The amount of detected baryons in the local Universe is at least a factor of two smaller than measured at high redshift. It is believed that a significant fraction of the baryons in the current Universe is "hiding" in a hot filamentary structure filling the intergalactic space, the Warm-Hot Intergalactic Medium ($WHIM$). We found evidence of the missing baryons in the $WHIM$ by detecting their signature on the angular correlation of diffuse X-ray emission with the XMM-Newton satellite. Our result indicates that $(12\pm 5)$% of the total diffuse X-ray emission in the energy range 0.4-0.6 keV is due to intergalactic filaments. The statistical significance of our detection is several sigmas ($χ^2>136$ N=19). The error bar in the X-ray flux is dominated, instead, by cosmic variation and model uncertainties.
The global missing baryon problem - that the sum of observed baryons falls short of the number expected form BBN - is well known. In addition to this, there is also a local missing baryon problem that applies to individual dark matter halos. This halo by halo missing baryon problem is such that the observed mass fraction of baryons in individual galaxies falls short of the cosmic baryon fraction. This deficit is a strong function of circular velocity. I give an empirical estimate of this function, and note the presence of a critical scale of ~900 km/s therein. I also briefly review Omega_b fro…
About 90% of baryons in the universe have thus far escaped direct observation. This is known as the {\it missing baryon problem}. The Sunyaev Zel'dovich effect (SZ effect) has the potential to directly measure the state of the majority of these missing baryons. The next generation CMB experiments such as AMIBA will provide an unbiased sample of the intergalactic medium through the SZ effect. The existing and upcoming simulations and analytical studies provide a quantitative understanding of the SZ effect. All these make the SZ effect a powerful baryon probe. We present an overview of this prob…
Stars and gas in galaxies, hot intracluster medium, and intergalactic photo-ionized gas make up at most half of the baryons that are expected to be present in the universe. The majority of baryons are still missing and are expected to be hidden in a web of warm-hot intergalactic medium. This matter was shock-heated during the collapse of density perturbations that led to the formation of the relaxed structures that we see today. Finding the missing baryons and thereby producing a complete inventory of possibly the only detectable component of the energy-mass budget of the universe is crucial t…
Approximately 30 to 50 percent of the total baryons in the present universe is supposed to take a form of warm/hot intergalactic medium (WHIM) whose X-ray continuum emission is very weak. In order to carry out a direct and homogeneous survey of elusive cosmic missing baryons, we propose a dedicated soft-X-ray mission, {\it DIOS} (Diffuse Intergalactic Oxygen Surveyor). The unprecedented energy resolution ($\sim 2$eV) of the XSA (X-ray Spectrometer Array) on-board {\it DIOS} enables us to identify WHIM with gas temperature $T=10^{6-7}$K and overdensity $δ=10-100$ located at $z<0.3$ through emis…
The cluster baryon fraction is estimated from the CMB-scattering leptonic component of the intracluster medium (ICM); however, the observed cluster baryon fraction is less than the cosmic one. Understanding the origin of this discrepancy is necessary for correctly describing the structure of the ICM. We estimate the baryonic mass in the outskirts of galaxy clusters which is difficult to observe because of low electron temperature and density in these regions. The time scale for the electrons and protons to reach equipartition in the outskirts is longer than the cluster age. Since thermal equil…
Recent Cosmological measurements indicate that baryons comprise about four percent of the total mass-energy density of the Universe, which is in accord with the predictions arising from studies of the production of the lightest elements. It also is in agreement with the actual number of baryons detected at early times (redshifts>2). However, close to our own epoch (z<2), the number of baryons actually detected add up to just over half (~55 percent) of the number seen at z>2, meaning that about 45 percent are 'missing'. Here we report a determination of the mass-density of a previously undetect…
The recent constraints on the cosmological parameters put from the observations of the WMAP satellite limit the cosmic baryon fraction in a range that is larger than, and marginally consistent with, what is measured in galaxy clusters. This rises the question whether or not we are considering all the ingredients of cluster baryonic budget. Making a careful weight of the baryons in X-ray emitting plasma and stars in cluster galaxies, I conclude that the cluster baryonic pie is made by 13 (with a 1 sigma range of 8-19) per cent of stars, 70 (56-89) per cent of intracluster hot medium and 17 (0-3…
At low redshift (z<2), almost half of the baryons in the Universe are not found in bound structures like galaxies and clusters and therefore most likely reside in a Warm-Hot Intergalactic Medium (WHIM), as predicted by simulations. Attempts to detect WHIM filaments at cosmological distances in absorption towards bright background sources have yielded controversial results that I review here. I argue that a secure detection of absorption features by the WHIM is at the limit of the XMM-Newton capabilities, but feasible. A proper characterisation of the whole WHIM belongs to the realm of future X…
