Robert S. Priddey

The SCUBA Half-Degree Extragalactic Survey - II. Submillimetre maps, catalogue and number counts

We present maps, source catalogue and number counts of the largest, most complete and unbiased extragalactic submillimetre survey: the 850-μm SCUBA Half-Degree Extragalactic Survey (SHADES). Using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT), SHADES mapped two separate regions of sky: the Subaru/XMM–Newton Deep Field (SXDF) and the Lockman Hole East (LH). Encompassing 93 per cent of the overall acquired data (i.e. data taken up to 2004 February 1), these SCUBA maps cover 720 arcmin2 with a rms noise level of about 2 mJy and have uncovered >100 submillimetre galaxies. In order to ensure the utmost robustness of the resulting source catalogue, data reduction was independently carried out by four subgroups within the SHADES team, providing an unprecedented degree of reliability with respect to other SCUBA catalogues available from the literature. Individual source lists from the four groups were combined to produce a robust 120-object SHADES catalogue; an invaluable resource for follow-up campaigns aiming to study the properties of a complete and consistent sample of submillimetre galaxies. For the first time, we present deboosted flux densities for each submillimetre galaxy found in a large survey. Extensive simulations and tests were performed separately by each group in order to confirm the robustness of the source candidates and to evaluate the effects of false detections, completeness and flux density boosting. Corrections for these effects were then applied to the data to derive the submillimetre galaxy source counts. SHADES has a high enough number of detected sources that meaningful differential counts can be estimated, unlike most submillimetre surveys which have to consider integral counts. We present differential and integral source number counts and find that the differential counts are better fit with a broken power law or a Schechter function than with a single power law; the SHADES data alone significantly show that a break is required at several mJy, although the precise position of the break is not well constrained. We also find that a 850-μm survey complete down to 2 mJy would resolve 20–30 per cent of the far-infrared background into point sources.

A mean redshift of 2.8 for swift gamma-ray bursts

The exceptionally high luminosities of gamma-ray bursts (GRBs), gradually emerging as extremely useful probes of star formation, make them promising tools for exploration of the high-redshift Universe. Here we present a carefully selected sample of Swift GRBs, intended to estimate in an unbiased way the GRB mean redshift (z(mean)), constraints on the fraction of high-redshift bursts and an upper limit on the fraction of heavily obscured afterglows. We find that z(mean) = 2.8 and that at least 7% of GRBs originate at z > 5. In addition, consistent with pre-Swift observations, at most 20% of afterglows can be heavily obscured. The redshift distribution of the sample is qualitatively consistent with models where the GRB rate is proportional to the star formation rate in the Universe. We also report optical, near-infrared and X-ray observations of the afterglow of GRB 050814, which was seen to exhibit very red optical colours. By modelling its spectral energy distribution we find that z = 5.3 +/- 0.3. The high mean redshift of GRBs and their wide redshift range clearly demonstrates their suitability as efficient probes of galaxies and the intergalactic medium over a significant fraction of the history of the Universe. (Less)

Probing cosmic chemical evolution with gamma-ray bursts: GRB 060206 at z = 4.048

Aims.We present early optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 with the aim of determining the metallicity of the GRB absorber and the physical conditions in the circumburst medium. We also discuss how GRBs may be important complementary probes of cosmic chemical evolution. Methods.Absorption line study of the GRB afterglow spectrum. Results.We determine the redshift of the GRB to be z=4.04795±0.00020. Based on the measurement of the neutral hydrogen column density from the damped Lyman-alpha line and the metal content from weak, unsaturated S II lines we derive a metallicity of [S/H]=-0.84±0.10. This is one of the highest metallicities measured from absorption lines at z~4. From the very high column densities for the forbidden Si II*, O I*, and O I** lines we infer very high densities and low temperatures in the system. There is evidence for the presence of H2 molecules with log N(H_2)~17.0, translating into a molecular fraction of log{f}≈ -3.5 with f=2N(H2)/(2N(H2) + N(H I)). Even if GRBs are only formed by single massive stars with metallicities below ~0.3 Zo, they could still be fairly unbiased tracers of the bulk of the star formation at z>2. Hence, metallicities as derived for GRB 060206 here for a complete sample of GRB afterglows will directly show the distribution of metallicities for representative star-forming galaxies at these redshifts.

