25,272 research outputs found
NoSOCS in SDSS. VI. The Environmental Dependence of AGN in Clusters and Field in the Local Universe
We investigated the variation in the fraction of optical active galactic
nuclei (AGN) hosts with stellar mass, as well as their local and global
environments. Our sample is composed of cluster members and field galaxies at
and we consider only strong AGN. We find a strong variation in the
AGN fraction () with stellar mass. The field population comprises a
higher AGN fraction compared to the global cluster population, especially for
objects with log . Hence, we restricted our analysis to more
massive objects. We detected a smooth variation in the with local
stellar mass density for cluster objects, reaching a plateau in the field
environment. As a function of clustercentric distance we verify that
is roughly constant for R R, but show a steep decline inwards. We
have also verified the dependence of the AGN population on cluster velocity
dispersion, finding a constant behavior for low mass systems ( km s). However, there is a strong decline in
for higher mass clusters ( 700 km s). When comparing the in
clusters with or without substructure we only find different results for
objects at large radii (R R), in the sense that clusters with
substructure present some excess in the AGN fraction. Finally, we have found
that the phase-space distribution of AGN cluster members is significantly
different than other populations. Due to the environmental dependence of
and their phase-space distribution we interpret AGN to be the result
of galaxy interactions, favored in environments where the relative velocities
are low, typical of the field, low mass groups or cluster outskirts.Comment: 11 pages, 10 figures, Accepted to MNRA
Black hole entropy functions and attractor equations
The entropy and the attractor equations for static extremal black hole
solutions follow from a variational principle based on an entropy function. In
the general case such an entropy function can be derived from the reduced
action evaluated in a near-horizon geometry. BPS black holes constitute special
solutions of this variational principle, but they can also be derived directly
from a different entropy function based on supersymmetry enhancement at the
horizon. Both functions are consistent with electric/magnetic duality and for
BPS black holes their corresponding OSV-type integrals give identical results
at the semi-classical level. We clarify the relation between the two entropy
functions and the corresponding attractor equations for N=2 supergravity
theories with higher-derivative couplings in four space-time dimensions. We
discuss how non-holomorphic corrections will modify these entropy functions.Comment: 21 pages,LaTeX,minor change
Non-Gaussian velocity distributions - The effect on virial mass estimates of galaxy groups
We present a study of 9 galaxy groups with evidence for non-Gaussianity in
their velocity distributions out to 4R200. This sample is taken from 57 groups
selected from the 2PIGG catalog of galaxy groups. Statistical analysis
indicates that non-Gaussian groups have masses significantly higher than
Gaussian groups. We also have found that all non-Gaussian systems seem to be
composed of multiple velocity modes. Besides, our results indicate that
multimodal groups should be considered as a set of individual units with their
own properties. In particular, we have found that the mass distribution of such
units are similar to that of Gaussian groups. Our results reinforce the idea of
non-Gaussian systems as complex structures in the phase space, likely
corresponding to secondary infall aggregations at a stage before virialization.
The understanding of these objects is relevant for cosmological studies using
groups and clusters through the mass function evolution.Comment: 5 pages, 4 figures and 2 tables. Accepted for publication in the
MNRA
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