Eli Dwek

THE COSMIC INFRARED BACKGROUND: Measurements and Implications ⁄

▪ Abstract The cosmic infrared background records much of the radiant energy released by processes of structure formation that have occurred since the decoupling of matter and radiation following the Big Bang. In the past few years, data from the Cosmic Background Explorer (COBE) mission provided the first measurements of this background, with additional constraints coming from studies of the attenuation of TeV γ-rays. At the same time, there has been rapid progress in resolving a significant fraction of this background with the deep galaxy counts at infrared wavelengths from the Infrared Space Observatory (ISO) instruments and at submillimeter wavelengths from the Submillimeter Common User Bolometer Array (SCUBA) instrument. This article reviews the measurements of the infrared background and sources contributing to it and discusses the implications for past and present cosmic processes.

The Evolution of the Elemental Abundances in the Gas and Dust Phases of the Galaxy

We present models for the evolution of the elemental abundances in the gas and dust phases of the interstellar medium (ISM) of our Galaxy by generalizing standard models for its dynamical and chemical evolution. In these models, the stellar birthrate history is determined by the infall rate of primordial gas and by its functional dependence on the mass surface density of the stars and gas. We adopt a two-component Galaxy consisting of a central bulge and an exponential disk with different infall rates and stellar birthrate histories. Condensation in stellar winds, Type Ia and Type II supernovae, and the accretion of refractory elements onto preexisting grains in dense molecular clouds are the dominant contributors to the abundance of elements locked up in the dust. Grain destruction by sputtering and evaporative grain-grain collisions in supernova remnants are the most important mechanisms that return these elements back to the gas phase. Guided by observations of dust formation in various stellar sources, and by the presence of isotopic anomalies in meteorites, we calculate the production yield of silicate and carbon dust as a function of stellar mass. We find that Type II supernovae are the main source of silicate dust in the Galaxy. Carbon dust is produced primarily by low-mass stars in the ~2-5 M☉ range. Type Ia SNe can be important sources of metallic iron dust in the ISM. We also analyze the origin of the elemental depletion pattern and find that the observed core + mantle depletion must reflect the efficiency of the accretion process in the ISM. We also find that grain destruction is very efficient, leaving only ~10% of the refractory elements in grain cores. Observed core depletions are significantly higher, requiring significant UV, cosmic ray, or shock processing of the accreted mantle into refractory core material. Adopting the current grain destruction lifetimes from Jones et al., we formulate a prescription for its evolution in time. We make a major assumption, that the accretion timescale evolves in a similar fashion, so that the current ratio between these quantities is preserved over time. We then calculate the evolution of the dust abundance and composition at each Galactocentric radius as a function of time. We find that the dust mass is linearly proportional to the ISM metallicity and is equal to about 40% of the total mass of heavy elements in the Galaxy, independent of Galactocentric radius. The derived relation of dust mass with metallicity is compared to the observed Galactic dust abundance gradient, and to the Mdust versus log (O/H) relation that is observed in external dwarf galaxies. The dependence of dust composition on the mass of the progenitor star and the delayed recycling of newly synthesized dust by low-mass stars back to the ISM give rise to variations in the dust composition as a function of time. We identify three distinct epochs in the evolution of the dust composition, characterized by different carbon-to-silicate mass ratios. Two such epochs are represented by the Galaxy and the SMC. The third is characterized by an excess of carbon dust (compared to the Milky Way Galaxy), and should be observed in galaxies or star-forming regions in which the most massive carbon stars are just evolving off the main sequence. Our models provide a framework for the self-consistent inclusion of dust in population synthesis models for various pre-galactic and galactic systems, allowing for the calculation of their UV to far-infrared spectral energy distribution at various stages of their evolution.

