arXiv subjects

James S. Dunlop

Publications and source records attributed to James S. Dunlop.

At least 127 records · Page 7Linked to original sources

arXiv2010

Evidence of different star formation histories for high- and low-luminosity radio galaxies

We present the results of our investigation into the stellar populations of 24 radio galaxies at z~0.5 drawn from four complete, low-frequency selected radio surveys. We use the strength of the 4000A break as an indicator of recent star formation, and compare this with radio luminosity, optical spectral classification and morphological classification. We find evidence of different star formation histories for high- and low-luminosity radio sources; our group of low radio luminosity sources (typically FRI-type sources) has systematically older stellar populations than the higher radio luminosity group. Our sample is also fairly well divided by optical spectral classification. We find that galaxies classified as having low excitation spectra (LEGs) possess older stellar populations than high excitation line objects (HEGs), with the HEGs showing evidence for recent star formation. We also investigate the link between radio morphology, as used by Owen & Laing (1989), and the stellar populations. We find that there is a preference for the "fat-double" sources to have older stellar populations than the "classical double" sources, although this is also linked to these sources lying predominantly in the LEG and HEG categories respectively. These results are consistent with the hypothesis that HEGs are powered by accretion of cold gas, which could be supplied, for example, by recent mergers, secular instabilities, or filamentary cold flows. These processes could also trigger star formation in the host galaxy. The host galaxies of the LEGs do not show evidence for recent star formation and an influx of cold gas, and are consistent with being powered by the accretion of the hot phase of the inter-stellar medium.

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arXiv2009

Obscured star formation at z = 0.84 with HiZELS: the relationship between star formation rate and H-alpha or ultra-violet dust extinction

[Abridged] We compare H-alpha, ultraviolet (UV) and infrared (IR) indicators of star formation rate (SFR) for a sample of z = 0.84 galaxies from the High-z Emission Line Survey (HiZELS). Using multi-wavelength diagnostics, we estimate that 5 - 11 per cent of H-alpha emitters at this redshift are active galactic nuclei. We detect 35 per cent of the H-alpha emitters individually at 24 microns, and stack the star-forming emitters on deep 24-micron images in order to calculate the typical SFRs of our galaxies. These are compared to the observed H-alpha line fluxes in order to estimate the extinction at z = 0.84, and we find a significant increase in dust extinction for galaxies with higher SFRs. We demonstrate that the local relationship between SFR and extinction is suitable for our sample, and attribute the overall increase in dust extinction for z = 0.84 galaxies to an increase in the average SFR, rather than to a change in dust properties. We calculate the UV extinction, and find a similar dependence on SFR, but no evidence for a 2175 angstrom UV bump in the dust attenuation law. We calculate the conversion between the dust attenuation of nebular and stellar radiation, gamma, and show that gamma = 0.50 +/- 0.14. The extinction / SFR relationship is shown to be applicable to galaxies with a range of morphologies and bulge-to-disk ratios, to both merging and non-merging galaxies, and to galaxies within high- and low-density environments, implying that it is a fundamental property of star-forming regions. In order to allow future studies to correct for a SFR-dependent amount of dust extinction, we present an equation to predict the extinction of a galaxy, based solely upon its observed H-alpha luminosity, and use this to recalculate the H-alpha luminosity function and star formation rate density at z = 0.84.

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arXiv2009

Radio and mid-infrared identification of BLAST source counterparts in the Chandra Deep Field South

We have identified radio and/or mid-infrared counterparts to 198 out of 350 sources detected at >=5 sigma over ~ 9 square degrees centered on the Chandra Deep Field South (CDFS) by the Balloon-borne Large Aperture Submillimeter Telescope (BLAST) at 250, 350 and 500 um. We have matched 114 of these counterparts to optical sources with previously derived photometric redshifts and fitted SEDs to the BLAST fluxes and fluxes at 70 and 160 um acquired with the Spitzer Space Telescope. In this way, we have constrained dust temperatures, total far-infrared/sub-millimeter luminosities and star formation rates for each source. Our findings show that on average, the BLAST sources lie at significantly lower redshifts and have significantly lower rest-frame dust temperatures compared to submm sources detected in surveys conducted at 850 um. We demonstrate that an apparent increase in dust temperature with redshift in our sample arises as a result of selection effects. Finally, we provide the full multi-wavelength catalog of >= 5 sigma BLAST sources contained within the complete ~ 9 square degree survey area.

