arXiv subjects

Richard G. McMahon

Publications and source records attributed to Richard G. McMahon.

At least 55 records · Page 3Linked to original sources

arXiv2002

The afterglow and complex environment of the optically dim burst GRB 980613

We report the identification of the optical afterglow of GRB 980613 in R- and I-band images obtained between 16 and 48 hours after the gamma-ray burst. Early near-infrared (NIR) H and K' observations are also reported. The afterglow was optically faint (R ~ 23) at discovery but did not exhibit an unusually rapid decay (power-law decay slope alpha < 1.8 at 2 sigma). The optical/NIR spectral index (beta_RH < 1.1) was consistent with the optical-to-X-ray spectral index (beta_RX ~ 0.6), indicating a maximal reddening of the afterglow of ~0.45 mag in R. Hence the dimness of the optical afterglow was mainly due to the fairly flat spectral shape rather than internal reddening in the host galaxy. We also present late-time HST/STIS images of the field in which GRB 980613 occurred, obtained 799 days after the burst. These images show that GRB 980613 was located close to a very compact, blue V = 26.1 object inside a complex region consisting of star-forming knots and/or interacting galaxy fragments. Therefore, GRB 980613 constitutes a strong case for the association of cosmological gamma-ray bursts with star-forming regions.

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arXiv2002

A Redshift z = 6.56 Galaxy Behind the Cluster Abell 370

We report the discovery of a redshift z=6.56 galaxy lying behind the cluster Abell 370. The object HCM 6A was found in a narrowband imaging survey using a 118 Angstrom bandpass filter centered at 9152 Ang in the LRIS camera on the 10 m Keck II Telescope. Candidate Lyman alpha emitters were identified by the equivalent width of the emission and the absence of lower wavelength flux in ultradeep broadband images. HCM6A is the first galaxy to be confirmed at redshift z>6, and has W_lambda(observed)=190 Angstroms, flux = 2.7 x 10^-17 erg cm^-2 s^-1. Spectra obtained with LRIS confirm the emission line and the continuum break across the line, and show an asymmetric line profile with steep fall-off on the blue side. Deep Subaru near-infrared CISCO images in J, H and K' which extend the sampled continuum to longer wavelengths give a consistent estimate of the continuum flux density in these line-free regions of 2.6+/-0.7 x 10^-30 erg cm^-2 s^-1 Hz^-1. The line width and strength, asymmetric profile, and very deep spectral break are only consistent with the interpretation of the line as a redshifted Lyman alpha feature. From the detailed lensing model of this cluster, we estimate a lensing amplification of 4.5 for this galaxy, which is located slightly over an arcminute from the center of the cluster, for an unlensed flux of 6.5 x 10^-18 erg cm^-2 s^-1. The presence of such a galaxy suggests that the reionizing epoch is beyond z=6.6.

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arXiv2001

Optical Counterparts for 70,000 Radio Sources: APM Identifications for the FIRST Radio Survey

We describe a program to identify optical counterparts to radio sources from the VLA FIRST survey using the Cambridge APM scans of the POSS-I plates. We use radio observations covering 4150 square degrees of the north Galactic cap to a 20 cm flux density threshold of 1.0 mJy; the 382,892 sources detected all have positional uncertainties of <1" (radius of 90% confidence). Our description of the APM catalog, derived from the 148 POSS-I O and E plates covering this region, includes an assessment of its astrometric and photometric accuracy, a photometric recalibration using the Minnesota APS catalog, a discussion of the classification algorithm, and quantitative tests of the catalog's reliability and completeness. We go on to show how the use of FIRST sources as astrometric standards allows us to improve the absolute astrometry of the POSS plates by nearly an order of magnitude to ~0.15" rms. Matching the radio and optical catalogs yields counterparts for over 70,000 radio sources; we include detailed discussions of the reliability and completeness of these identifications as a function of optical and radio morphology, optical magnitude and color, and radio flux density. An analysis of the problem of radio sources with complex morphologies (e.g., double-lobed radio galaxies) is included. We conclude with a brief discussion of the source classes represented among the radio sources with identified counterparts.

