Extragalactic X-ray Background Research Articles

Abundance and temperature of the outer hot circumgalactic medium

Context. Despite their vital importance to understanding galaxy evolution and our own Galactic ecosystem, our knowledge of the physical properties of the hot X-ray emitting phase of the Milky Way is still inadequate. However, sensitive SRG/eROSITA large area surveys are now providing us with the long-sought data needed to mend this state of affairs. Aims. Our aim is to constrain the properties of the Milky Way hot halo emission toward intermediate Galactic latitudes close to the Galactic anti-center. Methods. We analyzed the spectral properties of the integrated soft X-ray emission observed by eROSITA in the relatively deep eFEDS field. Results. We observe a flux of 12.6 and 5.1 × 10−12 erg cm−2 s−1 deg−2 in the total (0.3–2) and soft (0.3–0.6 keV) band. We measure the temperature and metal (oxygen) abundance of the hot circumgalactic medium (CGM) to be within kTCGM = 0.153–0.178 keV and ZCGM = 0.052–0.072 Z⊙, depending on the contribution of solar wind charge exchange (SWCX). Slightly higher CGM abundances ZCGM = 0.05–0.10 Z⊙ are possible, considering the uncertain extrapolation of the extragalactic cosmic X-ray background (CXB) emission below ~1 keV. To recover CGM abundances as high as ZCGM = 0.3 Z⊙, the presence of an additional component must be postulated, likely associated with the warm-hot intergalactic medium, providing ~15–20% of the flux in the soft X-ray band. We observe line widths of the CGM plasma smaller than Δυ ≤ 500 km s−1. The emission in the soft band is dominated (~47%) by the circumgalactic medium (CGM), whose contribution reduces to ~30% if heliospheric SWCX contributes at the level of ~15% also during solar minimum. The remaining flux is provided by the CXB (~33%) and the local hot bubble (~18%). Moreover, the eROSITA data require the presence of an additional component associated with the elusive Galactic corona plus a possible contribution from unresolved M dwarf stars. This component has a temperature of kT ~ 0.4– 0.7 keV, a considerable (~ kiloparsec) scale height, and might be out of thermal equilibrium. It contributes ~9% to the total emission in the 0.6—2 keV band, and is therefore a likely candidate to produce part of the unresolved CXB flux observed in X-ray ultra-deep fields. We also observe a significant contribution to the soft X-ray flux due to SWCX, during periods characterized by stronger solar wind activity, and causing the largest uncertainty on the determination of the CGM temperature. Conclusions. We constrain temperature, emission measure, abundances, thermal state, and spectral shape of the outer hot CGM of the Milky Way.

AGN and QSOs in the eROSITA All-Sky Survey

Context. The main element of the observing program of the Spectrum-Roentgen-Gamma orbital observatory is a four-year all-sky survey, in the course of which the entire sky will be scanned eight times. Aims. We analyze the statistical properties of AGN and QSOs that are expected to be detected in the course of the eROSITA all-sky survey (eRASS). Methods. According to the currently planned survey strategy and based on the parameters of the Galactic and extragalactic X-ray background as well as on the results of the recent calculations of the eROSITA instrumental background, we computed a sensitivity map of the eRASS. Using the best available redshift-dependent AGN X-ray luminosity function (XLF), we computed various characteristics of the eRASS AGN sample, such as their luminosity- and redshift distributions, and the brightness distributions of their optical counterparts. Results. After four years of the survey, a sky-average sensitivity of ~1x10^(-14) erg/s/cm^2 will be achieved in the 0.5-2.0keV band. With this sensitivity, eROSITA is expected to detect ~3 million AGN on the extragalactic sky (|b|>10deg). The median redshift of the eRASS AGN will be z~1 with ~40% of the objects in the z=1-2 redshift range. About 10^4 - 10^5 AGN are predicted beyond redshift z=3 and about 2 000 - 30 000 AGN beyond redshift z=4, the exact numbers depend on the poorly known behavior of the AGN XLF in the high-redshift and luminosity regimes. Of the detected AGN, the brightest 10% will be detected with more than ~38 counts per PSF HEW, while the faintest 10% will have fewer than ~9 counts. The optical counterparts of ~95% of the AGN will be brighter than I_(AB)=22.5mag. The planned scanning strategy will allow one to search for transient events on a timescale of half a year and a few hours with a 0.5-2.0keV sensitivity of ~2x10^(-14) to ~2x10^(-13) erg/s/cm^2, respectively.

