1,199 research outputs found
A possible observational bias in the estimation of the virial parameter in virialized clumps
The dynamics of massive clumps, the environment where massive stars
originate, is still unclear. Many theories predict that these regions are in a
state of near-virial equilibrium, or near energy equi-partition, while others
predict that clumps are in a sub-virial state. Observationally, the majority of
the massive clumps are in a sub-virial state with a clear anti-correlation
between the virial parameter and the mass of the clumps ,
which suggests that the more massive objects are also the more gravitationally
bound. Although this trend is observed at all scales, from massive clouds down
to star-forming cores, theories do not predict it. In this work we show how,
starting from virialized clumps, an observational bias is introduced in the
specific case where the kinetic and the gravitational energies are estimated in
different volumes within clumps and how it can contribute to the spurious
anti-correlation in these data. As a result, the observed
effective virial parameter , and in some
circumstances it might not be representative of the virial state of the
observed clumps.Comment: A&A letter, accepte
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
Non-linear Stochastic Galaxy Biasing in Cosmological Simulations
We study the biasing relation between dark-matter halos or galaxies and the
underlying mass distribution, using cosmological -body simulations in which
galaxies are modelled via semi-analytic recipes. The nonlinear, stochastic
biasing is quantified in terms of the mean biasing function and the scatter
about it as a function of time, scale and object properties. The biasing of
galaxies and halos shows a general similarity and a characteristic shape, with
no galaxies in deep voids and a steep slope in moderately underdense regions.
At \sim 8\hmpc, the nonlinearity is typically \lsim 10 percent and the
stochasticity is a few tens of percent, corresponding to percent
variations in the cosmological parameter . Biasing
depends weakly on halo mass, galaxy luminosity, and scale. The time evolution
is rapid, with the mean biasing larger by a factor of a few at
compared to , and with a minimum for the nonlinearity and stochasticity at
an intermediate redshift. Biasing today is a weak function of the cosmological
model, reflecting the weak dependence on the power-spectrum shape, but the time
evolution is more cosmology-dependent, relecting the effect of the growth rate.
We provide predictions for the relative biasing of galaxies of different type
and color, to be compared with upcoming large redshift surveys. Analytic models
in which the number of objects is conserved underestimate the evolution of
biasing, while models that explicitly account for merging provide a good
description of the biasing of halos and its evolution, suggesting that merging
is a crucial element in the evolution of biasing.Comment: 27 pages, 21 figures, submitted to MNRA
The dependence of the pairwise velocity dispersion on galaxy properties
(abridged) We present measurements of the pairwise velocity dispersion (PVD)
for different classes of galaxies in the Sloan Digital Sky Survey. For a sample
of about 200,000 galaxies, we study the dependence of the PVD on galaxy
properties such as luminosity, stellar mass (M_*), colour (g-r), 4000A break
strength (D4000), concentration index (C), and stellar surface mass density
(\mu_*). The luminosity dependence of the PVD is in good agreement with the
results of Jing & B\"orner (2004) for the 2dFGRS catalog. The value of
\sigma_{12} measured at k=1 h/Mpc decreases as a function of increasing galaxy
luminosity for galaxies fainter than L*, before increasing again for the most
luminous galaxies in our sample. Each of the galaxy subsamples selected
according to luminosity or stellar mass is divided into two further subsamples
according to colour, D4000, C and \mu_*. We find that galaxies with redder
colours and higher D4000, C, and \mu_* values have larger PVDs on all scales
and at all luminosities/stellar masses. The dependence of the PVD on parameters
related to recent star formation(colour, D4000) is stronger than on parameters
related to galaxy structure (C, \mu_*), especially on small scales and for
faint galaxies. The reddest galaxies and galaxies with high surface mass
densities and intermediate concentrations have the highest pairwise peculiar
velocities, i.e. these move in the strongest gravitational fields. We conclude
that the faint red population located in rich clusters is responsible for the
high PVD values that are measured for low-luminosity galaxies on small scales.Comment: 14 pages, 13 figures; reference updated and text slightly changed to
match the published version; data of measurements of power spectrum and PVD
available at http://www.mpa-garching.mpg.de/~leech/papers/clustering
The Expected Mass Function for Low Mass Galaxies in a CDM Cosmology: Is There a Problem?
