50 research outputs found
The fate of cannibalized fundamental-plane ellipticals
Evolution and disruption of galaxies orbiting in the gravitational field of a
larger cluster galaxy are driven by three coupled mechanisms: 1) the heating
due to its time dependent motion in the primary; 2) mass loss due to the tidal
strain field; and 3) orbital decay. Previous work demonstrated that tidal
heating is effective well inside the impulse approximation limit. Not only does
the overall energy increase over previous predictions, but the work is done
deep inside the secondary galaxy, e.g. at or inside the half mass radius in
most cases. Here, these ideas applied to cannibalization of elliptical galaxies
with fundamental-plane parameters. In summary, satellites which can fall to the
center of a cluster giant by dynamical friction are evaporated by internal
heating by the time they reach the center. This suggests that true
merger-produced multiple nuclei giants should be rare. Specifically,
secondaries with mass ratios as small as 1\% on any initial orbit evaporate and
those on eccentric orbits with mass ratios as small as 0.1\% evolve
significantly and nearly evaporate in a galaxian age. Captured satellites with
mass ratios smaller than roughly 1\% have insufficient time to decay to the
center. After many accretion events, the model predicts that the merged system
has a profile similar to that of the original primary with a weak increase in
concentration.Comment: 19 pages, 10 Postscript figures, uses aaspp4.sty. Submitted to
Astrophysical Journa
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
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 High Resolution IRAS Galaxy Atlas
An atlas of the Galactic plane (-4.7 deg < b < 4.7 deg) plus the molecular
clouds in Orion, Rho Oph, and Taurus-Auriga has been produced at 60 and 100
micron from IRAS data. The Atlas consists of resolution-enhanced coadded images
having 1 arcmin -- 2 arcmin resolution as well as coadded images at the native
IRAS resolution. The IRAS Galaxy Atlas, together with the DRAO HI line / 21 cm
continuum and FCRAO CO (1-0) line Galactic plane surveys, both with similar
(approx. 1 arcmin) resolution, provide a powerful venue for studying the
interstellar medium, star formation and large scale structure in our Galaxy.
This paper documents the production and characteristics of the Atlas.Comment: To appear in Astrophysical Journal Supplement Series. Replaced June
2, 1997. Text unchanged. Missing tables added. Wrong figure sequence
corrected. The Atlas images can now be accessed on line at
http://crystal.ipac.caltech.edu:8001/applications/IGA
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
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
Bayesian Inference in Processing Experimental Data: Principles and Basic Applications
This report introduces general ideas and some basic methods of the Bayesian
probability theory applied to physics measurements. Our aim is to make the
reader familiar, through examples rather than rigorous formalism, with concepts
such as: model comparison (including the automatic Ockham's Razor filter
provided by the Bayesian approach); parametric inference; quantification of the
uncertainty about the value of physical quantities, also taking into account
systematic effects; role of marginalization; posterior characterization;
predictive distributions; hierarchical modelling and hyperparameters; Gaussian
approximation of the posterior and recovery of conventional methods, especially
maximum likelihood and chi-square fits under well defined conditions; conjugate
priors, transformation invariance and maximum entropy motivated priors; Monte
Carlo estimates of expectation, including a short introduction to Markov Chain
Monte Carlo methods.Comment: 40 pages, 2 figures, invited paper for Reports on Progress in Physic
Analysis of the Interaction Effects in the Southern Galaxy Pair Tol1238-364 and ESO381-G009
In the context of the connection among galaxy-galaxy interaction, starbursts
and nuclear activity, we present and discuss a quantitative morphological
analysis based on BVR images and a detailed spectroscopic investigation of two
interacting galaxies, the Seyfert 2 Tol1238-364 (IC 3639) and its companion
ESO381-G009, forming a triple system with ESO381-G006. Broad-band optical
photometry is complemented by Halpha imaging, which provides information about
the distribution of star forming regions across the galaxies. Long-slit
spectroscopic data obtained at different position angles of the slit are
employed to determine the physical conditions of circumnuclear and extranuclear
regions. A mixture of thermal and non-thermal ionizing radiation is found in
the surroundings of the nucleus of Tol1238-364 and the energy budget supports
the presence of a circumnuclear starburst.Several regions in both the galaxies
show anomalous line ratios: additional ionization by shock-heating and low
ionization of some extranuclear HII regions are suggested as possible
explanations. An analysis of the emission-line profiles reveals the presence of
a broad Halpha component in the nuclear region of Tol1238-364. Independent
estimates of the star formation rates (SFR) were obtained through
flux-calibrated Halpha-images and FIR emission in the four IRAS bands. Overall
SFR densities have been compared with the SFR densities derived from Halpha
emission in the individual regions of the galaxies sampled by long-slit
spectra. In both galaxies an enhancement of the star formation activity with
respect to isolated galaxies is revealed. The prevalence of starburst or
nuclear activity has been examined through FIR color indices. The interaction
scenario is discussed on the basis of the observed galaxy properties.Comment: Tentatively scheduled for publication in ApJS, v184 n2 October 2003
issue. A version with full resolution figures is available as unedited
preprint at http://www.journals.uchicago.edu/ApJ/future.htm
Use of model systems to understand the etiology of fragile X-associated primary ovarian insufficiency (FXPOI)
Fragile X-associated primary ovarian insufficiency (FXPOI) is among the family of disorders caused by the expansion of a CGG repeat sequence in the 5' untranslated region of the X-linked gene FMR1. About 20% of women who carry the premutation allele (55 to 200 unmethylated CGG repeats) develop hypergonadotropic hypogonadism and cease menstruating before age 40. Some proportion of those who are still cycling show hormonal profiles indicative of ovarian dysfunction. FXPOI leads to subfertility and an increased risk of medical conditions associated with early estrogen deficiency. Little progress has been made in understanding the etiology of this clinically significant disorder. Understanding the molecular mechanisms of FXPOI requires a detailed knowledge of ovarian FMR1 mRNA and FMRP’s function. In humans, non-invasive methods to discriminate the mechanisms of the premutation on ovarian function are not available, thus necessitating the development of model systems. Vertebrate (mouse and rat) and invertebrate (Drosophila melanogaster) animal studies for the FMR1 premutation and ovarian function exist and have been instrumental in advancing our understanding of the disease phenotype. For example, rodent models have shown that FMRP is highly expressed in oocytes where it is important for folliculogenesis. The two premutation mouse models studied to date show evidence of ovarian dysfunction and, together, suggest that the long repeat in the transcript itself may have some pathological effect quite apart from any effect of the toxic protein. Further, ovarian morphology in young animals appears normal and the primordial follicle pool size does not differ from that of wild-type animals. However, there is a progressive premature decline in the levels of most follicle classes. Observations also include granulosa cell abnormalities and altered gene expression patterns. Further comparisons of these models are now needed to gain insight into the etiology of the ovarian dysfunction. Premutation model systems in non-human primates and those based on induced pluripotent stem cells show particular promise and will complement current models. Here, we review the characterization of the current models and describe the development and potential of the new models. Finally, we will discuss some of the molecular mechanisms that might be responsible for FXPOI