The nature of the unresolved extragalactic cosmic soft X-ray background

In this paper we investigate the power spectrum of the unresolved 0.5-2 keV CXB with deep Chandra 4 Ms observations in the CDFS. We measured a signal which, on scales >30", is significantly higher than the Shot-Noise and is increasing with the angular scale. We interpreted this signal as the joint contribution of clustered undetected sources like AGN, Galaxies and Inter-Galactic-Medium (IGM). The power of unresolved cosmic sources fluctuations accounts for \sim 12% of the 0.5-2 keV extragalactic CXB. Overall, our modeling predicts that \sim 20% of the unresolved CXB flux is made by low luminosity AGN, \sim 25% by galaxies and \sim 55% by the IGM (Inter Galactic Medium). We do not find any direct evidence of the so called Warm Hot Intergalactic Medium (i.e. matter with 10^5K<T<10^7K and density contrast {\delta} <1000), but we estimated that it could produce about 1/7 of the unresolved CXB. We placed an upper limit to the space density of postulated X-ray-emitting early black hole at z>7.5 and compared it with SMBH evolution models.

CanChandraResolve the Remaining Cosmic X‐Ray Background?

The deepest extragalactic X-ray observation, the 2 Ms Chandra Deep Field North (CDF-N), resolves ~80% of the total extragalactic cosmic X-ray background (CXB) in the 1-2 keV band. Recent work has shown that 70% of the remaining CXB flux is associated with sources detected by the Hubble Space Telescope (HST). This paper uses the existing CDF-N data to constrain the X-ray flux distribution of these X-ray-undetected HST sources by comparing the number of 0.5-2 keV X-ray counts at the HST positions to those expected for model flux distributions. In the simple case where all the undetected HST X-ray sources have the same 0.5-2 keV flux, the data are best fit by 1.5-3 counts per source in 2 Ms, compared to a detection limit (at 10% completeness) of 9 counts. Assuming a more realistic power-law log N − log S distribution [N(> S) ∝ S−α], the data favor a relatively steep flux distribution, with α = 1.1−0.3+0.5 (limits are 99% confidence). This slope is very similar to that previously found for faint normal and starburst galaxies in the CDF-N. These results suggest deeper Chandra observations will detect a new population of faint X-ray sources, but extremely deep exposures are needed to resolve the remainder of the soft CXB. In the most optimistic scenario, when the HST sources have the flattest allowed flux distribution and all the sources without HST counterparts are detected, observations 5 times more sensitive than the existing ones would resolve at most ~60% of the remaining soft CXB.

Does the AGN Unified Model Evolve with Redshift? Using the X‐Ray Background to Predict the Mid‐Infrared Emission of AGNs

Deep X-ray surveys by Chandra and XMM-Newton have resolved about 80% of the 2-10 keV cosmic extragalactic X-ray background (CXRB) into point sources, the majority of which are obscured AGNs. The obscuration might be connected to processes within the host galaxy, possibly the star formation rate. Here we use the results of CXRB synthesis calculations as input to detailed CLOUDY simulations in order to predict the evolution of AGN properties at several mid-IR wavelengths. Computations were performed for three different evolutions of the AGN type 2/type 1 ratio between z = 0 and 1, where the ratio increased as (1 + z)0.9, as (1 + z)0.3, and with one model having no redshift evolution. Models were calculated with the inner radius of the absorbing gas and dust at 1 or at 10 pc. Comparing the results of the calculations to combined X-ray and Spitzer data of AGNs show that the predicted spectral energy distributions are a good description of average AGNs found in the deep surveys. The existing data indicate that the mid-IR emission from an average AGN is best described by models in which the attenuating material is ~10 pc from the central engine. We present the expected Spitzer cumulative number count distributions and the evolution of the total AGN (type 1+type 2) luminosity function (LF) between z = 0 and 1 at rest frame 8 and 30 μm for the three evolutionary scenarios. The mid-IR AGN LF will be an excellent tool to measure the evolution of the covering factor of the gas and dust from z ~ 0 to 1.

