125 research outputs found
Optical Monitoring of Quasars: I. Variability
We present an analysis of quasar variability from data collected during a
photometric monitoring of 50 objects carried out at CNPq/Laboratorio Nacional
de Astrofisica, Brazil, between March 1993 and July 1996. A distinctive feature
of this survey is its photometric accuracy, ~ 0.02 V mag, achieved through
differential photometry with CCD detectors, what allows the detection of faint
levels of variability. We find that the relative variability, delta = sigma /
L, observed in the V band is anti-correlated with both luminosity and redshift,
although we have no means of discovering the dominant relation, given the
strong coupling between luminosity and redshift for the objects in our
sample.We introduce a model for the dependence of quasar variability on
frequency that is consistent with multi-wavelength observations of the nuclear
variability of the Seyfert galaxy NGC 4151. We show that correcting the
observed variability for this effect slightly increases the significance of the
trends of variability with luminosity and redshift. Assuming that variability
depends only on the luminosity, we show that the corrected variability is
anti-correlated with luminosity and is in good agreement with predictions of a
simple Poissonian model. The energy derived for the hypothetical pulses, ~
10^50 erg, agrees well with those obtained in other studies. We also find that
the radio-loud objects in our sample tend to be more variable than the
radio-quiet ones, for all luminosities and redshifts.Comment: 17 pages, 12 figures, accepted for publication in MNRAS (uses MNRAS
style
The dynamical state of massive galaxy clusters
We study the mass distribution of a sample of 24 X-ray bright Abell clusters
through weak gravitational lensing. This method is independent of the dynamical
state of the galaxy cluster. Hence, by comparing dynamical and lensing mass
estimators, we can access the dynamical state of these clusters. We have found
that clusters with ICM temperatures above 8 keV show strong deviations from the
relaxation, as well as the presence of prominent sub-structures. For the
remaining clusters (the majority of the sample) we have found agreement among
the several mass estimators, which indicates that most of the clusters are in
or close to a state of dynamical equilibrium.Comment: 2 pages, 2 figures. Contributed talk, XIth IAU Latin-American
Regional Meeting (Dec. 2005, Pucon, Chile), to appear in RMxA
A moving cold front in the intergalactic medium of A3667
We present results from a Chandra observation of the central region of the
galaxy cluster A3667, with emphasis on the prominent sharp X-ray brightness
edge spanning 0.5 Mpc near the cluster core. Our temperature map shows
large-scale nonuniformities characteristic of the ongoing merger, in agreement
with earlier ASCA results. The brightness edge turns out to be a boundary of a
large cool gas cloud moving through the hot ambient gas, very similar to the
"cold fronts" discovered by Chandra in A2142. The higher quality of the A3667
data allows the direct determination of the cloud velocity. At the leading edge
of the cloud, the gas density abruptly increases by a factor of 3.9+-0.8, while
the temperature decreases by a factor of 1.9+-0.2 (from 7.7 keV to 4.1 keV).
The ratio of the gas pressures inside and outside the front shows that the
cloud moves through the ambient gas at near-sonic velocity, M=1+-0.2 or
v=1400+-300 km/s. In front of the cloud, we observe the compression of the
ambient gas with an amplitude expected for such a velocity. A smaller surface
brightness discontinuity is observed further ahead, ~350 kpc in front of the
cloud. We suggest that it corresponds to a weak bow shock, implying that the
cloud velocity may be slightly supersonic. Given all the evidence, the cold
front appears to delineate the remnant of a cool subcluster that recently has
merged with A3667. The cold front is remarkably sharp. The upper limit on its
width, 3.5 arcsec or 5 kpc, is several times smaller than the Coulomb mean free
path. This is a direct observation of suppression of the transport processes in
the intergalactic medium, most likely by magnetic fields.Comment: Submitted to ApJ. 9 pages with embedded color figures, uses
emulateapj5. Postscript with higher quality figures is available at
http://hea-www.harvard.edu/~alexey/a3667-hydro.ps.g
A Robust Classification of Galaxy Spectra: Dealing with Noisy and Incomplete Data
Over the next few years new spectroscopic surveys (from the optical surveys
of the Sloan Digital Sky Survey and the 2 degree Field survey through to
space-based ultraviolet satellites such as GALEX) will provide the opportunity
and challenge of understanding how galaxies of different spectral type evolve
with redshift. Techniques have been developed to classify galaxies based on
their continuum and line spectra. Some of the most promising of these have used
the Karhunen and Loeve transform (or Principal Component Analysis) to separate
galaxies into distinct classes. Their limitation has been that they assume that
the spectral coverage and quality of the spectra are constant for all galaxies
within a given sample. In this paper we develop a general formalism that
accounts for the missing data within the observed spectra (such as the removal
of sky lines or the effect of sampling different intrinsic rest wavelength
ranges due to the redshift of a galaxy). We demonstrate that by correcting for
these gaps we can recover an almost redshift independent classification scheme.
From this classification we can derive an optimal interpolation that
reconstructs the underlying galaxy spectral energy distributions in the regions
of missing data. This provides a simple and effective mechanism for building
galaxy spectral energy distributions directly from data that may be noisy,
incomplete or drawn from a number of different sources.Comment: 20 pages, 8 figures. Accepted for publication in A
Empirical modeling of the stellar spectrum of galaxies
An empirical method of modeling the stellar spectrum of galaxies is proposed,
based on two successive applications of Principal Component Analysis (PCA). PCA
is first applied to the newly available stellar library STELIB, supplemented by
the J, H and K magnitudes taken mainly from the 2 Micron All Sky Survey
(2MASS). Next the resultant eigen-spectra are used to fit the observed spectra
of a sample of 1016 galaxies selected from the Sloan Digital Sky Survey Data
Release One (SDSS DR1). PCA is again applied, to the fitted spectra to
construct the eigen-spectra of galaxies with zero velocity dispersion. The
first 9 galactic eigen-spectra so obtained are then used to model the stellar
spectrum of the galaxies in SDSS DR1, and synchronously to estimate the stellar
velocity dispersion, the spectral type, the near-infrared SED, and the average
reddening. Extensive tests show that the spectra of different type galaxies can
be modeled quite accurately using these eigen-spectra. The method can yield
stellar velocity dispersion with accuracies better than 10%, for the spectra of
typical S/N ratios in SDSS DR1.Comment: 34 pages with 18 figures, submitted to A
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