18,651 research outputs found
Major Merging: The Way to Make a Massive, Passive Galaxy
We analyze the projected axial ratio distribution, p(b/a), of galaxies that
were spectroscopically selected from the Sloan Digital Sky Survey (DR6) to have
low star-formation rates. For these quiescent galaxies we find a rather abrupt
change in p(b/a) at a stellar mass of ~10^{11} M_sol: at higher masses there
are hardly any galaxies with b/a<0.6, implying that essentially none of them
have disk-like intrinsic shapes and must be spheroidal. This transition mass is
~3-4 times higher than the threshold mass above which quiescent galaxies
dominate in number over star-forming galaxies, which suggests these mass scales
are unrelated. At masses lower than ~10^{11} M_sol, quiescent galaxies show a
large range in axial ratios, implying a mix of bulge- and disk-dominated
galaxies. Our result strongly suggests that major merging is the most
important, and perhaps only relevant, evolutionary channel to produce massive
(>10^{11} M_sol), quiescent galaxies, as it inevitably results in spheroids.Comment: Minor changes to match published version in ApJ Letter
The merger-driven evolution of massive galaxies
We explore the rate and impact of galaxy mergers on the massive galaxy
population using the amplitude of the two-point correlation function on small
scales for M > 5e10 M_sun galaxies from the COSMOS and COMBO-17 surveys. Using
a pair fraction derived from the Sloan Digital Sky Survey as a low-redshift
benchmark, the large survey area at intermediate redshifts allows us to
determine the evolution of the close pair fraction with unprecedented accuracy
for a mass-selected sample: we find that the fraction of galaxies more massive
than 5e10M_sun in pairs separated by less than 30 kpc in 3D space evolves as
F(z) = (0.0130+/-0.0019)x(1+z)^1.21+/-0.25 between z = 0 and z = 1.2. Assuming
a merger time scale of 0.5 Gyrs, the inferred merger rate is such that galaxies
with mass in excess of 1e11 M_sun have undergone, on average, 0.5 (0.7) mergers
involving progenitor galaxies both more massive than 5e10 M_sun since z = 0.6
(1.2). We also study the number density evolution of massive red sequence
galaxies using published luminosity functions and constraints on the M/L
evolution from the fundamental plane. Moreover, we demonstrate that the
measured merger rate of massive galaxies is sufficient to explain this observed
number density evolution in massive red sequence galaxies since z = 1.Comment: Accepted in Ap
M31 Transverse Velocity and Local Group Mass from Satellite Kinematics
We present several different statistical methods to determine the transverse
velocity vector of M31. The underlying assumptions are that the M31 satellites
on average follow the motion of M31 through space, and that the galaxies in the
outer parts of the Local Group on average follow the motion of the Local Group
barycenter through space. We apply the methods to the line-of-sight velocities
of 17 M31 satellites, to the proper motions of the 2 satellites M33 and IC 10,
and to the line-of-sight velocities of 5 galaxies near the Local Group turn
around radius, respectively. This yields 4 independent but mutually consistent
determinations of the heliocentric M31 transverse velocities in the West and
North directions, with weighted averages = -78+/-41 km/s and =
-38+/-34 km/s. The Galactocentric tangential velocity of M31 is 42 km/s, with
1-sigma confidence interval V_tan <= 56 km/s. The implied M31-Milky Way orbit
is bound if the total Local Group mass M exceeds 1.72^{+0.26}_{-0.25}x10^{12}
solar masses. If indeed bound, then the timing argument combined with the known
age of the Universe implies that M = 5.58^{+0.85}_{-0.72}x10^{12} solar masses.
