80 research outputs found
Dark Matter Halo Mergers I: Dependence on Environment & Redshift Evolution
This paper presents a study of the specific merger rate as a function of
group membership, local environment, and redshift in a very large, , cosmological N-body simulation, the \textit{Millennium Simulation}. The
goal is to provide environmental diagnostics of major merger populations in
order to test simulations against observations and provide further constraints
on major merger driven galaxy evolution scenarios. A halo sample is defined
using the maximum circular velocity, which is both well defined for subhalos
and closely correlated with galaxy luminosity. Subhalos, including the
precursors of major mergers, are severely tidally stripped. Major mergers
between subhalos are therefore extremely rare. Tidal stripping also suppresses
dynamical friction, resulting in long major merger time scales when the more
massive halo does not host other subhalos. In contrast, when other subhalos are
present major merger time scales are several times shorter. This enhancement is
likely due to inelastic unbound collisions between subhalos. Following these
results, we predict that major mergers in group environments are dominated by
mergers involving the central galaxy, that the specific merger rate is
suppressed in groups, and that the frequency of fainter companions is enhanced
for mergers and their remnants. We also observe an `assembly bias' in the major
merger rate in that mergers of galaxy-like halos are slightly suppressed in
overdense environments while mergers of group-like halos are slightly enhanced.
A dynamical explanation for this trend is advanced which calls on both tidal
effects and interactions between bound halos beyond the virial radii of locally
dynamically dominant halos.Comment: 44 pages, 8 figures, Preprint Submitted to Ap
Dark Matter Halo Mergers I: Dependence on Environment & Redshift Evolution
This paper presents a study of the specific merger rate as a function of
group membership, local environment, and redshift in a very large, , cosmological N-body simulation, the \textit{Millennium Simulation}. The
goal is to provide environmental diagnostics of major merger populations in
order to test simulations against observations and provide further constraints
on major merger driven galaxy evolution scenarios. A halo sample is defined
using the maximum circular velocity, which is both well defined for subhalos
and closely correlated with galaxy luminosity. Subhalos, including the
precursors of major mergers, are severely tidally stripped. Major mergers
between subhalos are therefore extremely rare. Tidal stripping also suppresses
dynamical friction, resulting in long major merger time scales when the more
massive halo does not host other subhalos. In contrast, when other subhalos are
present major merger time scales are several times shorter. This enhancement is
likely due to inelastic unbound collisions between subhalos. Following these
results, we predict that major mergers in group environments are dominated by
mergers involving the central galaxy, that the specific merger rate is
suppressed in groups, and that the frequency of fainter companions is enhanced
for mergers and their remnants. We also observe an `assembly bias' in the major
merger rate in that mergers of galaxy-like halos are slightly suppressed in
overdense environments while mergers of group-like halos are slightly enhanced.
A dynamical explanation for this trend is advanced which calls on both tidal
effects and interactions between bound halos beyond the virial radii of locally
dynamically dominant halos.Comment: 44 pages, 8 figures, Preprint Submitted to Ap
Two Body Relaxation in Simulated Cosmological Haloes
This paper aims at quantifying discreetness effects, born of finite particle
number, on the dynamics of dark matter haloes forming in the context of
cosmological simulations. By generalising the standard calculation of two body
relaxation to the case when the size and mass distribution are variable, and
parametrising the time evolution using established empirical relations, we find
that the dynamics of a million particle halo is noise-dominated within the
inner percent of the final virial radius. Far larger particle numbers (~ 10^8)
are required for the RMS perturbations to the velocity to drop to the 10 %
level there. The radial scaling of the relaxation time is simple and strong:
t_relax ~ r^2, implying that numbers >> 10^8 are required to faithfully model
the very inner regions; artificial relaxation may thus constitute an important
factor, contributing to the contradictory claims concerning the persistence of
a power law density cusp to the very centre. The cores of substructure haloes
can be many relaxation times old. Since relaxation first causes their expansion
before recontraction occurs, it may render them either more difficult or easier
to disrupt, depending on their orbital parameters. It may thus modify the
characteristics of the subhalo distribution and effects of interactions with
the parent. We derive simple closed form formulas for the characteristic
relaxation times, as well as for the weak N-scaling reported by Diemand et al.
