46 research outputs found
Stellar populations in superclusters of galaxies
A catalogue of superclusters of galaxies is used to investigate the influence
of the supercluster environment on galaxy populations, considering galaxies
brighter than M-21+5 h. Empirical spectral synthesis techniques are
applied to obtain the stellar population properties of galaxies which belong to
superclusters and representative values of stellar population parameters are
attributed to each supercluster. We show that richer superclusters present
denser environments and older stellar populations. The galaxy populations of
superclusters classified as filaments and pancakes are statistically similar,
indicating that the morphology of superclusters does not have a significative
influence on the stellar populations. Clusters of galaxies within superclusters
are also examined in order to evaluate the influence of the supercluster
environment on their galaxy properties. Our results suggest that the
environment affects galaxy properties but its influence should operate on
scales of groups and clusters, more than on the scale of superclusters.Comment: 7 pages, 4 figures; accepted to MNRA
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
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
How Stochastic is the Relative Bias Between Galaxy Types?
Examining the nature of the relative clustering of different galaxy types can
help tell us how galaxies formed. To measure this relative clustering, I
perform a joint counts-in-cells analysis of galaxies of different spectral
types in the Las Campanas Redshift Survey (LCRS). I develop a
maximum-likelihood technique to fit for the relationship between the density
fields of early- and late-type galaxies. This technique can directly measure
nonlinearity and stochasticity in the biasing relation. At high significance, a
small amount of stochasticity is measured, corresponding to a correlation
coefficient of about 0.87 on scales corresponding to 15 Mpc/h spheres. A large
proportion of this signal appears to derive from errors in the selection
function, and a more realistic estimate finds a correlation coefficient of
about 0.95. These selection function errors probably account for the large
stochasticity measured by Tegmark & Bromley (1999), and may have affected
measurements of very large-scale structure in the LCRS. Analysis of the data
and of mock catalogs shows that the peculiar geometry, variable flux limits,
and central surface-brightness selection effects of the LCRS do not seem to
cause the effect.Comment: 38 pages, 14 figures. Submitted to Apj. Modified from a chapter of my
Ph.D. Thesis at Princeton University, available at
http://www-astro-theory.fnal.gov/Personal/blanton/thesis/index.htm
Semi-empirical analysis of Sloan Digital Sky Survey galaxies III. How to distinguish AGN hosts
We consider the techniques to distinguish normal star forming (NSF) galaxies
and active galactic nuclei (AGN) hosts using optical spectra. The observational
data base is a set of 20000 galaxies extracted from the Sloan Digital Sky
Survey, for which we have determined the emission line intensities after
subtracting the stellar continuum obtained from spectral synthesis. Our
analysis is based on photoionization models computed using the stellar ionizing
radiation predicted by Starburst 99 and, for the AGNs, a broken power-law
spectrum. We explain why, among the four classical emission line diagnostic
diagrams, the [OIII]/Hb vs [NII]/Ha one works best. We show however, that none
of these diagrams is efficient in detecting AGNs in metal poor galaxies, should
such cases exist. We propose a new divisory line between ``pure'' NSF galaxies
and AGN hosts. We also show that a classification into NSF and AGN galaxies
using only [NII]/Ha is feasible and useful. Finally, we propose a new
classification diagram, the DEW diagram, plotting D_n(4000) vs
max(EW[OII],EW[NeIII]). This diagram can be used with optical spectra for
galaxies with redshifts up to z = 1.3, meaning an important progress over
classifications proposed up to now. Since the DEW diagram requires only a small
range in wavelength, it can also be used at even larger redshifts in suitable
atmospheric windows. It also has the advantage of not requiring stellar
synthesis analysis to subtract the stars and of allowing one to see ALL the
galaxies in the same diagram, including passive galaxies.Comment: 14 pages, 9 figures, accepted for publication in MNRAS (replaced on
august 3, 2006, eqs 6 and 7 corrected
Environment and the cosmic evolution of star formation
We present a mark correlation analysis of the galaxies in the Sloan Digital
Sky Survey using weights provided by MOPED. The large size of the sample
permits statistically significant statements about how galaxies with different
metallicities and star formation histories are spatially correlated. Massive
objects formed a larger fraction of their stars at higher redshifts and over
shorter timescales than did less massive objects (sometimes called
down-sizing). We find that those galaxies which dominated the cosmic star
formation at z~3 are predominantly in clusters today, whereas galaxies which
dominate the star formation at z~0 inhabit substantially lower mass objects in
less dense regions today. Hence, our results indicate that star formation and
chemical enrichment occured first in the denser regions of the Universe, and
moved to less dense regions at later times.Comment: 4 pages, 4 figures, submitted to ApJ