107 research outputs found
CG J1720-67.8: A Detailed Analysis of Optical and Infrared Properties of a New Ultracompact Group of Galaxies
We present here optical spectroscopy and BVRJHK(s) photometry of the recently
discovered ultra-compact group of galaxies CG J1720-67.8. This work represents
a considerable extension of the preliminary results we presented in a previous
paper. Despite the complicated morphology of the group, a quantitative
morphological classification of the three brightest members of the group is
attempted based on photometric analysis. We find that one galaxy is consistent
with a morphological type S0, while the other two are most probably late-type
spirals that are already losing their identity due tothe interaction process.
Information on the star formation activity and dust content derived from both
spectroscopic data and optical and near-infrared colors are complemented with a
reconstruction of far-infrared (FIR) maps from IRAS raw data. Enhanced star
formation activity is revealed in all the group's members, including the
early-type galaxy and the extended tidal tail, along which several tidal dwarf
galaxy candidates are identified. The metallicity of the gaseous component is
investigated and photoionization models are applied to the three main galaxies
of the group, while a detailed study of the tidal dwarf candidates will appear
in a companion paper. Subsolar metal abundances are found for all the three
galaxies, the highest values being shown by the early-type galaxy (Z ~ 0.5
Zsolar).Comment: Accepted for publication in The Astrophysical Journa
Dissipationless Merging and the Assembly of Central Galaxies
We reanalyze the galaxy-mass correlation function measured by the Sloan
Digital Sky Survey to obtain host dark matter halo masses at galaxy and galaxy
group scales. We extend the data to galaxy clusters in the 2MASS catalog and
study the relation between central galaxy luminosity and halo mass. While the
central galaxy luminosity scales as ~M^{0.7-0.8} at low masses, the relation
flattens to ~M^{<0.3} above ~4x10^{13} M_sun. The total luminosity of galaxies
in the halo, however, continues to grow as a power-law ~M^{0.8-0.9}. Starting
from the hypothesis that the central galaxies grow by merging ("galactic
cannibalism"), we develop a simple model for the evolution of their
luminosities as a consequence of the accretion of satellite galaxies. The
luminosity-mass relation flattens when the time scale on which dynamical
friction induces orbital decay in the satellite galaxies exceeds the age of the
dark matter halo. Then, the growth of the central galaxy is suppressed as it
can cannibalize only the rare, massive satellite galaxies. The model takes the
dependence of the total luminosity of galaxies in a halo on its mass and the
global galaxy luminosity function as input, and reproduces the observed central
galaxy luminosity-mass relation over three decades in halo mass,
(10^{12}-10^{15}) M_sun.
The success of the model suggests that gas cooling and subsequent star
formation did not play an important role in the final assembly of central
galaxies from sub-L_star precursors.Comment: 4 pages, 2 figures, submitte
The Role of Gas in the Merging of Massive Black Holes in Galactic Nuclei. I. Black Hole Merging in a Spherical Gas Cloud
Using high-resolution SPH numerical simulations, we investigate the effects
of gas on the inspiral and merger of a massive black hole binary. This study is
motivated by both observational and theoretical work that indicate the presence
of large amounts of gas in the central regions of merging galaxies. N-body
simulations have shown that the coalescence of a massive black hole binary
eventually stalls in a stellar background. However, our simulations suggest
that the massive black hole binary will finally merge if it is embedded in a
gaseous background. Here we present results in which the gas is assumed to be
initially spherical with a relatively smooth distribution. In the early
evolution of the binary, the separation dimishes due to the gravitational drag
exerted by the background gas. In the later stages, when the binary dominates
the gravitational potential in its vicinity, the medium responds by forming an
ellipsoidal density enhancement whose axis lags behind the binary axis, and
this offset produces a torque on the binary that causes continuing loss of
angular momentum and is able to reduce the binary separation to distances where
gravitational radiation is efficient. Assuming typical parameters from
observations of Ultra Luminous Infrared Galaxies, we predict that a black hole
binary will merge within yrs; therefore these results imply that in a
merger of gas-rich galaxies, any massive central black holes will coalescence
soon after the galaxies merge. Our work thus supports scenarios of massive
black hole evolution and growth where hierarchical merging plays an important
role. The final coalescence of the black holes leads to gravitational radiation
emission that would be detectable up to high redshift by LISA. We show that
similar physical effects are important for the formation of close binary stars.Comment: 38 pages, 14 figures, submitted to Ap
Accretion of a satellite onto a spherical galaxy. II. Binary evolution and orbital decay
We study the dynamical evolution of a satellite orbiting outside of a
companion spherical galaxy. The satellite is subject to a back-reaction force
resulting from the density fluctuations excited in the primary stellar system.