Deep optical and near-infrared galaxy counts are utilized to estimate the extragalactic background light (EBL) coming from normal galactic light in the universe. Although the slope of number-magnitude relation of the faintest counts is flat enough for the count integration to converge, considerable fraction of EBL from galaxies could still have been missed in deep galaxy surveys because of various selection effects including the cosmological dimming of surface brightness of galaxies. Here we give an estimate of EBL from galaxy counts, in which these selection effects are quantitatively taken i…
From clusters to groups of galaxies, the powerful bremsstrahlung radiation L_X emitted in X rays by the intracluster plasma is observed to decline sharply with lowering virial temperatures T (i.e., at shallower depths of the gravitational wells) after a steep local L_X-T correlation; this implies increasing scarcity of diffuse baryons relative to dark matter, well under the cosmic fraction. We show how the widely debated issue concerning these `missing baryons' is solved in terms of the thermal and/or dynamical effects of the kinetic (at low redshifts z) and radiative (at high z) energy inputs…
Several popular cosmological models predict that most of the baryonic mass in the local universe is located in filamentary and sheet-like structures associated with galaxy overdensities. This gas is expected to be gravitationally heated to ~10^6 K and therefore emitting in the soft X-rays. We have detected diffuse soft X-ray structures in a high Galactic latitude ROSAT field after point source subtraction and correction for Galactic absorption. These diffuse structures have an X-ray energy distribution that is much softer than expected from clusters, groups or unresolved emission from AGNs, bu…
At low redshift, only about one-tenth of the known baryons lie in galaxies or the hot gas seen in galaxy clusters and groups. Models posit that these "missing baryons" are in gaseous form in overdense filaments that connect the much denser virialized groups and clusters. About 30% are cool (<1E5 K) and are detected in Ly alpha absorption studies, but about half is predicted to lie in the 1E5-1E7 K regime. Gas is detected in the 2-5E5 K range through OVI absorption studies (7% of the baryons) and possibly near 1E5 K from broad Ly absorption (20% of the baryons). Hotter gas (0.5-2E6 K) is detect…
We discuss physical properties and the baryonic content of the Warm-hot Intergalactic Medium (WHIM) at low redshifts. Cosmological simulations predict that the WHIM contains a large fraction of the baryons at z=0 in the form of highly-ionized gas at temperatures between 10^5 and 10^7 K. Using high-resolution ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph (STIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) we have studied the WHIM at low redshifts by searching for intervening OVI and thermally broadened Lyman alpha (BL) absorption toward a number of quasars …
We review the current high-significance X-ray detections of Warm-Hot Intergalactic Medium (WHIM) filaments at z>0 along the lines of sight to the two blazars Mrk 421 (z=0.03) and 1ES 1028+511 (z=0.361). For these WHIM filaments, we derive ionization corrections and, when possible, metallicity estimates. This allows us to obtain refined estimates of the number density of O VII WHIM systems down to the O VII column density sensitivity of our observations, and most importantly, a measurement of the cosmological mass density Omega_b^{WHIM} in the WHIM, at redshift z<0.361. These estimates agree we…
To predict the X-ray observables associated to the diffuse baryons in clusters of galaxies, we develop a new physical approach to model such a hot intra-cluster plasma. Such approach is based on punctuated equilibria. and comprises the following blocks: Monte Carlo ``merging histories'' of dark matter potential wells; the central hydrostatic disposition for the ICP, reset to a new equilibrium after each merging episode; conditions of shock, or of closely adiabatic compression at the boundary with the external gas, preheated by stellar energy feedbacks. We predict the L-T relation, consistent w…
The existence of macroscopic regions with antibaryon excess in the baryon asymmetric Universe with general baryon excess is the possible consequence of practically all models of baryosynthesis. Diffusion of matter and antimatter to the border of antimatter domains defines the minimal scale of the antimatter domains surviving to the present time. A model of diffused antiworld is considered, in which the density within the surviving antimatter domains is too low to form gravitationally bound objects. The possibility to test this model by measurements of cosmic gamma ray fluxes is discussed. The …
The thermodynamics of the diffuse, X-ray emitting gas in clusters and groups of galaxies are affected by a certain amount of non-gravitational energy input, as indicated by the scaling properties of X-ray halos. Such a view has been recently confirmed by the detection of an excess entropy in the center of groups. It is not easy, however, to identify unambiguously the source and the epoch of such excess entropy. Here we show that the observations of high z clusters will help in reconstructing both the dynamic and the thermodynamic history of the diffuse cosmic baryons.
