578 research outputs found
The Mass Function and Average Mass Loss Rate of Dark Matter Subhaloes
We present a simple, semi-analytical model to compute the mass functions of
dark matter subhaloes. The masses of subhaloes at their time of accretion are
obtained from a standard merger tree. During the subsequent evolution, the
subhaloes experience mass loss due to the combined effect of dynamical
friction, tidal stripping, and tidal heating. Rather than integrating these
effects along individual subhalo orbits, we consider the average mass loss
rate, where the average is taken over all possible orbital configurations. This
allows us to write the average mass loss rate as a simple function that depends
only on redshift and on the instantaneous mass ratio of subhalo and parent
halo. After calibrating the model by matching the subhalo mass function (SHMF)
of cluster-sized dark matter haloes obtained from numerical simulations, we
investigate the predicted mass and redshift dependence of the SHMF.We find
that, contrary to previous claims, the subhalo mass function is not universal.
Instead, both the slope and the normalization depend on the ratio of the parent
halo mass, M, and the characteristic non-linear mass M*. This simply reflects a
halo formation time dependence; more massive parent haloes form later, thus
allowing less time for mass loss to operate. We analyze the halo-to-halo
scatter, and show that the subhalo mass fraction of individual haloes depends
most strongly on their accretion history in the last Gyr. Finally we provide a
simple fitting function for the average SHMF of a parent halo of any mass at
any redshift and for any cosmology, and briefly discuss several implications of
our findings.Comment: Replaced to match version accepted for publication in MNRAS. Small
section added that discusses higher-order moments of subhalo occupation
distribution (including a new figure). Otherwise, few small change
Can giant radio halos probe the merging rate of galaxy clusters?
Radio and X-ray observations of galaxy clusters probe a direct link between
cluster mergers and giant radio halos (RH), suggesting that these sources can
be used as probes of the cluster merging rate with cosmic time. In this paper
we carry out an explorative study that combines the observed fractions of
merging clusters (fm) and RH (fRH) with the merging rate predicted by
cosmological simulations and attempt to infer constraints on merger properties
of clusters that appear disturbed in X-rays and of clusters with RH. We use
morphological parameters to identify merging systems and analyze the currently
largest sample of clusters with radio and X-ray data (M500>6d14 Msun, and
0.2<z<0.33, from the Planck SZ cluster catalogue). We found that in this sample
fm~62-67% while fRH~44-51%. The comparison of the theoretical f_m with the
observed one allows to constrain the combination (xi_m,tau_m), where xi_m and
tau_m are the minimum merger mass ratio and the timescale of merger-induced
disturbance. Assuming tau_m~ 2-3 Gyr, as constrained by simulations, we find
that the observed f_m matches the theoretical one for xi_m~0.1-0.18. This is
consistent with optical and near-IR observations of clusters in the sample
(xi_m~0.14-0.16). The fact that RH are found only in a fraction of merging
clusters may suggest that merger events generating RH are characterized by
larger mass ratio; this seems supported by optical/near-IR observations of RH
clusters in the sample (xi_min~0.2-0.25). Alternatively, RH may be generated in
all mergers but their lifetime is shorter than \tau_m (by ~ fRH/fm). This is an
explorative study, however it suggests that follow up studies using the
forthcoming radio surveys and adequate numerical simulations have the potential
to derive quantitative constraints on the link between cluster merging rate and
RH at different cosmic epochs and for different cluster masses.Comment: 10 pages, 3 figures, accepted for publication in A&
Selecting background galaxies in weak-lensing analysis of galaxy clusters
In this paper, we present a new method to select the faint, background
galaxies used to derive the mass of galaxy clusters by weak lensing.
The method is based on the simultaneous analysis of the shear signal, that
should be consistent with zero for the foreground, unlensed galaxies, and of
the colors of the galaxies: photometric data from the COSMic evOlution Survey
are used to train the color selection. In order to validate this methodology,
we test it against a set of state-of-the-art image simulations of mock galaxy
clusters in different redshift [] and mass
[] ranges, mimicking medium-deep multicolor
imaging observations (e.g. SUBARU, LBT).