We evaluate this force using the linear response theory developed in Colpi and
Pallavicini (1997). The force is computed in the reference frame comoving with
the primary galaxy and is expanded in multipoles. To capture the relevant
features of the physical process determining the evolution of the detached
binary, we introduce in the Hamiltonian the harmonic potential as interaction
potential among stars. The dynamics of the satellite is computed
self-consistently. We determine the conditions for tidal capture of a satellite
from an asymptotic free state. If the binary comes to existence as a bound
pair, stability against orbital decay is lost near resonance. The time scale of
binary coalescence is computed as a function of the eccentricity and mass
ratio. In a comparison with Weinberg's perturbative technique we demonstrate
that pinning the center of mass of the galaxy would induce a much larger torque
on the satellite.Comment: 13 pages, Tex,+ 10 .ps figures Submitted to The Astrophysical Journa
Історія польських поселень Володарсько-Волинського району
В даній роботі описано 10 сіл з переважаючим польським населенням, на що вказують архівні матеріали і опитування жителів сіл
Ejection of Supermassive Black Holes from Galaxy Cores
[Abridged] Recent numerical relativity simulations have shown that the
emission of gravitational waves during the merger of two supermassive black
holes (SMBHs) delivers a kick to the final hole, with a magnitude as large as
4000 km/s. We study the motion of SMBHs ejected from galaxy cores by such kicks
and the effects on the stellar distribution using high-accuracy direct N-body
simulations. Following the kick, the motion of the SMBH exhibits three distinct
phases. (1) The SMBH oscillates with decreasing amplitude, losing energy via
dynamical friction each time it passes through the core. Chandrasekhar's theory
accurately reproduces the motion of the SMBH in this regime if 2 < ln Lambda <
3 and if the changing core density is taken into account. (2) When the
amplitude of the motion has fallen to roughly the core radius, the SMBH and
core begin to exhibit oscillations about their common center of mass. These
oscillations decay with a time constant that is at least 10 times longer than
would be predicted by naive application of the dynamical friction formula. (3)
Eventually, the SMBH reaches thermal equilibrium with the stars. We estimate
the time for the SMBH's oscillations to damp to the Brownian level in real
galaxies and infer times as long as 1 Gyr in the brightest galaxies. Ejection
of SMBHs also results in a lowered density of stars near the galaxy center;
mass deficits as large as five times the SMBH mass are produced for kick
velocities near the escape velocity. We compare the N-body density profiles
with luminosity profiles of early-type galaxies in Virgo and show that even the
largest observed cores can be reproduced by the kicks, without the need to
postulate hypermassive binary SMBHs. Implications for displaced AGNs and
helical radio structures are discussed.Comment: 18 pages, The Astrophysical Journal, in press. Replaced with revised
versio
The construction of non-spherical models of quasi-relaxed stellar systems
Spherical models of collisionless but quasi-relaxed stellar systems have long
been studied as a natural framework for the description of globular clusters.