An Exploratory Chandra Survey of a Well-defined Sample of 35 Large Bright Quasar Survey Broad Absorption Line Quasars

We present 4–7 ks Chandra observations of 35 broad absorption line (BAL) quasars from the Large Bright Quasar Survey, the largest sample of sensitive, 0.5–8.0 keV X-ray observations of this class of quasars to date. The limitedrangesinbothredshift(z ¼ 1:42 2:90)andultravioletluminosity(afactorof � 12)ofthesamplealsomake itrelativelyuniform.Of 35 targets,27 aredetected fora detectionfractionof 77%, and we confirmprevious studies that find BAL quasars to be generally X-ray-weak. Five of the eight nondetections are known low-ionization BAL quasars, confirming reports of extreme X-ray weakness in this subset (� 10% of optically selected BAL quasars). ThoseBALquasarswiththehardestX-rayspectraarealsotheX-rayweakest,consistentwiththeinterpretationthat intrinsic absorption is the primary cause of X-ray weakness in this class of quasars as a whole. Furthermore, the observed trend is not consistent with simple neutral absorption, supporting findings from spectroscopic observations of individual targets that BAL quasars typically exhibit complex X-ray absorption (e.g., partially covering orionizedabsorbers).AssumingnormalquasarX-raycontinuaandusingthehard-band(observed-frame2–8keV) X-ray flux to ‘‘correct’’ for the effects of intrinsic absorption at softer energies increases the relative X-ray to optical flux ratios to much closer to the range for normal quasars, further indicating that typically neither are highionizationBALquasarsintrinsicallyX-ray-weak,nordotheysufferfromCompton-thickabsorption.Ingeneral,we find no evidence for correlations between X-ray weakness and ultraviolet absorption-line properties, with the exception of a likely correlation between the maximum outflow velocity of C iv absorption and the magnitude of X-ray weakness. We discuss the implications of our results for disk-wind models of BAL outflows in quasars. Subject headingg galaxies: active — quasars: absorption lines — quasars: general — X-rays: galaxies

H I column densities of z > 2 Swift gamma-ray bursts

Context: .Before the launch of the Swift satellite, the majority of the gamma-ray burst (GRB) afterglows for which Lyalpha was redshifted into the observable spectrum showed evidence for a damped Lyalpha absorber. This small sample indicated that GRBs explode either in galaxies, or regions within them, having high neutral hydrogen column densities. Aims: .To increase the spectroscopic sample of GRBs with z>2 and hence establish the N(H I) distribution along GRB lines-of-sight. Methods: .We have obtained six z > 2 GRB afterglow spectra and fitted the Lyalpha absorption line in each case to determine N(H I). This has been complemented with 12 other Swift N(H I) values from the literature. Results: .We show that the peak of the GRB N(H I) distribution is qualitatively consistent with a model where GRBs originate in Galactic-like molecular clouds. However, a systematic difference, in particular an excess of low column-density systems compared to the predictions, indicates that selection effects and conditions within the cloud (e.g. strong ionization) influence the observed N(H I) range. We also report the discovery of Lyalpha emission from the GRB 060714 host, corresponding to a star-formation rate of approximately 0.8 Mo yr-1. Finally, we present accurate redshifts of the six bursts: z = 3.240 ± 0.001 (GRB 050319), z = 2.198 ± 0.002 (GRB 050922C), z = 3.221 ± 0.001 (GRB 060526), z = 3.425 ± 0.002 (GRB 060707), z = 2.711 ± 0.001 (GRB 060714) and z = 3.686 ± 0.002 (GRB 060906).