when the main contribution comes from relaxing subhaloes (abridged).Comment: 11 Pages, 7 figs, Monthly Notices styl
Grid of Lya radiation transfer models for the interpretation of distant galaxies
Lya is a key diagnostic for numerous observations of distant star-forming
galaxies. It's interpretation requires, however, detailed radiation transfer
models. We provide an extensive grid of 3D radiation transfer models simulating
the Lya and UV continuum radiation transfer in the interstellar medium of
star-forming galaxies. We have improved our Monte Carlo MCLya code, and have
used it to compute a grid of 6240 radiation transfer models for homogeneous
spherical shells containing HI and dust surrounding a central source. The
simulations cover a wide range of parameter space. We present the detailed
predictions from our models including in particular the Lya escape fraction
fesc, the continuum attenuation, and detailed Lya line profiles. The Lya escape
fraction is shown to depend strongly on dust content, but also on other
parameters (HI column density and radial velocity). The predicted line profiles
show a great diversity of morphologies ranging from broad absorption lines to
emission lines with complex features. The results from our simulations are
distributed in electronic format. Our models should be of use for the
interpretation of observations from distant galaxies, for other simulations,
and should also serve as an important base for comparison for future, more
refined, radiation transfer models.Comment: Accepted for publication in Astronomy & Astrophysics. Results from
simulations available at http://obswww.unige.ch/sf
Galaxy-Mass Correlations on 10 Mpc Scales in the Deep Lens Survey
We examine the projected correlation of galaxies with mass from small scales
(<few hundred kpc) where individual dark matter halos dominate, out to 15 Mpc
where correlated large-scale structure dominates. We investigate these profiles
as a function of galaxy luminosity and redshift. Selecting 0.8 million galaxies
in the Deep Lens Survey, we use photometric redshifts and stacked weak
gravitational lensing shear tomography out to radial scales of 1 degree from
the centers of foreground galaxies. We detect correlated mass density from
multiple halos and large-scale structure at radii larger than the virial
radius, and find the first observational evidence for growth in the galaxy-mass
correlation on 10 Mpc scales with decreasing redshift and fixed range of
luminosity. For a fixed range of redshift, we find a scaling of projected halo
mass with rest-frame luminosity similar to previous studies at lower redshift.
We control systematic errors in shape measurement and photometric redshift,
enforce volume completeness through absolute magnitude cuts, and explore
residual sample selection effects via simulations.Comment: 13 pages, 9 figures, re-submitted to ApJ after addressing referee
comment
Constraining halo occupation properties of X-ray AGNs using clustering of Chandra sources in the Bootes survey region
We present one of the most precise measurement to date of the spatial
clustering of X-ray selected AGNs using a sample derived from the Chandra X-ray
Observatory survey in the Bootes field. The real-space two-point correlation
function over a redshift interval from z=0.17 to z~3 is well described by the
power law, xi(r)=(r/r0)^-gamma, for comoving separations r<~20h^-1 Mpc. We find
gamma=1.84+-0.12 and r0 consistent with no redshift trend within the sample
(varying between r0=5.5+-0.6 h^-1 Mpc for =0.37 and r0=6.9+-1.0 h^-1 Mpc for
=1.28). Further, we are able to measure the projections of the two-point
correlation function both on the sky plane and in the line of sight. We use
these measurements to show that the Chandra/Bootes AGNs are predominantly
located at the centers of dark matter halos with the circular velocity Vmax>320
km/s or M_200 > 4.1e12 h^-1 Msun, and tend to avoid satellite galaxies in halos
of this or higher mass. The halo occupation properties inferred from the
clustering properties of Chandra/Bootes AGNs --- the mass scale of the parent
dark matter halos, the lack of significant redshift evolution of the clustering
length, and the low satellite fraction --- are broadly consistent with the
Hopkins et al. scenario of quasar activity triggered by mergers of
similarly-sized galaxies.Comment: Accepted to ApJ. The revision matches the accepted version. The most
significant changes include the recalculation of uncertainties using mock
catalogs and explicit comparison with the AGN HOD studies based on projected
correlation function, w(rp
Faint extended Lyalpha emission due to star formation at the centre of high-column density QSO absorption systems
We use detailed Lyalpha radiative transfer calculations to further test the
claim of Rauch et al. (2008) that they have detected spatially extended faint
Lyalpha emission from the elusive host population of Damped Lyalpha Absorption
systems (DLAs) in their recent ultra-deep spectroscopic survey. We investigate
the spatial and spectral distribution of Lyalpha emission due to star-formation
at the centre of DLAs, and its dependence on the spatial and velocity structure
of the gas. Our model simultaneously reproduces the observed properties of DLAs
and the faint Lyalpha emitters, including the velocity width and column density
distribution of DLAs and the large spatial extent of the emission of the faint
emitters. Our modelling confirms previous suggestions that DLAs are
predominately hosted by Dark Matter (DM) halos in the mass range
10^{9.5}-10^{12} M_sun, and are thus of significantly lower mass than those
inferred for L_* Lyman Break Galaxies (LBGs). Our modelling suggests that DM
halos hosting DLAs retain up to 20% of the cosmic baryon fraction in the form
of neutral hydrogen, and that star formation at the centre of the halos is
responsible for the faint Lyalpha emission. The scattering of a significant
fraction of the Lyalpha emission to the observed radii, which can be as large
as 50 kpc or more, requires the amplitude of the bulk motions of the gas at the
centre of the halos to be moderate. The observed space density and size
distribution of the emitters together with the incidence rate of DLAs suggests
that the Lyalpha emission due to star formation has a duty cycle of ~ 25%.Comment: 17 pages, 13 figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
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