Constraints on sterile neutrinos as dark matter candidates from the diffuse X-ray background

Sterile neutrinos with masses in the kilo-electronvolt range are viable candidates for the warm dark matter. We analyse existing data for the extragalactic diffuse X-ray background for signatures of sterile neutrino decay. The absence of a detectable signal within current uncertainties of background measurements puts model-independent constraints on allowed values of the sterile neutrino mass and mixing angle, which we present in this work.

Masses of active neutrinos in the νMSM from x-ray astronomy

In an extention of the Standard Model by three relatively light right-handed neutrinos (the nuMSM model) the role of the dark matter particle is played by the lightest sterile neutrino. We demonstrate that the observations of the extragalactic X-ray background allow to put a strong upper bound on the mass of the lightest active neutrino and predict the absolute values of the mass of the two heavier active neutrinos in the nuMSM, provided that the mass of the dark matter sterile neutrino is larger than 1.8 keV.

The Fall of Active Galactic Nuclei and the Rise of Star-forming Galaxies: A Close Look at the Chandra Deep Field X-Ray Number Counts

We investigate the X-ray number counts in the 1?2 Ms Chandra Deep Fields (CDFs) to determine the contributions of faint X-ray source populations to the extragalactic X-ray background (XRB). X-ray sources were separated into active galactic nuclei (AGNs), star-forming galaxies, and Galactic stars primarily on the basis of their X-ray?to?optical flux ratios, optical spectral classifications, X-ray spectra, and intrinsic X-ray luminosities. Number count slopes and normalizations below 2 ? 10-15 ergs cm-2 s-1 were calculated in each band for all source types assuming a single power-law model. We find that AGNs continue to dominate the number counts in the 0.5?2.0 and 2?8 keV bands. At flux limits of ?2.5 ? 10-17 ergs cm-2 s-1 (0.5?2.0 keV) and ?2.0 ? 10-16 ergs cm-2 s-1 (2?8 keV), the overall AGN source densities are 7166 and 4558 sources deg-2, respectively; these are factors of ~10?20 higher than those found in the deepest optical spectroscopic surveys. Although still a minority, the number counts of star-forming galaxies climb steeply, such that they eventually achieve source densities of 1727 sources deg-2 (0.5?2.0 keV) and 711 sources deg-2 (2?8 keV) at the CDF flux limits. The number of star-forming galaxies will likely overtake the number of AGNs at ~1 ? 10-17 ergs cm-2 s-1 (0.5?2.0 keV) and dominate the overall number counts thereafter. Adopting XRB flux densities of (7.52 ? 0.35) ? 10-12 ergs cm-2 s-1 deg-2 for 0.5?2.0 keV and (1.79 ? 0.11) ? 10-11 ergs cm-2 s-1 deg-2 for 2?8 keV, the CDFs resolve a total of 89.5 percent and 92.6 percent of the extragalactic 0.5?2.0 and 2?8 keV XRBs, respectively. AGNs as a whole contribute ?83% and ?95% to these resolved XRB fractions, respectively, whereas star-forming galaxies comprise only ?3% and ?2%, respectively, and Galactic stars comprise the remainder. Extrapolation of the number count slopes can easily account for the entire 0.5?2.0 and 2?8 keV XRBs to within statistical errors. We also examine the X-ray number counts as functions of intrinsic X-ray luminosity and absorption, finding that sources with L0.5?8 keV > 1043.5 ergs s-1 and NH < 1022 cm-2 are the dominant contributors to the 0.5?2.0 keV XRB flux density, whereas sources with L0.5?8 keV = 1042.5?1044.5 ergs s-1 and a broad range of absorption column densities primarily contribute to the 2?8 keV XRB flux density. This trend suggests that even less intrinsically luminous, more highly obscured AGNs may dominate the number counts at higher energies, where the XRB intensity peaks. Finally, we revisit the reported differences between the CDF-North and CDF-South number counts, finding that the two fields are consistent with each other except for sources detected at 2?8 keV below F2?8 keV ? 1 ? 10-15 ergs cm-2 s-1, for which deviations gradually increase to ?3.9 ?.