The performance of our method in terms of contamination by unlensed sources
is comparable to a selection based on photometric redshifts, which however
requires a good spectral coverage and is thus much more observationally
demanding. The application of our method to simulations gives an average ratio
between estimated and true masses of . As a further test,
we finally apply our method to real data, and compare our results with other
weak lensing mass estimates in the literature: for this purpose we choose the
cluster Abell 2219 (), for which multi-band (BVRi) data are publicly
available.Comment: MNRAS, Accepted 2016 February 2
The mass function and average mass-loss rate of dark matter subhaloes
We present a simple, semi-analytical model to compute the mass functions of dark matter subhaloes. The masses of subhaloes at their time of accretion are obtained from a standard merger tree. During the subsequent evolution, the subhaloes experience mass loss due to the combined effect of dynamical friction, tidal stripping, and tidal heating. Rather than integrating these effects along individual subhalo orbits, we consider the average mass-loss rate, where the average is taken over all possible orbital configurations. Under the Ansatz that the average distribution of orbits is independent of parent halo mass, this allows us to write the average mass-loss rate as a simple function that depends only on redshift and on the instantaneous mass ratio of subhalo and parent halo. After calibrating this model by matching the subhalo mass function (SHMF) of cluster-sized dark matter haloes obtained from high-resolution, numerical simulations, we investigate the predicted mass and redshift dependence of the SHMF. We find that, contrary to previous claims, the subhalo mass function is not universal. Instead, both the slope and the normalization depend on the ratio of the parent halo mass, M, and the characteristic non-linear mass M*. This simply reflects a halo formation time dependence; more massive parent haloes form later, thus allowing less time for mass loss to operate. We predict that galaxy-sized haloes, with a present-day mass of M≃ 1012 h−1 M⊙, have an average mass fraction of dark matter subhaloes that is a factor of 3 lower than for massive clusters with M≃ 1015 h−1 M⊙. We also analyse the halo-to-halo scatter in SHMFs, and show that the subhalo mass fraction of individual haloes depends most strongly on their accretion history in the last ∼1 Gyr. Finally we provide a simple fitting function for the average SHMF of a parent halo of any mass at any redshift and for any cosmology, and briefly discuss several implications of our finding
The Substructure Hierarchy in Dark Matter Haloes
We present a new algorithm for identifying the substructure within simulated
dark matter haloes. The method is an extension of that proposed by Tormen et
al. (2004) and Giocoli et al. (2008a), which identifies a subhalo as a group of
self-bound particles that prior to being accreted by the main progenitor of the
host halo belonged to one and the same progenitor halo (hereafter satellite).
However, this definition does not account for the fact that these satellite
haloes themselves may also have substructure, which thus gives rise to
sub-subhaloes, etc. Our new algorithm identifies substructures at all levels of
this hierarchy, and we use it to determine the mass function of all
substructure (counting sub-haloes, sub-subhaloes, etc.). On average, haloes
which formed more recently tend to have a larger mass fraction in substructure
and to be less concentrated than average haloes of the same mass. We provide
quantitative fits to these correlations. Even though our algorithm is very
different from that of Gao et al. (2004), we too find that the subhalo mass
function per unit mass at redshift z = 0 is universal. This universality
extends to any redshift only if one accounts for the fact that host haloes of a
given mass are less concentrated at higher redshifts, and concentration and
substructure abundance are anti-correlated. This universality allows a simple
parametrization of the subhalo mass function integrated over all host halo
masses, at any given time. We provide analytic fits to this function which
should be useful in halo model analyses which equate galaxies with halo
substructure when interpreting clustering in large sky surveys. Finally, we
discuss systematic differences in the subhalo mass function that arise from
different definitions of (host) halo mass.Comment: 18 pages, 24 figures, accepted for publication on MNRA
Turbulent pressure support and hydrostatic mass-bias in the intracluster medium
The degree of turbulent pressure support by residual gas motions in galaxy
clusters is not well known. Mass modelling of combined X-ray and Sunyaev
Zel'dovich observations provides an estimate of turbulent pressure support in
the outer regions of several galaxy clusters. Here, we test two different
filtering techniques to disentangle bulk from turbulent motions in
non-radiative high-resolution cosmological simulations of galaxy clusters using
the cosmological hydro code ENZO. We find that the radial behavior of the ratio
of non-thermal pressure to total gas pressure as a function of cluster-centric
distance can be described by a simple polynomial function. The typical
non-thermal pressure support in the centre of clusters is 5%, increasing
to 15% in the outskirts, in line with the pressure excess found in recent
X-ray observations. While the complex dynamics of the ICM makes it impossible
to reconstruct a simple correlation between turbulent motions and hydrostatic
bias, we find that a relation between them can be established using the median
properties of a sample of objects. Moreover, we estimate the contribution of
radial accelerations to the non-thermal pressure support and conclude that it
decreases moving outwards from 40% (in the core) to 15% (in the cluster's
outskirts). Adding this contribution to one provided by turbulence, we show
that it might account for the entire observed hydrostatic bias in the innermost
