31 research outputs found
Clumping factors of HII, HeII and HeIII
Estimating the intergalactic medium ionization level of a region needs proper
treatment of the reionization process for a large representative volume of the
universe. The clumping factor, a parameter which accounts for the effect of
recombinations in unresolved, small-scale structures, aids in achieving the
required accuracy for the reionization history even in simulations with low
spatial resolution.
In this paper, we study for the first time the redshift evolution of clumping
factors of different ionized species of H and He in a small but very high
resolution simulation of the reionization process. We investigate the
dependence of the value and redshift evolution of clumping factors on their
definition, the ionization level of the gas, the grid resolution, box size and
mean dimensionless density of the simulations.Comment: 12 pages, 10 figures, 1 table. Accepted by MNRA
Empirical Constraints on the Star Formation & Redshift Dependence of the Lyman Alpha `Effective' Escape Fraction
We derive empirical constraints on the volume averaged `effective' escape
fraction of Lyman Alpha (Lya) photons from star forming galaxies as a function
of redshift, by comparing star formation functions inferred directly from
observations, to observed Lya luminosity functions. Our analysis shows that the
effective escape fraction increases from f_esc^eff ~ 1-5 % at z=0, to f_esc^eff
~ 10 % at z=3-4, and to f_esc^eff=30-50 % at z=6. Our constraint at z=6 lies
above predictions by models that do not include winds, and therefore hints at
the importance of winds in the Lya transfer process (even) at this redshift. We
can reproduce Lya luminosity functions with an f_esc^eff that does not depend
on the galaxies star formation rates (SFR) over up to ~2 orders of magnitude in
Lya luminosity. It is possible to reproduce the luminosity functions with an
f_esc^eff that decreases with SFR - which appears favored by observations of
drop-out galaxies - in models which include a large scatter (~ 1.0 dex) in
f_esc^eff, and/or in which star forming galaxies only have a non-zero f_esc^eff
for a fraction of their life-time or a fraction of sightlines. We provide a
fitting formula that summarizes our findings.Comment: 10 pages, 3 figures, Accepted to MNRAS. Changed functional form of
fitting formula. Some minor textual changes & added reference
Dark Matter Halo Environment for Primordial Star Formation
We study the statistical properties (such as shape and spin) of high-z halos
likely hosting the first (PopIII) stars with cosmological simulations including
detailed gas physics. In the redshift range considered () the
average sphericity is , and for more than 90% of halos the
triaxiality parameter is , showing a clear preference for
oblateness over prolateness. Larger halos in the simulation tend to be both
more spherical and prolate: we find and , with and at z = 11.
The spin distributions of dark matter and gas are considerably different at
, with the baryons rotating slower than the dark matter. At lower
redshift, instead, the spin distributions of dark matter and gas track each
other almost perfectly, as a consequence of a longer time interval available
for momentum redistribution between the two components. The spin of both the
gas and dark matter follows a lognormal distribution, with a mean value at z=16
of , virtually independent of halo mass. This is in good
agreement with previous studies. Using the results of two feedback models (MT1
and MT2) by McKee & Tan (2008) and mapping our halo spin distribution into a
PopIII IMF, we find that at high- the IMF closely tracks the spin lognormal
distribution. Depending on the feedback model, though, the distribution can be
centered at (MT1) or (MT2). At later
times, model MT1 evolves into a bimodal distribution with a second prominent
peak located at as a result of the non-linear relation between
rotation and halo mass. We conclude that the dark matter halo properties might
be a key factor shaping the IMF of the first stars.Comment: 10 pages, 6 figures, accepted for publication in MNRA
Disentangling galaxy environment and host halo mass
[Abridged] The properties of observed galaxies and dark matter haloes in
simulations depend on their environment. The term environment has been used to
describe a wide variety of measures that may or may not correlate with each
other. Popular measures of environment include the distance to the N'th nearest
neighbour, the number density of objects within some distance, or the mass of
the host dark matter halo. We use results from the Millennium simulation and a
semi-analytic model for galaxy formation to quantify the relations between
environment and halo mass. We show that the environmental parameters used in
the observational literature are in effect measures of halo mass, even if they
are measured for a fixed stellar mass. The strongest correlation between
environment and halo mass arises when the number of objects is counted out to a
distance of 1.5-2 times the virial radius of the host halo and when the
galaxies/haloes are required to be relatively bright/massive. For observational
studies the virial radius is not easily determined, but the number of
neighbours out to 1-2 Mpc/h gives a similarly strong correlation. For the
distance to the N'th nearest neighbour the correlation with halo mass is nearly
as strong provided N>2. We demonstrate that this environmental parameter
becomes insensitive to halo mass if it is constructed from dimensionless
quantities. This can be achieved by scaling the minimum luminosity/mass of
neighbours to that of the object in question and by dividing the distance to a
length scale associated with either the neighbour or the galaxy under
consideration. We show how such a halo mass independent environmental parameter
can be defined for observational and numerical studies. The results presented
here will help future studies to disentangle the effects of halo mass and
