1,504 research outputs found
Structure and spatial distribution of Ge nanocrystals subjected to fast neutron irradiation
The influence of fast neutron irradiation on the structure and spatial
distribution of Ge nanocrystals (NC) embedded in an amorphous SiO2 matrix has
been studied. The investigation was conducted by means of laser Raman
Scattering (RS), High Resolution Transmission Electron Microscopy (HR-TEM) and
X-ray photoelectron spectroscopy (XPS). The irradiation of NC-Ge samples by a
high dose of fast neutrons lead to a partial destruction of the nanocrystals.
Full reconstruction of crystallinity was achieved after annealing the radiation
damage at 800 deg. C, which resulted in full restoration of the RS spectrum.
HR-TEM images show, however, that the spatial distributions of NC-Ge changed as
a result of irradiation and annealing. A sharp decrease in NC distribution
towards the SiO2 surface has been observed. This was accompanied by XPS
detection of Ge oxides and elemental Ge within both the surface and subsurface
region
Does environment affect the star formation histories of early-type galaxies?
Differences in the stellar populations of galaxies can be used to quantify
the effect of environment on the star formation history. We target a sample of
early-type galaxies from the Sloan Digital Sky Survey in two different
environmental regimes: close pairs and a general sample where environment is
measured by the mass of their host dark matter halo. We apply a blind source
separation technique based on principal component analysis, from which we
define two parameters that correlate, respectively, with the average stellar
age (eta) and with the presence of recent star formation (zeta) from the
spectral energy distribution of the galaxy. We find that environment leaves a
second order imprint on the spectra, whereas local properties - such as
internal velocity dispersion - obey a much stronger correlation with the
stellar age distribution.Comment: 5 pages, 2 figures. Proceedings of JENAM 2010, Symposium 2:
"Environment and the formation of galaxies: 30 years later
Extremely Red Objects from the NICMOS/HST Parallel Imaging Survey
We present a catalog of extremely red objects discovered using the NICMOS/HST
parallel imaging database and ground-based optical follow-up observations.
Within an area of 16 square arc-minutes, we detect 15 objects with and . We have also obtained K-band photometry for
a subset of the 15 EROs. All of the selected EROs imaged at
K-band have . Our objects have colors in the
range of 1.3 - 2.1, redder than the cluster ellipticals at and
nearly 1 magnitude redder than the average population selected from the F160W
images at the same depth. In addition, among only 22 NICMOS pointings, we
detected two groups or clusters in two fields, each contains 3 or more EROs,
suggesting that extremely red galaxies may be strongly clustered. At bright
magnitudes with , the ERO surface density is similar to what
has been measured by other surveys. At the limit of our sample, F160W = 21.5,
our measured surface density is 0.94 arcmin^{-2}. Excluding the two
possible groups/clusters and the one apparently stellar object, reduces the
surface density to 0.38 arcmin^{-2}.Comment: To appear in the AJ August issue. Replaced with the published versio
The Las Campanas/AAT Rich Cluster Survey III: Spectroscopic Studies of X-ray Bright Galaxy Clusters at z~0.1
[abridged] We present the analysis of the spectroscopic and photometric
catalogues of 11 X-ray luminous clusters at z=0.07-0.16 from the Las Campanas /
Anglo-Australian Telescope Rich Cluster Survey. Our spectroscopic dataset
consists of over 1600 galaxy cluster members, of which two thirds are outside
r_200. We assign cluster membership using a detailed mass model and expand on
our previous work on the cluster colour-magnitude relation where membership was
inferred statistically. We confirm that the modal colours of galaxies on the
colour magnitude relation become progressively bluer with increasing radius and
decreasing local galaxy density. Interpreted as an age effect, we hypothesize
that these trends in galaxy colour should be reflected in mean Hdelta
equivalent width. We confirm that passive galaxies in the cluster increase in
Hdelta line strength as dHdelta / d r_p = 0.35 +/- 0.06. A variation of star
formation rate, as measured by [OII], with increasing local density of the
environment is discernible and is shown to be in broad agreement with previous
studies from 2dFGRS and SDSS. We find that clusters at z~0.1 are less active
than their higher redshift analogues. We also investigate unusual populations
of blue and very red nonstarforming galaxies and we suggest that the former are
likely to be the progenitors of galaxies which will lie on the colour-magnitude
relation, while the colours of the latter possibly reflect dust reddening. The
cluster galaxies at large radii consist of both backsplash ones and those that
are infalling to the cluster for the first time. We make a comparison to the
field population at z~0.1 and examine broad differences between the two
populations. Individually, the clusters show significant variation in their
galaxy populations which reflects their recent infall histories.Comment: 25 pages, 16 figures. Accepted for publication in MNRA
The Evolutionary Status of Clusters of Galaxies at z ~ 1
Combined HST, X-ray, and ground-based optical studies show that clusters of
galaxies are largely "in place" by , an epoch when the Universe was
less than half its present age. High resolution images show that elliptical,
S0, and spiral galaxies are present in clusters at redshifts up to . Analysis of the CMDs suggest that the cluster ellipticals formed their
stars several Gyr earlier, near redshift 3. The morphology--density relation is
well established at , with star-forming spirals and irregulars residing
mostly in the outer parts of the clusters and E/S0s concentrated in dense
clumps. The intracluster medium has already reached the metallicity of
present-day clusters. The distributions of the hot gas and early-type galaxies
are similar in clusters, indicating both have largely virialized in
the deepest potentials wells. In spite of the many similarities between
and present-day clusters, there are significant differences. The
morphologies revealed by the hot gas, and particularly the early-type galaxies,
are elongated rather than spherical. We appear to be observing the clusters at
an epoch when the sub-clusters and groups are still assembling into a single
regular cluster. Support for this picture comes from CL0152 where the gas
appears to be lagging behind the luminous and dark mass in two merging
sub-components. Moreover, the luminosity difference between the first and
second brightest cluster galaxies at is smaller than in 93% of
present-day Abell clusters, which suggests that considerable luminosity
evolution through merging has occurred since that epoch. Evolution is also seen
in the bolometric X-ray luminosity function.Comment: 18 pages, 12 figures, to appear in Penetrating Bars through Masks of
Cosmic Dust: the Hubble Tuing Fork Strikes a New Note, eds. D.L. Block, K.C.