The Chandra Deep Field North Survey. XV. Optically Bright, X-Ray-Faint Sources

We have analyzed optically bright, X-ray faint [OBXF; i.e., log(fX/fR) < -2] sources identified in an 178.9 square arcminute area within the Chandra Deep Field-North (CDF-N) 2 Ms survey. We find 43 OBXF sources in this area, comprising ~15% of the X-ray sources above a 0.5--2 keV flux of 2.3e-17 erg cm^-2 s^-1. We present spectroscopic identifications for 42 of the OBXF sources and optical spectra for 25, including 5 previously unpublished redshifts. Deep optical imaging data (either HST or ground-based) are presented for all the OBXF sources. The OBXF population consists mainly of and starburst galaxies detected out to cosmologically significant distances (i.e., to a median redshift of z=0.297 and a full redshift range z=0.06-0.845). This is notable since these distances equate to look-back times of up to ~8 Gyr; we are thus provided with a window on the X-ray emission from galaxies at redshifts much closer to the cosmic star formation peak than was possible prior to Chandra. The X-ray luminosity distribution of OBXF sources extends to higher luminosity than does that of normal galaxies indicating that a significant fraction are likely dominated by low-luminosity AGN (LLAGN) or vigorous star formation. By combining the detected X-ray counts, we find the average OBXF X-ray spectrum to be consistent with a Gamma=2.0 power law. The 0.5--2 keV log N-log S for the OBXF galaxies is much steeper (alpha=-1.7) than for the general X-ray source population. Indeed, the number of OBXF sources has doubled between the 1~Ms and 2~Ms survey, rising sharply in numbers at faint fluxes. The extragalactic OBXF sources are found to contribute ~1-2% of the soft extragalactic X-ray background.

Radiative feedback from an early X-ray background

The first generation of stars (commonly known as population III) are expected to form in low-mass protogalaxies in which molecular hydrogen is the dominant coolant. Radiation from these stars will rapidly build up an extragalactic ultraviolet (UV) background capable of photodissociating H2, and it is widely believed that this background will suppress further star formation in low-mass systems. However, star formation will also produce an extragalactic X-ray background. This X-ray background, by increasing the fractional ionization of protogalactic gas, promotes H2 formation and reduces the effectiveness of ultraviolet feedback. In this paper, we examine which of these backgrounds has the dominant effect. Using a simple model for the growth of the UV and X-ray backgrounds, together with a detailed one-dimensional model of protogalactic chemical evolution, we examine the effects of the X-ray backgrounds produced by a number of likely source models. We show that in several cases, the resulting X-ray background is strong enough to offset UV photodissociation in large H2-cooled protogalaxies. On the other hand, small protogalaxies (those with virial temperatures Tvir < 2000 K) remain dominated by the UV background in all of the models we examine. We also briefly investigate the effects of the X-ray background upon the thermal and chemical evolution of the diffuse intergalactic medium.

The soft X‐ray background towards the northern sky. A detailed analysis of the Milky Way halo.

