374 research outputs found
Detecting shocked intergalactic gas with X-ray and radio observations
Detecting the thermal and non-thermal emission from the shocked cosmic gas
surrounding large-scale structures represents a challenge for observations, as
well as a unique window into the physics of the warm-hot intergalactic medium.
In this work, we present synthetic radio and X-ray surveys of large
cosmological simulations in order to assess the chances of jointly detecting
the cosmic web in both frequency ranges. We then propose best observing
strategies tailored for existing (LOFAR, MWA and XMM) or future instruments
(SKA-LOW and SKA-MID, ATHENA and eROSITA). We find that the most promising
targets are the extreme peripheries of galaxy clusters in an early merging
stage, where the merger causes the fast compression of warm-hot gas onto the
virial region. By taking advantage of a detection in the radio band, future
deep X-ray observations will probe this gas in emission, and help us to study
plasma conditions in the dynamic warm-hot intergalactic medium with
unprecedented detail.Comment: 22 pages, 25 Figures. A\&A accepted, in press. Moderate revision
compared to version 1, with a few new figure
Simulated X-ray galaxy clusters at the virial radius: slopes of the gas density, temperature and surface brightness profiles
Using a set of hydrodynamical simulations of 9 galaxy clusters with masses in
the range 1.5 10^{14} M_sun < M_vir < 3.4 10^{15} M_sun, we have studied the
density, temperature and X-ray surface brightness profiles of the intracluster
medium in the regions around the virial radius. We have analyzed the profiles
in the radial range well above the cluster core, the physics of which are still
unclear and matter of tension between simulated and observed properties, and up
to the virial radius and beyond, where present observations are unable to
provide any constraints. We have modeled the radial profiles between 0.3 R_200
and 3 R_200 with power laws with one index, two indexes and a rolling index.
The simulated temperature and [0.5-2] keV surface brightness profiles well
reproduce the observed behaviours outside the core. The shape of all these
profiles in the radial range considered depends mainly on the activity of the
gravitational collapse, with no significant difference among models including
extraphysics. The profiles steepen in the outskirts, with the slope of the
power-law fit that changes from -2.5 to -3.4 in the gas density, from -0.5 to
-1.8 in the gas temperature, and from -3.5 to -5.0 in the X-ray soft surface
brightness. We predict that the gas density, temperature and [0.5-2] keV
surface brightness values at R_200 are, on average, 0.05, 0.60, 0.008 times the
measured values at 0.3 R_200. At 2 R_200, these values decrease by an order of
magnitude in the gas density and surface brightness, by a factor of 2 in the
temperature, putting stringent limits on the detectable properties of the
intracluster-medium (ICM) in the virial regions.Comment: 13 pages, 6 figures; added reference and other minor change
Large-scale inhomogeneities of the intracluster medium: improving mass estimates using the observed azimuthal scatter
Using a set of hydrodynamical simulations of 62 galaxy clusters and groups we
study the ICM of inhomogeneities, focusing on the ones on the large scale that,
unlike clumps, are the most difficult to identify. To this purpose we introduce
the concept of residual clumpiness, C_R, that quantifies the large-scale
inhomogeneity of the ICM. After showing that this quantity can be robustly
defined for relaxed systems, we characterize how it varies with radius, mass
and dynamical state of the halo. Most importantly, we observe that it
introduces an overestimate in the determination of the density profile from the
X-ray emission, which translates into a systematic overestimate of 6 (12)% in
the measurement of M_gas at R_200 for our relaxed (perturbed) cluster sample.
At the same time, the increase of C_R with radius introduces also a ~2%
systematic underestimate in the measurement of the hydrostatic-equilibrium mass
(M_he), which adds to the previous one generating a systematic ~8.5%
overestimate in f_gas in our relaxed sample. Since the residual clumpiness of
the ICM is not directly observable, we study its correlation with the azimuthal
scatter in the X-ray surface brightness of the halo and in the y-parameter
profiles. We find that their correlation is highly significant (r_S = 0.6-0.7),
allowing to define the azimuthal scatter measured in the X-ray surface
brightness profile and in the y-parameter as robust proxies of C_R. After
providing a function that connects the two quantities, we obtain that
correcting the observed gas density profiles using the azimuthal scatter
eliminates the bias in the measurement of M_gas for relaxed objects, which
becomes (0+/-2)% up to 2R_200, and reduces it by a factor of 3 for perturbed
ones. This method allows also to eliminate the systematics on the measurements
of M_he and f_gas, although a significant halo to halo scatter remains.
(abridged)Comment: 18 pages, 17 figures, 3 tables. Submitted to MNRAS, revised after
referee's comment
Gas clumping in galaxy clusters
The reconstruction of galaxy cluster's gas density profiles is usually
performed by assuming spherical symmetry and averaging the observed X-ray
emission in circular annuli. In the case of a very inhomogeneous and asymmetric
gas distribution, this method has been shown to return biased results in
numerical simulations because of the dependence of the X-ray emissivity.
