Warm-hot Intergalactic Medium Research Articles

The Dark Matter filament between Abell 222/223

AbstractWeak lensing detections and measurements of filaments have been elusive for a long time. The reason is that the low density contrast of filaments generally pushes the weak lensing signal to unobservably low scales. To nevertheless map the dark matter in filaments exquisite data and unusual systems are necessary. SuprimeCam observations of the supercluster system Abell 222/223 provided the required combination of excellent seeing images and a fortuitous alignment of the filament with the line-of-sight. This boosted the lensing signal to a detectable level and led to the first weak lensing mass measurement of a large-scale structure filament. The filament connecting Abell 222 and Abell 223 is now the only one traced by the galaxy distribution, dark matter, and X-ray emission from the hottest phase of the warm-hot intergalactic medium. The combination of these data allows us to put the first constraints on the hot gas fraction in filaments.

SHOCK WAVES AND COSMIC RAY ACCELERATION IN THE OUTSKIRTS OF GALAXY CLUSTERS

The outskirts of galaxy clusters are continuously disturbed by mergers and gas infall along filaments, which in turn induce turbulent flow motions and shock waves. We examine the properties of shocks that form within $r_{200}$ in sample galaxy clusters from structure formation simulations. While most of these shocks are weak and inefficient accelerators of cosmic rays (CRs), there are a number of strong, energetic shocks which can produce large amounts of CR protons via diffusive shock acceleration. We show that the energetic shocks reside mostly in the outskirts and a substantial fraction of them are induced by infall of the warm-hot intergalactic medium from filaments. As a result, the radial profile of the CR pressure in the intracluster medium is expected to be broad, dropping off more slowly than that of the gas pressure, and might be even temporarily inverted, peaking in the outskirts. The volume-integrated momentum spectrum of CR protons inside $r_{200}$ has the power-law slope of $4.25 - 4.5$, indicating that the average Mach number of the shocks of main CR production is in the range of $\left< M_s \right>_{\rm CR} \approx 3 - 4$. We suggest that some radio relics with relatively flat radio spectrum could be explained by primary electrons accelerated by energetic infall shocks with $M_s >~ 3$ induced in the cluster outskirts.

X-RAY ABSORPTION BY THE WARM-HOT INTERGALACTIC MEDIUM IN THE HERCULES SUPERCLUSTER

The "missing baryons", in the form of warm-hot intergalactic medium (WHIM), are expected to reside in cosmic filamentary structures that can be traced by signposts such as large-scale galaxy superstructures. The clear detection of an X-ray absorption line in the Sculptor Wall demonstrated the success of using galaxy superstructures as a signpost to search for the WHIM. Here we present an XMM-Newton Reflection Grating Spectrometer (RGS) observation of the blazar Mkn 501, located in the Hercules Supercluster. We detected an O VII K$\alpha$ absorption line at the 98.7 % level ($2.5\sigma$) at the redshift of the foreground Hercules Supercluster. The derived properties of the absorber are consistent with theoretical expectations of the WHIM. We discuss the implication of our detection for the search for the "missing baryons". While this detection shows again that using signposts is a very effective strategy to search for the WHIM, follow-up observations are crucial both to strengthen the statistical significance of the detection and to rule out other interpretations. A local, $z\sim0$ O VII K$\alpha$ absorption line was also clearly detected at the $4\sigma$ level, and we discuss its implications for our understanding of the hot gas content of our Galaxy.