external environment on the properties of galaxies and dark matter haloes.Comment: 15 pages, 9 figures, 2 tables. Accepted by MNRA
Influence of baryons on the orbital structure of dark matter haloes
We explore the dynamical signatures imprinted by baryons on dark matter
haloes during the formation process using the OverWhelmingly Large Simulations
(OWLS), a set of state-of-the-art high-resolution cosmological hydrodynamical
simulations. We present a detailed study of the effects of the implemented
feedback prescriptions on the orbits of dark matter particles, stellar
particles and subhaloes, analysing runs with no feedback, with stellar feedback
and with feedback from supermassive black holes. We focus on the central
regions (0.25 r_{200}) of haloes with virial masses ~ 6 x 10^{13} (~ 7 x
10^{11}) Msun/h at z = 0(2). We also investigate how the orbital content
(relative fractions of the different orbital types) of these haloes depends on
several key parameters such as their mass, redshift and dynamical state. The
results of spectral analyses of the orbital content of these simulations are
compared, and the change in fraction of box, tube and irregular orbits is
quantified. Box orbits are found to dominate the orbital structure of dark
matter haloes in cosmological simulations. There is a strong anticorrelation
between the fraction of box orbits and the central baryon fraction. While
radiative cooling acts to reduce the fraction of box orbits, strong feedback
implementations result in a similar orbital distribution to that of the dark
matter only case. The orbital content described by the stellar particles is
found to be remarkably similar to that drawn from the orbits of dark matter
particles, suggesting that either they have forgotten their dynamical history,
or that subhaloes bringing in stars are not biased significantly with respect
to the main distribution. The orbital content of the subhaloes is in broad
agreement with that seen in the outer regions of the particle distributions.Comment: 18 pages, 13 figures, 3 tables. Accepted for publication in MNRA
Properties of Dark Matter Haloes and their Correlations: the Lesson from Principal Component Analysis
We study the correlations between the structural parameters of dark matter
haloes using Principal Component Analysis (PCA). We consider a set of eight
parameters, six of which are commonly used to characterize dark matter halo
properties: mass, concentration, spin, shape, overdensity, and the angle
() between the major axis and the angular momentum vector. Two
additional parameters (\x_{off} and ) are used to describe the
degree of `relaxedness' of the halo. We find that we can account for much of
the variance of these properties with halo mass and concentration, on the one
hand, and halo relaxedness on the other. Nonetheless, three principle
components are usually required to account for most of the variance. We argue
that halo mass is not as dominant as expected, which is a challenge for halo
occupation models and semi-analytic models that assume that mass determines
other halo (and galaxy) properties. In addition, we find that the angle
is not significantly correlated with other halo parameters, which may
present a difficulty for models in which galaxy disks are oriented in haloes in
a particular way. Finally, at fixed mass, we find that a halo's environment
(quantified by the large-scale overdensity) is relatively unimportant.Comment: 14 pages, 8 figures; minor revisions; MNRAS, in pres
Multimodality of rich clusters from the SDSS DR8 within the supercluster-void network
We study the relations between the multimodality of galaxy clusters drawn
from the SDSS DR8 and the environment where they reside. As cluster environment
we consider the global luminosity density field, supercluster membership, and
supercluster morphology. We use 3D normal mixture modelling, the
Dressler-Shectman test, and the peculiar velocity of cluster main galaxies as
signatures of multimodality of clusters. We calculate the luminosity density
field to study the environmental densities around clusters, and to find
superclusters where clusters reside. We determine the morphology of
superclusters with the Minkowski functionals and compare the properties of
clusters in superclusters of different morphology. We apply principal component
analysis to study the relations between the multimodality parametres of
clusters and their environment simultaneously. We find that multimodal clusters
reside in higher density environment than unimodal clusters. Clusters in
superclusters have higher probability to have substructure than isolated
clusters. The superclusters can be divided into two main morphological types,
spiders and filaments. Clusters in superclusters of spider morphology have
higher probabilities to have substructure and larger peculiar velocities of
their main galaxies than clusters in superclusters of filament morphology. The
most luminous clusters are located in the high-density cores of rich
superclusters. Five of seven most luminous clusters, and five of seven most
multimodal clusters reside in spider-type superclusters; four of seven most
unimodal clusters reside in filament-type superclusters. Our study shows the
importance of the role of superclusters as high density environment which
affects the properties of galaxy systems in them.Comment: 16 pages, 12 figures, 2 online tables, accepted for publication in
Astronomy and Astrophysic
Multimodality in galaxy clusters from SDSS DR8: substructure and velocity distribution
We search for the presence of substructure, a non-Gaussian, asymmetrical
velocity distribution of galaxies, and large peculiar velocities of the main
galaxies in galaxy clusters with at least 50 member galaxies, drawn from the
SDSS DR8. We employ a number of 3D, 2D, and 1D tests to analyse the
distribution of galaxies in clusters: 3D normal mixture modelling, the
Dressler-Shectman test, the Anderson-Darling and Shapiro-Wilk tests and others.
