arXiv subjects

David H. Hughes

Publications and source records attributed to David H. Hughes.

81 records · Page 5Linked to original sources

arXiv2000

Simulated Submillimetre Galaxy Surveys

Current submillimetre surveys are hindered in their ability to reveal detailed information on the epoch of galaxy formation and the evolutionary history of a high-redshift starburst galaxy population. The difficulties are due to the small primary apertures (D < 15 m) of existing submillimetre telescopes and the limited sensitivities of their first generation of bolometer cameras. This situation is changing rapidly due to a variety of powerful new ground-based, airborne and satellite FIR to millimetre wavelength facilities. Improving our understanding of the luminosity and clustering evolution provides the motivation for conducting cosmological submillimetre and millimetre surveys. It is therefore important that we quantify the limitations of the future surveys and the significance of the results that can be drawn from them. In this paper we present simulated surveys which are made as realistic as possible in order to address some key issues confronting existing and forthcoming surveys. We discuss the results from simulations with a range of wavelengths (200 um - 1.1 mm), spatial resolutions (6 arcsec - 27 arcsec) and flux densities (0.01-310 mJy). We address how the measured source-counts could be affected by resolution and confusion, by the survey sensitivity and noise, and by the sampling variance due to clustering and shot-noise.

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arXiv2000

Cosmological Surveys at Submillimetre Wavelengths

One of the major goals of observational cosmology is to acquire empirical data that has the diagnostic power to develop the theoretical modelling of the high-redshift universe, ultimately leading to an accurate understanding of the processes by which galaxies and clusters form and subsequently evolve. New bolometer arrays operating on the world's largest submillimetre telescopes now offer a unique view of the high-redshift universe through unbiassed surveys with unprecedented sensitivity. For brevity, except when there is a need to be more specific, the FIR to millimetre wavelength regime (100um < lambda < 6000um) will be referred to as the ``submillimetre'' (submm). One of the major challenges in this field is to accurately quantify the star-formation history of submm-selected galaxies, particularly those at redshifts > 1, and determine their contribution to the submm extragalactic background. The field of observational cosmology will be revolutionized during the course of the next 10 years due to the variety of powerful new ground-based, airborne and satellite facilities, particularly those operating at FIR to millimetre wavelengths. This review summarises the results from the recent blank-field submm surveys, and describes the future observations that will provide accurate source-counts over wider ranges of wavelength and flux-density, constrain the spectral energy distributions of the submm-selected galaxies and accurately constrain the redshift distribution and submm luminosity function by removing the current ambiguities in the optical, IR and radio counterparts.

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arXiv2000

Optical off-nuclear spectra of quasar hosts and radio galaxies

We present optical (~3200A to ~9000A) off-nuclear spectra of 26 powerful active galaxies in the redshift range 0.1 < z < 0.3, obtained with the Mayall and William Herschel 4-meter class telescopes. The sample consists of radio-quiet quasars, radio-loud quasars (all with -23 > M_V > -26) and radio galaxies of Fanaroff & Riley Type II (with extended radio luminosities and spectral indices comparable to those of the radio-loud quasars). The spectra were all taken approximately 5 arcseconds off-nucleus, with offsets carefully selected so as to maximise the amount of galaxy light falling into the slit, whilst simultaneously minimising the amount of scattered nuclear light. The majority of the resulting spectra appear to be dominated by the integrated stellar continuum of the underlying galaxies rather than by light from the non-stellar processes occurring in the active nuclei, and in many cases a 4000A break feature can be identified. The individual spectra are described in detail, and the importance of the various spectral components is discussed. Stellar population synthesis modelling of the spectra will follow in a subsequent paper (Nolan et al. 2000).

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arXiv2000

Infrared Properties of High Redshift and X-ray Selected AGN Samples

The NASA/ISO Key Project on active galactic nuclei (AGN) seeks to better understand the broad-band spectral energy distributions (SEDs) of these sources from radio to X-rays, with particular emphasis on infrared properties. The ISO sample includes a wide variety of AGN types and spans a large redshift range. Two subsamples are considered herein: 8 high-redshift (1 < z < 4.7) quasars; and 22 hard X-ray selected sources. The X-ray selected AGN show a wide range of IR continuum shapes, extending to cooler colors than the optical/radio sample of Elvis et al. (1994). Where a far-IR turnover is clearly observed, the slopes are < 2.5 in all but one case so that non-thermal emission remains a possibility. The highest redshift quasars show extremely strong, hot IR continua requiring ~ 100 solar masses of 500 - 1000 Kelvin dust with ~ 100 times weaker optical emission. Possible explanations for these unusual properties include: reflection of the optical light from material above/below a torus; strong obscuration of the optical continuum; or an intrinsic deficit of optical emission.