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arXiv2009

Discovery of a Giant Lya Emitter Near the Reionization Epoch

We report the discovery of a giant Lya emitter (LAE) with a Spitzer/IRAC counterpart near the reionization epoch at z=6.595. The giant LAE is found from the extensive 1 deg^2 Subaru narrow-band survey for z=6.6 LAEs in the Subaru/XMM-Newton Deep Survey (SXDS) field, and subsequently identified by deep spectroscopy of Keck/DEIMOS and Magellan/IMACS. Among our 207 LAE candidates, this LAE is not only the brightest narrow-band object with L(Lya) = 3.9+/-0.2 x 10^43 erg/s in our survey volume of 10^6 Mpc^3, but also a spatially extended Lya nebula with the largest isophotal area whose major axis is at least ~3''. This object is more likely to be a large Lya nebula with a size of >~17 kpc than to be a strongly-lensed galaxy by a foreground object. Our Keck spectrum with medium-high spectral and spatial resolutions suggests that the velocity width is v(FWHM)=251+/-21 km/s, and that the line-center velocity changes by ~60 km/s in a 10-kpc range. The stellar mass and star-formation rate are estimated to be 0.9-5.0 x 10^10 Mo and >34 Mo/yr, respectively, from the combination of deep optical to infrared images of Subaru, UKIDSS-Ultra Deep Survey, and Spitzer/IRAC. Although the nature of this object is not yet clearly understood, this could be an important object for studying cooling clouds accreting onto a massive halo, or forming-massive galaxies with significant outflows contributing to cosmic reionization and metal enrichment of inter-galactic medium.

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arXiv2007

The SCUBA HAlf Degree Extragalactic Survey (SHADES) - IV: Radio-mm-FIR photometric redshifts

We present the redshift distribution of the SHADES galaxy population based on the rest-frame radio-mm-FIR colours of 120 robustly detected 850um sources in the Lockman Hole East (LH) and Subaru XMM-Newton Deep Field (SXDF). The redshift distribution derived from the full SED information is shown to be narrower than that determined from the radio-submm 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.

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arXiv2007

Radio-mm-FIR Photometric Redshifts for (sub-)mm Galaxies

We present a comparison between the published optical, IR and CO spectroscopic redshifts of 86 (sub-)mm galaxies and their photometric redshifts as derived from long-wavelength radio-mm-FIR photometric data. The redshift accuracy measured for 13 sub-mm galaxies with at least one robustly determined colour in the radio-mm-FIR regime and additional constraining upper limits is z \~0.3. This accuracy degrades to z~0.65 when only the 1.4GHz/850um spectral index is used, as derived from the analysis of a subsample of 58 galaxies with robustly determined redshifts. Despite the wide range of spectral energy distributions in the local galaxies that are used in an un-biased manner as templates, this analysis demonstrates that photometric redshifts can be effciently derived for sub-mm galaxies with a precision of Delta z < 0.5 using only the rest-frame FIR to radio wavelength data, suficient to guide the tuning of broad-band heterodyne observations (e.g. 100m GBT, 50m LMT, ALMA) or aid their determination in the case of a single line detection by these experiments.

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arXiv2005

The extragalactic sub-mm population: predictions for the SCUBA Half-Degree Extragalactic Survey (SHADES)

We present predictions for the angular correlation function and redshift distribution for SHADES, the SCUBA HAlf-Degree Extragalactic Survey, which will yield a sample of around 300 sub-mm sources in the 850 micron waveband in two separate fields. Complete and unbiased photometric redshift information on these sub-mm sources will be derived by combining the SCUBA data with i) deep radio imaging already obtained with the VLA, ii) guaranteed-time Spitzer data at mid-infrared wavelengths, and iii) far-infrared maps to be produced by BLAST, the Balloon-borne Large-Aperture Sub-millimeter Telescope. Predictions for the redshift distribution and clustering properties of the final anticipated SHADES sample have been computed for a wide variety of models, each constrained to fit the observed number counts. Since we are dealing with around 150 sources per field, we use the sky-averaged angular correlation function to produce a more robust fit of a power-law shape w(theta)=(theta/A)^{-delta} to the model data. Comparing the predicted distributions of redshift and of the clustering amplitude A and slope delta, we find that models can be constrained from the combined SHADES data with the expected photometric redshift information.

