2,431 research outputs found
Ionized Gas in Damped Lyman Alpha Protogalaxies: II. Comparison Between Models and the Kinematic Data
We test semi-analytic models for galaxy formation with accurate kinematic
data of damped Lyman alpha protogalaxies (DLAs) presented in the companion
paper I. The models envisage centrifugally supported exponential disks at the
centers of dark matter halos which are filled with ionized gas undergoing
radial infall to the disks. The halo masses are drawn from cross-section
weighted mass distributions predicted by CDM cosmogonies, or by the null
hypothesis (TF model) that the dark matter mass distribution has not evolved
since z ~ 3. In our models, C IV absorption lines detected in DLAs arise in
infalling ionized clouds while the low-ion absorption lines arise from neutral
gas in the disks. Using Monte Carlo methods we find: (a) The CDM models are
incompatible with the low-ion statistics at more than 99% confidence whereas
some TF models cannot be excluded at more than 88% confidence. (b) Both CDM and
TF models agree with the observed distribution of C IV velocity widths. (c) The
CDM models generate differences between the mean velocities of C IV and low ion
profiles in agreement with the data, while the TF model produces differences in
the means that are too large. (d) Both CDM and TF models produce ratios of C IV
to low-ion velocity widths that are too large. (e) Both CDM and TF models
generate C IV versus low-ion cross-correlation functions incompatible with the
data.
While it is possible to select model parameters resulting in consistency with
the data, the disk-halo configuration assumed in both cosmogonies still does
not produce significant overlap in velocity space between C IV low-ion velocity
profiles. We conjecture that including angular momentum of the infalling clouds
will increase the overlap between C IV and low-ion profiles.Comment: 18 pages, 12 Figures, Accepted for publication in the Dec. 20 issue
of the Astrophysical Journa
Infalling Faint [OII] Emitters in Abell 851. I. Spectroscopic Confirmation of Narrowband-Selected Objects
We report on a spectroscopic confirmation of narrowband-selected [OII]
emitters in Abell 851 catalogued by Martin et al. (2000). The optical spectra
obtained from the Keck I Low Resolution Imaging Spectrometer (LRIS) and Keck II
Deep Imaging Multi-Object Spectrograph (DEIMOS) have confirmed [OII]3727
emission in narrowband-selected cluster [OII] candidates at a 85% success rate
for faint (i <~ 25) blue (g-i < 1) galaxies. The rate for the successful
detection of [OII] emission is a strong function of galaxy color, generally
proving the efficacy of narrowband [OII] search supplemented with broadband
colors in selecting faint cluster galaxies with recent star formation. Balmer
decrement-derived reddening measurements show a high degree of reddening
[E(B-V) >~ 0.5] in a significant fraction of this population. Even after
correcting for dust extinction, the [OII]/Ha line flux ratio for the
high-E(B-V) galaxies remains generally lower by a factor of ~2 than the mean
[OII]/Ha ratios reported by the studies of nearby galaxies. The strength of
[OII] equivalent width shows a negative trend with galaxy luminosity while the
Ha equivalent width does not appear to depend as strongly on luminosity. This
in part is due to the high amount of reddening observed in luminous galaxies.
Furthermore, emission line ratio diagnostics show that AGN-like galaxies are
abundant in the high luminosity end of the cluster [OII]-emitting sample, with
only moderately strong [OII] equivalent widths, consistent with a scenario of
galaxy evolution connecting AGNs and suppression of star-forming activity in
massive galaxies.Comment: 11 pages (LaTeX emulateapj), 8 figures, to appear in ApJ. A version
with high resolution figures available from the lead autho
Spectral and morphological properties of quasar hosts in SPH simulations of AGN feeding by mergers
We present a method for generating virtual observations from
smoothed-particle-hydrodynamics (SPH) simulations. This method includes stellar
population synthesis models and the reprocessing of starlight by dust to
produce realistic galaxy images. We apply this method and simulate the merging
of two identical giant Sa galaxies. The merger remnant is an elliptical galaxy.
The merger concentrates the gas content of the two galaxies into the nuclear
region. The gas that flows into the nuclear region refuels the central black
holes of the merging galaxies. We follow the refuelling of the black holes
during the merger semi-analytically.