Astronomische NachrichtenVolume 324, Issue 1-2 p. 150-150 Original Paper The soft X-ray background towards the northern sky. A detailed analysis of the Milky Way halo. J. Pradas, J. Pradas [email protected] Search for more papers by this authorJ. Kerp, J. Kerp Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, GermanySearch for more papers by this authorP.M.W. Kalberla, P.M.W. Kalberla Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, GermanySearch for more papers by this author J. Pradas, J. Pradas [email protected] Search for more papers by this authorJ. Kerp, J. Kerp Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, GermanySearch for more papers by this authorP.M.W. Kalberla, P.M.W. Kalberla Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, GermanySearch for more papers by this author First published: 14 February 2003 https://doi.org/10.1002/asna.200310049Citations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article.Citing Literature Volume324, Issue1-2February 2003Pages 150-150 RelatedInformation

The Chandra Deep Field–North Survey. XI. X-Ray Emission from Luminous Infrared Starburst Galaxies

Using the 1 Ms Chandra Deep Field-North and 15 μm ISOCAM Hubble Deep Field-North surveys, we find a tight correlation between the population of strongly evolving starburst galaxiess discovered in faint 15 μm ISOCAM surveys and the apparently normal galaxy population detected in deep X-ray surveys. Up to 100% of the X-ray-detected emission-line galaxies (ELGs) have 15 μm counterparts, in contrast to 10%-20% of the X-ray-detected absorption-line galaxies and AGN-dominated sources. None of the X-ray-detected ELGs are detected in the hard band (2-8 keV), and their stacked-average X-ray spectral slope of Γ ≈ 2.0 suggests a low fraction of obscured AGN activity within the X-ray-detected ELG population. The characteristics of the z = 0.4-1.3 X-ray-detected ELGs are consistent with those expected for M82- and NGC 3256-type starburst galaxies; these X-ray-detected ELGs contribute ≈2% of the 0.5-8.0 keV extragalactic X-ray background. The only statistical difference between the X-ray-detected and X-ray-undetected 15 μm-selected ELGs is that a much larger fraction of the former have radio emission.

Hydrodynamic Simulation of the Cosmological X‐Ray Background

We use a hydrodynamic simulation of an inflationary cold dark matter model with a cosmological constant to predict properties of the extragalactic X-ray background (XRB). We focus on emission from the intergalactic medium (IGM), with particular attention to diffuse emission from warm-hot gas that lies in relatively smooth filamentary structures between galaxies and galaxy clusters. We also include X-rays from point sources associated with galaxies in the simulation, and we make maps of the angular distribution of the emission. Although much of the X-ray luminous gas has a filamentary structure, the filaments are not evident in the simulated maps because of projection effects. In the soft (0.5-2 keV) band, our calculated mean intensity of radiation from intergalactic and cluster gas is 2.3 × 10-12 ergs-1 cm-2 deg-2, 35% of the total softband emission. This intensity is compatible at the ~1 σ level with estimates of the unresolved soft background intensity from deep ROSAT and Chandra measurements. Only 4% of the hard (2-10 keV) emission is associated with intergalactic gas. Relative to active galactic nuclei flux, the IGM component of the XRB peaks at a lower redshift (median z ~ 0.45) and spans a narrower redshift range, so its clustering makes an important contribution to the angular correlation function of the total emission. The clustering on the scales accessible to our simulation (01-10') is significant, with an amplitude roughly consistent with an extrapolation of recent ROSAT results to small scales. A cross-correlation analysis of the XRB against nearby galaxies taken from a simulated redshift survey also yields a strong signal from the IGM. Our conclusions about the soft background intensity differ from those of some recent papers that have argued that the expected emission from gas in galaxy, group, and cluster halos would exceed the observed background unless much of the gas is expelled by supernova feedback. We obtain reasonable compatibility with current observations in a simulation that incorporates cooling, star formation, and only modest feedback. A clear prediction of our model is that the unresolved portion of the soft XRB will remain mostly unresolved even as observations reach deeper point-source sensitivity.