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arXiv2005

SHADES: The Scuba HAlf Degree Extragalactic Survey

SHADES is a new, major, extragalactic sub-mm survey currently being undertaken with SCUBA on the JCMT. The aim of this survey is to map 0.5 square degrees of sky at a depth sufficient to provide the first, major (~300 source), unbiased sample of bright (S(850) > 8 mJy) sub-mm sources. Combined with extensive multi-frequency supporting observations already in hand, we aim to measure the redshift distribution, clustering and AGN content of the sub-mm population. Currently 40% complete, the survey is due to run until early 2006. Here I provide some early example results which demonstrate the potential power of our combined data set, and highlight a series of forthcoming papers which will present results based on the current interim sample of ~130 850-micron sources detected within the Lockman Hole and SXDF SHADES survey fields.

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arXiv2004

Dust and gas obscuration in ELAIS Deep X-ray Survey reddened quasars

Hard X-ray surveys have uncovered a large population of heavily obscured AGN. They also reveal a population of quasars with moderate obscuration at both visible and X-ray wavelengths. We use Chandra selected samples of quasars from the ELAIS Deep X-ray Survey (EDXS) and CDF-N to investigate the obscuration towards the nuclei of moderately obscured AGN. We find an inverse correlation between the optical to X-ray flux ratio and the X-ray hardness ratio which can be interpreted as due to obscuration at visible and X-ray wavelengths. We present detailed optical and near-infrared data for a sample of optically-faint (R>23) quasars from the EDXS. These are used to constrain the amount of rest-frame UV/optical reddening towards these quasars. It is found that optically-faint quasars are mostly faint due to obscuration, not because they are intrinsically weak. After correcting for reddening, the optical magnitudes of most of these quasars are similar to the brighter quasars at these X-ray fluxes. Combining with gas column densities inferred from the X-ray observations we consider the gas-to-dust ratios of the obscuring matter. We find that the quasars generally have higher gas-to-dust absorption than that seen in the Milky Way - similar to what has been found for nearby Seyfert galaxies. We consider the possible existence of a large population of X-ray sources which have optical properties of Type 1 (unobscured) quasars, but X-ray properties of Type 2 (obscured) quasars. We show that the observed distribution of optical-to-X-ray flux ratios of quasars at z>1 is skewed to low values compared to the intrinsic distribution...

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arXiv2004

Submillimetre photometry of typical high-redshift radio quasars

We present submillimetre (SCUBA) photometry of a sample of eight high-redshift (2.5 < z < 3.5) radio quasars from two redshift surveys: the TEXOX-1000, or TOOT, survey; and the 7C Redshift Survey (7CRS). Unlike the powerful high-redshift radio sources observed previously in the submillimetre, these radio sources are typical of those dominating the radio luminosity density of the population. We detect just two of the TOOT/7CRS targets at 850 microns, and one of these detections is probably due to synchrotron emission rather than dust. The population represented by the other six objects is detected in a statistical sense with their average 850-micron flux density implying that they are similar to low-redshift, far-infrared-luminous quasars undergoing at most moderate (< 200 solar masses per year) starbursts. By considering all the SCUBA data available for radio sources, we conclude that positive correlations between rest-frame far-infrared luminosity, 151-MHz luminosity and redshift, although likely to be present, are hard to interpret because of subtle selection and classification biases, small number statistics and uncertainties concerning synchrotron contamination and K-correction. We argue that there is not yet any compelling evidence for significant differences in the submillimetre properties of radio-loud and radio-quiet quasars at high redshift.

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arXiv2004

A sample of radio galaxies spanning three decades in radio luminosity - I. The host-galaxy properties and black-hole masses

The results of analysis of HST I-band imaging of a sample of 41 radio galaxies spanning three orders of magnitude in radio luminosity at redshift z~0.5 are presented. The full sample is drawn from four complete, low-frequency selected radio samples with progressively fainter flux-density limits (3CRR, 6CE, 7CRS and the new TexOx-1000 sample). Modelling of the HST imaging data shows that the host galaxies have surface-brightness distributions consistent with those expected for classic ellipticals (Sersic parameter, beta~0.25), with beta in the range 0.17 =0.23\pm0.01. The host-galaxy luminosities are comparable with those of galaxies drawn from the bright end of the local cluster galaxy luminosity function, spanning the range 0.7L^{\star} =10^{8.87\pm 0.04}\Msun. Finally, a significant (~3 sigma) correlation is found between black-hole mass and 151-MHz radio luminosity for those objects in the sample with either high-excitation nuclear spectra or classical double radio structures. (abridged)

