22 research outputs found
On a novel approach using massive clusters at high redshifts as cosmological probe
In this work we propose a novel method for testing the validity of the
fiducial LCDM cosmology by measuring the cumulative distribution function of
the most massive haloes in a sample of subvolumes of identical size tiled on
the sky at a fixed redshift. The fact that the most massive clusters probe the
high-mass tail of the mass function, where the difference between LCDM and
alternative cosmological models is strongest, makes our method particularly
interesting as a cosmological probe. We utilise general extreme value
statistics (GEV) to obtain a cumulative distribution function of the most
massive objects in a given volume. We sample this distribution function
according to the number of patches covered by the survey area for a range of
different "test cosmologies" and for differently accurate mass estimations of
the haloes. By fitting this sample with the GEV distribution function, we can
study which parameters are the most sensitive with respect to the test
cosmologies. We find that the peak of the probability distribution function of
the most massive halo is well suited to test the validity of the fiducial LCDM
model, once we are able to establish a sufficiently complete large-area survey
with M_lim=10^14.5 M_sun/h (M_lim=10^14 M_sun/h) at redshifts above z=1
(z=1.5). Being of cumulative nature the proposed measure is robust and an
accuracy of 20-30% in the cluster masses would be sufficient to test for
alternative models. Since one only needs the most massive system in each
angular patch, this method would be ideally suited as a first fast consistency
check before going into a more complex statistical analysis of the observed
halo sample.Comment: 11 pages, 13 figures, 1 Table, MNRAS accepted versio
On the modelling of the excesses of galaxy clusters over high-mass thresholds
In this work we present for the first time an application of the Pareto
approach to the modelling of the excesses of galaxy clusters over high-mass
thresholds. The distribution of those excesses can be described by the
generalized Pareto distribution (GPD), which is closely related to the
generalized extreme value (GEV) distribution. After introducing the formalism,
we study the impact of different thresholds and redshift ranges on the
distributions, as well as the influence of the survey area on the mean excess
above a given mass threshold. We also show that both the GPD and the GEV
approach lead to identical results for rare, thus high-mass and high-redshift,
clusters. As an example, we apply the Pareto approach to ACT-CL J0102-4915 and
SPT-CL J2106-5844 and derive the respective cumulative distribution functions
of the exceedance over different mass thresholds. We also study the possibility
to use the GPD as a cosmological probe. Since in the maximum likelihood
estimation of the distribution parameters all the information from clusters
above the mass threshold is used, the GPD might offer an interesting
alternative to GEV-based methods that use only the maxima in patches. When
comparing the accuracy with which the parameters can be estimated, it turns out
that the patch-based modelling of maxima is superior to the Pareto approach. In
an ideal case, the GEV approach is capable to estimate the location parameter
with a percent level precision for less than 100 patches. This result makes the
GEV based approach potentially also interesting for cluster surveys with a
smaller area.Comment: 10 pages, 8 figures, MNRAS accepted, minor modifications to match the
accepted versio
The strongest gravitational lenses: II. Is the large Einstein radius of MACS J0717.5+3745 in conflict with LCDM?
Can the standard cosmological model be questioned on the basis of a single
observed extreme galaxy cluster? Usually, the word extreme refers directly to
cluster mass, which is not a direct observable and thus subject to substantial
uncertainty. Hence, it is desirable to extend studies of extreme clusters to
direct observables, such as the Einstein radius (ER). We aim to evaluate the
occurrence probability of the large observed ER of MACS J0717.5 within the
standard LCDM cosmology. In particular, we want to model the distribution
function of the single largest ER in a given cosmological volume and to study
which underlying assumptions and effects have the strongest impact on the
results. We obtain this distribution by a Monte Carlo approach, based on the
semi-analytic modelling of the halo population on the past lightcone. After
sampling the distribution, we fit the results with the general extreme value
(GEV) distribution which we use for the subsequent analysis. We find that the
distribution of the maximum ER is particularly sensitive to the precise choice
of the halo mass function, lens triaxiality, the inner slope of the halo
density profile and the mass-concentration relation. Using the distributions so
obtained,we study the occurrence probability of the large ER of MACS J0717.5,
finding that this system is not in tension with LCDM. We also find that the GEV
distribution can be used to fit very accurately the sampled distributions and
that all of them can be described by a Frechet distribution. With a multitude
of effects that strongly influence the distribution of the single largest ER,
it is more than doubtful that the standard LCDM cosmology can be ruled out on
the basis of a single observation. If, despite the large uncertainties in the
underlying assumptions, one wanted to do so, a much larger ER (> 100 arcsec)
than that of MACS J0717.5 would have to be observed.Comment: 15 pages, 11 figures, accepted for publication in Astronomy and
Astrophysics, minor corrections to match the accepted version, added
discussion of the distribution of the largest Einstein radii for the MACS
survey area, extended Fig.
