The far-infrared–submillimetre spectral energy distribution of high-redshift quasars

We combine photometric observations of high-redshift     quasars, obtained at submillimetre to millimetre wavelengths, to obtain a mean far-infrared (rest-frame) spectral energy distribution (SED) of the thermal emission from dust, parametrized by a single temperature ( T ) and power-law emissivity index ( β ). The best-fitting values are     and     . Our method exploits the redshift spread of this set of quasars, which allows us to sample the SED at a larger number of rest wavelengths than is possible for a single object: the wavelength range extends down to ∼60 μm, and therefore samples the turnover in the greybody curve for these temperatures. This parametrization is of use to any studies that extrapolate from a flux at a single wavelength, for example to infer dust masses and far-infrared luminosities.
We interpret the cool, submillimetre component as arising from dust heated by star formation in the host galaxy of the quasar, although we do not exclude the presence of dust heated directly by the active galactic nucleus (AGN). Applying the mean SED to the data, we derive consistent star formation rates ∼1000 M_⊙ yr⁻¹ and dust masses ∼10⁹ M_⊙, and investigate a simple scheme of AGN and host galaxy co-evolution to account for these quantities. The time-scale for formation of the host galaxy is     , and the luminous quasar phase occurs towards the end of this period, just before the reservoir of cold gas is depleted. Given the youth of the Universe at     (1.6 Gyr), the coexistence of a massive black hole and a luminous starburst at high redshifts is a powerful constraint on models of quasar host galaxy formation.     

The HELLAS2XMM survey – XII. The infrared/submillimetre view of an X-ray selected type 2 quasar at z≈ 2

We present multiwavelength observations (from optical to submillimetre, including Spitzer and Submillimetre Common-User Bolometer Array) of H2XMMJ 003357.2−120038 (also GD 158_19), an X-ray selected, luminous narrow-line (type 2) quasar at   z = 1.957  selected from the HELLAS2XMM survey. Its broad-band properties can be reasonably well modelled assuming three components: a stellar component to account for the optical and near-infrared (IR) emission; an active galactic nucleus (AGN) component (i.e. dust heated by an accreting active nucleus), dominant in the mid-IR, with an optical depth at 9.7      along the line of sight (close to the equatorial plane of the obscuring matter) of  τ(9.7) = 1  and a full covering angle of the reprocessing matter (torus) of 140° and a far-IR starburst component (i.e. dust heated by star formation) to reproduce the wide bump observed longward of 70      .
The derived star formation rate is  ≈1500 M_⊙ yr⁻¹  . The overall modelling indicates that GD 158_19 is a high-redshift X-ray luminous, obscured quasar with coeval powerful AGN activity and intense star formation. It is probably caught before the process of expelling the obscuring gas has started, thus quenching the star formation.     

Implications for unified schemes from submillimetre and far-infrared follow-up of radio-selected samples

We extend our previous analysis which used generalized luminosity functions (GLFs) to predict the number of quasars and galaxies in low-radio-frequency-selected samples as a function of redshift, radio luminosity, narrow-emission-line luminosity and type of unified scheme. Our extended analysis incorporates the observed submillimetre (850-μm) flux densities of radio sources, employs a new method which allows us to deal with non-detections, and focuses on the high-luminosity population. First, we conclude that the submillimetre luminosity L ₈₅₀ of low-frequency-selected radio sources is correlated with the bolometric luminosity L _bol of their quasar nuclei via an approximate scaling relation   L ₈₅₀∝ L ^0.7±0.2_bol  . Secondly, we conclude that there is quantitative evidence for a receding-torus-like physical process for the high-luminosity population within a two-population unified scheme for radio sources; this evidence comes from the fact that radio quasars are brighter in both narrow emission lines and submillimetre luminosity than radio galaxies matched in radio luminosity and redshift. Thirdly, we note that the combination of a receding-torus-like scheme and the assumption that the observed submillimetre emission is dominated by quasar-heated dust yields a scaling relation   L ₈₅₀∝ L ^1/2_bol  which is within the errors of that determined here for radio-selected quasars, and consistent with that inferred for radio-quiet quasars.     

Infrared and submillimetre studies of Vega-excess stars

We report new infrared and submillimetre observations of a sample of 24 candidate Vega-excess stars, and derive CO masses, dust masses, gas to dust ratios and the strengths of various emission lines. Most of these stars have dustier discs than the class archetypes (Vega, Fomalhaut, β Pic, etc.), yet, like the archetypes, all the stars observed in CO show the gas content of their discs to be depleted compared with molecular cloud values. We discuss how the extra dust content might imply that these stars are less evolved than the archetypes, and use other infrared and submillimetre characteristics to support this contention.     