We propose a method to recover the true density profiles in the presence of
inhomogeneities, based on the derivation of the azimuthal median of the surface
brightness in concentric annuli. We demonstrate the performance of this method
with numerical simulations, and apply it to a sample of 31 galaxy clusters in
the redshift range 0.04-0.2 observed with ROSAT/PSPC. The clumping factors
recovered by comparing the mean and the median are mild and show a slight trend
of increasing bias with radius. For , we measure a clumping factor
, which indicates that the thermodynamic properties and
hydrostatic masses measured in this radial range are only mildly affected by
this effect. Comparing our results with three sets of hydrodynamical numerical
simulations, we found that non-radiative simulations significantly overestimate
the level of inhomogeneities in the ICM, while the runs including cooling, star
formation, and AGN feedback reproduce the observed trends closely. Our results
indicate that most of the accretion of X-ray emitting gas is taking place in
the diffuse, large-scale accretion patterns rather than in compact structures.Comment: 12 pages, 11 figures, accepted for publication in MNRAS.
Largely-improved version compared to v1, method and comparison with
simulations update
The kinematic Sunyaev-Zel'dovich effect of the large-scale structure (II): the effect of modified gravity
The key to understand the nature of dark energy relies in our ability to
probe the distant Universe. In this framework, the recent detection of the
kinematic Sunyaev-Zel'dovich (kSZ) effect signature in the cosmic microwave
background obtained with the South Pole Telescope (SPT) is extremely useful
since this observable is sensitive to the high-redshift diffuse plasma. We
analyse a set of cosmological hydrodynamical simulation with 4 different
realisations of a Hu & Sawicki gravity model, parametrised by the values
of , to compute the
properties of the kSZ effect due to the ionized Universe and how they depend on
and on the redshift of reionization, . In
the standard General Relativity limit (=0) we obtain an
amplitude of the kSZ power spectrum of K (=8.8), close to the limit
of the K measurement by
SPT. This corresponds to an upper limit on the kSZ contribute from patchy
reionization of K (95 per
cent confidence level). Modified gravity boosts the kSZ signal by about 3, 12
and 50 per cent for ,
respectively, with almost no dependence on the angular scale. This means that
with modified gravity the limits on patchy reionization shrink significantly:
for we obtain K. Finally, we provide an analytical formula
for the scaling of the kSZ power spectrum with and
at different multipoles: at we obtain
.Comment: 11 pages, 5 figures, 2 table
Resolving the unresolved cosmic X-ray background in the Chandra Deep Fields
We present a measurement of the surface brightness of the cosmic X-ray
background (CXB) in the Chandra Deep Fields, after excluding all detected
X-ray, optical and infrared sources. The work is motivated by a recent X-ray
stacking analysis by Worsley and collaborators, which showed that galaxies
detected by HST but not by Chandra may account for most of the unresolved CXB
at E>1 keV. We find that after excluding HST and Spitzer IRAC sources, some CXB
still remains, but it is marginally significant: (3.4+/-1.4)x10^-13 ergs cm^-2
s^-1 deg^2 in the 1-2 keV band and (4+/-9)x10^-13 ergs cm^-2 s^-1 deg^2 in the
2-5 keV band, or 7%+/-3% and 4%+/-9% of the total CXB, respectively. Of the 1-2
keV signal resolved by the HST sources, 34%+/-2% comes from objects with
optical colors typical of ``normal'' galaxies (which make up 25% of the HST
sources), while the remaining flux comes from objects with colors of starburst
and irregular galaxies. In the 0.65-1 keV band (just above the bright Galactic
O VII line) the remaining diffuse intensity is (1.0+/-0.2)x10^-12 ergs cm^-2
s^-1 deg^2. This flux includes emission from the Galaxy as well as from the
hypothetical warm-hot intergalactic medium (WHIM), and provides a conservative
upper limit on the WHIM signal that comes interestingly close to theoretical
predictions.Comment: 5 emulateapj pages, 4 figures, 1 table, accepted to ApJL (mostly
minor improvements in response to referee's comments, in particular further
treatment of uncertainties on resolved fluxes
The imprints of local superclusters on the Sunyaev-Zel'dovich signals and their detectability with Planck
We use high-resolution hydrodynamical simulations of large-scale structure
formation to study the imprints of the local superclusters onto the full-sky
Sunyaev-Zel'dovich (SZ) signals. Following (Mathis et al. 2002), the initial
conditions have been statistically constrained to reproduce the density field
within a sphere of 110 Mpc around the Milky Way, as observed in the IRAS 1.2-Jy
all-sky redshift survey. As a result, the positions and masses of prominent
galaxy clusters and superclusters in our simulations coincide closely with
their real counterparts in the local universe. We present the results of two
different runs, one with adiabatic gas physics only, and one also including
cooling, star formation and feedback. By analysing the full-sky maps for the
thermal and kinetic SZ signals extracted from these simulations, we find that
for multipoles with l<100 the power spectrum is dominated by the prominent
local superclusters, and its amplitude at these scales is a factor of two
higher than that obtained from unconstrained simulations; at lower multipoles
(l<20) this factor can even reach one order of magnitude. We check the
influence of the SZ effect from local superclusters on the cosmic microwave
background (CMB) power spectrum at small multipoles and find it negligible and
with no signs of quadrupole-octopole alignment. However, performing simulations
of the CMB radiation including the experimental noise at the frequencies which
will be observed by the Planck satellite, we find results suggesting that an
estimate of the SZ power spectrum at large scales can be extracted.Comment: revised version, MNRAS, accepte
The effect of feedback on the emission properties of the Warm-Hot Intergalactic Medium
At present, 30-40 per cent of the baryons in the local Universe is still
undetected. According to theoretical predictions, this gas should reside in
filaments filling the large-scale structure (LSS) in the form of a Warm-Hot
Intergalactic Medium (WHIM), at temperatures of 10^5 - 10^7 K, thus emitting in
the soft X-ray energies via free-free interaction and line emission from heavy
elements. In this work we characterize the properties of the X-ray emission of
the WHIM, and the LSS in general, focusing on the influence of different
physical mechanisms, namely galactic winds (GWs), black-hole feedback and
star-formation, and providing estimates of possible observational constraints.