Spherical collapse model in dark energy cosmologies
We study the spherical collapse model for several dark energy scenarios using
the fully nonlinear differential equation for the evolution of the density
contrast within homogeneous spherical overdensities derived from Newtonian
hydrodynamics. While mathematically equivalent to the more common approach
based on the differential equation for the radius of the perturbation, this
approach has substantial conceptual as well as numerical advantages. Among the
most important are that no singularities at early times appear, which avoids
numerical problems in particular in applications to cosmologies with dynamical
and early dark energy, and that the assumption of time-reversal symmetry can
easily be dropped where it is not strictly satisfied. We use this approach to
derive the two parameters characterising the spherical-collapse model, i.e.~the
linear density threshold for collapse and the virial
overdensity , for a broad variety of dark-energy models and
to reconsider these parameters in cosmologies with early dark energy. We find
that, independently of the model under investigation, and
are always very close to the values obtained for the
standard CDM model, arguing that the abundance of and the mean density
within non-linear structures are quite insensitive to the differences between
dark-energy cosmologies. Regarding early dark energy, we thus arrive at a
different conclusion than some earlier papers, including one from our group,
and we explain why.Comment: 11 pages, 7 figures, accepted for publications on MNRA
The strongest gravitational lenses: I. The statistical impact of cluster mergers
For more than a decade now, it has been controversial whether or not the high
rate of giant gravitational arcs and the largest observed Einstein radii are
consistent with the standard cosmological model. Recent studies indicate that
mergers provide an efficient mechanism to substantially increase the
strong-lensing efficiency of individual clusters. Based on purely semi-analytic
methods, we investigated the statistical impact of cluster mergers on the
distribution of the largest Einstein radii and the optical depth for giant
gravitational arcs of selected cluster samples. Analysing representative
all-sky realizations of clusters at redshifts z < 1 and assuming a constant
source redshift of z_s = 2.0, we find that mergers increase the number of
Einstein radii above 10 arcsec (20 arcsec) by ~ 35 % (~ 55 %). Exploiting the
tight correlation between Einstein radii and lensing cross sections, we infer
that the optical depth for giant gravitational arcs with a length-to-width
ratio > 7.5 of those clusters with Einstein radii above 10 arcsec (20 arcsec)
increases by ~ 45 % (85 %). Our findings suggest that cluster mergers
significantly influence in particular the statistical lensing properties of the
strongest gravitational lenses. We conclude that semi-analytic studies must
inevitably take these events into account before questioning the standard
cosmological model on the basis of the largest observed Einstein radii and the
statistics of giant gravitational arcs.Comment: 23 pages, 18 figures; accepted for publication in Astronomy and
Astrophysics; v2: minor corrections (added clarifying comments; added Fig.