Most of the baryons in the present-day universe are thought to reside in intergalactic space at temperatures of 10^5-10^7 K. X-ray emission from these baryons contributes a modest (~10%) fraction of the ~ 1 keV background whose prominence within the large-scale cosmic web depends on the amount of non-gravitational energy injected into intergalactic space by supernovae and AGNs. Here we show that the virialized regions of groups and clusters cover over a third of the sky, creating a source-confusion problem that may hinder X-ray searches for individual intercluster filaments and contaminate obs…
We briefly review the use of UV absorption lines in the spectra of low-redshift QSOs for the study of the physical conditions, metallicity, and baryonic content of the low-z IGM, with emphasis on the missing baryons problem. Current results on the statistics and baryonic content of intervening, low-z O VI and Lya absorption-line systems are presented with some comments on overlap between these two classes of absorbers and consequent baryon double-counting problems. From observations of a sample of 16 QSOs observed with the E140M echelle mode of STIS, we find 44 intervening O VI absorbers and 1…
We present an analysis of the Lyman alpha forests of five quasar spectra in the near UV. Properties of the intergalactic medium (IGM) at an intermediate redshift interval (0.9 < z < 1.9) are studied. The amount of baryons in the diffuse photoionised IGM and the warm-hot intergalactic medium (WHIM) are traced to get constraints on the redshift evolution of the different phases of the intergalactic gas. The baryon density of the diffuse IGM is determined with photoionisation calculations under the assumption of local hydrostatic equilibrium. We assume that the gas is ionised by a metagalactic ba…
The number of detected baryons in the low-redshift Universe (z < 1) is far small er than the corresponding number of baryons observed at higher redshift. According to hydrodynamical simulations for the formation of structure in the Universe, up to 2/3 of the 'missing baryons' may have escaped identification so far, due to their high temperature and low density. One of the few ways to detect this matter is to look for its UV absorption signature on the light from background sources. Here we show that the amplitude of the average velocity vector of 'high velocity' OVI absorbers detected in a UV …
The X-ray background intensity around Lick count galaxies and rich clusters of galaxies is investigated in three ROSAT energy bands. It is found that the X-ray enhancements surrounding concentrations of galaxies exhibit significantly softer spectrum than the standard cluster emission and the average extragalactic background. The diffuse soft emission accompanying the galaxies is consistent with the thermal emission of the hot gas postulated first by the Cen & Ostriker hydrodynamic simulations. Our estimates of the gas temperature - although subject to large uncertainties - averaged over severa…
Based on 19 high-resolution N-body/gas-dynamical galaxy formation simulations in the LCDM cosmology it is shown, that for a galaxy like the Milky Way, in addition to the baryonic mass of the galaxy itself, about 70% extra baryonic mass should reside around the galaxy (inside of the virial radius), chiefly in the form of hot gas. Averaging over the entire field galaxy population, this ``external'' component amounts to 64-85% of the baryonic mass of the population itself. These results are supported by the recent detection of very extended, soft X-ray emission from the halo of the quiescent, mas…
Nucleosynthetic signatures in common between the gas responsible for the high redshift Lyman alpha forest and a subsample of extremely metal poor stars are found. A simple mass loss model of chemical evolution with physically motivated parameters provides a consistent picture in which the gas is identified with that lost by supernova-driven winds during the first generation of star formation. Substantial mass loss occurs which can account for a diffuse IGM with up to 80% of the total baryon content and a peak [C-O/H] abundance of \~-2.9. This mass loss component differs from one produced later…
If the peculiar motion of galaxy groups and clusters indeed resembles that of the surrounding baryons, then the kinetic Sunyaev-Zel'dovich (kSZ) pattern of those massive halos should be closely correlated to the kSZ pattern of all surrounding electrons. Likewise, it should also be correlated to the CMB E-mode polarization field generated via Thomson scattering after reionization. We explore the cross-correlation of the kSZ generated in groups and clusters to the all sky E-mode polarization in the context of upcoming CMB experiments like Planck, ACT, SPT or APEX. We find that this cross-correla…
If gamma ray bursts produce a total energy of 10^54 ergs and this energy is concentrated on the high energy tail of the spectrum E> 1 TeV, then they may account for the observed diffuse extragalactic gamma ray emission for energies > 100 MeV. Such an energy could be released if the GRB's are produced by the burning of the total mass of a neutron star in a phase transition which violates baryon number.