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arXiv2001

The SCUBA Bright Quasar Survey (SBQS): 850micron observations of the z>4 sample

We present initial results of a new, systematic search for massive star-formation in the host galaxies of the most luminous and probably most massive z>=4 radio-quiet quasars (M(B) <= -27.5; nuL(nu)(1450Ang)>10^13Lsun). A total of 38 z>=4 radio-quiet quasars have been observed at the JCMT using SCUBA at 850microns: 8 were detected (>3sigma) with S(850microns)>~ 10mJy (submillimetre-loud). The new detections almost triple the number of optically selected, submillimetre-loud z>~4 radio-quiet quasars known to date. We include a detailed description of how our quasar sample is defined in terms of radio and optical properties. There is no strong evidence for trends in either detectability or 850microns flux with absolute magnitude, M(B). We find that the weighted mean flux of the undetected sources is 2.0 +/- 0.6mJy, consistent with an earlier estimate of \~3mJy based on more sensitive observations of a sample z>~4 radio-quiet quasars (McMahon et al., 1999). This corresponds to an inferred starformation rate of \~1000Msun/yr, similar to Arp220. The typical starformation timescale for the submillimetre-bright sources is ~1Gyr, 10 times longer than the typical accretion-driven e-folding timescale of ~5x10^7 years. Our 850micron detection of the z=4.4 quasar PSS J1048+4407 when analysed in conjunction with 1.2mm single-dish and interferometric observations suggests that this source is resolved on angular scales of 1-2" (6-12 kpc). In addition, we present a new optical spectrum of this source, identifying it as a broad absorption line (BAL) quasar. The new redshift is outside that covered in a recent CO line search by Guilloteau et al., (1999), highlighting the need for accurate redshifts for the obervation and interpretation of high-redshift line studies.

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arXiv2001

Statistical Properties of DLAs and sub-DLAs

Quasar absorbers provide a powerful observational tool with which to probe both galaxies and the intergalactic medium up to high redshift. We present a study of the evolution of the column density distribution, f(N,z), and total neutral hydrogen mass in high-column density quasar absorbers using data from a recent high-redshift survey for damped Lyman-alpha (DLA) and Lyman limit system (LLS) absorbers. Whilst in the redshift range 2 to 3.5, ~90% of the neutral HI mass is in DLAs, we find that at z>3.5 this fraction drops to only 55% and that the remaining 'missing' mass fraction of the neutral gas lies in sub-DLAs with N(HI) 10^{19} - 2 * 10^{20} cm^{-2}.

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arXiv2001

The far-infrared--submm spectral energy distribution of high-redshift quasars

We combine submm photometric data of z>4 quasars, to obtain a mean far-infrared (FIR) (rest-frame) spectral energy distribution (SED) of thermal emission from dust, parameterised by a single temperature (T) and power-law emissivity index (beta). Best-fit values are T=41+/-5K, beta=1.95+/-0.3. The redshift spread of this set of quasars is exploited to allow us to sample the SED at a greater number of rest wavelengths than is possible for a single object. This parameterisation is of use to any study that extrapolates from a flux at a single submm wavelength, for example to infer dust masses and FIR luminosities. We then interpret the submm component as arising from dust heated by star-formation in the quasar's host galaxy, and investigate a simple scheme of AGN--host coevolution, in which the timescale for formation of the host galaxy is c.0.5-1.0Gyr, with star formation proceeding at a constant rate c.1000Msol/yr. The luminous quasar phase occurs towards the end of the star-forming period, just before the galaxy's reservoir of cold gas is depleted. Given the youth of the Universe at z=4 (1.6Gyr), the coexistence of a massive black hole and a luminous starburst can be a powerful constraint on models of quasar host-galaxy formation.