INTERSTELLAR DUST MODELS CONSISTENT WITH EXTINCTION, EMISSION, AND ABUNDANCE CONSTRAINTS

We present new interstellar dust models that have been derived by simultaneously fitting the far-ultraviolet to near-infrared extinction, the diffuse infrared (IR) emission and, unlike previous models, the elemental abundance constraints on the dust for different interstellar medium abundances, including solar, F and G star, and B star abundances. The fitting problem is a typical ill-posed inversion problem, in which the grain size distribution is the unknown, which we solve by using the method of regularization. The dust model contains various components: polycyclic aromatic hydrocarbons (PAHs), bare silicate, graphite, and amorphous carbon particles, as well as composite particles containing silicate, organic refractory material, water ice, and voids. The optical properties of these components were calculated using physical optical constants. As a special case, we reproduce the Li & Draine results; however, their model requires an excessive amount of silicon, magnesium, and iron to be locked up in dust: about 50 ppm (atoms per million of H atoms), significantly more than the upper limit imposed by solar abundances of these elements, about 34, 35, and 28 ppm, respectively. A major conclusion of this paper is that there is no unique interstellar dust model that simultaneously fits the observed extinction, diffuse IR emission, and abundance constraints. We find several classes of acceptable interstellar dust models that comply with these constraints. The first class is identical in composition to the Li & Draine model, consisting of PAHs, bare graphite and silicate grains, but with a different size distribution that is optimized to comply with the abundance constraints. The second class of models contains in addition to PAHs bare graphite and silicate grains also composite particles. Other classes contain amorphous carbon instead of graphite particles, or no carbon at all, except for that in PAHs. All classes are consistent with solar and F and G star abundances but have greater difficulty fitting the B star carbon abundance, which is better fitted with the latter (no carbon) models. Additional observational constraints, such as the interstellar polarization, or X-ray scattering may be able to discriminate between the various interstellar dust models.

Herschel detects a massive dust reservoir in supernova 1987A.

The large amount of dust produced by this supernova may help explain the dust observed in young galaxies. We report far-infrared and submillimeter observations of supernova 1987A, the star whose explosion was observed on 23 February 1987 in the Large Magellanic Cloud, a galaxy located 160,000 light years away. The observations reveal the presence of a population of cold dust grains radiating with a temperature of about 17 to 23 kelvin at a rate of about 220 times the luminosity of the Sun. The intensity and spectral energy distribution of the emission suggest a dust mass of about 0.4 to 0.7 times the mass of the Sun. The radiation must originate from the supernova ejecta and requires the efficient precipitation of all refractory material into dust. Our observations imply that supernovae can produce the large dust masses detected in young galaxies at very high redshifts.

HerMES: The SPIRE confusion limit

We report on the sensitivity of SPIRE photometers on the Herschel Space Observatory. Specifically, we measure the confusion noise from observations taken during the Science Demonstration Phase of the Herschel Multi-tiered Extragalactic Survey. Confusion noise is defined to be the spatial variation of the sky intensity in the limit of infinite integration time, and is found to be consistent among the different fields in our survey at the level of 5.8, 6.3 and 6.8 mJy/beam at 250, 350 and 500 microns, respectively. These results, together with the measured instrument noise, may be used to estimate the integration time required for confusion-limited maps, and provide a noise estimate for maps obtained by SPIRE.

HerMES : SPIRE galaxy number counts at 250, 350, and 500 μm

Emission at far-infrared wavelengths makes up a significant fraction of the total light detected from galaxies over the age of Universe. Herschel provides an opportunity for studying galaxies at the peak wavelength of their emission. Our aim is to provide a benchmark for models of galaxy population evolution and to test pre-existing models of galaxies. With the Herschel Multi-tiered Extra-galactic survey, HerMES, we have observed a number of fields of different areas and sensitivity using the SPIRE instrument on Herschel. We have determined the number counts of galaxies down to ~20 mJy. Our constraints from directly counting galaxies are consistent with, though more precise than, estimates from the BLAST fluctuation analysis. We have found a steep rise in the Euclidean normalised counts <100 mJy. We have directly resolved ~15% of the infrared extra-galactic background at the wavelength near where it peaks.

Evolution of Dust in Primordial Supernova Remnants: Can Dust Grains Formed in the Ejecta Survive and Be Injected into the Early Interstellar Medium?

We investigate the evolution of dust that formed at Population III supernova (SN) explosions and its processing through the collisions with the reverse shocks resulting from the interaction of the SN ejecta with the ambient medium. In particular, we investigate the transport of the shocked dust within the SNR and its effect on the chemical composition, the size distribution, and the total mass of dust surviving in SNRs. We find that the evolution of the reverse shock, and hence its effect on the processing of the dust, depends on the thickness of the envelope retained by the progenitor star. Furthermore, the transport and survival of the dust grains depend on their initial radius, aini, and composition: for Type II SNRs expanding into the ISM with a density of nH,0 = 1 cm-3, small grains with aini 0.05 μm are completely destroyed by sputtering in the postshock flow, while grains with aini = 0.05-0.2 μm are trapped into the dense shell behind the forward shock. Very large grains of aini 0.2 μm are ejected into the ISM without decreasing their sizes significantly. We find that the total mass fraction of dust that is destroyed by the reverse shock ranges from 0.2 to 1.0, depending on the energy of the explosion and the density of the ambient ISM. The results of our calculations have significant impact on the abundance pattern of the second-generation stars that form in the dense shell of primordial SNRs.

