6,034 research outputs found
VLA Observations of Single Pulses from the Galactic Center Magnetar
We present the results of a 7-12 GHz phased-array study of the Galactic
center magnetar J1745-2900 with the Karl G. Jansky Very Large Array (VLA).
Using data from two 6.5 hour observations from September 2014, we find that the
average profile is comprised of several distinct components at these epochs and
is stable over day timescales and GHz frequencies. Comparison with
additional phased VLA data at 8.7 GHz shows significant profile changes on
longer timescales. The average profile at 7-12 GHz is dominated by the jitter
of relatively narrow pulses. The pulses in each of the four main profile
components seen in September 2014 are uncorrelated in phase and amplitude,
though there is a small but significant correlation in the occurrence of pulses
in two of the profile components. Using the brightest pulses, we measure the
dispersion and scattering parameters of J1745-2900. A joint fit of 38 pulses
gives a 10 GHz pulse broadening time of and a dispersion measure of . Both of these results are consistent with previous measurements,
which suggests that the scattering and dispersion measure of J1745-2900 may be
stable on timescales of several years.Comment: 20 pages, 10 figures, published in Ap
B3 0003+387: AGN Marked Large-Scale Structure at z=1.47?
We present evidence for a significant overdensity of red galaxies, as much as
a factor of 14 over comparable field samples, in the field of the z=1.47 radio
galaxy B3 0003+387. The colors and luminosities of the brightest red galaxies
are consistent with their being at z>0.8. The radio galaxy and one of the red
galaxies are separated by 5" and show some evidence of a possible interaction.
However, the red galaxies do not show any strong clustering around the radio
galaxy nor around any of the brighter red galaxies. The data suggest that we
are looking at a wall or sheet of galaxies, possibly associated with the radio
galaxy at z=1.47. Spectroscopic redshifts of these red galaxies will be
necessary to confirm this large-scale structure.Comment: 19 pages, 7 figures, LaTeX2e/AASTeX v5.0.2. The full photometric
catalog is included as a separate deluxetable file. To appear in the
Astronomical Journal (~Nov 00
Photometric Properties of 47 Clusters of Galaxies: I. The Butcher-Oemler Effect
We present gri CCD photometry of 44 Abell clusters and 4 cluster candidates.
Twenty one clusters in our sample have spectroscopic redshifts. Fitting a
relation between mean g, r and i magnitudes, and redshift for this subsample,
we have calculated photometric redshifts for the remainder with an estimated
accuracy of 0.03. The resulting redshift range for the sample is 0.03<z<0.38.
Color-magnitude diagrams are presented for the complete sample and used to
study evolution of the galaxy population in the cluster environment. Our
observations show a strong Butcher-Oemler effect (Butcher & Oemler 1978, 1984),
with an increase in the fraction of blue galaxies (f_B) with redshift that
seems more consistent with the steeper relation estimated by Rakos and
Schombert (1995) than with the original one by Butcher & Oemler (1984).
However, in the redshift range between ~ 0.08 and 0.2, where most of our
clusters lie, there is a wide range of f_B values, consistent with no redshift
evolution of the cluster galaxy population. A large range of f_B values is also
seen between ~ 0.2 and 0.3, when Smail at al. (1998) x-ray clusters are added
to our sample. The discrepancies between samples underscore the need for an
unbiased sample to understand how much of the Butcher-Oemler effect is due to
evolution, and how much to selection effects. We also tested the idea proposed
by Garilli et al. (1996) that there is a population of unusually red galaxies
which could be associated either with the field or clusters, but we find that
these objects are all near the limiting magnitude of the images (20.5<r<22) and
have colors that are consistent with those expected for stars or field galaxies
at z ~ 0.7.Comment: 35 pages including 8 figures, submitted to A
Age, Metallicity and Star Formation History of Cluster Galaxies at z~0.3 F
We investigate the color-magnitude distribution in the rich cluster AC 118 at
z=0.31. The sample is selected by the photometric redshift technique, allowing
to study a wide range of properties of stellar populations, and is complete in
the K-band, allowing to study these properties up to a given galaxy mass. We
use galaxy templates based on population synthesis models to translate the
physical properties of the stellar populations - formation epoch, time-scale of
star formation, and metallicity - into observed magnitudes and colors. In this
way we show that a sharp luminosity-metallicity relation is inferred without
any assumption on the galaxy formation scenario (either monolithic or
hierarchical). Our data exclude significant differences in star formation
histories along the color-magnitude relation, and therefore confirm a pure
metallicity interpretation for its origin, with an early (z~5) formation epoch
for the bulk of stellar populations. The dispersion in the color-magnitude
diagram implies that fainter galaxies in our sample (K~18) ceased to form stars
as late as z~0.5, in agreement with the picture that these galaxies were
recently accreted into the cluster environment. The trend with redshift of the
total stellar mass shows that half of the luminous mass in AC 118 was already
formed at $z~2, but also that 20% of the stars formed at z<1.Comment: 16 pages, 10 figures. ApJ in pres
Galaxy bimodality versus stellar mass and environment
We analyse a z<0.1 galaxy sample from the Sloan Digital Sky Survey focusing
on the variation of the galaxy colour bimodality with stellar mass and
projected neighbour density Sigma, and on measurements of the galaxy stellar
mass functions. The characteristic mass increases with environmental density
from about 10^10.6 Msun to 10^10.9 Msun (Kroupa IMF, H_0=70) for Sigma in the
range 0.1--10 per Mpc^2. The galaxy population naturally divides into a red and
blue sequence with the locus of the sequences in colour-mass and