The temperatures of dust-enshrouded active galactic nuclei

A high density of massive dark objects (MDOs), probably supermassive black holes, in the centres of nearby galaxies has been inferred from recent observations. There are various indications that much of the accretion responsible for producing these objects took place in dust-enshrouded active galactic nuclei (AGNs). If so, then measurements of the intensity of background radiation and the source counts in the far-infrared and submillimetre wavebands constrain the temperature of dust in these AGNs. An additional constraint comes from the hard X-ray background, if this is produced by accretion. One possibility is that the dust shrouds surrounding the accreting AGNs are cold, about 30 K. In this event, the dusty AGNs could be some subset of the population of luminous distant sources discovered at 850 μm using the SCUBA array on the James Clerk Maxwell Telescope, as proposed by Almaini, Lawrence & Boyle. An alternative is that the dust shrouds surrounding the accreting AGNs are much hotter (>60 K). These values are closer to the dust temperatures of a number of well-studied low-redshift ultraluminous galaxies that are thought to derive their power from accretion. If the local MDO density is close to the maximum permitted, then cold sources cannot produce this density without the submillimetre background being overproduced if they accrete at high radiative efficiency, and thus a hot population is required. If the dust-enshrouded accretion occurred at similar redshifts to that taking place in unobscured optical quasars, then a significant fraction of the far-infrared background radiation measured by COBE at 140 μm, but very little of the submillimetre background at 850 μm, may have been produced by hot dust-enshrouded AGNs which may have already been seen in recent Chandra X-ray surveys.     

Hyperluminous infrared galaxies

39 galaxies are now known, from follow-up of faint IRAS sources and from submillimetre observations of high-redshift AGN, with far-infrared luminosities >10¹³ L_⊙. 13 of these, which have been found in 60- or 850-μm surveys, form an important unbiased subsample. 12 have been found by comparison of 60-μm surveys with quasar or radio galaxy catalogues, or from infrared surveys with colour selection biased towards AGN, while a further 14 have been found through submillimetre observations of known high-redshift AGN. In this paper I argue, on the basis of detailed modelling of the spectral energy distributions of hyperluminous galaxies with accurate radiative transfer models, and from evidence of high gas mass in several cases, that the bulk of the emission from these galaxies at rest frame wavelengths ≥50 μm is caused by star formation. Even after correction for the effects of lensing, hyperluminous galaxies with emission peaking at rest frame wavelengths ≥50 μm are therefore undergoing star formation at rates >10³ M_⊙ yr⁻¹ and are strong candidates for being primeval galaxies, in the process of a major episode of star formation.     

类星体SDSS J0916+2921的类银河系2175 ?尘埃消光特征

光谱观测显示类星体SDSS J091613.60+292106.1 (简称J0916+2921,系统红移zem=1.1418±0.0018)中有特殊的2175?尘埃消光特征,其强度显著大于银河系平均强度.光谱同时探测到与尘埃成协的丰富气体吸收线,确定吸收线系统红移为zabs=1.1413±0.0002,和类星体红移一致.气态金属离子柱密度相对太阳丰度的比例为[Al/Zn]=-1.68±0.10,[Cr/Zn]=-0.49±0.10,[Fe/Zn]=-0.81±0.18.尘埃耗散作用显著,说明该系统中尘埃十分丰富,与观测的强尘埃消光特征吻合.类银河系的2175?尘埃消光特征在类星体光谱中多见于中间插入吸收线系统,至今未明确认证内禀吸收线系统出现该特征,类星体SDSS J0916+2921是目前仅有的数个候选者之一.该类星体X射线辐射相较一般类星体更强,后续可用作研究2175?尘埃在强高能射线照射条件下的形成与离解平衡,以揭示2175?尘埃的化学成分、物理性质和起源.     

A census of metals at high and low redshift and the connection between submillimetre sources and spheroid formation

Deep surveys in many wavebands have shown that the rate at which stars were forming was at least a factor of 10 higher at redshifts >1 than today. Heavy elements ('metals') are produced by stars, and the star formation history deduced by these surveys implies that a significant fraction of all metals in the Universe today should already exist at   z ∼ 2–3  . However, only 10 per cent of the total metals expected to exist at this redshift have so far been accounted for (in damped Lyman α absorbers and the Lyman forest). In this paper, we use the results of submillimetre surveys of the local and high-redshift Universe to show that there was much more dust in galaxies in the past. We find that a large proportion of the missing metals are traced by this dust, bringing the metals implied from the star formation history and observations into agreement. We also show that the observed distribution of dust masses at high redshift can be reproduced remarkably well by a simple model for the evolution of dust in spheroids, suggesting that the descendants of the dusty galaxies found in deep submillimetre surveys are the relatively dust-free spiral bulges and ellipticals in the Universe today.     