It is well known that the mass function for_halos_ in CDM cosmology is a
relatively steep power law for low masses, possibly too steep to be consistent
with observations. But how steep is the_galaxy_ mass function? We have analyzed
the stellar and gas mass functions of the first massive luminous objects formed
in a \Lambda CDM universe, as calculated in the numerical simulation described
in Gnedin (2000ab). We found that while the dark matter mass function is steep,
the stellar and gas mass functions are flatter for low mass objects. The
stellar mass function is consistently flat at the low mass end. Moreover, while
the gas mass function follows the dark matter mass function until reionization
at z~7, between z=7 and z=4, the gas mass function also flattens considerably
at the low mass end. At z=4, the gas and stellar mass functions are fit by a
Schechter function with \alpha ~ -1.2 +/- 0.1, significantly shallower than the
dark matter halo mass function and consistent with some recent observations.
The baryonic mass functions are shallower because (a) the dark matter halo mass
function is consistent with the Press-Schechter formulation at low masses n(M)
M^-2 and (b) heating/cooling and ionization processes appear to cause baryons
to collect in halos with the relationship M_b M_d^4 at low masses. Combining
(a) and (b) gives n(M_b) M_b^-5/4, comparable to the simulation results. Thus,
the well known observational fact that low mass galaxies are underabundant as
compared to expectations from numerical dark matter simulations or
Press-Schechter modeling of CDM universes emerges naturally from these results,
implying that perhaps no ``new physics'' beyond the standard model is needed.Comment: Submitted to ApJ, 17 pages including 6 figure
Time Evolution of Galaxy Formation and Bias in Cosmological Simulations
The clustering of galaxies relative to the mass distribution declines with
time because: first, nonlinear peaks become less rare events; second, the
densest regions stop forming new galaxies because gas there becomes too hot to
cool and collapse; third, after galaxies form, they are gravitationally
``debiased'' because their velocity field is the same as the dark matter. To
show these effects, we perform a hydrodynamic cosmological simulation and
examine the density field of recently formed galaxies as a function of
redshift. We find the bias b_* of recently formed galaxies (the ratio of the
rms fluctuations of these galaxies and mass), evolves from 4.5 at z=3 to around
1 at z=0, on 8 h^{-1} Mpc comoving scales. The correlation coefficient r_*
between recently formed galaxies and mass evolves from 0.9 at z=3 to 0.25 at
z=0. As gas in the universe heats up and prevents star formation, star-forming
galaxies become poorer tracers of the mass density field. After galaxies form,
the linear continuity equation is a good approximation to the gravitational
debiasing, even on nonlinear scales. The most interesting observational
consequence of the simulations is that the linear regression of the
star-formation density field on the galaxy density field evolves from about 0.9
at z=1 to 0.35 at z=0. These effects also provide a possible explanation for
the Butcher-Oemler effect, the excess of blue galaxies in clusters at redshift
z ~ 0.5. Finally, we examine cluster mass-to-light ratio estimates of Omega,
finding that while Omega(z) increases with z, one's estimate Omega_est(z)
decreases. (Abridged)Comment: 31 pages of text and figures; submitted to Ap
Formation time distribution of dark matter haloes: theories versus N-body simulations
This paper uses numerical simulations to test the formation time distribution
of dark matter haloes predicted by the analytic excursion set approaches. The
formation time distribution is closely linked to the conditional mass function
and this test is therefore an indirect probe of this distribution. The
excursion set models tested are the extended Press-Schechter (EPS) model, the
ellipsoidal collapse (EC) model, and the non-spherical collapse boundary (NCB)
model. Three sets of simulations (6 realizations) have been used to investigate
the halo formation time distribution for halo masses ranging from dwarf-galaxy
like haloes (, where is the characteristic non-linear mass
scale) to massive haloes of . None of the models can match the
simulation results at both high and low redshift. In particular, dark matter
haloes formed generally earlier in our simulations than predicted by the EPS
model. This discrepancy might help explain why semi-analytic models of galaxy