The X‐Ray Surface Brightness Distribution from Diffuse Gas

We use simulations to predict the X-ray surface brightness distribution arising from hot, cosmologically distributed diffuse gas. The distribution is computed for two bands, 0.5-2 keV and 0.1-0.4 keV, using a cosmological-constant-dominated cosmology that fits many other observations. We examine a number of numerical issues such as resolution, simulation volume, and pixel size and show that the predicted mean background is sensitive to resolution such that higher resolution systematically increases the mean predicted background. Although this means that we can compute only lower bounds to the predicted level, these bounds are already quite restrictive. Since the observed extragalactic X-ray background is mostly accounted for by compact sources, the amount of the observed background attributable to diffuse gas is tightly constrained. We show that without physical processes in addition to those included in the simulations (such as radiative cooling or nongravitational heating), both bands exceed observational limits. In order to examine the effect of nongravitational heating we explore a simple model of energy injection and show that if this is the only mechanism operating to suppress the background then substantial amounts of heating would be required (i.e., 5 keV per particle when averaged over all baryons). Finally, we also compute the distribution of surface brightness on the sky and show that it has a well-resolved characteristic shape. This shape is substantially modified by nongravitational heating and can be used as a probe of such energy injection.

TheChandraDeep Survey of the Hubble Deep Field–North Area. II. Results from the Caltech Faint Field Galaxy Redshift Survey Area

We present results from a 221.9 ks Chandra exposure of the HDF-N and its vicinity, concentrating on the 8.6' X 8.7' area covered by the Caltech Faint Field Galaxy Redshift Survey (the `Caltech area'). The minimum detectable fluxes in the 0.5-2 keV and 2-8 keV bands are 1.3e-16 cgs and 6.5e-16 cgs, respectively and a total of 82 sources are detected. More than 80% of the extragalactic X-ray background in the 2-8 keV band is resolved. Redshifts are available for 96% of the sources with R<23; the redshift range is 0.1-3.5 with most sources having z < 1.5. Eight of the X-ray sources are located in the HDF-N itself, including two not previously reported. A population of X-ray faint, optically bright, nearby galaxies emerges at soft-band fluxes of ~< 3e-16 cgs. We set the tightest constraints to date on the X-ray emission properties of microJy radio sources, mid-infrared sources detected by ISO, and very red (R-K_s > 5.0) objects. Where both the infrared and the X-ray coverage are deepest, 75% of the X-ray sources are detected by ISO; the high X-ray to infrared matching rate bodes well for future sensitive infrared observations of faint X-ray sources. Four of the 33 very red objects that have been identified in the Caltech area by Hogg et al. (2000) are detected in X-rays; these four are among our hardest Chandra sources, and we argue that they contain moderately luminous obscured AGN. Overall, however, the small Chandra detection fraction suggests a relatively small AGN content in the optically selected very red object population. (Abridged)

The soft X-ray background: evidence for widespread disruption of the gas haloes of galaxy groups

Almost all of the extragalactic X-ray background (XRB) at 0.25 keV can be accounted for by radio-quiet quasars, allowing us to derive an upper limit of 4 keV cm−2 s−1 sr−1 keV−1 for the remaining background at 0.25 keV. However, the XRB from the gas haloes of groups of galaxies, with gas removal resulting from cooling accounted for, exceeds this upper limit by an order of magnitude if non-gravitational heating is not included. We calculate this using simulations of halo merger trees and realistic gas density profiles, which we require to reproduce the observed gas fractions and abundances of X-ray clusters. In addition, we find that the entire mass range of groups, from ∼5×1012 to ∼1014 M⊙, contributes to the 0.25-keV background in this case. In a further study, we reduce the luminosities of groups by maximally heating their gas haloes while maintaining the same gas fractions. This reduces the XRB by only a factor of 2 or less. We thus argue that most of the gas associated with groups must be outside their virial radii. This conclusion is supported by X-ray studies of individual groups. The properties of both groups and X-ray clusters can be naturally explained by a model in which the gas is given excess specific energies of ∼1 keV per particle by non-gravitational heating. With this excess energy, the gas is gravitationally unbound from groups, but recollapses with the formation of a cluster of temperature ≳1 keV. This is similar to a model proposed by Pen, but is contrary to the evolution of baryons described by Cen & Ostriker. In addition to the soft XRB spectrum, we simulate source counts in two bands, 0.1–0.4 keV and 0.5–2 keV, for comparison with present and future data.