This is on the high end of the allowed mass range suggested by cosmologically
motivated models for the individual structure and dynamics of M31 and the Milky
Way, respectively. It is therefore possible that the timing mass is an
overestimate of the true mass, especially if one takes into account recent
results from the Millennium Simulation that show that there is also a
theoretical uncertainty of 41 percent (Gaussian dispersion) in timing mass
estimates. The M31 transverse velocity implies that M33 is in a tightly bound
orbit around M31. This may have led to some tidal deformation of M33. It will
be worthwhile to search for observational evidence of this.Comment: ApJ in press, 14 pages, including 3 figures (has minor revisions with
respect to previously posted version to address referee comments
Testing a prediction of the merger origin of early-type galaxies: a correlation between stellar populations and asymmetry
One of the key predictions of the merger hypothesis for the origin of
early-type (elliptical and lenticular) galaxies is that tidally-induced
asymmetric structure should correlate with signatures of a relatively young
stellar population. Such a signature was found by Schweizer and Seitzer (1992;
AJ, 104, 1039) at roughly 4sigma confidence. In this paper, we revisit this
issue with a nearly ten-fold larger sample of 0.01<z<0.03 galaxies selected
from the Two Micron All-Sky Survey and the Sloan Digital Sky Survey. We
parameterize tidal structure using a repeatable algorithmic measure of
asymmetry, and correlate this with color offset from the early-type galaxy
color-magnitude relation. We recover the color offset-asymmetry correlation;
furthermore, we demonstrate observationally for the first time that this effect
is driven by a highly-significant trend towards younger ages at higher
asymmetry values. We present a simple model for the evolution of early-type
galaxies through gas-rich major and minor mergers that reproduces their
observed build-up from z=1 to the present day and the distribution of
present-day colors and ages. We show using this model that if both stellar
populations and asymmetry were ideal `clocks' measuring the time since last
major or minor gas-rich interaction, then we would expect a rather tight
correlation between age and asymmetry. We suggest that the source of extra
scatter is natural diversity in progenitor star formation history, gas content,
and merger mass ratio, but quantitative confirmation of this conjecture will
require sophisticated modeling. We conclude that the asymmetry-age correlation
is in basic accord with the merger hypothesis, and indicates that an important
fraction of the early-type galaxy population is affected by major or minor
mergers at cosmologically-recent times.Comment: Astrophysical Journal, in press. 20 pages, 18 figure
Statistical Mechanics of Quantum-Classical Systems with Holonomic Constraints
The statistical mechanics of quantum-classical systems with holonomic
constraints is formulated rigorously by unifying the classical Dirac bracket
and the quantum-classical bracket in matrix form.
The resulting Dirac quantum-classical theory, which conserves the holonomic
constraints exactly, is then used to formulate time evolution and statistical
mechanics. The correct momentum-jump approximation for constrained system
arises naturally from this formalism. Finally, in analogy with what was found
in the classical case, it is shown that the rigorous linear response function
of constrained quantum-classical systems contains non-trivial additional terms
which are absent in the response of unconstrained systems.Comment: Submitted to Journal of Chemical Physic
On the Size and Comoving Mass Density Evolution of Early-Type Galaxies
We present a simple, empirically motivated model that simultaneously predicts
the evolution of the mean size and the comoving mass density of massive
early-type galaxies from z=2 to the present. First we demonstrate that some
size evolution of the population can be expected simply due to the continuous
emergence of early-type galaxies. SDSS data reveal that in the present-day
universe more compact early-type galaxies with a given dynamical mass have
older stellar populations. In contrast, at a given stellar velocity dispersion,
SDSS data show that there is no relation between size and age, which implies
that the velocity dispersion can be used to estimate the epoch at which
galaxies stopped forming stars, turning into early-type galaxies. Applying such
a 'formation' criterion to a large sample of nearby early-type galaxies, we
predict the redshift evolution in the size distribution and the comoving mass
density. The resulting evolution in the mean size is roughly half of the
observed evolution. Then we include a prescription for the merger histories of
galaxies between the 'formation' redshift and the present, based on
cosmological simulations of the assembly of dark matter halos. Such mergers
after the transformation into an early-type galaxy are presumably
dissipationless ('dry'), where the increase in size is expected to be
approximately proportional to the increase in mass. This model successfully