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arXiv2003

Quasar Hosts and the Black Hole-Spheroid Connection

I review our current understanding of the structures and ages of the host galaxies of quasars, and the masses of their central black holes. At low redshift, due largely to the impact of the Hubble Space Telescope, there is now compelling evidence that the hosts of quasars with M_V < -24 mag are virtually all massive ellipticals, with basic properties indistinguishable from those displayed by their quiescent counterparts. The masses of these spheroids are as expected given the relationship between black hole and spheroid mass now established for nearby galaxies, as is the growing prevalence of significant disk components in the hosts of progressively fainter active nuclei. In fact, from spectroscopic measurements of the velocity of the broad-line region in quasars, it has now proved possible to obtain an independent dynamical estimate of the masses of the black holes that power quasars. I summarize recent results from this work, which can be used to demonstrate that the black hole-spheroid mass ratio in quasars is the same as that found for quiescent galaxies, namely M_bh = 0.0012 M_sph. These results offer the exciting prospect of using observations of quasars and their hosts to extend the study of the black hole-spheroid mass ratio out to very high redshifts (z > 2). Moreover, there is now good evidence that certain ultraviolet quasar emission lines can provide robust estimates of black hole masses from the observed optical spectra of quasars out to z > 2, and perhaps even at z > 4. By combining such information with deep, high-resolution infrared imaging of high-redshift quasar hosts on 8-m class telescopes, there is now a real prospect of clarifying the evolution of the black hole spheroid connection over cosmological time scales.

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arXiv2002

The clustering of halo mergers

We analyse the spatial distribution of halo merger sites using four numerical simulations with high time resolution of structure growth in a Lambda-CDM cosmology. We find no evidence for any large-scale relative bias between mergers and randomly selected haloes of the same mass at high redshift. Given a sample of galaxies that form a Poisson sampling of halo mergers, then the amplitude of the measured clustering ought to lead to a robust estimate of the median host halo mass. In the hierarchical picture of structure growth, mergers between galaxies at high redshifts are thought to create dust enshrouded starbursts leading to increased emission in the sub-mm wavebands. Hence, a measurement of the clustering strength of SCUBA galaxies, for example, can determine whether or not they are giant elliptical galaxies in the process of formation.

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arXiv2002

F stars, metallicity, and the ages of red galaxies at z > 1

We explore whether the rest-frame near-UV spectral region, observable in high-redshift galaxies via optical spectroscopy, contains sufficient information to allow the degeneracy between age and metallicity to be lifted. We do this by testing the ability of evolutionary synthesis models to reclaim the correct metallicity when fitted to the near-UV spectra of F stars of known (sub-solar and super-solar) metallicity. F stars are of particular interest because the rest-frame near-UV spectra of the oldest known elliptical galaxies at z > 1 appear to be dominated by F stars near to the main-sequence turnoff. We find that, in the case of the F stars, where the HST ultraviolet spectra have high signal:noise, model-fitting with metallicity allowed to vary as a free parameter is rather successful at deriving the correct metallicity. As a result, the estimated turnoff ages of these stars yielded by the model fitting are well constrained. Encouraged by this we have fitted these same variable- metallicity models to the deep, optical spectra of the z \simeq 1.5 mJy radio galaxies 53W091 and 53W069 obtained with the Keck telescope. While the age-metallicity degeneracy is not so easily lifted for these galaxies, we find that even when metallicity is allowed as a free parameter, the best estimates of their ages are still \geq 3 Gyr, with ages younger than 2 Gyr now strongly excluded. Furthermore, we find that a search of the entire parameter space of metallicity and star formation history using MOPED (Heavens et al., 2000) leads to the same conclusion. Our results therefore continue to argue strongly against an Einstein-de Sitter universe, and favour a lambda-dominated universe in which star formation in at least these particular elliptical galaxies was completed somewhere in the redshift range z = 3 - 5.