类星体SDSSJ0916+2921的类银河系2175A 尘埃消光特征

光谱观测显示类星体SDSS J091613.60+292106.1 (简称J0916+2921, 系统红移zem=1.1418\pm 0.0018)中有特殊的2175A尘埃消光特征, 其强度显著大于银河系平均强度. 光谱同时探测到与尘埃成协的丰富气体吸收线, 确定吸收线系统红移为zbs = 1.1413 \pm 0.0002, 和类星体红移一致. 气态金属离子柱密度相对太阳丰度的比例为Al/Zn =-1.68 \pm 0.10, Cr/Zn=-0.49 \pm0.10, Fe/Zn = -0.81 pm 0.18. 尘埃耗散作用显著, 说明该系统中尘埃十分丰富, 与观测的强尘埃消光特征吻合. 类银河系的2175A尘埃消光特征在类星体光谱中多见于中间插入吸收线系统, 至今未明确认证内禀吸收线系统出现该特征, 类星体SDSS J0916+2921是目前仅有的数个候选者之一. 该类星体X射线辐射相较一般类星体更强, 后续可用作研究2175A}尘埃在强高能射线照射条件下的形成与离解平衡, 以揭示2175A尘埃的化学成分、物理性质和起源.     

The initial conditions of isolated star formation – IV. C18O observations and modelling of the pre-stellar core L1689B

We present C¹⁸O observations of the pre-stellar core L1689B, in the J =3→2 and 2→1 rotational transitions, taken at the James Clerk Maxwell Telescope in Hawaii. We use a λ -iteration radiative transfer code to model the data. We adopt a similar form of radial density profile to that which we have found in all pre-stellar cores, with a 'flat' inner profile, steepening towards the edge, but we make the gradient of the 'flat' region a free parameter. We find that the core is close to virial equilibrium, but there is tentative evidence for core contraction. We allow the temperature to vary with a power-law form and find that we can consistently fit all of the CO data with an inverse temperature gradient that is warmer at the edge than at the centre. However, when we combine the CO data with the previously published millimetre data we fail to find a simultaneous fit to both data sets without additionally allowing the CO abundance to decrease towards the centre. This effect has been observed qualitatively many times before, as the CO freezes out on to the dust grains at high densities, but we quantify the effect. Hence we show that the combination of millimetre/submillimetre continuum and spectral line data is a very powerful method of constraining the physical parameters of cores on the verge of forming stars.     

Dust in the solar system and in extra-solar planetary systems

Among the observed circumstellar dust envelopes a certain population, planetary debris disks, is ascribed to systems with optically thin dust disks and low gas content. These systems contain planetesimals and possibly planets and are believed to be systems that are most similar to our solar system in an early evolutionary stage. Planetary debris disks have been identified in large numbers by a brightness excess in the near-infrared, mid-infrared and/or submillimetre range of their stellar spectral energy distributions. In some cases, spatially resolved observations are possible and reveal complex spatial structures. Acting forces and physical processes are similar to those in the solar system dust cloud, but the observational approach is obviously quite different: overall spatial distributions for systems of different ages for the planetary debris disks, as opposed to detailed local information in the case of the solar system. Comparison with the processes of dust formation and evolution observed in the solar system therefore helps understand the planetary debris disks. In this paper, we review our present knowledge of observations, acting forces, and major physical interactions of the dust in the solar system and in similar extra-solar planetary systems.     

Observational limits to source confusion in the millimetre/submillimetre waveband

The first observations to detect a population of distant galaxies directly in the submillimetre waveband have recently been made using the new Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). The results indicate that a large number of distant galaxies are radiating strongly in this waveband. Here we discuss their significance for source confusion in future millimetre/submillimetre-wave observations of both distant galaxies and cosmic microwave background radiation (CMBR) anisotropies. Earlier estimates of such confusion involved significant extrapolation of the results of observations of galaxies at low redshifts; our new estimates do not, as they are derived from direct observations of distant galaxies in the submillimetre waveband. The results have important consequences for the design and operation of existing and proposed millimetre/submillimetre-wave telescopes: the Planck Surveyor survey will be confusion-limited at frequencies greater than 350 GHz, even in the absence of Galactic dust emission; a 1σ confusion noise limit of about 0.44 mJy beam⁻¹ is expected for the JCMT/SCUBA at a wavelength of 850 μm; and the subarcsecond resolution of large millimetre/submillimetre-wave interferometer arrays will be required in order to execute very deep galaxy surveys.     

