62 research outputs found
A Search for Optical Variability of Type 2 Quasars in SDSS Stripe 82
Hundreds of Type 2 quasars have been identified in Sloan Digital Sky Survey
(SDSS) data, and there is substantial evidence that they are generally galaxies
with highly obscured central engines, in accord with unified models for active
galactic nuclei (AGNs). A straightforward expectation of unified models is that
highly obscured Type 2 AGNs should show little or no optical variability on
timescales of days to years. As a test of this prediction, we have carried out
a search for variability in Type 2 quasars in SDSS Stripe 82 using
difference-imaging photometry. Starting with the Type 2 AGN catalogs of
Zakamska et al. (2003) and Reyes et al. (2008), we find evidence of significant
g-band variability in 17 out of 173 objects for which light curves could be
measured from the Stripe 82 data. To determine the nature of this variability,
we obtained new Keck spectropolarimetry observations for seven of these
variable AGNs. The Keck data show that these objects have low continuum
polarizations (p<~1% in most cases) and all seven have broad H-alpha and/or
MgII emission lines in their total (unpolarized) spectra, indicating that they
should actually be classified as Type 1 AGNs. We conclude that the primary
reason variability is found in the SDSS-selected Type 2 AGN samples is that
these samples contain a small fraction of Type 1 AGNs as contaminants, and it
is not necessary to invoke more exotic possible explanations such as a
population of "naked" or unobscured Type 2 quasars. Aside from misclassified
Type 1 objects, the Type 2 quasars do not generally show detectable optical
variability over the duration of the Stripe 82 survey.Comment: 14 pages, 8 figures. Accepted for publication in A
X-ray Observations of Optically Selected Giant Elliptical-Dominated Galaxy Groups
We present a combined optical and X-ray analysis of three optically selected
X-ray bright groups with giant elliptical galaxies in the center. These massive
ellipticals were targeted for XMM-Newton X-ray observations based on their
large velocity dispersions and their proximity to a nearby ROSAT X-ray source.
Additionally, these targets are significantly brighter in the optical than
their nearest neighbors. We show that one of these systems meets the standard
criteria for a fossil group. While the other two systems have a prominent
magnitude gap in the E/S0 ridgeline, they do not appear to have reached the
fossil-like final stage of group evolution.Comment: 8 pages, 6 figures, Accepted for publication in Ap
Soft X-Ray Sources at the Centers of the Elliptical Galaxies NGC 4472 and NGC 4649
Analysis of recent Chandra observations of the elliptical galaxies NGC 4472
and NGC 4649 has revealed faint soft X-ray sources at their centers. The
sources are located to within 1'' of the optical centers of the galaxies. They
are most likely associated with the central supermassive black holes. Interest
in these and several other similar objects stems from the unusually low
luminosity of the supermassive black holes embedded in dense interstellar
medium. Our Chandra sources have very soft spectra. They are detectable only
below ~0.6 keV and have luminosities in the 0.2-2.5 keV energy band of ~ 6 *
10^{37} erg/s and ~1.7 * 10^{38} erg/s in NGC 4649 and NGC 4472, respectively.Comment: Shortened version of the paper published in Astronomy Letter
Chandra Cluster Cosmology Project II: Samples and X-ray Data Reduction
We discuss the measurements of the galaxy cluster mass functions at z=~0.05
and z=~0.5 using high-quality Chandra observations of samples derived from the
ROSAT PSPC All-Sky and 400deg^2 surveys. We provide a full reference for the
data analysis procedures, present updated calibration of relations between the
total cluster mass and its X-ray indicators (T_X, Mgas, and Y_X) based on a
subsample of low-z relaxed clusters, and present a first measurement of the
evolving L_X-Mtot relation (with Mtot estimated from Y_X) obtained from a
well-defined statistically complete cluster sample and with appropriate
corrections for the Malmquist bias applied. Finally, we present the derived
cluster mass functions, estimate the systematic uncertainties in this
measurement, and discuss the calculation of the likelihood function. We
confidently measure the evolution in the cluster comoving number density at a
fixed mass threshold, e.g., by a factor of 5.0 +- 1.2 at M_500=2.5e14 h^-1 Msun
between z=0 and 0.5. This evolution reflects the growth of density
perturbations and can be used for the cosmological constraints complementing
those from the distance-redshift relation.Comment: ApJ in press (Feb 10, 2009 issue); replacement to match accepted
version, includes revisions in response to referee's and community comment
Cosmological constraints from evolution of cluster baryon mass function at z~0.5
We present a new method for deriving cosmological constraints based on the
evolution of the baryon mass function of galaxy clusters, and implement it
using 17 distant clusters from our 160deg2 ROSAT survey. The method uses the
cluster baryon mass as a proxy for the total mass, thereby avoiding the large
uncertainties of the M_tot-T or M_tot-L_X relations used in all previous
studies. Instead, we rely on a well-founded assumption that the M_b/M_tot ratio
is a universal quantity, which should result in a much smaller systematic
uncertainty. Taking advantage of direct and accurate Chandra measurements of
the gas masses for distant clusters, we find strong evolution of the baryon
mass function between z>0.4 and the present. The observed evolution defines a
narrow band in the Omega_m-Lambda plane, Omega_m + 0.23Lambda = 0.41+-0.10 at
68% confidence, which intersects with constraints from the Cosmic Microwave
Background and supernovae Ia near Omega_m=0.3 and Lambda=0.7.Comment: ApJ in press, 11 pages; new emulateapj.cls. Better treatment of the
mass measurement scatter increased the final Omega,Lambda uncertainties by
20
The XMM Cluster Survey: The Stellar Mass Assembly of Fossil Galaxies
This paper presents both the result of a search for fossil systems (FSs)
within the XMM Cluster Survey and the Sloan Digital Sky Survey and the results
of a study of the stellar mass assembly and stellar populations of their fossil
galaxies. In total, 17 groups and clusters are identified at z < 0.25 with
large magnitude gaps between the first and fourth brightest galaxies. All the
information necessary to classify these systems as fossils is provided. For
both groups and clusters, the total and fractional luminosity of the brightest
galaxy is positively correlated with the magnitude gap. The brightest galaxies
in FSs (called fossil galaxies) have stellar populations and star formation
histories which are similar to normal brightest cluster galaxies (BCGs).