In the simulation presented in this article, the black holes grow from 3 x
10^7 to 1.8X 10^8 Solar masses, with a peak AGN luminosity of M_B ~ -23.7. We
study how the morphological and spectral properties of the system evolve during
the merger and work out the predictions of this scenario for the properties of
host galaxies during the active phase. The peak of AGN activity coincides with
the merging of the two galactic nuclei and occurs at a stage when the remnant
looks like a lenticular galaxy. The simulation predicts the formation of a
circumnuclear starburst ring/dusty torus with an opening angle of 30-40 degrees
and made of clouds with n_H=10^24 cm^-2. The average optical depth of the torus
is quite high, but the obscuring medium is patchy, so that there still exist
lines of sight where the AGN is visible in a nearly edge-on view. For the same
reason, there are lines of sight where the AGN is completely obscured in the
face-on view.Comment: 14 pages, 11 figure
NIR Luminosity Function of Galaxies in Close Major-Merger Pairs and Mass Dependence of Merger Rate
A sample of close major-merger pairs (projected separation kpc, band magnitude difference mag) is selected from the matched 2MASS-2dFGRS catalog of Cole et al.
(2001). The pair primaries are brighter than mag. After
corrections for various biases, the comparison between counts in the paired
galaxy sample and counts in the parent sample shows that for the local `M*
galaxies' sampled by flux limited surveys, the fraction of galaxies in the
close major-merger pairs is 1.70. Using 38 paired galaxies in the
sample, a band luminosity function (LF) is calculated. This is the
first unbiased LF for a sample of objectively defined interacting/merging
galaxies in the local universe, while all previously determined LFs of paired
galaxies are biased by mistreating paired galaxies as singles. A stellar mass
function (MF) is translated from the LF. Compared to the LF/MF of 2MASS
galaxies, a differential pair fraction function is derived. The results suggest
a trend in the sense that less massive galaxies may have lower chance to be
involved in close major-merger pairs than more massive galaxies. The algorithm
presented in this paper can be easily applied to much larger samples of 2MASS
galaxies with redshifts in near future.Comment: Accepted by ApJL, 16 pages, 2 figure
The Spatial and Kinematic Distributions of Cluster Galaxies in a LCDM Universe -- Comparison with Observations
We combine dissipationless N-body simulations and semi-analytic models of
galaxy formation to study the spatial and kinematic distributions of cluster
galaxies in a LCDM cosmology. We investigate how the star formation rates,
colours and morphologies of galaxies vary as a function of distance from the
cluster centre and compare our results with the CNOC1 survey of galaxies from
15 X-ray luminous clusters in the redshift range 0.18 to 0.55. In our model,
gas no longer cools onto galaxies after they fall into the cluster and their
star formation rates decline on timescales of 1-2 Gyr. Galaxies in cluster
cores have lower star formation rates and redder colours than galaxies in the
outer regions because they were accreted earlier. Our colour and star formation
gradients agree with those those derived from the data. The difference in
velocity dispersions between red and blue galaxies observed in the CNOC1
clusters is also well reproduced by the model. We assume that the morphologies
of cluster galaxies are determined solely by their merging histories.