A submillimetre survey of the star formation history of radio galaxies

We present the results of the first major systematic submillimetre survey of radio galaxies spanning the redshift range 1< z <5. The primary aim of this work is to elucidate the star formation history of this sub class of elliptical galaxies by tracing the cosmological evolution of dust mass. Using SCUBA on the JCMT, we have obtained 850-μm photometry of 47 radio galaxies to a consistent rms depth of 1 mJy, and have detected dust emission in 14 cases. The radio galaxy targets have been selected from a series of low-frequency radio surveys of increasing depth (3CRR, 6CE, etc.), in order to allow us to separate the effects of increasing redshift and increasing radio power on submillimetre luminosity. Although the dynamic range of our study is inevitably small, we find clear evidence that the typical submillimetre luminosity (and hence dust mass) of a powerful radio galaxy is a strongly increasing function of redshift; the detection rate rises from ≃15 per cent at z <2.5 to ≳75 per cent at z >2.5, and the average submillimetre luminosity rises at a rate ∝(1+ z )³ out to z ≃4. Moreover, our extensive sample allows us to argue that this behaviour is not driven by underlying correlations with other radio galaxy properties such as radio power, radio spectral index, or radio source size/age. Although radio selection may introduce other more subtle biases, the redshift distribution of our detected objects is in fact consistent with the most recent estimates of the redshift distribution of comparably bright submillimetre sources discovered in blank field surveys. The evolution of submillimetre luminosity found here for radio galaxies may thus be representative of massive ellipticals in general.     

The chemical evolution of the Universe – I. High column density absorbers

We construct a simple, robust model of the chemical evolution of galaxies from high to low redshift, and apply it to published observations of damped Lyman α quasar absorption line systems (DLAs). The elementary model assumes quiescent star formation and isolated galaxies (no interactions, mergers or gas flows). We consider the influence of dust and chemical gradients in the galaxies, and hence explore the selection effects in quasar surveys. We fit individual DLA systems to predict some observable properties of the absorbing galaxies, and also indicate the expected redshift behaviour of chemical element ratios involving nucleosynthetic time delays.
Despite its simplicity, our 'monolithic collapse' model gives a good account of the distribution and evolution of the metallicity and column density of DLAs, and of the evolution of the global star formation rate and gas density below redshifts z ∼3. However, from the comparison of DLA observations with our model, it is clear that star formation rates at higher redshifts ( z >3) are enhanced. Galaxy interactions and mergers, and gas flows very probably play a major role.     

Confirming a population of hot-dust dominated, star-forming, ultraluminous galaxies at high redshift

We identify eight   z > 1  radio sources undetected at 850 μm but robustly detected at 70 μm, confirming that they represent ultraluminous infrared galaxies (ULIRGs) with hotter dust temperatures  (〈 T _d〉= 52 ± 10 K)  than submillimetre galaxies (SMGs) at similar luminosities and redshifts. These galaxies share many properties with SMGs: ultraviolet spectra consistent with starbursts, high stellar masses and radio luminosities. We can attribute their radio emission to star formation since high-resolution Multi-Element Radio Linked Interferometer Network (MERLIN) radio maps show extended emission regions (with characteristic radii of 2–3 kpc), which are unlikely to be generated by active galactic nucleus (AGN) activity. These observations provide the first direct confirmation of hot, dusty ULIRGs which are missed by current submillimetre surveys. They have significant implications for future observations from the Herschel Space Observatory and Submillimetre Common-User Bolometer Array 2 (SCUBA2), which will select high-redshift luminous galaxies with less selection biases.     

The complete submillimetre spectrum of NGC 891

Submillimetre maps of NGC 891 have been obtained with the Programme National d'Observations Submillimétriques (PRONAOS) balloon-borne telescope and with the Infrared Space Observatory Photopolarimeter (ISOPHOT) on board the ISO satellite. In this article, we also gather data from the Infrared Astronomical Satellite ( IRAS ) and the Submillimetre Common-User Bolometer Array (SCUBA) to present the complete submillimetre spectrum of this nearby edge-on spiral galaxy. We derive submillimetre emission profiles along the major axis. The modified blackbody fits, assuming a single dust component, lead to temperatures of 19–24 K towards the centre and 18–20 K towards the edges, with possible variations in the dust spectral index from 1.4 to 2. The two-component fits lead to a warm-component temperature of 29 K all along the galaxy, with a cold component at 16 K. The interstellar medium (ISM) masses derived by these two methods are quite different:  4.6×10⁹ M_⊙  in the case of the one-component model and  12 × 10⁹ M_⊙  in the case of the two-component one. This two-component fit indicates that the cold dust to warm dust ratio is between 20 and 40, the highest values being in the wings of this galaxy. Compared to dust mass estimates, both estimates of the ISM mass are consistent with a gas to dust mass ratio of 240, which is close to the Milky Way value. Our results illustrate the importance of accurate submillimetre spectra in deriving masses of the ISM in galaxies.     