X-Ray Constraints on the Warm-Hot Intergalactic Medium

Three observational constraints can be placed on a warm-hot intergalactic medium (WHIM) using ROSAT Position Sensitive Proportional Counter (PSPC) pointed and survey data, the emission strength, the energy spectrum, and the fluctuation spectrum. The upper limit to the emission strength of the WHIM is 7.5 +/- 1.0 keV/(s*sq cm*sr*keV) in the 3/4 keV band, an unknown portion of which value may be due to our own Galactic halo. The spectral stape of the WHIM emission can be described as thermal emission with logT = 6.42, although the true spectrum is more likely to come from a range of temperatures. The values of emission strength and spectral shape are in reasonable agreement with hydrodynamical cosmological models. The autocorrelation function in the 0.44 keV < E < 1.21 keV band range, w(theta), for the extragalactic soft X-ray background (SXRB) which includes both the WHIM and contributions due to point sources, is approx. < 0.002 for 10 min < 0 < 20 min in the 3/4 keV band. This value is lower than the Croft et al. (2000) cosmological model by a factor of approx. 5, but is still not inconsistent with cosmological models. It is also found that the normalization of the extragalactic power law component of the soft X-ray background spectrum must be 9.5 +/- 0.9 keV/(s*sq cm*sr*keV) to be consistent with the ROSAT All-Sky Survey.

On the intensity of the extragalactic X-ray background

Measurements of the intensity of the cosmic X-ray background (XRB) carried out over small solid angles are subject to spatial variations due to the discrete nature of the XRB. We find variations of the order of 10 per cent for solid angles under 1 deg2 and, therefore, cosmic variance often dominates over the statistical uncertainties quoted in the various estimates of the XRB intensity. Taking into account this fact, we have computed Bayesian estimates for the XRB intensity at 1 keV using different measurements of the intensity carried out with the imaging instruments onboard the ASCA, BeppoSAX and ROSAT missions. Our corresponding estimates are 9.8−1.0 +0.7 (ASCA), 11.1−2.3 +1.0 (BeppoSAX), and 12.7−1.9 +0.9 keV cm−2s−1 sr−1 keV−1 (ROSAT) (90 per cent errors).

Resolving the extragalactic hard X-ray background

The origin of the hard (2-10 keV) X-ray background has been a mystery for over 35 years. Most of the soft X-ray background has been resolved into individual sources (mainly quasars), but these sources do not have the spectral energy distribution required to match the spectrum of the X-ray background as a whole. Here we report the results of a deep survey, using the Chandra satellite, in which the detected hard X-ray sources account for at least 75 per cent of the hard X-ray background. The mean X-ray spectral energy distribution of these sources is in good agreement with that of the background. Moreover, most of those hard X-ray sources are associated unambiguously with either the nuclei of otherwise normal bright galaxies, or with optically faint sources. The latter could be active nuclei in dust-enshrouded galaxies or a population of quasars at extremely high redshift.

Where Are the Baryons?

New, high resolution, large-scale, cosmological hydrodynamic galaxy formation simulations of a standard cold dark matter model (with a cosmological constant) are utilized to predict the distribution of baryons at the present and at moderate redshift. It is found that the average temperature of baryons is an increasing function of time, with most of the baryons at the present time having a temperature in the range 10^{5-7} K. Thus, not only is the universe dominated by dark matter, but more than one half of the normal matter is yet to be detected. Detection of this warm/hot gas poses an observational challenge, requiring sensitive EUV and X-ray satellites. Signatures include a soft, cosmic X-ray background, apparent warm components in hot clusters due to both intrinsic warm intra-cluster gas and warm inter-cluster gas projected onto clusters along the line of sight, absorption lines in X-ray and UV quasar spectra [e.g., O VI (1032,1038)A lines, OVII 574 eV line], strong emission lines (e.g., O VIII 653 eV line) and low redshift, broad, low column density $\lya$ absorption lines. We estimate that approximately 1/4 of the extragalactic soft X-ray background (SXRB) (at 0.7 keV) arises from the warm/hot gas, half of it coming from $z<0.65$ and three-quarters from $z<1.00$, so the source regions should be identifiable on deep optical images.