However, at fixed group/cluster mass, the stellar masses of the fossil galaxies
are larger compared to normal BCGs, a fact that holds true over a wide range of
group/cluster masses. Moreover, the fossil galaxies are found to contain a
significant fraction of the total optical luminosity of the group/cluster
within 0.5R200, as much as 85%, compared to the non-fossils, which can have as
little as 10%. Our results suggest that FSs formed early and in the highest
density regions of the universe and that fossil galaxies represent the end
products of galaxy mergers in groups and clusters. The online FS catalog can be
found at http://www.astro.ljmu.ac.uk/~xcs/Harrison2012/XCSFSCat.html.Comment: 30 pages, 50 figures. ApJ published version, online FS catalog added:
http://www.astro.ljmu.ac.uk/~xcs/Harrison2012/XCSFSCat.htm
Cosmological parameters constraints from galaxy cluster mass function measurements in combination with other cosmological data
We present the cosmological parameters constraints obtained from the
combination of galaxy cluster mass function measurements (Vikhlinin et al.,
2009a,b) with new cosmological data obtained during last three years: updated
measurements of cosmic microwave background anisotropy with Wilkinson Microwave
Anisotropy Probe (WMAP) observatory, and at smaller angular scales with South
Pole Telescope (SPT), new Hubble constant measurements, baryon acoustic
oscillations and supernovae Type Ia observations.
New constraints on total neutrino mass and effective number of neutrino
species are obtained. In models with free number of massive neutrinos the
constraints on these parameters are notably less strong, and all considered
cosmological data are consistent with non-zero total neutrino mass \Sigma m_\nu
\approx 0.4 eV and larger than standard effective number of neutrino species,
N_eff \approx 4. These constraints are compared to the results of neutrino
oscillations searches at short baselines.
The updated dark energy equation of state parameters constraints are
presented. We show that taking in account systematic uncertainties, current
cluster mass function data provide similarly powerful constraints on dark
energy equation of state, as compared to the constraints from supernovae Type
Ia observations.Comment: Accepted for publication in Astronomy Letter
Evidence for new physics from clusters ?
The abundance of local clusters is a traditional way to derive the amplitude
of matter fluctuations. In the present work, by assuming that the observed
baryon content of clusters is representative of the universe, we show that the
mass temperature relation (M-T) can be specified for any cosmological model.
This approach allows one to remove most of the uncertainty coming from M-T
relation, and to provide an estimation of sigma\_8 whose uncertainty is
essentially statistical. The values we obtain are fortuitously almost
independent of the matter density of the Universe (sigma\_8 ~ 0.6-0.63) with an
accuracy better than 5%. Quite remarkably, the amplitude of matter fluctuations
can be also tightly constrained to similar accuracy from existing CMB
measurements alone. However, the amplitude inferred in this way in a
concordance model (Lambda-CDM) is significantly larger than the value derived
from the above method based on X-ray clusters. Such a discrepancy would almost
disappear if the actual optical thickness of the Universe was 0 but could also
be alleviated from more exotic solutions: the existence of a new dark component
in the Universe as massive neutrinos. However, recent other indications of
sigma\_8 favor a high normalization. In this case, the assumption that the
baryonic content observed in clusters actually reflects the primordial value
has to be relaxed : either there exists a large baryonic dark component in the
Universe or baryons in clusters have undergone a large depletion during the
formation of these structures. We concluded that the baryon fraction in
clusters is not representative and therefore that an essential piece of the
physics of baryons in clusters is missing in standard structure formation
scenario.Comment: New version including changes and typos corrected, accepted for
publication in A&
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