A Direct Upper Limit on the Density of Cosmological Dust from the Absence of an X‐Ray Scattering Halo around thez = 4.3 Quasar QSO 1508 \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $+$ \end{document} 5714

In this paper we examine the allowed amount of intergalactic (IG) dust, which is constrained by extinction and reddening of distant Type Ia supernovae (SNe Ia) and the thermal history of the intergalactic medium (IGM) affected by dust photoelectric heating. Based on the observational cosmic star formation history, we find an upper bound of χ, the mass ratio of the IG dust to the total metal in the Universe, as χ≲ 0.1 for 10 Å ≲a≲ 0.1 μm and χ≲ 0.1(a/0.1 μm) for 0.1 ≲a≲ 1 μm, where a is a characteristic grain size of the IG dust. This upper bound of χ∼ 0.1 suggests that the dust-to-metal ratio in the IGM is smaller than the current Galactic value. The corresponding allowed density of the IG dust increases from ∼10−34 g cm−3 at z= 0 to ∼10−33 g cm−3 at z∼ 1, and keeps almost the value toward higher redshift. This causes IG extinction of ≲0.2 mag at the observer's B band for z∼ 1 sources and that of ≲1 mag for higher redshift sources. Furthermore, if E(B–V) ∼ 0.1 mag at the observer's frame against z≳ 1 sources is detected, we can conclude that a typical size of the IG dust is ≲100Å. The signature of the 2175-Å feature of small graphite may be found as a local minimum at z∼ 2.5 in a plot of the observed E(B–V) as a function of the source redshift. Finally, the IGM mean temperature at z≲ 1 can be still higher than 104 K, provided the size of the IG dust is ≲100 Å.

Estimates of the cosmic star formation rate and of cluster metallicities independently imply that at z 0.5 the gas in the universe has substantial average metallicity: 1/10 Z/Z☉ 1/3 for Ωgas = 0.05. This metal density probably cannot be contained in known solar-metallicity galaxies of density parameter Ω* ≈ 0.004, implying significant enrichment of the intergalactic medium (IGM) by ejection of metals and dust from galaxies via winds, in mergers or in dust efflux driven by radiation pressure. Galaxies have a dust-to-metal ratio of ~0.5 in their interstellar media, but some fraction (1 - f) > 0 of this must be destroyed in the IGM or during the ejection process. Assuming the Draine & Lee dust model and preferential destruction of small grains (as destruction by sputtering would provide), I calculate the reddening and extinction of a uniform cosmological dust component in terms of f and the minimum grain size amin. Very small grains provide most of the reddening but less than half of the opacity for optical extinction. For f 0.3 and amin 0.1 μm, the intergalactic dust would be too gray to have been detected by its reddening, yet dense enough to be cosmologically important: it could account for the recently observed Type Ia supernova dimming at z ~ 0.5 without cosmic acceleration. It would also have implications for galaxy counts and evolutionary studies and would contribute significantly to the cosmic infrared background (CIB). The importance of gray intergalactic dust of the described type can be tested by observations of z = 0.5 supernovae in (rest) R-band or longer wavelengths and by the fluxes of a large sample of supernovae at z > 1.

The amount and properties of high-redshift galactic and intergalactic (IG) dust are largely unknown, but could be investigated using multi-wavelength photometry of high-z objects that have a known intrinsic spectrum. Observations of gamma-ray burst (GRB) afterglows appear to support the theoretical model of an adiabatic blast wave expanding into an external medium. In this model, the synchrotron peak flux is independent of frequency, providing a flat spectrum when observed over time, and therefore allowing straightforward measurement of the relative attenuation of afterglow flux in widely separated bands. Applying this method to dust extinction, we show that for a sample of afterglows which have been corrected by galactic extinction, comparison between the number counts of peak fluxes in X-ray versus optical can provide constraints on an intergalactic component of dust. A similar technique can probe the redshift-dependence of extinction in GRB-forming regions without requiring an assumed relation between extinction and reddening by the dust. Probing systematic changes in extinction with redshift - particularly in IG and/or non-reddening dust - is crucial to a proper interpretation of the Type Ia Supernova Hubble diagram and similar observations, and useful in understanding GRB progenitor environments.