Constraints on the Sunyaev–Zel'dovich signal from the warm–hot intergalactic medium from WMAP and SPT data

The fraction of ionized gas in the Warm Hot Intergalactic Medium induces temperature anisotropies on the Cosmic Microwave Background similar to those of clusters of galaxies. The Sunyaev-Zel'dovich anisotropies due to these low density, weakly non-linear, baryon filaments can not be distinguished from that of clusters using frequency information, but they can be separated since their angular scales are very different. To determine the relative contribution of the WHIM SZ signal to the radiation power spectrum of temperature anisotropies, we explore the parameter space of the concordance LCDM model using Monte Carlo Markov Chains and the Wilkinson Microwave Anisotropy Probe 7yr and South Pole Telescope data. We find marginal evidence of a contribution by diffuse gas, with amplitudes of A_WHIM=10-20 muK^2, but the results are also compatible with a null contribution from the WHIM, allowing to set an upper limit of A_WHIM < 43 muK^2 (95.4% C.L.). The signal produced by galaxy clusters remains at A_CL=4.5 muK^2, a value similar to what is obtained when no WHIM is included. From the measured WHIM amplitude we constrain the temperature-density phase diagram of the diffuse gas, and find it to be compatible with numerical simulations. The corresponding baryon fraction in the WHIM varies from 0.43 to 0.47, depending on model parameters. Planck data could set tighter constraints on the temperature-density relation.

Turbulence driven by structure formation in the circumgalactic medium

The injection of turbulence in the circum-galactic medium at redshift z = 2 is investigated using the mesh-based hydrodynamic code Enzo and a subgrid-scale (SGS) model for unresolved turbulence. Radiative cooling and heating by a uniform Ultraviolet (UV) background are included in our runs and compared with the effect of turbulence modelling. Mechanisms of gas exchange between galaxies and the surrounding medium, as well as metal enrichment, are not taken into account, and turbulence is here driven solely by structure formation (mergers and shocks). We find that turbulence, both at resolved and SGS scales, impacts mostly the warm-hot intergalactic medium (WHIM), with temperature between 10^5 and 10^7 K, mainly located around collapsed and shock heated structures, and in filaments. Typical values of the ratio of turbulent to thermal pressure is 0.1 in the WHIM, corresponding to a volume-weighted average of the SGS turbulent to thermal Doppler broadening b_t / b_therm = 0.26, on length scales below the grid resolution of 25 kpc/h. In the diffuse intergalactic medium (IGM), defined in a range of baryon overdensity \delta\ between 1 and 50, the importance of turbulence is smaller, but grows as a function of gas density, and the Doppler broadening ratio is fitted by the function b_t / b_therm = 0.023 \delta^{0.58}.

X‐ray study of clusters at the outer edge and beyond

AbstractWe report on recent Suzaku results on cluster outer regions. Relaxed clusters show a monotonous temperature decline to the virial radii, reaching 1/3 to 1/4 of the central levels. Significant temperature jumps have been confirmed for a number of radio relics, including A 3667, A 3376, and CIZA J2242.8+5301. This confirms that the radio relics generally correspond to shock fronts with the Mach numbers around 3. New filament junctions are identified by a new algorithm applied to the SDSS galaxy distribution and show significant X‐ray emission with Suzaku. The emission agrees with the LX‐kT relation for elliptical galaxies and galaxy groups, and the junctions can be a important baryon reservoir. We propose a small X‐ray mission DIOS (Diffuse Intergalactic Oxygen Surveyor), equipped with microcalorimeters with a wide field of view (∼50′), which will detect the emission from warm‐hot intergalactic medium (WHIM) and measure detailed thermal and dynamical properties of the gas accreting onto clusters from the filaments. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Giant radio galaxies – I. Intergalactic barometers