19) to match the accepted versio
A Comparison of Algorithms for the Construction of SZ Cluster Catalogues
We evaluate the construction methodology of an all-sky catalogue of galaxy
clusters detected through the Sunyaev-Zel'dovich (SZ) effect. We perform an
extensive comparison of twelve algorithms applied to the same detailed
simulations of the millimeter and submillimeter sky based on a Planck-like
case. We present the results of this "SZ Challenge" in terms of catalogue
completeness, purity, astrometric and photometric reconstruction. Our results
provide a comparison of a representative sample of SZ detection algorithms and
highlight important issues in their application. In our study case, we show
that the exact expected number of clusters remains uncertain (about a thousand
cluster candidates at |b|> 20 deg with 90% purity) and that it depends on the
SZ model and on the detailed sky simulations, and on algorithmic implementation
of the detection methods. We also estimate the astrometric precision of the
cluster candidates which is found of the order of ~2 arcmins on average, and
the photometric uncertainty of order ~30%, depending on flux.Comment: Accepted for publication in A&A: 14 pages, 7 figures. Detailed
figures added in Appendi
An application of extreme value statistics to the most massive galaxy clusters at low and high redshifts
In this work we present an application of general extreme value statistics
(GEV) to very massive single clusters at high and low redshifts. After
introducing the formalism, we apply this statistics to four very massive high
redshift clusters. Those clusters comprise ACT-CL J0102-4915 with a mass of
M_200m=(2.16+/-0.32)x10^{15} M_sun at a redshift of z=0.87, SPT-CL J2106-5844
with a mass of M_200m=(1.27+/-0.21)x10^{15} M_sun at z=1.132 and two clusters
found by the XMM-Newton Distant Cluster Project survey: XMMU J2235.32557 with a
mass of M_200c= (7.3+/-1.3)x10^{14} M_sun located at a redshift of z=1.4 and
XMMU J0044.0-2033 having a mass in the range of M_200c= (3.5-5.0)x10^{14} M_sun
at z=1.579. By relating those systems to their corresponding distribution
functions of being the most massive system in a given survey area, we find that
none of the systems alone is in extreme tension with LCDM. We confront these
results with a GEV analysis of four very massive low redshift clusters: A2163,
A370, RXJ1347-1145 and 1E0657-558, finding no tendency of the high-z systems to
be more extreme than the low-z ones. In addition, we study the extreme
quantiles of single clusters at high-z and present contour plots for fixed
quantiles in the mass vs. survey area plane for four redshift intervals,
finding that, in order to be significantly in conflict with LCDM, cluster
masses would have to be substantially higher than the currently observed ones.Comment: 10 pages, 5 figures, 2 tables, MNRAS accepted, major revision, added
discussion of the impact of the bias arising from the posterior choice of the
redshift intervals, modified figures and structure of manuscrip
Impact of early dark energy on the Planck SZ cluster sample
Context. One science goal of the upcoming Planck mission is to perform a
full-sky cluster survey based on the Sunyaev-Zel'dovich (SZ) effect, which
leads to the question of how such a survey would be affected by cosmological
models with a different history of structure formation than LCDM. One class of
these models are early dark energy (EDE) cosmologies, where the dark energy
contribution does not vanish at early times.
Aims. Since structures grow slower in the presence of EDE, one expects an
increase in the number of galaxy clusters compared to LCDM at intermediate and
high redshifts, which could explain the reported excess of the angular CMB
power spectrum on cluster scales via an enhanced SZ contribution. We study the
impact of EDE on Planck's expected cluster sample.
Methods. To obtain realistic simulations, we constructed full-sky SZ maps for
EDE and LCDM cosmologies, taking angular cluster correlation into account.
Using these maps, we simulated Planck observations with and without Galactic
foregrounds and fed the results into our filter pipeline based on the spherical
multi-frequency matched filters.
Results. For the case of EDE cosmologies, we clearly find an increase in the
detected number of clusters compared to the fiducial LCDM case. This shows that
the spherical multi-frequency matched filter is sensitive enough to find
deviations from the LCDM sample, being caused by EDE. In addition we find an
interesting effect of EDE on the completeness of the cluster sample, such that
EDE helps to obtain cleaner samples.Comment: 12 pages, 10 figures, accepted for publication in A&A, minor language
corrections. Notable changes include an added subsection on collapse
parameters for EDE models and a discussion of the consequent SZ power spectr