R. J. Ivison

The All-wavelength Extended Groth Strip International Survey (AEGIS) Data Sets

In this the first of a series of Letters, we present a panchromatic data set in the Extended Groth Strip region of the sky. Our survey, the All-Wavelength Extended Groth Strip International Survey (AEGIS), aims to study the physical properties and evolutionary processes of galaxies at z ~ 1. It includes the following deep, wide-field imaging data sets: Chandra/ACIS X-ray, GALEX ultraviolet, CFHT/MegaCam Legacy Survey optical, CFHT/CFH12K optical, Hubble Space Telescope/ACS optical and NICMOS near-infrared, Palomar/WIRC near-infrared, Spitzer/IRAC mid-infrared, Spitzer/MIPS far-infrared, and VLA radio continuum. In addition, this region of the sky has been targeted for extensive spectroscopy using the Deep Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II 10 m telescope. Our survey is compared to other large multiwavelength surveys in terms of depth and sky coverage.

SHARC-2 350 μm Observations of Distant Submillimeter-selected Galaxies

We present 350 μm observations of 15 Chapman et al. submillimeter galaxies (SMGs) with radio counterparts and optical redshifts. We detect 12 and obtain sensitive upper limits for three, providing direct, precise measurements of their far-infrared luminosities and characteristic dust temperatures. With these, we verify the linear radio-far-infrared correlation at redshifts of z ~ 1-3 and luminosities of 10^(11)-10^(13) L_☉, with a power-law index of 1.02 ± 0.12 and rms scatter of 0.12 dex. However, either the correlation constant q or the dust emissivity index β is lower than measured locally. The best-fitting q ≃2.14 is consistent with SMGs being predominantly starbust galaxies, without significant AGN contribution, at far-infrared wavelengths. Gas-to-dust mass ratios are estimated at 54^(+14)_(-11)(κ_(850μm)/0.15 m^2 kg^(-1)), depending on the absoption efficiency κ_ν, with intrinsic dispersion ≃40% around the mean value. Dust temperatures consistent with 34.6 ± 3 K (1.5/β)^(0.71), at z ~ 1.5-3.5, suggest that far-infrared photometric redshifts may be viable, and perhaps accurate to 10% ≲ dz/(1 + z), for up to 80% of the SMG population in this range, if the above temperature characterizes the full range of SMGs. However, observed temperature evolution of T_d ∝ (1 + z) is also plausible and could result from selection effects. From the observed luminosity-temperature (L-T) relation, L ∝ T^(2.82±0.29)_(obs), we derive scaling relations for dust mass versus dust temperature, and we identify expressions to interrelate the observed quantities. These suggest that measurements at a single wavelength, in the far-infrared, submillimeter, or radio wave bands, might constrain dust temperatures and far-infrared luminosities for most SMGs with redshifts at z ~ 0.5-4.

A median redshift of 2.4 for galaxies bright at submillimetre wavelengths

A significant fraction of the energy emitted in the early Universe came from very luminous galaxies that are largely hidden at optical wavelengths (because of interstellar dust grains); this energy now forms part of the cosmic background radiation at wavelengths near 1 mm (ref. 1). Some submillimetre (submm) galaxies have been resolved from the background radiation, but they have been difficult to study because of instrumental limitations. This has impeded the determination of their redshifts (z), which is a crucial element in understanding their nature and evolution. Here we report spectroscopic redshifts for ten submm galaxies that were identified using high-resolution radio observations. The median redshift for our sample is 2.4, with a quartile range of 1.9–2.8. This population therefore coexists with the peak activity of quasars, suggesting a close relationship between the growth of massive black holes and luminous dusty galaxies. The space density of submm galaxies at redshifts over 2 is about 1,000 times greater than that of similarly luminous galaxies in the present-day Universe, so they represent an important component of star formation at high redshifts.

Intense star formation within resolved compact regions in a galaxy at z = 2.3

Massive galaxies in the early Universe have been shown to be forming stars at surprisingly high rates. Prominent examples are dust-obscured galaxies which are luminous when observed at sub-millimetre wavelengths and which may be forming stars at a rate of 1,000 solar masses (M⊙) per year. These intense bursts of star formation are believed to be driven by mergers between gas-rich galaxies. Probing the properties of individual star-forming regions within these galaxies, however, is beyond the spatial resolution and sensitivity of even the largest telescopes at present. Here we report observations of the sub-millimetre galaxy SMMJ2135-0102 at redshift z = 2.3259, which has been gravitationally magnified by a factor of 32 by a massive foreground galaxy cluster lens. This magnification, when combined with high-resolution sub-millimetre imaging, resolves the star-forming regions at a linear scale of only 100 parsecs. We find that the luminosity densities of these star-forming regions are comparable to the dense cores of giant molecular clouds in the local Universe, but they are about a hundred times larger and 107 times more luminous. Although vigorously star-forming, the underlying physics of the star-formation processes at z ≈ 2 appears to be similar to that seen in local galaxies, although the energetics are unlike anything found in the present-day Universe.