We present new wideband radio observations with the Australia Telescope Compact Array of a sample of 12 giant radio galaxies. The radio observations are part of a larger radio-optical study aimed at relating the radio structures with the ambient medium on large scales. With projected linear sizes larger than 0.7 Mpc, these objects are ideal candidates for the study of the Warm-Hot Intergalactic Medium (WHIM). The sample includes sources with sizes spanning 0.8 to 3.2 Mpc and total powers of 1.2*10^24 to 4.0*10^26 W Hz^-1 at 2.1 GHz. Redshifts were limited to z<0.15 to permit spectroscopic observations of the hosts and neighbouring galaxies, which were obtained using the AAOmega spectrograph on the Anglo-Australian Telescope. We derive lobe energy densities from the radio observations via equipartition arguments. The inferred pressures in the lobes of the giant radio sources, which range from 1.1*10^-15 to 2.0*10^-14 Pa (80 to 1500 cm^-3 K), are lower than previously inferred from X-ray observations of dense filaments. Comparison with the OverWhelmingly Large Simulations (OWLS) suggests that the WHIM in pressure balance with the radio lobes has a temperature in excess of ~10^6.5 K or a particle overdensity in the range 50 to 500. This study highlights the capability of next generation surveys, such as the Evolutionary Map of the Universe (EMU) survey with the Australian Square Kilometre Array Pathfinder (ASKAP), to study populations of giant radio sources at lower surface brightness and thereby discriminate between models for the cosmological evolution of the intergalactic medium and examine the validity of cosmological hydrodynamical simulations.

X-ray absorption evolution in gamma-ray bursts: intergalactic medium or evolutionary signature of their host galaxies

The intrinsic X-ray emission of gamma-ray bursts (GRBs) is often found to be absorbed over and above the column density through our own Galaxy. The extra component is usually assumed to be due to absorbing gas lying within the host galaxy of the GRB itself. There is an apparent correlation between the equivalent column density of hydrogen, NH,intrinsic (assuming it to be at the GRB redshift), and redshift, z, with the few z > 6 GRBs showing the greatest intrinsic column densities. We investigate the NH, intrinsic-z relation using a large sample of Swift GRBs, as well as active galactic nuclei and quasar samples, paying particular attention to the spectral energy distributions of the two highest redshift GRBs. Various possible sample biases and systematics that might produce such a correlation are considered, and we conclude that the correlation is very likely to be real. This may indicate either an evolutionary effect in the host galaxy properties, or a contribution from gas along the line of sight, in the diffuse intergalactic medium (IGM) or intervening absorbing clouds. Employing a more realistic model for IGM absorption than in previous works, we find that this may explain much of the observed opacity at z ≳ 3 providing it is not too hot, likely between 105 and 106.5 K, and moderately metal enriched, Z ∼ 0.2 Z⊙. This material could therefore constitute the warm–hot intergalactic medium. However, a comparable level of absorption is also expected from the cumulative effect of intervening cold gas clouds, and given current uncertainties it is not possible to say which, if either, dominates. At lower redshifts, we conclude that gas in the host galaxies must be the dominant contributor to the observed X-ray absorption.

Motion induced second order temperature and y-type anisotropies after the subtraction of linear dipole in the CMB maps

y-type spectral distortions of the cosmic microwave background allow usto detect clusters and groups of galaxies, filaments of hot gas and thenon-uniformities in the warm hot intergalactic medium. Several CMBexperiments (on small areas of sky) and theoretical groups (for full sky)have recently published y-type distortion maps. We propose to search fortwo artificial hot spots in such y-type maps resulting from theincomplete subtraction of the effect of the motion induced dipole on the cosmic microwavebackground sky. This dipole introduces, at second order, additionaltemperature and y-distortion anisotropy on the sky of amplitude few μK which couldpotentially be measured by Planck HFI and Pixie experiments and can be used as a source of crosschannel calibration by CMB experiments. This y-type distortion is present inevery pixel and is not the result of averaging the whole sky. Thisdistortion, calculated exactly from the known linear dipole, can besubtracted from the final y-type maps, if desired.