formation, based on EPS merger trees, under-predict the number of high redshift
galaxies compared with recent observations.Comment: 7 pages, 5 figures, accepted for publication in MNRA
Expected Number and Flux Distribution of Gamma-Ray-Burst Afterglows with High Redshifts
If Gamma-Ray-Bursts (GRBs) occur at high redshifts, then their bright
afterglow emission can be used to probe the ionization and metal enrichment
histories of the intervening intergalactic medium during the epoch of
reionization. In contrast to other sources, such as galaxies or quasars, which
fade rapidly with increasing redshift, the observed infrared flux from a GRB
afterglow at a fixed observed age is only a weak function of its redshift. This
results from a combination of the spectral slope of GRB afterglows and the
time-stretching of their evolution in the observer's frame. Assuming that the
GRB rate is proportional to the star formation rate and that the characteristic
energy output of GRBs is ~10^{52} ergs, we predict that there are always ~15
GRBs from redshifts z>5 across the sky which are brighter than ~100 nJy at an
observed wavelength of ~2 \mu m. The infrared spectrum of these sources could
be taken with the future Next Generation Space Telescope, as a follow-up on
their early X-ray localization with the Swift satellite.Comment: 29 pages, 14 figures; submitted to Ap
The Physical Connections Among IR QSOs, PG QSOs and Narrow-Line Seyfert 1 Galaxies
We study the properties of infrared-selected QSOs (IR QSOs),
optically-selected QSOs (PG QSOs) and Narrow Line Seyfert 1 galaxies (NLS1s).
We compare their properties from the infrared to the optical and examine
various correlations among the black hole mass, accretion rate, star formation
rate and optical and infrared luminosities. We find that the infrared excess in
IR QSOs is mostly in the far infrared, and their infrared spectral indices
suggest that the excess emission is from low temperature dust heated by
starbursts rather than AGNs. The infrared excess is therefore a useful
criterion to separate the relative contributions of starbursts and AGNs. We
further find a tight correlation between the star formation rate and the
accretion rate of central AGNs for IR QSOs. The ratio of the star formation
rate and the accretion rate is about several hundred for IR QSOs, but decreases
with the central black hole mass. This shows that the tight correlation between
the stellar mass and the central black hole mass is preserved in massive
starbursts during violent mergers. We suggest that the higher Eddington ratios
of NLS1s and IR QSOs imply that they are in the early stage of evolution toward
classical Seyfert 1's and QSOs, respectively.Comment: 32 pages, 6 figures, accepted by Ap
Nonlinear Stochastic Biasing of Galaxies and Dark Halos in Cosmological Hydrodynamic Simulations
We perform an extensive analysis of nonlinear and stochastic biasing of
galaxies and dark halos in spatially flat low-density CDM universe using
cosmological hydrodynamic simulations. We compare their biasing properties with
the predictions of an analytic halo biasing model. Dark halos in our
simulations exhibit reasonable agreement with the predictions only on scales
larger than 10h^{-1}Mpc, and on smaller scales the volume exclusion effect of
halos due to their finite size becomes substantial. Interestingly the biasing
properties of galaxies are better described by extrapolating the halo biasing
model predictions.
We also find the clear dependence of galaxy biasing on their formation epoch;
the distribution of old populations of galaxies tightly correlates with the
underlying mass density field, while that of young populations is slightly more
stochastic and anti-biased relative to dark matter. The amplitude of two-point
correlation function of old populations is about 3 times larger than that of
the young populations. Furthermore, the old population of galaxies reside
within massive dark halos while the young galaxies are preferentially formed in
smaller dark halos. Assuming that the observed early and late-type galaxies
correspond to the simulated old and young populations of galaxies,
respectively, all of these segregations of galaxies are consistent with
observational ones for the early and late-type of galaxies such as the
morphology--density relation of galaxies.Comment: 28 pages, 14 figures, accepted for publication in ApJ, Abstract
abridged. For preprint with higher-resolution PS files, see
ftp://www.kusastro.kyoto-u.ac.jp/pub/kohji/ytjs2001
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