149 research outputs found

### Analytic model for the matter power spectrum, its covariance matrix, and baryonic effects

We develop a model for the matter power spectrum as the sum of Zeldovich
approximation and even powers of $k$, i.e., $A_0 - A_2k^2 + A_4k^4 - ...$,
compensated at low $k$. With terms up to $k^4$ the model can predict the true
power spectrum to a few percent accuracy up to $k\sim 0.7 h \rm{Mpc}^{-1}$,
over a wide range of redshifts and models. The $A_n$ coefficients contain
information about cosmology, in particular amplitude of fluctuations. We write
a simple form of the covariance matrix as a sum of Gaussian part and $A_0$
variance, which reproduces the simulations remarkably well. In contrast, we
show that one needs an N-body simulation volume of more than 1000 $({\rm
Gpc}/h)^3$ to converge to 1\% accuracy on covariance matrix. We investigate the
super-sample variance effect and show it can be modeled as an additional
parameter that can be determined from the data. This allows a determination of
$\sigma_8$ amplitude to about 0.2\% for a survey volume of 1$({\rm Gpc}/h)^3$,
compared to 0.4\% otherwise. We explore the sensitivity of these coefficients
to baryonic effects using hydrodynamic simulations of van Daalen (2011). We
find that because of baryons redistributing matter inside halos all the
coefficients $A_{2n}$ for $n>0$ are strongly affected by baryonic effects,
while $A_0$ remains almost unchanged, a consequence of halo mass conservation.
Our results suggest that observations such as weak lensing power spectrum can
be effectively marginalized over the baryonic effects, while still preserving
the bulk of the cosmological information contained in $A_0$ and Zeldovich
terms.Comment: 21 pages,11 figures, 1 table; Accepted for publication in MNRA

### Perturbative approach to covariance matrix of the matter power spectrum

We evaluate the covariance matrix of the matter power spectrum using
perturbation theory up to dominant terms at 1-loop order and compare it to
numerical simulations. We decompose the covariance matrix into the disconnected
(Gaussian) part, trispectrum from the modes outside the survey (beat coupling
or super-sample variance), and trispectrum from the modes inside the survey,
and show how the different components contribute to the overall covariance
matrix. We find the agreement with the simulations is at a 10\% level up to $k
\sim 1 h {\rm Mpc^{-1}}$. We show that all the connected components are
dominated by the large-scale modes ($k<0.1 h {\rm Mpc^{-1}}$), regardless of
the value of the wavevectors $k,\, k'$ of the covariance matrix, suggesting
that one must be careful in applying the jackknife or bootstrap methods to the
covariance matrix. We perform an eigenmode decomposition of the connected part
of the covariance matrix, showing that at higher $k$ it is dominated by a
single eigenmode. The full covariance matrix can be approximated as the
disconnected part only, with the connected part being treated as an external
nuisance parameter with a known scale dependence, and a known prior on its
variance for a given survey volume. Finally, we provide a prescription for how
to evaluate the covariance matrix from small box simulations without the need
to simulate large volumes.Comment: 22 pages, 17 figures, 1 tabl

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### The biasing of baryons on the cluster mass function and cosmological parameter estimation

We study the effect of baryonic processes on the halo mass function in the
galaxy cluster mass range using a catalogue of 153 high resolution cosmological
hydrodynamical simulations performed with the AMR code ramses. We use the
results of our simulations within a simple analytical model to gauge the
effects of baryon physics on the halo mass function. Neglect of AGN feedback
leads to a significant boost in the cluster mass function similar to that
reported by other authors. However, including AGN feedback not only gives rise
to systems that are similar to observed galaxy clusters, but they also reverse
the global baryonic effects on the clusters. The resulting mass function is
closer to the unmodified dark matter halo mass function but still contains a
mass dependent bias at the 5-10% level. These effects bias measurements of the
cosmological parameters, such as $\sigma_8$ and $\Omega_m$. For current cluster
surveys baryonic effects are within the noise for current survey volumes, but
forthcoming and planned large SZ, X-ray and multi-wavelength surveys will be
biased at the percent level by these processes. The predictions for the halo
mass function including baryonic effects need to be carefully studied with
larger and improved simulations. However, simulations of full cosmological
boxes with the resolution we achieve and including AGN feedback are still
computationally challenging.Comment: 12 pages, 3 tables, 6 figures, accepted for publication in MNRA