We have examined dust photoelectric heating in the intergalactic medium (IGM). The heating rate in a typical radiation field of the IGM is represented by Γpe=1.2x10-34 erg s-1 cm-3 (D/10-4)(nH/10-5 cm-3)43(JL/10-21 erg s-1 cm-2 HZ-1 sr-1)2/3(T104 K)-1/6), where D is the dust-to-gas mass ratio, nH is the hydrogen number density, JL is the mean intensity at the hydrogen Lyman limit of the background radiation, and T is the gas temperature, if we assume the new X-ray photoelectric yield model by Weingartner et al. (2006) and the dust size distribution in the Milky Way by Mathis et al. (1977). This heating rate dominates the HI and HeII photoionization heating rates when the hydrogen number density is less than ∼10-6 cm-3 if D = 10-4 which is 1% of that in the Milky Way, although the heating rate is a factor of 2–4 smaller than that with the old yield model by Weingartner and Draine (2001). The grain size distribution is very important. If only large (≥0.1 μm) grains exist in the IGM, the heating rate is reduced by a factor of ≏5. Since dust heating is more efficient in a lower density medium relative to the photoionization heating, it may cause an inverted temperature-density relation in the low-density IGM, as suggested by Bolton et al. (2008). Finally, we have found that dust heating is not very important in the mean IGM before the cosmic reionization.

The observations of the reddening of the distant galaxies and the weak diffuse radiation in the clusters of galaxies can be interpreted as a consequence of the presence of dust grains in the intergalactic medium. When allowance is made for the destruction of the grains in collision with particles of the hot gas, its lifetime is about 107–108 yr at a gas concentrationng≈10−3 cm−3. The detection of the infrared (IR) emission from the galaxy clusters might be the test for the proof of the presence of dust grains in the intergalactic medium. In this paper the estimates of the expected intensities and fluxes of IR emission for the spectral region 50–300μ are presented for two galaxy clusters in Coma and Perseus. The parameters of the hot gas spatial distribution are chosen from X-ray observations. Having assumed that intergalactic dust can be ejected only from the galaxies, we used such a model for intergalactic dust grains which explains very well the interstellar dust effects. It is shown that the dust temperature, which is determined from the general energetic balance of the dust grains, can achieve some scores of degrees of Kelvin. Two models of the dust spatial distribution are considered. It is found that the maximum of IR flux for the Coma cluster lies near λ=100μ and the same for the Perseus cluster near λ≈50–70μ. The total fluxes of IR emission from these clusters are about 105–106 Jy and can be detected by modern observational methods.

Recent observations of Type Ia SNe at redshifts 0 < z <1 reveal a progressive dimming which has been interpreted as evidence for a cosmological constant of Omega_Lambda ~ 0.7. An alternative explanation of the SN results is an open universe with Omega_Lambda=0 and the presence of > 0.1 micron dust grains with a mass density of Omega_dust ~ (few) * 10^{-5} in the intergalactic (IG) medium. The same dust that dims the SNe absorbs the cosmic UV/optical background radiation around ~ 1 micron, and re-emits it at far infrared (FIR) wavelengths. Here we compare the FIR emission from IG dust with observations of the cosmic microwave (CMB) and cosmic far infrared backgrounds (FIRB) by the DIRBE/FIRAS instruments. We find that the emission would not lead to measurable distortion to the CMB, but would represent a substantial fraction (> 75%) of the measured value of the FIRB in the 300-1000 micron range. This contribution would be marginally consistent with the present unresolved fraction of the observed FIRB in an open universe. However, we find that IG dust probably could not reconcile the standard Omega=1 CDM model with the SN observations, even if the necessary quantity of dust existed. Future observations able to reliably resolve the FIRB to a flux limit of ~0.5 mJy, along with a more precise measure of the coarse-grained FIRB, will provide a definitive test of the IG dust hypothesis in all cosmologies.

Infrared emission from intergalactic dust might compromise the ability of future experiments to detect subtle spectral distortions in the Cosmic Microwave Background (CMB) from the early universe. We provide the first estimate of foreground contamination of the CMB signal due to diffuse dust emission in the intergalactic medium. We use models of the extragalactic background light to calculate the intensity of intergalactic dust emission and find that emission by intergalactic dust at z ≲ 0.5 exceeds the sensitivity of the planned Primordial Inflation Explorer to CMB spectral distortions by 1–3 orders of magnitude. In the frequency range ν = 150–2400 GHz, we place an upper limit of 0.06% on the contribution to the far-infrared background from intergalactic dust emission.