Morphology gradients in clusters arise naturally, with the fraction of bulge-
dominated galaxies highest in cluster cores. We compare these gradients with
the CNOC1 data and find excellent agreement for bulge-dominated galaxies. The
simulated clusters contain too few galaxies of intermediate bulge-to-disk
ratio, suggesting that additional processes may influence the morphological
evolution of disk-dominated galaxies in clusters. Although the properties of
the cluster galaxies in our model agree extremely well with the data, the same
is not true of field galaxies. Both the star formation rates and the colours of
bright field galaxies appear to evolve much more strongly from redshift 0.2 to
0.4 in the CNOC1 field sample than in our simulations.Comment: 17 pages, sumitted to MNRAS. Simulation outputs, halo catalogs,
merger trees and galaxy catalogs are now available at
http://www.mpa-garching.mpg.de/GIF
Cosmic Voids and Galaxy Bias in the Halo Occupation Framework
(Abridged) We investigate the power of void statistics to constrain galaxy
bias and the amplitude of dark matter fluctuations. We use the halo occupation
distribution (HOD) framework to describe the relation between galaxies and dark
matter. After choosing HOD parameters that reproduce the mean space density
n_gal and projected correlation function w_p measured for galaxy samples with
M_r<-19 and M_r<-21 from the Sloan Digital Sky Survey (SDSS), we predict the
void probability function (VPF) and underdensity probability function (UPF) of
these samples by populating the halos of a large, high-resolution N-body
simulation. If we make the conventional assumption that the HOD is independent
of large scale environment at fixed halo mass, then models constrained to match
n_gal and w_p predict nearly identical void statistics, independent of the
scatter between halo mass and central galaxy luminosity or uncertainties in HOD
parameters. Models with sigma_8=0.7 and sigma_8=0.9 also predict very similar
void statistics. However, the VPF and UPF are sensitive to environmental
variations of the HOD in a regime where these variations have little impact on
w_p. For example, doubling the minimum host halo mass in regions with large
scale (5 Mpc/h) density contrast delta<-0.65 has a readily detectable impact on
void probabilities of M_r<-19 galaxies, and a similar change for delta<-0.2
alters the void probabilities of M_r<-21 galaxies at a detectable level. The
VPF and UPF provide complementary information about the onset and magnitude of
density- dependence in the HOD. By detecting or ruling out HOD changes in low
density regions, void statistics can reduce systematic uncertainties in the
cosmological constraints derived from HOD modeling, and, more importantly,
reveal connections between halo formation history and galaxy properties.Comment: emulateapj, 16 pages, 13 figure
Twinning phenomena along and beyond the bain path
Twinning is a phenomenon that occurs, e.g., during deformation, martensitic transformation and film growth. The present study shows that the crystallography of twinning can be described by two twinning modes along the complete Bain transformation path and beyond connecting body-centered and face-centered cubic structures. To probe this concept, we used strained epitaxial films of the Fe-Pd magnetic shape memory system. As the substrate acts as an absolute reference frame, we could show by pole figure measurements that all observed twinning can be a body-centered and face-centered cubic twinning mode. This continuously transforms towards identity when approaching the complementary structure
The Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem
We present Keck/DEIMOS spectroscopy of stars in 8 of the newly discovered
ultra-faint dwarf galaxies around the Milky Way. We measure the velocity
dispersions of Canes Venatici I and II, Ursa Major I and II, Coma Berenices,
Hercules, Leo IV and Leo T from the velocities of 18 - 214 stars in each galaxy
and find dispersions ranging from 3.3 to 7.6 km/s. The 6 galaxies with absolute
magnitudes M_V < -4 are highly dark matter-dominated, with mass-to-light ratios
approaching 1000. The measured velocity dispersions are inversely correlated
with their luminosities, indicating that a minimum mass for luminous galactic
systems may not yet have been reached. We also measure the metallicities of the
observed stars and find that the 6 brightest of the ultra-faint dwarfs extend
the luminosity-metallicity relationship followed by brighter dwarfs by 2 orders
of magnitude in luminosity; several of these objects have mean metallicities as
low as [Fe/H] = -2.3 and therefore represent some of the most metal-poor known
stellar systems. We detect metallicity spreads of up to 0.5 dex in several
objects, suggesting multiple star formation epochs. Having established the
masses of the ultra-faint dwarfs, we re-examine the missing satellite problem.
After correcting for the sky coverage of the SDSS, we find that the ultra-faint
dwarfs substantially alleviate the discrepancy between the predicted and
observed numbers of satellites around the Milky Way, but there are still a
factor of ~4 too few dwarf galaxies over a significant range of masses. We show
that if galaxy formation in low-mass dark matter halos is strongly suppressed
after reionization, the simulated circular velocity function of CDM subhalos
can be brought into approximate agreement with the observed circular velocity
function of Milky Way satellite galaxies. [slightly abridged]Comment: 22 pages, 15 figures (12 in color), 6 tables, minor revisions in
response to referee report. Accepted for publication in Ap
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