Tracing the molecular gas in distant submillimetre galaxies via CO(1-0) imaging with the EVLA

We report the results of a pilot study with the Expanded Very Large Array (EVLA) of 12 CO J = 1 0 emission from four submillimetre-selected galaxies (SMGs) at z = 2.2‐2.5, each with an existing detection of 12 CO J = 3 2, one of which comprises two distinct spatial components. Using the EVLA’s most compact configuration we d etect strong, broad (medians: 990 km s −1 FWZI; 540 km s −1 FWHM) J = 1 0 line emission from all of our targets ‐ coincident in position and velocity with their J = 3 2 emission. The median line width ratio, �1−0/�3−2 = 1.15 ± 0.06, suggests that the J =1 0 is more spatially extended than the J =3 2 emission, a situation confirmed by our maps which reveal velo city structure in several cases and typical sizes of �16 kpc FWHM. The median brightness temperature (Tb) ratio is r3−2/1−0 = 0.55 ± 0.05, consistent with local galaxies with LIR > 10 11 L� , noting that our value may be biased high because of the J =3 2-based sample selection. Naively, this suggests gas masses roughly 2× higher than estimates made using higher-J transitions of CO, with the discrepency due entirely to the difference in assumed Tb ratio. We also estimate molecular gas masses using the 12 CO J =1 0 line and the observed global Tb ratios, assuming standard underlying Tb ratios for the non-star-forming and star-forming gas phases as well as a limiting star-formation efficiency (SFE) for the latter in all systems, i.e. without calling upon XCO (� �). Using this new method, we find a median molecular gas mass of (2.5±0.8)×10 10 M� , with a plausible range stretching up to 3× higher. Even larger masses cannot be ruled out, but are not favoured by dynamical constraints: the median dynamical mass within R � 7 kpc for our sample is (2.3 ± 1.4) × 10 11 M� , or �6× more massive than UV-selected galaxies at this epoch. We examine the Schmidt-Kennicutt (S-K) relation for all the distant galaxy populations for which CO J = 1 0 or J = 2 1 data are available, finding small systematic differences between galaxy p opulations. These have previously been interpreted as evidence for different modes of star for mation, but we argue that these differences are to be expected, given the still considerabl e uncertainties, certainly when considering the probable excitation biases due to the molecular lines used, and the possibility of sustained S-K offsets during the evolution of individual gas-rich systems. Finally, we discuss the morass of degeneracies surrounding molecular gas mass estimates, the possibilities for breaking them, and the future prospects for imaging and studying cold, quiescent molecular gas at high redshifts.

Rapid Growth of Black Holes in Massive Star-Forming Galaxies

The tight relationship between the masses of black holes and galaxy spheroids in nearby galaxies implies a causal connection between the growth of these two components. Optically luminous quasars host the most prodigious accreting black holes in the Universe, and can account for [gsims]30 per cent of the total cosmological black-hole growth. As typical quasars are not, however, undergoing intense star formation and already host massive black holes (> 108 M [circdot], where M [circdot] is the solar mass), there must have been an earlier pre-quasar phase when these black holes grew (mass range ∼(106–108)M [circdot]). The likely signature of this earlier stage is simultaneous black-hole growth and star formation in distant (redshift z > 1; >8 billion light years away) luminous galaxies. Here we report ultra-deep X-ray observations of distant star-forming galaxies that are bright at submillimetre wavelengths. We find that the black holes in these galaxies are growing almost continuously throughout periods of intense star formation. This activity appears to be more tightly associated with these galaxies than any other coeval galaxy populations. We show that the black-hole growth from these galaxies is consistent with that expected for the pre-quasar phase.

Interferometric observations of powerful CO emission from three submillimeter galaxies at z=2.39, 2.51, and 3.35

We report IRAM Plateau de Bure, millimeter interferometry of three z=~2.4 to 3.4, SCUBA deep field galaxies. Our CO line observations confirm the rest-frame UV/optical redshifts, thus more than doubling the number of confirmed, published redshifts of the faint submillimeter population and proving their high-z nature. In all three sources our measurements of the intrinsic gas and dynamical mass are large (1e10 to 1e11 Msun). In at least two cases the data show that the submillimeter sources are part of an interacting system. Together with recent information gathered in the X-ray, optical and radio bands our observations support the interpretation that the submm-population consists of gas rich (gas to dynamical mass ratio ~0.5) and massive, composite starburst/AGN systems, which are undergoing a major burst of star formation and are evolving into m*-galaxies.We report IRAM millimeter interferometry of three z ~ 2.4-3.4 Submillimeter Common-User Bolometric Array deep field galaxies. Our CO line observations confirm the rest-frame UV/optical redshifts, thus more than doubling the number of confirmed published redshifts of the faint submillimeter population and proving their high-z nature. In all three sources our measurements of the intrinsic gas and dynamical mass are large (1010-1011 M☉). In at least two cases the data show that the submillimeter sources are part of an interacting system. Together with recent information gathered in the X-ray, optical, and radio bands, our observations support the interpretation that the submillimeter population, at least the radio-detected ones, consists of gas-rich (gas-to-dynamical mass ratio ~0.5) and massive interacting starburst/active galactic nucleus systems.