We find the peculiar velocities of the main galaxies, and use principal
component analysis to characterise our results. More than 80% of the clusters
in our sample have substructure according to 3D normal mixture modelling, the
Dressler-Shectman (DS) test shows substructure in about 70% of the clusters.
The median value of the peculiar velocities of the main galaxies in clusters is
206 km/s (41% of the rms velocity). The velocities of galaxies in more than 20%
of the clusters show significant non-Gaussianity. While multidimensional normal
mixture modelling is more sensitive than the DS test in resolving substructure
in the sky distribution of cluster galaxies, the DS test determines better
substructure expressed as tails in the velocity distribution of galaxies.
Richer, larger, and more luminous clusters have larger amount of substructure
and larger (compared to the rms velocity) peculiar velocities of the main
galaxies. Principal component analysis of both the substructure indicators and
the physical parameters of clusters shows that galaxy clusters are complicated
objects, the properties of which cannot be explained with a small number of
parameters or delimited by one single test. The presence of substructure, the
non-Gaussian velocity distributions, as well as the large peculiar velocities
of the main galaxies, shows that most of the clusters in our sample are
dynamically young.Comment: 15 pages, 11 figures, 2 online tables, accepted for publication in
Astronomy and Astrophysic
Effect of Intergalactic Medium on the Observability of Lyman Alpha Emitters during Cosmic Reionization
We perform a systematic study of how the inhomogeneities in the
Inter-Galactic Medium (IGM) affect the observability of Lyman Alpha Emitters
(LAEs) around the Epoch of Reionization. We focus on the IGM close to the
galaxies as the detailed ionization distribution and velocity fields of this
region could significantly influence the scattering of Ly-alpha photons off
neutral H atoms as they traverse the IGM after escaping from the galaxy. We
simulate the surface brightness (SB) maps and spectra of more than 100 LAEs at
z=7.7 as seen by an observer at z=0. To achieve this, we extract the source
properties of galaxies and their surrounding IGM from cosmological simulations
of box sizes 5-30 Mpc/h and follow the coupled radiative transfer of ionizing
and Ly-alpha radiation through the IGM using CRASH-alpha. We find that the
simulated SB profiles are extended and their detailed structure is affected by
inhomogeneities in the IGM, especially at high neutral fractions. The
detectability of LAEs and the fraction of the flux observed depend heavily on
the shape of the SB profile and the SB threshold (SB_th) of the observational
campaign. Only ultradeep observations (e.g. SB_th ~ 10^-23
ergs/s/cm^2/arcsec^2) would be able to obtain the true underlying
mass-luminosity relation and luminosity functions of LAEs. The details of our
results depend on whether Ly-alpha photons are significantly shifted in the
galaxy to longer wavelengths, the mean ionization fraction in the IGM and the
clustering of ionizing sources. These effects can lead to an easier escape of
Ly-alpha photons with less scattering in the IGM and a concentrated SB profile
similar to the one of a point source. Finally, we show that the SB profiles are
steeper at high ionization fraction for the same LAE sample which can
potentially be observed from the stacked profile of a large number of LAEs.Comment: 22 pages, 23 figures, 2 tables, Accepted by MNRAS. Minor change