The contribution of the warm-hot intergalactic medium to the cosmic microwave background anisotropies via the Sunyaev–Zeldovich effect

Cosmological hydrodynamical simulations predict that a large fraction of all baryons reside within mildly non-linear structures with temperatures in the range $10^{5}-10^{7}$K. As the gas is highly ionized, it could be detected by the temperature anisotropies generated on the Cosmic Microwave Background radiation. We refine our previous estimates of the thermal Sunyaev-Zeldovich effect by introducing a non-polytropic equation of state to model the temperature distribution of the shock heated gas derived from temperature-density phase diagrams of different hydrodynamical simulations. Depending on the specific model, the Comptonization parameter varies in the range $10^{-7}\le y_c \le 2\times 10^{-6}$, compatible with the FIRAS upper limit. This amplitude is in agreement with a simple toy model constructed to estimate the average effect induced by filaments of ionized gas. Using the log-normal probability density function we calculate the correlation function and the power spectrum of the temperature anisotropies generated by the WHIM filaments. For a wide range of the parameter space, the maximum amplitude of the radiation power spectrum is $(\ell+1)\ell C_{\ell}/2\pi =0.7-70(\mu K)^2$ at $\ell \approx 200-500$. This amplitude scales with baryon density, Hubble constant and the amplitude of the matter power spectrum $\sigma_8$ as $[(\ell+1)\ell C_\ell]_{\mathrm{max}}/2\pi \propto \sigma_8^{2.6}(\Omega_b h)^2$. Since the thermal Sunyaev-Zeldovich effect has a specific frequency dependence, we analyze the possibility of detecting this component with the forthcoming Planck data.

WARM-HOT GAS IN GROUPS AND GALAXIES TOWARD H2356-309

We present a detailed analysis of the galaxy and group distributions around three reported X-ray absorption line systems in the spectrum of the quasar H2356-309. Previous studies associated these absorbers with known large-scale galaxy structures (i.e., walls and filaments) along the line of sight. Such absorption lines typically trace 10^{5-7} K gas, and may be evidence of the elusive warm-hot intergalactic medium (WHIM) thought to harbor the bulk of the low-redshift "missing baryons;" alternatively, they may be linked to individual galaxies or groups in the filaments. Here we combine existing galaxy survey data with new, multi-object Magellan spectroscopy to investigate the detailed galaxy distribution near each absorber. All of these three absorption systems are within the projected virial radii of nearby galaxies and/or groups, and could therefore arise in these virialized structures rather than (or in addition to) the WHIM. However, we find no additional galaxies near a fourth "void" absorber recently found in the spectrum, suggesting that this system may indeed trace gas unassociated with any individual halo. Though the number of known systems is still small, spatial coincidences suggest that some X-ray absorbers lie in galaxy and/or group environments, though others could still trace the large-scale filamentary WHIM gas predicted by simulations.

Evolution of the baryon fraction in the Local Group: accretion versus feedback at low and highz

Using hydrodynamical zoom simulations in the standard LCDM cosmology, we investigate the evolution of the distribution of baryons (gas and stars) in a local group-type universe. First, with standard star formation and supernova feedback prescriptions, we find that the mean baryonic fraction value estimated at the virial radius of the two main central objects (i.e. the Milky Way and Andromeda) is decreasing over time, and is 10-15% lower than the universal value, 0.166, at z=0. This decrease is mainly due to the fact that the amount of accretion of dissipative gas onto the halo, especially at low redshift, is in general much lower than that of the dissipationless dark matter. Indeed, a significant part of the baryons does not collapse onto the haloes and remains in their outskirts, mainly in the form of warm-hot intergalactic medium (WHIM). Moreover, during the formation of each object, some dark matter and baryons are also expelled through merger events via tidal disruption. In contrast to baryons, expelled dark matter can be more efficiently re-accreted onto the halo, enhancing both the reduction of fb inside Rv, and the increase of the mass of WHIM outside Rv. Varying the efficiency of supernovae feedback at low redshift does not seem to significantly affect these trends. Alternatively, when a significant fraction of the initial gas in the main objects is released at high redshifts by more powerful sources of feedback, such as AGN from intermediate mass black holes in lower mass galaxies, the baryonic fraction at the virial radius can have a lower value (fb~0.12) at low redshift. Hence physical mechanisms able to drive the gas out of the virial radius at high redshifts will have a stronger impact on the deficit of baryons in the mass budget of Milky Way type-galaxies at present times than those that expel the gas in the longer, late phases of galaxy formation.