regions of the clusters, and for less than 80% of it at .Comment: 20 pages; 21 figures; Substantial Revision; MNRAS in pres
Cosmic degeneracies III: N-body simulations of Interacting Dark Energy with non-Gaussian initial conditions
We perform for the first time N-body simulations of interacting dark energy assuming non-Gaussian initial conditions, with the aim of investigating possible degeneracies of these two theoretically independent phenomena in different observational probes. We focus on the large-scale matter distribution, as well as on the statistical and structural properties of collapsed haloes and cosmic voids. On very large scales, we show that it is possible to choose the interaction and non-Gaussian parameters such that their effects on the halo power spectrum cancel, and the power spectrum is indistinguishable from a \u39b cold dark matter (\u2060\u39bCDM) model. On small scales, measurements of the non-linear matter power spectrum, halo-matter bias, halo and subhalo mass function, and cosmic void number function validate the degeneracy determined on large scales. However, the internal structural properties of haloes and cosmic voids, namely halo concentration\u2013mass relation and void density profile, are very different from those measured in the \u39bCDM model, thereby breaking the degeneracy. In practice, the values of fNL required to cancel the effect of interaction are already ruled by observations. Our results show in principle that the combination of large- and small-scale probes is needed to constrain interacting dark energy and primordial non-Gaussianity separately
Cosmic degeneracies III: N-body simulations of interacting dark energy with non-Gaussian initial conditions
We perform for the first time N-body simulations of interacting dark energy assuming non- Gaussian initial conditions, with the aim of investigating possible degeneracies of these two theoretically independent phenomena in different observational probes.We focus on the largescale matter distribution, as well as on the statistical and structural properties of collapsed haloes and cosmic voids. On very large scales, we show that it is possible to choose the interaction and non-Gaussian parameters such that their effects on the halo power spectrum cancel, and the power spectrum is indistinguishable from a \u39b cold dark matter (\u39bCDM) model. On small scales, measurements of the non-linear matter power spectrum, halo-matter bias, halo and subhalomass function, and cosmic void number function validate the degeneracy determined on large scales. However, the internal structural properties of haloes and cosmic voids, namely halo concentration-mass relation and void density profile, are very different from those measured in the \u39bCDM model, thereby breaking the degeneracy. In practice, the values of fNLrequired to cancel the effect of interaction are already ruled by observations. Our results show in principle that the combination of large- and small-scale probes is needed to constrain interacting dark energy and primordial non-Gaussianity separately
The Population of Dark Matter Subhaloes: Mass Functions and Average Mass Loss Rates
Using a cosmological N-Body simulation and a sample of re-simulated
cluster-like haloes, we study the mass loss rates of dark matter subhaloes, and
interpret the mass function of subhaloes at redshift zero in terms of the
evolution of the mass function of systems accreted by the main halo progenitor.
When expressed in terms of the ratio between the mass of the subhalo at the
time of accretion and the present day host mass the unevolved subhalo mass
function is found to be universal. However, the subhalo mass function at
redshift zero clearly depends on , in that more massive host haloes host
more subhaloes. To relate the unevolved and evolved subhalo mass functions, we
measure the subhalo mass loss rate as a function of host mass and redshift. We
find that the average, specific mass loss rate of dark matter subhaloes depends
mainly on redshift. These results suggest a pleasingly simple picture for the
evolution and mass dependence of the evolved subhalo mass function. Less
massive host haloes accrete their subhaloes earlier, which are thus subjected
to mass loss for a longer time. In addition, their subhaloes are typically
accreted by denser hosts, which causes an additional boost of the mass loss
rate. To test the self-consistency of this picture, we use a merger trees
constructed using the extended Press-Schechter formalism, and evolve the
subhalo populations using the average mass loss rates obtained from our
simulations, finding the subhalo mass functions to be in good agreement with
the simulations. [abridged]Comment: 12 pages, 12 figures; submitted to MNRA
Cosmic voids in modified gravity models with massive neutrinos
Cosmic voids are progressively emerging as a new viable cosmological probe. Their abundance and density profiles are sensitive to modifications of gravity, as well as to dark energy and neutrinos. The main goal of this work is to investigate the possibility of exploiting cosmic void statistics to disentangle the degeneracies resulting from a proper combination of f(R) modified gravity and neutrino mass. We use N-body simulations to analyse the density profiles and size function of voids traced by both dark matter particles and haloes. We find clear evidence of the enhancement of gravity in f(R) cosmologies in the void density profiles at z = 1. However, these effects can be almost completely overridden by the presence of massive neutrinos because of their thermal free streaming. Despite the limited volume of the analysed simulations does not allow us to achieve a statistically relevant abundance of voids larger than 40 Mpc h-1, we find that the void size function at high redshifts and for large voids is potentially an effective probe to disentangle these degenerate cosmological models, which is key in the prospective of the upcoming wide-field redshift surveys
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