The SCUBA half degree extragalactic survey – IV. Radio–mm–FIR photometric redshifts

We present the redshift distribution of the Submillimetre Common-User Bolometer Array (SCUBA) Half Degree Survey (SHADES) galaxy population based on the rest-frame radio–mm–far-infrared (FIR) colours of 120 robustly detected 850 μm sources in the Lockman Hole East (LH) and Subaru XMM–Newton Deep Field (SXDF). The redshift distribution derived from the full spectral energy distribution (SED) information is shown to be narrower than that determined from the radio–sub-mm spectral index, as more photometric bands contribute to a higher redshift accuracy. The redshift distribution of sources derived from at least two photometric bands peaks at z≈ 2.4 and has a near-Gaussian distribution, with 50 per cent (interquartile range) of sources at z= 1.8–3.1 . We find a statistically significant difference between the measured redshift distributions in the two fields; the SXDF peaking at a slightly lower redshift (median z ≈ 2.2 ) than the LH (median z ≈ 2.7 ), which we attribute to the noise properties of the radio observations. We demonstrate, however, that there could also be field-to-field variations that are consistent with the measured differences in the redshift distributions and, hence, that the incomplete area observed by SHADES with SCUBA, despite being the largest sub-mm survey to date, may still be too small to fully characterize the bright sub-mm galaxy population. Finally, we present a brief comparison with the predicted, or assumed, redshift distributions of sub-mm galaxy formation and evolution models, and we derive the contribution of these SHADES sources and the general sub-mm galaxy population to the star formation rate density at different epochs.

The SCUBA half-degree extragalactic survey - I. Survey motivation, design and data processing

he Submillimetre Common-User Bolometer Array (SCUBA) Half-Degree Extragalactic Survey (SHADES) is a major new blank-field extragalactic submillimetre (submm) survey currently underway at the James Clerk Maxwell Telescope (JCMT). Ultimately, SHADES aims to cover half a square degree at 450 and 850m to a 4 depth of ~= 8mJy at 850m. Two fields are being observed, the Subaru/XMM-Newton Deep Field (SXDF) (02h18m- 05) and the Lockman Hole East (10h52m+ 57). The survey has three main aims: (i) to investigate the population of high-redshift submm galaxies and the cosmic history of massive dust-enshrouded star formation activity; (ii) to investigate the clustering properties of submm-selected galaxies in order to determine whether these objects could be progenitors of present-day massive ellipticals; and (iii) to investigate the fraction of submm-selected sources that harbour active galactic nuclei. To achieve these aims requires that the submm data be combined with co-spatial information spanning the radio-to-X-ray frequency range. Accordingly, SHADES has been designed to benefit from ultra-deep radio imaging obtained with the Very Large Array (VLA), deep mid-infrared observations from the Spitzer Space Telescope, submm mapping by the Balloon-borne Large Aperture Submillimetre Telescope (BLAST), deep near-infrared imaging with the United Kingdom Infrared Telescope, deep optical imaging with the Subaru Telescope and deep X-ray observations with the XMM-Newton observatory. It is expected that the resulting extensive multiwavelength data set will provide complete photometric redshift information accurate to as well as detailed spectral energy distributions for the vast majority of the submm-selected sources. In this paper, the first of a series on SHADES, we present an overview of the motivation for the survey, describe the SHADES survey strategy, provide a detailed description of the primary data-analysis pipeline and demonstrate the superiority of our adopted matched-filter source-extraction technique over, for example, Emerson-II style methods. We also report on the progress of the survey. As of 2004 February, 720arcmin2 had been mapped with SCUBA (about 40 per cent of the anticipated final total area) to a median 1 depth of 2.2mJy per beam at 850m (25mJy per beam at 450m), and the source-extraction routines give a source density of 650 +/- 50 sources deg-2 > 3 at 850m. Although uncorrected for Eddington bias, this source density is more than sufficient for providing enough sources to answer the science goals of SHADES, once half a square degree is observed. A refined reanalysis of the original 8-mJy survey Lockman hole data was carried out in order to evaluate the new data-reduction pipeline. Of the 17 most secure sources in the original sample, 12 have been reconfirmed, including 10 of the 11 for which radio identifications were previously secured.