THE BARYON CENSUS IN A MULTIPHASE INTERGALACTIC MEDIUM: 30% OF THE BARYONS MAY STILL BE MISSING

For low-redshift cosmology and galaxy formation rates, it is important to account for all the baryons synthesized in the Big Bang. Although galaxies and clusters contain 10% of the baryons, many more reside in the photoionized Lyman-alpha forest and shocked-heated warm-hot intergalactic medium (WHIM) at T = 10^5 to 10^7 K. Current tracers of WHIM at 10^5 to 10^6 K include the O VI 1032, 1038 absorption lines, together with broad Lyman-alpha absorbers (BLAs) and EUV/X-ray absorption lines from Ne VIII, O VII, and O VIII. We improve the O VI baryon surveys with corrections for oxygen metallicity (Z/Zsun) and O VI ionization fraction (f_OVI) using cosmological simulations of heating, cooling, and metal transport in a density-temperature structured medium. Statistically, their product correlates with column density, (Z/Zsun)(f_OVI) = (0.015)(N_OVI/10^{14} cm^-2)^0.70. The N_OVI-weighted mean is 0.01, which doubles previous estimates of WHIM baryon content. We also reanalyze H I data from the Hubble Space Telescope, applying redshift corrections for absorber density, photoionizing background, and proper length, dl/dz. We find substantial baryon fractions in the photoionized Lya forest (28 +/- 11%), O VI/BLA-traced WHIM (25 +/- 8%), and collapsed phase (18 +/- 4%) in galaxies, groups, clusters, and circumgalactic gas. The baryon shortfall is 29 +/- 13%, which may be detected in X-ray absorbers from hotter WHIM or in weaker Lya and O VI absorbers. Further progress will require higher-precision baryon surveys of weak absorbers at column densities N_HI > 10^{12.0} cm^-2, N_OVI > 10^{12.5} cm^-2, and N_OVII > 10^{14.5} cm^-2, with moderate-resolution UV and X-ray spectrographs.

Absorption signatures of warm-hot gas at low redshift: broad H i Lyα absorbers

We investigate the physical state of HI absorbing gas at low redshift (z = 0.25) using a subset of cosmological, hydrodynamic simulations from the OWLS project, focusing in particular on broad (b_HI > 40 km/s) HI Lyman-Alpha absorbers (BLAs), which are believed to originate in shock-heated gas in the warm-hot intergalactic medium (WHIM). Our fiducial model, which includes radiative cooling by heavy elements and feedback by supernovae and active galactic nuclei, predicts that by z = 0.25 nearly 60 per cent of the gas mass ends up at densities and temperatures characteristic of the WHIM and we find that half of this fraction is due to outflows. The standard HI observables (distribution of HI column densities N_HI, distribution of Doppler parameters b_HI, b_HI - N_HI correlation) and the BLA line number density predicted by our simulations are in remarkably good agreement with observations. BLAs arise in gas that is hotter, more highly ionised and more enriched than the gas giving rise to typical Lyman-Alpha forest absorbers. The majority of the BLAs arise in warm-hot (log (T/K) ~ 5) gas at low (log Delta < 1.5) overdensities. On average, thermal broadening accounts for at least 60 per cent of the BLA line width, which in turn can be used as a rough indicator of the thermal state of the gas. Detectable BLAs account for only a small fraction of the true baryon content of the WHIM at low redshift. In order to detect the bulk of the mass in this gas phase, a sensitivity at least one order of magnitude better than achieved by current ultraviolet spectrographs is required. We argue that BLAs mostly trace gas that has been shock-heated and enriched by outflows and that they therefore provide an important window on a poorly understood feedback process.