ISO observations of the dusty quasar BR 1202−0725

We present mid- and far-infrared photometry of the high-redshift     dusty quasar BR 1202−0725. The quasar was detected in the near-infrared, at a flux level     consistent with an average radio-quiet quasar at its redshift. Only upper limits for the emission were obtained in the far-infrared. These upper limits, when combined with data from ground-based telescopes, are the first direct evidence for a turnover in the far-infrared emission, and hence confirm that a blackbody dominates the spectral energy distribution at far-infrared wavelengths. This blackbody is most probably cool dust, constrained to have a temperature below 80 K, for a β of 1.5.     

Millimetric properties of gamma-ray burst host galaxies

We present millimetre (mm) and submillimetre (submm) photometry of a sample of five host galaxies of gamma-ray bursts (GRBs), obtained using the Max Planck Millimetre Bolometer (MAMBO2) array and Submillimetre Common-User Bolometer Array (SCUBA). These observations were obtained as part of an ongoing project to investigate the status of GRBs as indicators of star formation. Our targets include two of the most unusual GRB host galaxies, selected as likely candidate submm galaxies: the extremely red  ( R − K ≈ 5)  host of GRB 030115, and the extremely faint  ( R > 29.5)  host of GRB 020124. Neither of these galaxies is detected, but the deep upper limits for GRB 030115 impose constraints on its spectral energy distribution, requiring a warmer dust temperature than is commonly adopted for submillimetre galaxies (SMGs).
As a framework for interpreting these data, and for predicting the results of forthcoming submm surveys of Swift -derived host samples, we model the expected flux and redshift distributions based on luminosity functions of both submm galaxies and GRBs, assuming a direct proportionality between the GRB rate density and the global star formation rate density. We derive the effects of possible sources of uncertainty in these assumptions, including (1) introducing an anticorrelation between GRB rate and the global average metallicity, and (2) varying the dust temperature.     

Extended dust emission in NGC 7465

We present SCUBA 850-μm, JCMT  CO( J =2→1)  , B -band imaging and VLA H  i observations of the NGC 7465/4/3 group of galaxies. The 850-μm emission associated with NGC 7465 extends to at least ∼2 R ₂₅ and is well correlated with the H  i . We investigate a range of possible mechanisms by which dust beyond R ₂₅ may be heated to give the observed extended submillimetre emission. By modelling the dust heating by stars in two extreme geometries, we fail to find any reasonable star formation scenario that is consistent with both the 850-μm and optical data. Furthermore, we do not detect any  CO( J =2→1)  emission coincident with the extended dust and atomic gas as would be expected if significant star formation were occurring. We show that shock-heating of dust via cloud–cloud collisions in the stripped interstellar medium of NGC 7465 could be sufficient to explain the extended 850-μm emission and lack of optical emission in the stripped gas, and suggest that cloud–cloud collisions may be an important dust heating mechanism in gas-rich systems.     

A search for the submillimetre counterparts to Lyman break galaxies

We have carried out targeted submillimetre observations as part of a programme to explore the connection between the rest-frame ultraviolet and far-infrared properties of star-forming galaxies at high redshift, which is currently poorly understood. On the one hand, the Lyman break technique is very effective at selecting     galaxies. On the other, 'blank-field' imaging in the submillimetre seems to turn up sources routinely, amongst which some are star-forming galaxies at similar redshifts. Already much work has been done searching for optical identifications of objects detected using the SCUBA instrument. Here we have taken the opposite approach, performing submillimetre photometry for a sample of Lyman break galaxies, the ultraviolet properties of which imply high star formation rates. The total signal from our Lyman break sample is undetected in the submillimetre, at an rms level of ∼0.5 mJy, which implies that the population of Lyman break galaxies does not constitute a large part of the recently detected blank-field submillimetre sources. However, our one detection suggests that with reasonable SCUBA integrations we might expect to detect those few Lyman break galaxies that are far-infrared brightest.