colour-concentration index not varying strongly with environment. The fraction
of galaxies on the red sequence is determined in bins of 0.2 in log Sigma and
log mass (12 x 13 bins). The red fraction f_r generally increases continuously
in both Sigma and mass such that there is a unified relation: f_r =
F(Sigma,mass). Two simple functions are proposed which provide good fits to the
data. These data are compared with analogous quantities in semi-analytical
models based on the Millennium N-body simulation: the Bower et al. (2006) and
Croton et al. (2006) models that incorporate AGN feedback. Both models predict
a strong dependence of the red fraction on stellar mass and environment that is
qualitatively similar to the observations. However, a quantitative comparison
shows that the Bower et al. model is a significantly better match; this appears
to be due to the different treatment of feedback in central galaxies.Comment: 19 pages, 17 figures; accepted by MNRAS, minor change
Constraining dark energy models using the lookback time to galaxy clusters and the age of the universe
An impressive amount of different astrophysical data converges towards the
picture of a spatially flat universe undergoing a today phase of accelerated
expansion. The nature of the dark energy dominating the energy content of the
universe is still unknown and a lot of different scenarios are viable
candidates to explain cosmic acceleration. Most of the methods employed to test
these cosmological models are essentially based on distance measurements to a
particular class of objects. A different method, based on the lookback time to
galaxy clusters and the age of the universe, is used here. In particular, we
constrain the characterizing parameters of three classes of dark energy
cosmological models to see whether they are in agreement with this kind of
data, based on time measurements rather than distance observations.Comment: 13 pages, 8 figures, accepted for publication on Physical Review
Chandra Observations of low velocity dispersion groups
Deviations of galaxy groups from cluster scaling relations can be understood
in terms of an excess of entropy in groups. The main effect of this excess is
to reduce the density and thus luminosity of the intragroup gas. Given this,
groups should also should show a steep relationship between X-ray luminosity
and velocity dispersion. However, previous work suggests that this is not the
case with many measuring slopes flatter than the cluster relation.
Examining the group L_X:\sigma relation shows that much of the flattening is
caused by a small subset of groups which show very high X-ray luminosities for
their velocity dispersions (or vice versa).
Detailed Chandra study of two such groups shows that earlier ROSAT results
were subject to significant (~30-40%) point source contamination, but confirm
that a significant hot IGM is present in these groups, although these are two
of the coolest systems in which intergalactic X-ray emission has been detected.
Their X-ray properties are shown to be broadly consistent with those of other
galaxy groups, although the gas entropy in NGC 1587 is unusually low, and its
X-ray luminosity correspondingly high for its temperature, compared to most
groups.
This leads us to suggest that the velocity dispersion in these systems has
been reduced in some way, and we consider how this might have come about.Comment: Accepted for publication in Ap
Merging history as a function of halo environment
According to the hierarchical scenario, galaxies form via merging and
accretion of small objects. Using N-body simulations, we study the frequency of
merging events in the history of the halos. We find that at z<~2 the merging
rate of the overall halo population can be described by a simple power law
(1+z)^3. The main emphasis of the paper is on the effects of environment of
halos at the present epoch (z=0). We find that the halos located inside
clusters have formed earlier (dz \approx 1) than isolated halos of the same
mass. At low redshifts (z<1), the merger rate of cluster halos is 3 times lower
than that of isolated halos and 2 times lower than merger rate of halos that
end up in groups by z=0. At higher redshifts (z~1-4), progenitors of cluster
and group halos have 3--5 times higher merger rates than isolated halos. We
briefly discuss implications of our results for galaxy evolution in different
environments.Comment: submitted to the Astrophys. Journal; 11 pages, 9 figs., LaTeX (uses
emulateapj.sty
Gravitational Waves from Supermassive Black Hole Coalescence in a Hierarchical Galaxy Formation Model
We investigate the expected gravitational wave emission from coalescing
supermassive black hole (SMBH) binaries resulting from mergers of their host
galaxies. When galaxies merge, the SMBHs in the host galaxies sink to the
center of the new merged galaxy and form a binary system. We employ a
semi-analytic model of galaxy and quasar formation based on the hierarchical
clustering scenario to estimate the amplitude of the expected stochastic
gravitational wave background owing to inspiraling SMBH binaries and bursts
owing to the SMBH binary coalescence events. We find that the characteristic
strain amplitude of the background radiation is for just below the detection
limit from measurements of the pulsar timing provided that SMBHs coalesce
simultaneously when host galaxies merge. The main contribution to the total
strain amplitude of the background radiation comes from SMBH coalescence events
at . We also find that a future space-based gravitational wave
interferometer such as the planned \textit{Laser Interferometer Space Antenna}
({\sl LISA}) might detect intense gravitational wave bursts associated with
coalescence of SMBH binaries with total mass at
at a rate . Our model predicts that
burst signals with a larger amplitude correspond
to coalescence events of massive SMBH binary with total mass at low redshift at a rate whereas those with a smaller amplitude
correspond to coalescence events of less massive SMBH binary with total mass
at high redshift .Comment: Accepted for publication in ApJ. 11 pages, 7 figure
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