A filament of dark matter between two clusters of galaxies

It is a firm prediction of the concordance cold-dark-matter cosmological model that galaxy clusters occur at the intersection of large-scale structure filaments. The thread-like structure of this 'cosmic web' has been traced by galaxy redshift surveys for decades. More recently, the warm–hot intergalactic medium (a sparse plasma with temperatures of 10(5) kelvin to 10(7) kelvin) residing in low-redshift filaments has been observed in emission and absorption. However, a reliable direct detection of the underlying dark-matter skeleton, which should contain more than half of all matter, has remained elusive, because earlier candidates for such detections were either falsified or suffered from low signal-to-noise ratios and unphysical misalignments of dark and luminous matter. Here we report the detection of a dark-matter filament connecting the two main components of the Abell 222/223 supercluster system from its weak gravitational lensing signal, both in a non-parametric mass reconstruction and in parametric model fits. This filament is coincident with an overdensity of galaxies and diffuse, soft-X-ray emission, and contributes a mass comparable to that of an additional galaxy cluster to the total mass of the supercluster. By combining this result with X-ray observations, we can place an upper limit of 0.09 on the hot gas fraction (the mass of X-ray-emitting gas divided by the total mass) in the filament.

AN X-RAY WHIM METAL ABSORBER FROM A Mpc-SCALE EMPTY REGION OF SPACE

We report a detection of an absorption line at ~44.8 {\AA} in a > 500 ks Chandra HRC-S/LETG X-ray grating spectrum of the blazar H 2356-309. This line can be identified as intervening CV-K{\alpha} absorption, at z\approx0.112, produced by a warm (log T = 5.1 K) intergalactic absorber. The feature is significant at a 2.9{\sigma} level (accounting for the number of independent redshift trials). We estimate an equivalent hydrogen column density of log N_H=19.05 (Z/Zsun)^-1 cm^-2. Unlike other previously reported FUV/X-ray metal detections of warm-hot intergalactic medium (WHIM), this CV absorber lies in a region with locally low galaxy density, at ~2.2 Mpc from the closest galaxy at that redshift, and therefore is unlikely to be associated with an extended galactic halo. We instead tentatively identify this absorber with an intervening Warm-Hot Intergalactic Medium filament possibly permeating a large-scale, 30 Mpc extended, structure of galaxies whose redshift centroid, within a cylinder of 7.5 Mpc radius centered on the line of sight to H 2356-309, is marginally consistent (at a 1.8{\sigma} level) with the redshift of the absorber.

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.

COINCIDENCES BETWEEN O VI AND O VII LINES: INSIGHTS FROM HIGH-RESOLUTION SIMULATIONS OF THE WARM-HOT INTERGALACTIC MEDIUM

With high resolution (0.46kpc/h), adaptive mesh-refinement Eulerian cosmological hydrodynamic simulations we compute properties of O VI and O VII absorbers from the warm-hot intergalactic medium (WHIM). Our new simulations are in broad agreement with previous simulations, with ~40% of the intergalactic medium being in the WHIM at z=0. It is found (1) The amount of gas in the WHIM at temperature below and above 10^6K is about equal within uncertainties. (1) Our simulations are in excellent agreement with observed properties of O VI absorbers, with respect to the line incidence rate and Doppler width-column density relation. (2) Velocity structures within absorbing regions are a significant, and for large Doppler width clouds, a dominant contributor to the Doppler widths of both O VI and O VII absorbers. A non-negligible fraction (in number and mass) of O VI and O VII clouds can arise from gas of temperature lower than 10^5, until the Doppler width is well in excess of 100km/s. (3) Strong O VI absorbers are predominantly collisionally ionized. About (61%, 57%, 39%) of O VI absorbers in the column density ranges of log N(OVI) cm^2=(12.5-13,13-14,>14) have temperature lower than 10^5K. (4) Quantitative prediction is made for the presence of broad and shallow O VI lines, which current observations may have largely missed. Upcoming observations by COS may be able to provide a test. (5) The reported 3 sigma upper limit on the mean column density of coincidental O VII lines at the location of detected O VI lines by Yao et al is above the predicted value by a factor of 2.5-4. (6) The claimed observational detection of O VII lines by Nicastro et al, if true, is 2 sigma above what our simulations predict.