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arXiv2002

The near-infrared Hubble diagram for sub-mm galaxies

Determing the nature of the bright sub-mm sources and their role in the process of galaxy formation is likely to be a major focus of extra-galactic research over the next few years. In particular, we would like to know if these sources are the progenitors of today's massive elliptical galaxies, or alternatively arise from short-lived, albeit spectacular starbursts within haloes of unexceptional mass. This question can be addressed from a number of different directions, one of which is to compare the masses of sub-mm host galaxies with those of other known massive high-redshift objects. Here I make a first attempt to do this by exploring whether the few well-studied sub-mm/mm sources are consistent with the well-established K-z relation for powerful radio galaxies. Out to z = 3 this appears to be the case, providing evidence that bright sub-mm sources are indeed destined to be massive ellipticals. At higher redshifts there is a suggestion that sub-mm galaxies are signficantly fainter at K than their radio-selected counterparts, but at present it is unclear whether this indicates a significant difference in stellar mass or the increasing impact of dust obscuration on the rest-frame light from the sub-mm hosts.

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arXiv2002

The masses of AGN host galaxies & the origin of radio loudness

We highlight some of the principal results from our recent Hubble Space Telescope studies of quasars and radio galaxies. The hosts of these powerful AGN are normal massive ellipticals which lie on the region of the fundamental plane populated predominantly by massive ellipticals with boxy isophotes and distinct cores. The hosts of the radio-loud sources are on average 1.5 times brighter than their radio-quiet counterparts and appear to lie above a mass threshold of 4 x 10^11 solar masses. This suggests that black holes more massive than 5 x 10^8 solar masses are required to produce a powerful radio source. However we show that this apparent threshold appears to be a consequence of an upper bound on radio output which is a strong function of black-hole mass, L_5GHz proportional to M_bh^2.5. This steep mass dependence can explain why the hosts of the most powerful radio sources are good standard candles. Such objects were certainly fully assembled by z = 1, and appear to have formed the bulk of their stars prior to z = 3.

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arXiv2002

A submillimetre difference between radio galaxies and radio quasars: evidence for quasar-heated dust and synchronized submillimetre and radio source activity

We present submillimetre photometry of 11 3CR and 6CE radio quasars of similar 151-MHz radio luminosity and redshifts to the radio galaxies studied at 1.3 ~2-times brighter accounting for possible synchrotron contamination. This difference is in quantitative agreement with a receding-torus unified scheme for radio sources in which the torus opening angle depends on quasar optical luminosity, provided that there is a close relationship between the optical and submillimetre luminosities. The implication is that quasar-heated dust dominates the submillimetre emission for powerful radio quasars at z~1.5. We find a significant anti-correlation between submillimetre/far-infrared luminosity L_FIR and radio source age in the sense that hyperluminous quasars tend to be associated with young (<10^7 yr) sources. This suggests that the processes controlling L_FIR are synchronized with the jet-triggering event and that L_FIR declines on a ~10^7 yr timescale. There is evidence for a small amount of obscuration in the hyperluminous quasars from reddening and associated or broad absorption lines. We speculate that shortly after jet triggering, dust is more widely distributed than at later times and is heated by the quasar nucleus and/or a short-lived synchronized starburst.

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arXiv2001

The Host Galaxies of Radio-Loud and Radio-Quiet Quasars

I review our knowledge of the properties of the host galaxies of radio-loud and radio-quiet quasars, both in comparison to each other and in the context of the general galaxy population. It is now clear that the hosts of radio-loud and radio-quiet quasars with M_V < -23.5 are virtually all massive elliptical galaxies. The masses of these spheroids are as expected given the relationship between black-hole and spheroid mass found for nearby quiescent galaxies, as is the growing prevalence of disc components in the hosts of progressively fainter AGN. There is also now compelling evidence that quasar hosts are practically indistinguishable from normal ellipticals, both in their basic structural parameters and in the old age of their dominant stellar populations; at low z the nuclear activity is not associated with the formation of a significant fraction of the host galaxy. While the long-held view that quasar radio power might be a simple function of host morphology is now dead and buried, I argue that host-galaxy studies may yet play a crucial role in resolving the long-standing problem of the origin of radio loudness. Specifically there is growing evidence that radio-loud objects are powered by more massive black holes accreting at lower efficiency than their radio-quiet couterparts of comparable optical output. A black-hole mass > 10^9 solar masses appears to be a necessary (although perhaps not sufficient) condition for the production of radio jets of sufficient power to produce an FRII radio source within a massive galaxy halo.

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