$Λ$CDM numerical simulations predict that the "missing baryons" reside in a Warm-Hot gas phase in the over-dense cosmic filaments. However, there are now several theoretical and observational arguments that support the fact that galactic disks may be more massive than usually thought, containing a substantial fraction of the "missing baryons". Hereafter, we present new N-body simulations of galactic disks, where the gas content has been multiplied by a factor 5. The stability of the disk is ensured by assuming that the ISM is composed out of two partially coupled phases, a warm phase, correspo…
Approximately 30-40% of all baryons in the present day universe reside in a warm-hot intergalactic medium (WHIM), with temperatures between 10^5<T<10^7 K. This is a generic prediction from six hydrodynamic simulations of currently favored structure formation models having a wide variety of numerical methods, input physics, volumes, and spatial resolutions. Most of these warm-hot baryons reside in diffuse large-scale structures with a median overdensity around 10-30, not in virialized objects such as galaxy groups or galactic halos. The evolution of the WHIM is primarily driven by shock heating…
We study the soft X-ray emission (0.1 - 1 keV) from the Warm-Hot Intergalactic Medium (WHIM) in a hydrodynamic simulation of a Cold Dark Matter universe. Our main goal is to investigate how such emission can be explored with a combination of imaging and spectroscopy, and to motivate future X-ray missions. We first present high resolution images of the X-ray emission in several energy bands, in which emission from different ion species dominates. We pick three different areas to study the high resolution spectra of X-rays from the warm-hot IGM: (a) a galaxy group; (b) a filament and (c) an unde…
We present predictions for the cosmic metal budget in various phases of baryons from redshift z=6-0, taken from a cosmological hydrodynamic simulation that includes a well-constrained model for enriched galactic outflows. We find that substantial amounts of metals are found in every baryonic phase at all epochs, with diffuse intergalactic gas dominating the metal budget at early epochs and stars and halo gas dominating at recent epochs. We provide a full accounting of metals in the context of the missing metals problem at z~2.5, showing that ~40% of the metals are in galaxies, and the remainde…
Most of the baryons in the local universe are ``missing'' in that they are not in galaxies or in the previously detected gaseous phases. These missing baryons are predicted to be in a moderately hot phase, 10^5-10^7 K, largely in the form of giant cosmic filaments that connect the denser virialized clusters and groups of galaxies. Models show that the highest covering fraction of such filaments occurs in superclusters. To determine whether such filaments exist, we have begun a project to search for UV absorption against AGNs projected behind possible supercluster filaments. Using data from the…
It is proposed that when the era of the big-bang nucleosynthesis ended, almost all of the 75 percent of the observed total baryonic matter remained in the form of hydrogen and continued to exist in the form of protons and electrons. They are present today as baryonic dark matter in the form of intergalactic hydrogen plasma. To test our hypothesis we have investigated the effects of Thomson scattering by free electrons on the reported dimming of Type Ia supernovae. The quantitative results of our calculation suggest that the dimming of these supernovae, which are dimmer than expected and hence …
Using cosmological hydrodynamical simulations, we investigate the prospects of the thermal Sunyaev-Zel'dovich (tSZ) effect to detect the missing baryons in the local universe. We find that at least 80% of the tSZ luminosity is generated in collapsed structures, and that $\sim$ 70% of the remaining diffuse tSZ luminosity (i.e., $\sim 15$% of the total) comes from overdense regions with $δ_{gas}>$10, such as filaments and superclusters. The gas present in slightly overdense and underdense regions with $δ_{gas} < 10$, despite making up 50% of the total baryon budget, leaves very little tSZ signat…