We present an X-ray study of the massive edge-on Sa galaxy, Sombrero (M 104; NGC 4594), based on XMM–Newton and Chandra observations. A list of 62 XMM–Newton and 175 Chandra discrete X-ray sources is provided, the majority of which are associated with the galaxy. Spectral analysis is carried out for relatively bright individual sources and for an accumulated source spectrum. At energies ≳2 keV, the source-subtracted X-ray emission is distributed similarly as the stellar K-band light and is primarily due to the residual emission from discrete sources. At lower energies, however, a substantial fraction of the source-subtracted emission arises from diffuse hot gas extending to ∼20 kpc from the galactic centre. The galactic disc shows little X-ray emission and instead shadows part of the X-ray radiation from the bulge. The observed diffuse X-ray emission from the galaxy shows a steep spectrum that can be characterized by an optically thin thermal plasma with temperatures of ∼0.6–0.7 keV, varying little with radius. The diffuse emission has a total luminosity of ∼3 × 1039 erg s−1 in the 0.2–2 keV energy range. This luminosity is significantly smaller than the prediction by current numerical simulations for galaxies as massive as Sombrero. However, such simulations do not include the effect of quiescent stellar feedback (e.g. ejecta from evolving stars and Type Ia supernovae) against the accretion from intergalactic medium. We argue that the stellar feedback likely plays an essential role in regulating the physical properties of hot gas. Indeed, the observed diffuse X-ray luminosity of Sombrero accounts for at most a few per cent of the expected mechanical energy input from Type Ia supernovae. The inferred gas mass and metal content are also substantially less than those expected from stellar ejecta. We speculate that a galactic bulge wind, powered primarily by Type Ia supernovae, has removed much of the ‘missing’ energy and metal-enriched gas from the region revealed by the X-ray observations.

The advanced X-ray astrophysics facility (AXAF)

Evolved asymptotic giant branch (AGB) stars and Type Ia supernovae (SNe) are important contributors to the elements that form dust in the interstellar medium of galaxies, in particular, carbon and iron. However, they require at least a Gyr to start producing these elements, therefore, a change in dust quantity or properties may appear at high redshifts. In this work, we use extinction of gamma-ray burst (GRB) afterglows as a tool to look for variations in dust properties at z>3. We use a spectroscopically selected sample of GRB afterglows observed with the VLT/X-shooter instrument to determine extinction curves out to high redshifts. We present ten new z>3 X-shooter GRBs of which six are dusty. Combining these with individual extinction curves of three previously known z>3 GRBs, we find an average extinction curve consistent with the SMC-Bar. A comparison with spectroscopically selected GRBs at all redshifts indicates a drop in visual extinction (A_V) at z>3.5 with no moderate or high extinction bursts. We check for observational bias using template spectra and find that GRBs up to z~8 are detectable with X-shooter up to A_V~0.3 mag. Although other biases are noted, a uniformly low dust content above z>3.5 indicates a real drop, suggesting a transition in dust properties and/or available dust building blocks. The remarkable increase in dust content at z<3.5 could arise due to carbon and possibly iron production by the first carbon-rich AGB and Type Ia SNe, respectively. Alternatively, z>3.5 dust drop could be the result of low stellar masses of GRB host galaxies.

AXAF is an x-ray observatory designed to study x-ray emission from al categories of astronomical objects, from normal stars to quasars. AXAF has broad scientific objectives and outstanding capability to provide high resolution images, spectrometric imaging and high resolution dispersive spectroscopy over the energy bandwidth from 0.1 to 10-keV. This is a significant year in the development of AXAF, to be launched in late 1998. Major elements of the observatory, the optics and the scientific instruments, are now nearing completion in preparation for calibration later this year.