The SCUBA HAlf Degree Extragalactic Survey (SHADES) -VII. Optical/IR photometry and stellar masses of submillimetre galaxies

We present estimates of the photometric redshifts, stellar masses and star formation histories of sources in the Submillimetre Common-User Bolometer Array (SCUBA) HAlf Degree Extragalactic Survey (SHADES). This paper describes the 60 SCUBA sources detected in the Lockman Hole covering an area of ∼320 arcmin 2 . Using photometry spanning the B band to 8 μm, we find that the average SCUBA source forms a significant fraction of its stars in an early period of star formation and that most of the remainder forms in a shorter more intense burst around the redshift it is observed. This trend does not vary significantly with source redshift. However, the sources show a clear increase in stellar mass with redshift, consistent with downsizing. In terms of spectral energy distribution types, only two out of the 51 sources we have obtained photometric redshifts for are best fitted by a quasar-like spectrum, with approximately 80 per cent of the sources being best fitted with late-type spectra (Sc, Im and starburst). By including photometry at 850 μm, we conclude that the average SCUBA source is forming stars at a rate somewhere between 6 and 30 times the rate implied from the rest-frame optical in a dust obscured burst and that this burst creates 15‐65 per cent of the total stellar mass. Using a simplistic calculation, we estimate from the average star formation history that between one in five and one in 15 bright (L∗ + 2 < Loptical < L∗ − 1 mag) galaxies in the field over the interval 0 < z < 3 will at some point in their lifetime

An origin in the local Universe for some short |[gamma]|-ray bursts

Gamma-ray bursts (GRBs) divide into two classes: ‘long’, which typically have initial durations of T90 > 2 s, and ‘short’, with durations of T90 < 2 s (where T90 is the time to detect 90% of the observed fluence). Long bursts, which on average have softer γ-ray spectra, are known to be associated with stellar core-collapse events—in some cases simultaneously producing powerful type Ic supernovae. In contrast, the origin of short bursts has remained mysterious until recently. A subsecond intense ‘spike’ of γ-rays during a giant flare from the Galactic soft γ-ray repeater, SGR 1806–20, reopened an old debate over whether some short GRBs could be similar events seen in galaxies out to ∼70 Mpc (refs 6–10; redshift z ≈ 0.016). Shortly after that, localizations of a few short GRBs (with optical afterglows detected in two cases) have shown an apparent association with a variety of host galaxies at moderate redshifts. Here we report a correlation between the locations of previously observed short bursts and the positions of galaxies in the local Universe, indicating that between 10 and 25 per cent of short GRBs originate at low redshifts (z < 0.025).

A 1.2 mm MAMBO/IRAM-30 m study of dust emission from optically luminous 2 quasars

We report 250 GHz (1.2 mm) observations of redshift 1.8 < z < 2.8 optically luminous (M B < -27.0), radio quiet quasars using the Max-Planck Millimeter Bolometer (MAMBO) array at the IRAM 30-metre telescope. Nine quasars were detected and for 26 quasars 3 σ- flux density limits in the range 1.8 to 4 mJy were obtained. Adopting a typical dust temperature of 45 K, the millimeter emission implies far-infrared luminosities of order 10 1 3 L O .and dust masses of ∼10 8 M O .. Applying a statistical survival analysis to our total sample of 43 detected and 95 undetected quasars at z 2 and z ≥ 4, we find that there is no apparent difference in the far-infrared (FIR) luminosities, hence the star formation rates, of QSOs at z ≥ 2 and at 3.6 < z < 5. This differs from radio galaxies, for which the FIR luminosity was found to increase with redshift (Archibald et al. 2001). We furthermore find that there is no strong correlation between the far-infrared and optical luminosities, confirming previous results obtained on smaller samples.