To this purpose we use a set of cosmological hydrodynamical simulations that
include a self-consistent treatment of star-formation and chemical enrichment
of the intergalactic medium, that allows us to follow the evolution of
different metal species. We construct a set of simulated light-cones to make
predictions of the emission in the 0.3-10 keV energy range. We obtain that GWs
increase by a factor of 2 the emission of both galaxy clusters and WHIM. The
amount of oxygen at average temperature and, consequently, the amount of
expected bright Ovii and Oviii lines is increased by a factor of 3 due to GWs
and by 20 per cent when assuming a top-heavy IMF. We compare our results with
current observational constraints and find that the emission from faint groups
and WHIM should account from half to all of the unresolved X-ray background in
the 1-2 keV band.Comment: 15 pages, 8 figures, 4 tables. Accepted for publication in the MNRAS.
Minor changes after referee repor
Searching for galaxy clusters in the Kilo-Degree Survey
In this paper, we present the tools used to search for galaxy clusters in the
Kilo Degree Survey (KiDS), and our first results. The cluster detection is
based on an implementation of the optimal filtering technique that enables us
to identify clusters as over-densities in the distribution of galaxies using
their positions on the sky, magnitudes, and photometric redshifts. The
contamination and completeness of the cluster catalog are derived using mock
catalogs based on the data themselves. The optimal signal to noise threshold
for the cluster detection is obtained by randomizing the galaxy positions and
selecting the value that produces a contamination of less than 20%. Starting
from a subset of clusters detected with high significance at low redshifts, we
shift them to higher redshifts to estimate the completeness as a function of
redshift: the average completeness is ~ 85%. An estimate of the mass of the
clusters is derived using the richness as a proxy. We obtained 1858 candidate
clusters with redshift 0 < z_c < 0.7 and mass 13.5 < log(M500/Msun) < 15 in an
area of 114 sq. degrees (KiDS ESO-DR2). A comparison with publicly available
Sloan Digital Sky Survey (SDSS)-based cluster catalogs shows that we match more
than 50% of the clusters (77% in the case of the redMaPPer catalog). We also
cross-matched our cluster catalog with the Abell clusters, and clusters found
by XMM and in the Planck-SZ survey; however, only a small number of them lie
inside the KiDS area currently available.Comment: 13 pages, 15 figures. Accepted for publication on Astronomy &
Astrophysic
The Santa Fe Light Cone Simulation Project: I. Confusion and the WHIM in Upcoming Sunyaev-Zel'dovich Effect Surveys
We present the first results from a new generation of simulated large sky
coverage (~100 square degrees) Sunyaev-Zeldovich effect (SZE) cluster surveys
using the cosmological adaptive mesh refinement N-body/hydro code Enzo. We have
simulated a very large (512^3h^{-3}Mpc^3) volume with unprecedented dynamic
range. We have generated simulated light cones to match the resolution and
sensitivity of current and future SZE instruments. Unlike many previous studies
of this type, our simulation includes unbound gas, where an appreciable
fraction of the baryons in the universe reside.
We have found that cluster line-of-sight overlap may be a significant issue
in upcoming single-dish SZE surveys. Smaller beam surveys (~1 arcmin) have more
than one massive cluster within a beam diameter 5-10% of the time, and a larger
beam experiment like Planck has multiple clusters per beam 60% of the time. We
explore the contribution of unresolved halos and unbound gas to the SZE
signature at the maximum decrement. We find that there is a contribution from
gas outside clusters of ~16% per object on average for upcoming surveys. This
adds both bias and scatter to the deduced value of the integrated SZE, adding
difficulty in accurately calibrating a cluster Y-M relationship.
Finally, we find that in images where objects with M > 5x10^{13} M_{\odot}
have had their SZE signatures removed, roughly a third of the total SZE flux
still remains. This gas exists at least partially in the Warm Hot Intergalactic
Medium (WHIM), and will possibly be detectable with the upcoming generation of
SZE surveys.Comment: 14 pages, 13 figures, version accepted to ApJ. Major revisions mad
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