The Gunn-Peterson effect predicts that an absorption trough should be associated with any resonance line arising in the intergalactic medium (IGM), extending blueward of the line in the QSO's rest frame. We show that such an absorption trough will not generally have a sharp edge at the QSO's redshift but should develop gradually toward shorter wavelengths. This proximity profile of the Gunn-Peterson trough arises because diffuse intergalactic gas in the vicinity of the QSO is more highly ionized than the general IGM. We consider the case of a uniform IGM in approximate photoionization equilibrium with a metagalactic UV background and investigate the proximity profile of He II lambda 304, which might be observable in QSOs with Z(sub Q) approximately 3. Assuming the QSO continuum extends beyond the He II ionization edge, the proximity profile has a characteristic width of delta z(sub p) = delta lambda/304 is approximately 0.1 f is the QSO Lyman limit luminosity in units of 10(exp 31) ergs/s/Hz averaged over the past approximately 10(exp 7) yr, omega(sub I) is the IGM density near (is less than or equal to 10 Mpc) the QSO, omega(sub b) is the normalized baryon density predicted by standard big band necleosynthesis, and f is a factor of order unity which depends weakly on several other factors. Application of this result to the reported detection of the He II Gunn-Peterson effect in Q0302-003 (Jakobsen et al. 1994) suggests that omega(sub I) is approximately equal to omega(sub b), some three orders of magnitude larger than the minimum density that may be inferred from application of the ordinary Gunn-Peterson effect to this QSO. Future observations of the He II proximity profile at higher resolution and signal-to-noise ratio in serveral ASOs should provide the means to measure the IGM density accurately.

Starburst-driven superwinds in galaxies have the potential to transport interstellar material (ISM) to the intergalactic medium (IGM). The mechanism is expected to be a function of the gravitational potential of the host galaxy, low-mass galaxies being more likely to lose mass in an outflow. In order to investigate the possibility of dwarf galaxies shedding enriched, hot material into the IGM, we selected, in addition to data directly obtained by us, archival data on a sample of nearby dwarf starburst galaxies that were observed with the Chandra X-Ray Observatory (employing the ACIS-S3 CCD). The sample comprises eight objects: I Zw 18, VII Zw 403, IC 2574 (actually a nonstarburst galaxy but containing a bright starburst region), NGC 1569, NGC 3077, NGC 4449, NGC 5253, and He 2-10. Toward each of the targets we detected unresolved X-ray sources (at the 1 .1 resolution of Chandra), which are thought to be related to the galaxies. They are in general located in the vicinity of bright H ii regions or on the rims of supergiant shells. This, plus their spectral properties, points at high-mass stars being ultimately responsible. Diffuse X-ray emission, attributed to the emission from a hot thermal plasma, was detected in NGC 1569, NGC 3077, NGC 4449, NGC 5253, and He 2-10. The size of the diffuse component varies between 1 and 10 kpc. Averaged over the entire galaxy, we derive X-ray luminosities of the hot gas in the range of 10–10 ergs s , temperatures of ∼ K, 1 6 3 # 10 and densities of ∼0.02 cm (MeKaL models; is the 0.5 3 f f v v volume filling factor). The pressures were found to be P/k ≈ K cm . This is an order of magnitude higher than 5 0.5 3 10 fv typical pressures of the ISM in the Milky Way, hence the hot gas must be expanding. The values quoted critically depend on the plasma models used for the fits; a disconcerting result is that equally good “best” fits can be obtained that vary by up to an order of magnitude in their unabsorbed X-ray luminosities. One of the galaxies, NGC 3077, is studied in depth in this thesis. In this galaxy, the hot gas is found to be confined to expanding Ha superbubbles. As in the other galaxies with extended emission, the energy provided by the starburst is high enough, in principle, for the hot gas to escape from the gravitational potential of the respective host. However, additional work is needed to overcome an extended gaseous environment. We argue that in NGC 3077, at least some of the gas is lost toward the north but not toward the south. The situation is similar in NGC 4449, where the hot gas extends beyond the optical disk but seems to be confined by a large H i ring of tidal origin. In NGC 1569 and NGC 5253, the X-ray emission resembles a freely flowing wind. This picture is supported by steeper volume density profiles of the hot gas as compared to NGC 3077 and NGC 4449. Normalized, azimuthally averaged surface brightness profiles of the Ha and the diffuse X-ray emission are very similar, suggesting a common origin. A comparison of the hot-gas masses derived on the basis of the X-ray data with estimates of stellar mass loss reveals that, except for NGC 3077, additional material (of up to a factor of 100) must have become entrained in the expanding X-ray plasma (mass loading). If diffuse, hot gas exists in I Zw 18, VII Zw 403, or IC 2574 with similar properties as that in the other objects, the low metallicity of these galaxies can explain the fact that they remained below the detection threshold. However, alternative explanations such as larger than predicted internal absorption, substantially lower or higher temperatures of the plasma, or loss of the coronal gas through blowout cannot be excluded at present.