Cosmological simulations predict that a large fraction of the baryonic mass of the Universe exists as 10^5-10^7 K diffuse, X-ray emitting gas, tracing low density filament and sheet-like structures exterior to massive clusters of galaxies. If present, this gas helps reconcile the current shortfall in observed baryon counts relative to the predictions of the standard Big Bang model. We present here the discovery and analysis of a 5-sigma significance half-degree filamentary structure, present in both I-band galaxy surface density and unresolved X-ray emission in a deep ROSAT PSPC field. The est…
The standard treatment of cooling in Cold Dark Matter halos assumes that all of the gas within a ``cooling radius'' cools and contracts monolithically to fuel galaxy formation. Here we take into account the expectation that the hot gas in galactic halos is thermally unstable and prone to fragmentation during cooling and show that the implications are more far-reaching than previously expected: allowing multi-phase cooling fundamentally alters expectations about gas infall in halos and naturally explains the bright-end cutoff in the galaxy luminosity function. We argue that cooling should proce…
The Sunyaev-Zel'dovich (SZ) effect is a direct probe of thermal energy content of the Universe, induced in the cosmic microwave background (CMB) sky through scattering of CMB photons off hot electrons in the intracluster medium (ICM). We report a 9-sigma detection of the SZ signal in the CMB maps of Wilkinson Microwave Anisotropy Probe (WMAP) 3yr data, through study of a sample of 193 massive galaxy clusters with observed X-ray temperatures greater than 3 keV. For the first time, we make a model-independent measurement of the pressure profile in the outskirts of the ICM, and show that it close…
Recently, Dixon et al. have re-analyzed the EGRET data, finding a statistically significant diffuse $γ$-ray emission from the galactic halo. We show that this emission can naturally be explained within a previously-proposed model for baryonic dark matter, in which $γ$-rays are produced through the interaction of high-energy cosmic-ray protons with cold $H_2$ clouds clumped into dark clusters - these dark clusters supposedly populate the outer galactic halo and can show up in microlensing observations. Our estimate for the halo $γ$-ray flux turns out to be in remarkably good agreement with the …
At low redshifts, measurements of the total baryon content in stars, atomic and molecular hydrogen, and cluster gas fall a factor of two to four below the baryon density derived from observed light-element ratios and nucleosynthesis arguments. A possible hiding place for a significant fraction of the missing baryons is in the warm/hot gas at temperatures T = 10^{5}-10^{7}K. We present predictions of the contribution to the soft X-ray background from warm/hot gas emission calculated using new hydrodynamical simulations and discuss the possibility of detecting the spectral signature of this gas …
How structures on various scales formed and evolved from the early Universe up to present time is a fundamental question of astrophysical cosmology. EDGE will trace the cosmic history of the baryons from the early generations of massive star by Gamma-Ray Burst (GRB) explosions, through the period of cluster formation, down to very low redshifts, when between a third and one half of the baryons are expected to reside in cosmic filaments undergoing gravitational collapse by dark matter (Warm Hot Intragalactic Medium: WHIM). In addition EDGE, with its unprecedented observational capabilities, wil…
We study the expected diffuse cosmic neutrino flux produced by Population III (PopIII) stars during their nuclear burning phases as well as from their final stages of evolution (core collapse). Assuming a fraction f_III=10^(-3) of all baryons forms PopIII stars, our flux estimate is comparable to the diffuse neutrino flux produced by the ordinary stars and by the ordinary core-collapse supernovae in the universe, i.e. of order 1-10 cm^(-2) s^(-1). Due to the large cosmic redshift, however, the typical energies are in the MeV and sub-MeV range where the solar and geophysical neutrino fluxes are…
The existence of antimatter domains in baryon asymmetrical Universe can appear as the cosmological consequence of particle theory in inflationary models with non-homogeneous baryosynthesis. Such a domain can survive in the early Universe and form globular cluster of antimatter stars in our Galaxy. The model of antimatter pollution of Galaxy and annihilation with matter gas is developed. The proton-antiproton annihilation gamma flux is shown to reproduce the observed galactic gamma background measured by EGRET. From comparison with observational data the estimation on the maximally allowed amou…