The Evolution of Dust in the Early Universe with Applications to the Galaxy SDSS J1148+5251

Dusty hyperluminous galaxies in the early universe provide unique environments for studying the role of massive stars in the formation and destruction of dust. At redshifts above ~6, when the universe was less than ~1 Gyr old, dust could have only condensed in the explosive ejecta of Type II supernovae (SNe), since most of the progenitors of the asymptotic giant branch stars, the major alternative source of interstellar dust, did not have time to evolve off the main sequence since the onset of star formation. In this paper we present analytical models for the evolution of the gas, dust, and metals in high-redshift galaxies, with a special application to SDSS J1148+5251 (hereafter J1148+5251), a hyperluminous quasar at z = 6.4. We find that an average SN must condense at least 1 M☉ of dust to account for the observed dust mass in J1148+5251. Observationally, it is in excess of the largest dust yield of 0.02 M☉ found thus far in the ejecta of any SN. If future observations find this to be a typical SN dust yield, then additional processes, such as accretion onto preexisting grains or condensation around the active galactic nucleus, will need to be invoked to account for the large amount of dust in this and similar objects. The galaxys star formation history is still uncertain, and current observations of the gas, metal, and dust contents of J1148+5251 can be reproduced by either an intensive and short burst of star formation (ψ 103 M☉ yr-1) with a duration of 108 yr or a much lower star formation rate (ψ ≈ 100 M☉ yr-1) occurring over the lifetime of the galaxy.

Herschel and SCUBA-2 imaging and spectroscopy of a bright, lensed submillimetre galaxy at z = 2.3

We present a detailed analysis of the far-infrared (-IR) properties of the bright, lensed, z = 2.3, submillimetre-selected galaxy (SMG), SMM J2135-0102 (hereafter SMM J2135), using new observations with Herschel, SCUBA-2 and the Very Large Array (VLA). These data allow us to constrain the galaxys spectral energy distribution (SED) and show that it has an intrinsic rest-frame 8-1000-μm luminosity, Lbol, of (2.3±0.2) × 1012 and a likely star-formation rate (SFR) of ~400 yr-1. The galaxy sits on the far-IR/radio correlation for far-IR-selected galaxies. At 70 μm, the SED can be described adequately by dust components with dust temperatures, Td ~ 30 and 60 k. Using SPIREs Fourier- transform spectrometer (FTS) we report a detection of the [C ii] 158 μm cooling line. If the [C ii], CO and far-IR continuum arise in photo-dissociation regions (PDRs), we derive a characteristic gas density, n ~ 103 cm-3, and a far-ultraviolet (-UV) radiation field, G0, 103× stronger than the Milky Way. L[CII]/Lbol is significantly higher than in local ultra-luminous IR galaxies (ULIRGs) but similar to the values found in local star-forming galaxies and starburst nuclei. This is consistent with SMM J2135 being powered by starburst clumps distributed across ~2 kpc, evidence that SMGs are not simply scaled-up ULIRGs. Our results show that SPIREs FTS has the ability to measure the redshifts of distant, obscured galaxies via the blind detection of atomic cooling lines, but it will not be competitive with ground-based CO-line searches. It will, however, allow detailed study of the integrated properties of high-redshift galaxies, as well as the chemistry of their interstellar medium (ISM), once more suitably bright candidates have been found.

The Herschel Multi-Tiered Extragalactic Survey: source extraction and cross-identifications in confusion-dominated SPIRE images

We present the cross-identification and source photometry techniques used to process Herschel SPIRE imaging taken as part of the Herschel Multi-Tiered Extragalactic Survey (HerMES). Cross-identifications are performed in map-space so as to minimize source-blending effects. We make use of a combination of linear inversion and model selection techniques to produce reliable cross-identification catalogues based on Spitzer MIPS 24-mu m source positions. Testing on simulations and real Herschel observations shows that this approach gives robust results for even the faintest sources (S-250 similar to 10 mJy). We apply our new technique to HerMES SPIRE observations taken as part of the science demonstration phase of Herschel. For our real SPIRE observations, we show that, for bright unconfused sources, our flux density estimates are in good agreement with those produced via more traditional point source detection methods (SUSSEXtractor) by Smith et al. When compared to the measured number density of sources in the SPIRE bands, we show that our method allows the recovery of a larger fraction of faint sources than these traditional methods. However, this completeness is heavily dependent on the relative depth of the existing 24-mu m catalogues and SPIRE imaging. Using our deepest multiwavelength data set in the GOODS-N, we estimate that the use of shallow 24-mu m catalogues in our other fields introduces an incompleteness at faint levels of between 20-40 per cent at 250 mu m.