We present a study of the cosmological Lyα emission signal at &lt;it&gt;z&lt;/it&gt; &gt; 4. Our goal is to predict the power spectrum of the spatial fluctuations that could be observed by an intensity mapping survey. The model uses the latest data from the &lt;it&gt;Hubble Space Telescope&lt;/it&gt; (&lt;it&gt;HST&lt;/it&gt;) legacy fields and the abundance matching technique to associate UV emission and dust properties with the haloes, computing the emission from the interstellar medium (ISM) of galaxies and the intergalactic medium (IGM), including the effects of reionization, self-consistently. The Lyα intensity from the diffuse IGM emission is 1.3 (2.0) times more intense than the ISM emission at &lt;it&gt;z&lt;/it&gt; = 4(7); both components are fair tracers of the star-forming galaxy distribution. However the power spectrum is dominated by ISM emission on small scales (&lt;it&gt;k&lt;/it&gt; &gt; 0.01 &lt;it&gt;h&lt;/it&gt; Mpc−1) with shot noise being significant only above &lt;it&gt;k&lt;/it&gt; = 1 &lt;it&gt;h&lt;/it&gt; Mpc−1. At very large scales (&lt;it&gt;k&lt;/it&gt; &lt; 0.01 &lt;it&gt;h&lt;/it&gt; Mpc−1) diffuse IGM emission becomes important. The comoving Lyα luminosity density from IGM and galaxies, &lt;f&gt;$\\dot{\\rho }_{\\rm {Ly\\alpha } }^{\\rm IGM} = 8.73(6.51) {\\times} 10^{40}\\,{\\rm erg}\\,{\\rm s}^{-1 }\\,{\\rm Mpc}^{-3}$&lt;/f&gt; and &lt;f&gt;$\\dot{\\rho }_{\\rm {Ly\\alpha } }^{\\rm ISM} = 6.62(3.21) {\\times} 10^{40}\\,{\\rm erg}\\,{\\rm s}^{-1}\\,{\\rm Mpc}^{-3}$&lt;/f&gt; at &lt;it&gt;z&lt;/it&gt; = 4(7), is consistent with recent Sloan Digital Sky Survey determinations. We predict a power &lt;it&gt;k&lt;/it&gt;3&lt;it&gt;P&lt;/it&gt;Lyα(&lt;it&gt;k&lt;/it&gt;, &lt;it&gt;z&lt;/it&gt;)/2π2 = 9.76 × 10−4(2.09 × 10−5)nW2m−4 sr−2 at &lt;it&gt;z&lt;/it&gt; = 4(7) for &lt;it&gt;k&lt;/it&gt; = 0.1 &lt;it&gt;h&lt;/it&gt; Mpc−1.

Although the highest redshift QSOs (z > 6.1) are embedded in a significantly neutral background universe (mass-averaged neutral hydrogen fraction >1%) as suggested by the Gunn-Peterson absorption troughs in their spectra, the intergalactic medium in their vicinity is highly ionized. The highly ionized region is generally idealized as spherical and called the Stromgren sphere. In this paper, by combining the expected evolution of the Stromgren sphere with the rule that the speed of light is finite, we illustrate the apparent shape of the ionization fronts around the highest redshift QSOs and its evolution, which depends on the age, luminosity evolution, and environment of the QSO (e.g., the hydrogen reionization history). The apparent shape may systematically deviate from a spherical shape, unless the QSO age is significantly long compared to the hydrogen recombination process within the ionization front and the QSO luminosity evolution is significantly slow. Effects of anisotropy of QSO emission are also discussed. The apparent shape of the Stromgren sphere may be directly mapped by transmitted spectra of background sources behind or inside the ionized regions or by surveys of the hyperfine transition (21 cm) line emission of neutral hydrogen.