AzTEC half square degree survey of the SHADES fields - I. Maps, catalogues and source counts

We present the first results from the largest deep extragalactic mm-wavelength survey undertaken to date. These results are derived from maps covering over 0.7 deg2, made at λ= 1.1 mm, using the AzTEC continuum camera mounted on the James Clerk Maxwell Telescope. The maps were made in the two fields originally targeted at λ= 850 μm with the Submillimetre Common-User Bolometer Array (SCUBA) in the SCUBA Half-Degree Extragalactic Survey (SHADES) project, namely the Lockman Hole East (mapped to a depth of 0.9–1.3 mJy rms) and the Subaru/XMM–Newton Deep Field (mapped to a depth of 1.0–1.7 mJy rms). The wealth of existing and forthcoming deep multifrequency data in these two fields will allow the bright mm source population revealed by these new wide-area 1.1 mm images to be explored in detail in subsequent papers. Here, we present the maps themselves, a catalogue of 114 high-significance submillimetre galaxy detections, and a thorough statistical analysis leading to the most robust determination to date of the 1.1 mm source number counts. These new maps, covering an area nearly three times greater than the SCUBA SHADES maps, currently provide the largest sample of cosmological volumes of the high-redshift Universe in the mm or sub-mm. Through careful comparison, we find that both the Cosmic Evolution Survey (COSMOS) and the Great Observatories Origins Deep Survey (GOODS) North fields, also imaged with AzTEC, contain an excess of mm sources over the new 1.1 mm source-count baseline established here. In particular, our new AzTEC/SHADES results indicate that very luminous high-redshift dust enshrouded starbursts (S1.1mm > 3 mJy) are 25–50 per cent less common than would have been inferred from these smaller surveys, thus highlighting the potential roles of cosmic variance and clustering in such measurements. We compare number count predictions from recent models of the evolving mm/sub-mm source population to these sub-mm bright galaxy surveys, which provide important constraints for the ongoing refinement of semi-analytic and hydrodynamical models of galaxy formation, and find that all available models overpredict the number of bright submillimetre galaxies found in this survey.

The Evolutionary Sequence of Active Galactic Nuclei and Galaxy Formation Revealed

Today, almost every galaxy spheroid contains a massive black hole: a remnant of, and testament to, a period in its evolution when it contained an active galactic nucleus (AGN). However, the sequence and timescales of the formation of the black hole and surrounding spheroid of stars are completely unknown, leaving a large gap in our knowledge of how the universe attained its present appearance. Here we present submillimeter observations of matched samples of X-ray absorbed and unabsorbed AGNs that have luminosities and redshifts characteristic of the sources responsible for most of the mass in present-day black holes. Strong submillimeter emission, an isotropic signature of copious star formation, is found only in the X-ray–absorbed sample, ruling out orientation effects as the cause of the absorption. The space density and luminosity range of the X-ray–absorbed AGNs indicate that they are undergoing the transition between a hidden growth phase and an unabsorbed AGN phase and imply that the X-ray–absorbed period in the AGNs evolution coincides with the formation of the galaxy spheroid.

Herschel-ATLAS: counterparts from the UV--NIR in the science demonstration phase catalogue

We present a technique to identify optical counterparts of 250-μm-selected sources from the Herschel–ATLAS survey. Of the 6621 250 μm > 32-mJy sources in our science demonstration catalogue we find that ∼60 per cent have counterparts brighter than r  = 22.4 mag in the Sloan Digital Sky Survey. Applying a likelihood ratio technique we are able to identify 2423 of the counterparts with a reliability R > 0.8. This is approximately 37 per cent of the full 250-μm catalogue. We have estimated photometric redshifts for each of these 2423 reliable counterparts, while 1099 also have spectroscopic redshifts collated from several different sources, including the GAMA survey. We estimate the completeness of identifying counterparts as a function of redshift, and present evidence that 250-μm-selected Herschel–ATLAS galaxies have a bimodal redshift distribution. Those with reliable optical identifications have a redshift distribution peaking at z ≈ 0.25 ± 0.05, while submillimetre colours suggest that a significant fraction with no counterpart above the r-band limit have z  > 1. We also suggest a method for selecting populations of strongly lensed high-redshift galaxies. Our identifications are matched to UV–NIR photometry from the GAMA survey, and these data are available as part of the Herschel–ATLAS public data release.