Freeman, I. Puerari & R. Groess. Figures degraded to meet astroph size limit;
a version with higher resolution figures may be downloaded from:
http://acs.pha.jhu.edu/~jpb/z1clusters/ford_clusters.pd
The intermediate-redshift galaxy cluster CL 0048-2942. Stellar populations
We present a detailed study of the cluster CL 0048-2942, located at z~0.64,
based on a photometric and spectroscopic catalogue of 54 galaxies in a 5 x 5
square arcmin region centred in that cluster. Of these, 23 galaxies were found
to belong to the cluster. Based on this sample, the line-of-sight velocity
dispersion of the cluster is approximately 680 +- 140 km/s. We have performed
stellar population synthesis in the cluster members as well as in the field
galaxies of the sample and found that there are population gradients in the
cluster with central galaxies hosting mainly intermediate/old populations
whereas galaxies in the cluster outskirts show clearly an increase of younger
populations, meaning that star formation is predominantly taking place in the
outer regions of the cluster. In a general way, field galaxies seem to host
less evolved stellar populations than cluster members. In fact, in terms of
ages, young supergiant stars dominate the spectra of field galaxies whereas
cluster galaxies display a dominant number of old and intermediate age stars.
Following the work of other authors (e.g. Dressler et al. 1999) we have
estimated the percentage of K+A galaxies in our sample and found around 13% in
the cluster and 10% in the field. These values were estimated through means of
a new method, based on stellar population synthesis results, that takes into
account all possible absorption features in the spectrum and thus makes optimal
use of the data.Comment: Accepted by Astronomy & Astrophysics. 24 pages, 10 figures, 10 tables
(figures 3, 4, 5 and tables 1, 3, 4, 5, 6, 7, 8 will be available in
electronic format only in the A&A published version
Searching for tidal tails - investigating galaxy harassment
Galaxy harassment has been proposed as a physical process that
morphologically transforms low surface density disc galaxies into dwarf
elliptical galaxies in clusters. It has been used to link the observed very
different morphology of distant cluster galaxies (relatively more blue galaxies
with 'disturbed' morphologies) with the relatively large numbers of dwarf
elliptical galaxies found in nearby clusters. One prediction of the harassment
model is that the remnant galaxies should lie on low surface brightness tidal
streams or arcs. We demonstrate in this paper that we have an analysis method
that is sensitive to the detection of arcs down to a surface brightness of 29 B
mag/arcsec^2 and then use this method to search for arcs around 46 Virgo
cluster dwarf elliptical galaxies. We find no evidence for tidal streams or
arcs and consequently no evidence for galaxy harassment as a viable explanation
for the relatively large numbers of dwarf galaxies found in the Virgo cluster.Comment: Accepted for publication in MNRA
Spin-dependent twist-4 matrix elements from the instanton vacuum: Flavor-singlet and nonsinglet
We estimate the twist-4 spin-1 nucleon matrix element f_2 in an
instanton-based description of the QCD vacuum. In addition to the
flavor-nonsinglet we compute also the flavor-singlet matrix element, which
appears in next-to-leading order of the (1/N_c)-expansion. The corresponding
twist-3 spin-2 matrix elements d_2 are suppressed in the packing fraction of
the instanton medium, (\bar \rho)/(\bar R) << 1. We use our results to estimate
the leading (1/Q^2) power corrections to the first moment of the proton and
neutron spin structure functions G_1, as well as the intrinsic charm
contribution to the nucleon spin.Comment: 17 pages, 4 eps figures include
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