondialdehyde) and histopathology. Results: There was a significant rise of AST in both groups 1 hour after transplantation (from 51+27 IU/l to 357+152 IU/l in group 1 and from 29+10 IU/l to 359+198 IU/l in group 2). AST levels were marginally lower in group 1 at 2 and 4 hours after transplantation. There was also a rise in malondialdehyde levels in both groups at 5, 20, 40 and 60 minutes after transplantation. Levels of malondialdehyde were lower in the primed group at 5, 20 and 40 minutes, while the levels at 60 minutes after transplantation were comparable. Histological changes, as measured by vacuolisation, neutrophil infiltration and hepatic cell necrosis, were less in livers transplanted after ischaemic preconditioning, although the difference was not significant. Conclusions: Ischaemic preconditioning of the donor liver seems to decrease hepatocellular damage, reperfusion injury and histological changes in the liver after transplantation. Further studies with larger numbers are indicated
Constraints on radiative decay of the 17-keV neutrino from COBE Measurements
It is shown that, for a nontrivial radiative decay channel of the 17-keV
neutrino, the photons would distort the microwave background radiation through
ionization of the universe. The constraint on the branching ratio of such
decays from COBE measurements is found to be more stringent than that from
other considerations. The limit on the branching ratio in terms of the Compton
parameter is for an
universe.Comment: 7 pages. (figures will be sent on request) (To appear in Phys. Rev.
D.
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