### Constraining galaxy cluster temperatures and redshifts with eROSITA survey data

The nature of dark energy is imprinted in the large-scale structure of the
Universe and thus in the mass and redshift distribution of galaxy clusters. The
upcoming eROSITA mission will exploit this method of probing dark energy by
detecting roughly 100,000 clusters of galaxies in X-rays. For a precise
cosmological analysis the various galaxy cluster properties need to be measured
with high precision and accuracy. To predict these characteristics of eROSITA
galaxy clusters and to optimise optical follow-up observations, we estimate the
precision and the accuracy with which eROSITA will be able to determine galaxy
cluster temperatures and redshifts from X-ray spectra. Additionally, we present
the total number of clusters for which these two properties will be available
from the eROSITA survey directly. During its four years of all-sky surveys,
eROSITA will determine cluster temperatures with relative uncertainties of
Delta(T)/T<10% at the 68%-confidence level for clusters up to redshifts of
z~0.16 which corresponds to ~1,670 new clusters with precise properties.
Redshift information itself will become available with a precision of
Delta(z)/(1+z)<10% for clusters up to z~0.45. Additionally, we estimate how the
number of clusters with precise properties increases with a deepening of the
exposure. Furthermore, the biases in the best-fit temperatures as well as in
the estimated uncertainties are quantified and shown to be negligible in the
relevant parameter range in general. For the remaining parameter sets, we
provide correction functions and factors. The eROSITA survey will increase the
number of galaxy clusters with precise temperature measurements by a factor of
5-10. Thus the instrument presents itself as a powerful tool for the
determination of tight constraints on the cosmological parameters.Comment: accepted for publication in A&A; 17 pages, 20 figure

### Analytic model for the matter power spectrum, its covariance matrix and baryonic effects

We develop a model for the matter power spectrum as the sum of Zeldovich approximation and even powers of k, i.e. A0âˆ’A2k2+A4k4âˆ’ , compensated at low k. With terms up to k4, the model can predict the true power spectrum to a few perâ€‰cent accuracy up to k âˆ¼ 0.7â€‰hâ€‰Mpcâˆ’1, over a wide range of redshifts and models. The An coefficients contain information about cosmology, in particular amplitude of fluctuations. We write a simple form of the covariance matrix as a sum of Gaussian part and A0 variance, which reproduces the simulations remarkably well. In contrast, we show that one needs an N-body simulation volume of more than 1000 (Gpcâ€‰hâˆ’1)3 to converge to 1â€‰perâ€‰cent accuracy on covariance matrix. We investigate the supersample variance effect and show it can be modelled as an additional parameter that can be determined from the data. This allows a determination of Ïƒ8 amplitude to about 0.2â€‰perâ€‰cent for a survey volume of 1(Gpcâ€‰hâˆ’1)3, compared to 0.4â€‰perâ€‰cent otherwise. We explore the sensitivity of these coefficients to baryonic effects using hydrodynamic simulations of van Daalen et al. We find that because of baryons redistributing matter inside haloes all the coefficients A2n for n > 0 are strongly affected by baryonic effects, while A0 remains almost unchanged, a consequence of halo mass conservation. Our results suggest that observations such as weak lensing power spectrum can be effectively marginalized over the baryonic effects, while still preserving the bulk of the cosmological information contained in A0 and Zeldovich term

### Mass-Galaxy offsets in Abell 3827, 2218 and 1689: intrinsic properties or line-of-sight substructures?

We have made mass maps of three strong-lensing clusters, Abell 3827, Abell
2218 and Abell 1689, in order to test for mass-light offsets. The technique
used is GRALE, which enables lens reconstruction with minimal assumptions, and
specifically with no information about the cluster light being given. In the
first two of these clusters, we find local mass peaks in the central regions
that are displaced from the nearby galaxies by a few to several kpc. These
offsets {\em could\/} be due to line of sight structure unrelated to the
clusters, but that is very unlikely, given the typical levels of chance
line-of-sight coincidences in $\Lambda CDM$ simulations --- for Abell 3827 and
Abell 2218 the offsets appear to be intrinsic. In the case of Abell 1689, we
see no significant offsets in the central region, but we do detect a possible
line of sight structure: it appears only when sources at z\ga 3 are used for
reconstructing the mass. We discuss possible origins of the mass-galaxy offsets
in Abell 3827 and Abell 2218: these include pure gravitational effects like
dynamical friction, but also non-standard mechanisms like self-interacting
dark-matter.Comment: 14 pages, 9 figures; Accepted for publication in MNRA

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