Filaments and sheets of the warm-hot intergalactic medium

Filaments, forming in the context of cosmological structure formation, are not only supposed to host the majority of the baryons at low redshifts in the form of the WHIM, but also to supply forming galaxies at higher redshifts with a substantial amount of cold gas via cold steams. In order to get insight into the hydro- and thermodynamical characteristics of these structures, we performed a series of hydrodynamical simulations. Instead of analyzing extensive simulations of cosmological structure formation, we simulate certain well-defined structures and study the impact of different physical processes as well as of the scale dependencies. In this paper, we continue our work from Klar & M\"ucket (2010), and extend our simulations into three dimensions. Instead of a pancake structure, we now obtain a configuration consisting of well-defined sheets, filaments, and a gaseous halo. We use a set of simulations, parametrized by the length of the initial perturbation L, to obtain detailed information on the state of the gas and its evolution inside the filament. For L > 4 Mpc, we obtain filaments which are fully confined by an accretion shock. Additionally, they exhibit an isothermal core, which temperature is balanced by radiative cooling and heating due to the UV background. This indicates on a multiphase structure for the medium temperature WHIM. We obtain scaling relations for the main quantities of this core. In the vicinity of the halo, the filament's core can be attributed to the cold streams found in cosmological hydro-simulations. Thermal conduction can lead to a complete evaporation of the cold stream for L > 6 Mpc/h. This corresponds to halos more massive than M_halo = 10^13 M_Sun, and implies that star-formation in more massive galaxies can not be supplied by cold streams. For perturbations on scales L > 6 Mpc/h the filament does not longer exhibit a cold core.

The polytropic approximation and X-ray scaling relations: constraints on gas and dark matter profiles for galaxy groups and clusters

We constrain gas and dark matter (DM) parameters of galaxy groups and clusters, by comparing X-ray scaling relations to theoretical expectations, obtained assuming that the gas is in hydrostatic equilibrium with the DM and follows a polytropic relation. We vary four parameters: the gas polytropic index Gamma, its temperature at large radii T_xi, the DM logarithmic slope at large radii zeta and its concentration c_vir. When comparing the model to the observed mass-temperature (M-T) relation of local clusters, our results are independent of both T_xi and c_vir. We thus obtain constraints on Gamma, by fixing the DM profile, and on zeta, by fixing the gas profile. For an NFW DM profile, we find that 6/5<Gamma<13/10, which is consistent with numerical simulations and observations of individual clusters. Taking 6/5<Gamma<13/10 allows the DM profile to be slightly steeper than the NFW profile at large radii. Upon including local groups, we constrain the mass-dependence of Gamma and the value of T_xi. Interestingly, with Gamma=6/5 and zeta=-3, we reproduce the observed steepening/breaking of the M-T relation at low M, if 10^6 K<T_xi<10^7 K, consistent with simulations and observations of the warm-hot intergalactic medium. When extrapolated to high redshift z, the model with a constant Gamma reproduces the expected self-similar behaviour. We also account for the observed, non-self-similar relations provided by some high-z clusters, as they provide constraints on the evolution of Gamma. Comparing our model to the observed luminosity-temperature relation, we discriminate between different M-c_vir relations: a weak dependence of c_vir on M is currently preferred by data. This simple theoretical model accounts for much of the complexity of recent, improved X-ray scaling relations, provided that we allow for a mild dependence of Gamma on M or for T_xi consistent with intercluster values. [abridged]