reproduces the observed evolution since z~2 in the mean size and in the
comoving mass density of massive early-type galaxies. We conclude that the
recently measured, substantial size evolution of early-type galaxies can be
explained by the combined effect of the continuous emergence of galaxies as
early types and their subsequent growth through dry merging.Comment: Accepted for publication in ApJ (13 pages, 5 figures), small changes
to match journal versio
An Absence of Radio-Loud Active Galactic Nuclei in Geometrically Flat Quiescent Galaxies: Implications for Maintenance-Mode Feedback Models
Maintenance-mode feedback from low-accretion rate AGN, manifesting itself
observationally through radio-loudness, is invoked in all cosmological galaxy
formation models as a mechanism that prevents excessive star-formation in
massive galaxies (M 310 M). We
demonstrate that at a fixed mass the incidence of radio-loud AGN (L
10 WHz) identified in the FIRST and NVSS radio surveys among a
large sample of quiescent (non-star forming) galaxies selected from the SDSS is
much higher in geometrically round galaxies than in geometrically flat,
disk-like galaxies. As found previously, the RL AGN fraction increases steeply
with stellar velocity dispersion and stellar mass, but even at a
fixed velocity dispersion of 200-250 kms this fraction increases from
0.3% for flat galaxies (projected axis ratio of q 0.4) to 5% for round
galaxies (q 0.8). We rule out that this strong trend is due to projection
effects in the measured velocity dispersion. The large fraction of radio-loud
AGN in massive, round galaxies is consistent with the hypothesis that such AGN
deposit energy into their hot gaseous halos, preventing cooling and
star-formation. However, the absence of such AGN in disk-like quiescent
galaxies -- most of which are not satellites in massive clusters, raises
important questions: is maintenance-mode feedback a generally valid explanation
for quiescence; and, if so, how does that feedback avoid manifesting at least
occasionally as a radio-loud galaxy?Comment: 7 pages, 5 figures, accepted for publication in ApJ Letter
The Rise of Massive Red Galaxies: the color-magnitude and color-stellar mass diagrams for z < ~2 from the MUltiwavelength Survey by Yale-Chile (MUSYC)
We present the color-magnitude and color-stellar mass diagrams for galaxies
with z_phot < ~2, based on a K < 22 (AB) catalog of the Extended Chandra Deep
Field South (ECDFS) from the MUltiwavelength Survey by Yale-Chile (MUSYC). Our
main sample of 7840 galaxies contains 1297 M_* > 10^11 M_Sol galaxies in the
range 0.2 < z_phot < 1.8. We show empirically that this catalog is
approximately complete for M_* > 10^11 M_Sol galaxies for z_phot < 1.8. For
this mass-limited sample, we show that the locus of the red sequence
color-stellar mass relation evolves as Del(u-r) ~ (-0.44+/-0.02) z_phot for
z_phot ~1.3, however, we are no longer able to reliably
distinguish red and blue subpopulations from the observed color distribution;
we show that this would require much deeper near infrared data. At 1.5 < z_phot
10^11 M_Sol galaxies is ~50% of the
local value, with a red fraction of ~33%. Making a parametric fit to the
observed evolution, we find n_tot(z) ~ (1+z_phot)^(-0.52+/-0.12(+/-0.20)). We
find stronger evolution in the red fraction: f_red(z) ~
(1+z_phot)^(-1.17+/-0.18(+/-0.21)). Through a series of sensitivity analyses,
we show that the most important sources of systematic error are: 1. systematic
differences in the analysis of the z~0 and z>>0 samples; 2. systematic effects
associated with details of the photometric redshift calculation; and 3.
uncertainties in the photometric calibration. With this in mind, we show that
our results based on photometric redshifts are consistent with a completely
independent analysis which does not require redshift information for individual
galaxies. Our results suggest that, at most, 1/5 of local red sequence galaxies
with M_* >10^11 M_Sol were already in place at z ~ 2.Comment: Accepted for publication in ApJ. 31 pages in emulateapj format; 18
figues (14 in main text). Additional online data available through
http://www.strw.leidenuniv.nl/~ent
Alternative model of the Antonov problem
Astrophysical systems will never be in a real Thermodynamic equilibrium: they
undergo an evaporation process due to the fact that the gravity is not able to
confine the particles. Ordinarily, this difficulty is overcome by enclosing the
system in a rigid container which avoids the evaporation. We proposed an
energetic prescription which is able to confine the particles, leading in this
way to an alternative version of the Antonov isothermal model which unifies the
well-known isothermal and polytropic profiles. Besides of the main features of
the isothermal sphere model: the existence of the gravitational collapse and
the energetic region with a negative specific heat, this alternative model has
the advantage that the system size naturally appears as a consequence of the
particles evaporation.Comment: RevTex4, 9 pages, 10 figures, Version Submitted to PR
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