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arXiv1999

The Future of Millimetre and Submillimetre Cosmology

Using the submm array camera SCUBA on the 15-m JCMT it is now possible to conduct unbiased submm surveys and quantify the level of star-formation activity in the young Universe by observing the rest-frame FIR thermal emission from dust in high-redshift galaxies. However an accurate interpretation of these existing submm cosmological surveys is prevented by uncertainties in the redshifts of the submm-selected galaxies, the ambiguities in the identifications of their optical/IR and radio counterparts and the restricted range of flux density over which the submm source-counts are measured. This paper outlines the future observations required to overcome these deficiencies.

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arXiv1998

A Submillimetre Survey of the Hubble Deep Field: Unveiling Dust-Enshrouded Star Formation in the Early Universe

The advent of sensitive sub-mm array cameras now allows a proper census of dust-enshrouded massive star-formation in very distant galaxies, previously hidden activity to which even the deepest optical images are insensitive. We present the deepest sub-mm survey, taken with the SCUBA camera on the James Clerk Maxwell Telescope (JCMT) and centred on the Hubble Deep Field (HDF). The high source density on this image implies that the survey is confusion-limited below a flux density of 2 mJy. However within the central 80 arcsec radius independent analyses yield 5 reproducible sources with S(850um) > 2 mJy which simulations indicate can be ascribed to individual galaxies. These data lead to integral source counts which are completely inconsistent with a no evolution model, whilst the combined brightness of the 5 most secure sources in our map is sufficient to account for 30-50% of the previously unresolved sub-mm background, and statistically the entire background is resolved at about the 0.3 mJy level. Four of the five brightest sources appear to be associated with galaxies which lie in the redshift range 2 < z < 4. With the caveat that this is a small sample of sources detected in a small survey area, these submm data imply a star-formation density over this redshift range that is at least five times higher than that inferred from the rest-frame ultraviolet output of HDF galaxies.

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arXiv1998

Continuum Observations of the High-Redshift Universe at Sub-millimetre Wavelengths

New bolometer arrays operating on the world's largest sub-mm and mm telescopes offer a unique view of the high-redshift universe with unprecedented sensitivity. Recent sub-millimetre continuum studies show that the host galaxies of many luminous high-redshift AGN (radio galaxies and radio-quiet quasars) radiate strongly at rest-frame far-infrared wavelengths and thus contain substantial quantities of dust. In the majority of these high-redshift AGN-hosts, the inferred star formation is proceeding at a rate comparable to that found in local, interacting ultra-luminous FIR galaxies. This level of activity is an order of magnitude greater than the more modest star-formation rates apparently displayed by the recently-discovered Lyman-limit galaxies at z ~ 3, which have been argued to represent the era of spheroid formation (although the degree to which the effects of reddening by dust grains may have biased the interpretation of these optical/UV studies of high-z galaxies has yet to be properly determined). However, it is too early to say whether such bright FIR emission is a feature of all massive galaxies at z > 3, or whether it is in fact confined to the hosts of the most luminous AGN. In this paper we review the current status of cosmological observations at sub-mm and mm wavelengths, highlighting our own recent SCUBA observations of high-redshift radio galaxies. We also explain how observations over the next few years should allow the true level of star-formation activity in the high-z universe to be properly quantified, and we provide example predictions for the first deep sub-millimetre survey (of the Hubble Deep Field), which we and our colleagues are currently undertaking with SCUBA at the JCMT.

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arXiv1997

Infrared imaging and off-nuclear spectroscopy of quasar hosts

We present the results of two complementary ground-based programmes to determine the host galaxy properties of radio-quiet and radio-loud quasars and to compare them with those of radio galaxies. Both infrared images and optical off-nuclear spectra were obtained and we discuss the various strategies used to separate the quasar-related emission from that of the underlying galaxy. However, the key feature of this project is the use of carefully matched samples, which ensure that the data for different types of object are directly comparable.

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arXiv1997

The radio properties of radio-quiet quasars

Although radio-quiet quasars (RQQs), which constitute the majority of optically-identified quasar samples, are by no means radio silent the properties of their radio emission are only poorly understood. We present the results of a multi-frequency VLA study of 27 low-redshift RQQs. In general, we find that the properties of the radio sources in RQQs are consistent with them being weak, small-scale (~1 kpc) jets similar to those observed in nearby Seyfert galaxies. We conclude that a significant fraction of the radio emission in RQQs is directly associated with the central engine and is not a result of stellar processes in the surrounding galaxy. There appears to be no difference between the radio properties of RQQs in elliptical and disc galaxies, implying that the relationship between the host galaxy and the `radio loudness' of the active nucleus is not straightforward.

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