Here we consider the construction of self-consistent models under the same
physical conditions, but including explicitly the ingredients that lead to
departures from spherical symmetry. In particular, we focus on the effects of
the tidal field associated with the hosting galaxy. We then take a stellar
system on a circular orbit inside a galaxy represented as a "frozen" external
field. The equilibrium distribution function is obtained from the one
describing the spherical case by replacing the energy integral with the
relevant Jacobi integral in the presence of the external tidal field. Then the
construction of the model requires the investigation of a singular perturbation
problem for an elliptic partial differential equation with a free boundary, for
which we provide a method of solution to any desired order, with explicit
solutions to two orders. We outline the relevant parameter space, thus opening
the way to a systematic study of the properties of a two-parameter family of
physically justified non-spherical models of quasi-relaxed stellar systems. The
general method developed here can also be used to construct models for which
the non-spherical shape is due to internal rotation. Eventually, the models
will be a useful tool to investigate whether the shapes of globular clusters
are primarily determined by internal rotation, by external tides, or by
pressure anisotropy.Comment: AASTeX v5.2, 37 pages with 2 figures, accepted for publication in The
Astrophysical Journa
Weak Lensing Mass Reconstruction using Wavelets
This paper presents a new method for the reconstruction of weak lensing mass
maps. It uses the multiscale entropy concept, which is based on wavelets, and
the False Discovery Rate which allows us to derive robust detection levels in
wavelet space. We show that this new restoration approach outperforms several
standard techniques currently used for weak shear mass reconstruction. This
method can also be used to separate E and B modes in the shear field, and thus
test for the presence of residual systematic effects. We concentrate on large
blind cosmic shear surveys, and illustrate our results using simulated shear
maps derived from N-Body Lambda-CDM simulations with added noise corresponding
to both ground-based and space-based observations.Comment: Accepted manuscript with all figures can be downloaded at:
http://jstarck.free.fr/aa_wlens05.pdf and software can be downloaded at
http://jstarck.free.fr/mrlens.htm
Dynamical friction and the evolution of satellites in virialized halos: the theory of linear response
The evolution of a small satellite inside a more massive truncated isothermal
spherical halo is studied using both the Theory of Linear Response for
dynamical friction and N-Body simulations. The analytical approach includes the
effects of the gravitational wake, of the tidal deformation and the shift of
the barycenter of the primary, so unifying the local versus global
interpretation of dynamical friction. Sizes, masses, orbital energies and
eccentricities are chosen as expected in hierarchical clustering models. We
find that in general the drag force in self-gravitating backgrounds is weaker
than in uniform media and that the orbital decay is not accompanied by a
significant circularization. We also show that the dynamical friction time
scale is weakly dependent on the initial circularity. We provide a fitting
formula for the decay time that includes the effect of mass and angular
momentum loss. Live satellites with dense cores can survive disruption up to an
Hubble time within the primary, notwithstanding the initial choice of orbital
parameters. Dwarf spheroidal satellites of the Milky Way, like Sagittarius A
and Fornax, have already suffered mass stripping and, with their present
masses, the sinking times exceed 10 Gyr even if they are on very eccentric
orbits.Comment: 27 pages including 9 figures. Accepted for publication in the
Astrophysical Journal. Part 2, issue November 10 1999, Volume 52
Galaxies in N-body simulations: overcoming the overmerging problem
We present analysis of the evolution of dark matter halos in dense
environments of groups and clusters in dissipationless cosmological
simulations. The premature destruction of halos in such environments, known as
the overmerging, reduces the predictive power of N-body simulations and makes
difficult any comparison between models and observations. We analyze the
possible processes that cause the overmerging and assess the extent to which
this problem can be cured with current computer resources and codes. Using both
analytic estimates and high resolution numerical simulations, we argue that the
overmerging is mainly due to the lack of numerical resolution. We find that the
force and mass resolution required for a simulated halo to survive in galaxy
groups and clusters is extremely high and was almost never reached before: ~1-3
kpc and 10^8-10^9 Msun, respectively. We use the high-resolution Adaptive
Refinement Tree (ART) N-body code to run cosmological simulations with the
particle mass of \approx 2x10^8/h Msun} and the spatial resolution of \approx
1-2/h kpc, and show that in these simulations the halos do survive in regions
that would appear overmerged with lower force resolution. Nevertheless, the
halo identification in very dense environments remains a challenge even with
the resolution this high. We present two new halo finding algorithms developed
to identify both isolated and satellite halos that are stable (existed at
previous moments) and gravitationally bound. To illustrate the use of the
satellite halos that survive the overmerging, we present a series of halo
statistics, that can be compared with those of observed galaxies. (Abridged)Comment: Accepted for publication in ApJ, substantional revisions after the
first version, LaTeX 23 pages, 18 figs. (uses emulateapj.sty),
Full-resolution version of Fig.9 is available upon reques
- …