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arXiv2001

Infrared constraints on the dark mass concentration observed in the cluster Abell 1942

We present a deep H-band image of the region in the vicinity of the cluster Abell 1942 containing the puzzling dark matter concentration detected in an optical weak lensing study by Erben et al. (2000). We demonstrate that our limiting magnitude, H=22, would be sufficient to detect clusters of appropriate mass out to redshifts comparable with the mean redshift of the background sources. Despite this, our infrared image reveals no obvious overdensity of sources at the location of the lensing mass peak, nor an excess of sources in the I-H vs. H colour-magnitude diagram. We use this to further constrain the luminosity and mass-to-light ratio of the putative dark clump as a function of its redshift. We find that for spatially-flat cosmologies, background lensing clusters with reasonable mass-to-light ratios lying in the redshift range 0<z<1 are strongly excluded, leaving open the possibility that the mass concentration is a new type of truly dark object.

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arXiv2001

Cosmological Evolution of the Universe Neutral Gas Mass Measured by Quasar Absorption Systems

The cosmological evolution of neutral hydrogen is an efficient way of tracing structure formation with redshift. It indicates the rate of evolution of gas into stars and hence the gas consumption and rate star formation history of the Universe. In measuring HI, quasar absorbers have proven to be an ideal tool and we use observations from a recent survey for high-redshift quasar absorption systems together with data gathered from the literature to measure the cosmological comoving mass density of neutral gas. This paper assumes Omega_M=0.3, Omega_lambda=0.7 and h=0.65.

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arXiv2001

Infrared Imaging Data Reduction Software and Techniques

We describe the InfraRed Data Reduction (IRDR) software package, a small ANSI C library of fast image processing routines for automated pipeline reduction of infrared (dithered) observations. We developed the software to satisfy certain design requirements not met in existing packages (e.g., full weight map handling) and to optimize the software for large data sets (non-interactive tasks that are CPU and disk efficient). The software includes stand-alone C programs for tasks such as running sky frame subtraction with object masking, image registration and coaddition with weight maps, dither offset measurement using cross-correlation, and object mask dilation. Although we currently use the software to process data taken with CIRSI (a near-IR mosaic imager), the software is modular and concise and should be easy to adapt/reuse for other work. IRDR is available from anonymous ftp to ftp.ast.cam.ac.uk in pub/sabbey.

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arXiv2001

Absorption Systems in the Spectra of 66 z > 4 Quasars

We present high signal-to-noise, 5 A resolution (FWHM) spectra of 66 z \ga 4 bright quasars obtained with the 4 m Cerro Tololo Inter-American Observatory and 4.2 m William Hershel telescopes. The primary goal of these observations was to undertake a new survey for intervening absorption systems detected in the spectra of background quasars. We look for both Lyman-limit systems (column densities N(HI) > 1.6 * 10^{17} atoms cm-2) and damped Ly-alpha systems (column densities N(HI) > 2 * 10^{20} atoms cm-2). This work resulted in the discovery of 49 Lyman-limit systems, 15 of which are within 3000 km s-1 of the quasar emission and thus might be associated with the quasar itself, 26 new damped Ly-alpha absorption candidates, 15 of which have z>3.5 and numerous metal absorption systems. In addition ten of the quasars presented here exhibit intrinsic broad absorption lines.

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arXiv2000

The Second APM UKST Colour Survey for z>4 Quasars

We present the spectra, positions, and finding charts for 31 bright (R<19.3) colour-selected quasars covering the redshift range z=3.85-4.78, with 4 having redshifts z>4.5. The majority are in the southern sky (dec<-25 deg). The quasar candidates were selected for their red (B-R > 2.5) colours from UK or POSSII Schmidt Plates scanned at the Automated Plate Measuring facility in Cambridge. Low resolution (~> 10A) spectra were obtained to identify the quasars, primarily at the Las Campanas Observatory. The highest redshift quasar in our survey is at z ~ 4.8 (R = 18.7) and its spectrum shows a damped Lyman-alpha absorption system at z = 4.46. This is currently the highest redshift damped Lyman-alpha absorber detected. Five of these quasars exhibit intrinsic broad absorption line features. Combined with the previously published results from the first part of the APM UKST survey we have now surveyed a total of ~8000 squ deg of sky i.e. 40% of the high galactic latitude (|b|>30 deg) sky, resulting in 59 optically selected quasars in the redshift range 3.85 to 4.78; 49 of which have z>=4.00.

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arXiv2000

Long-term Optical Variability of Radio-Selected Quasars from the FIRST Survey

We have obtained single-epoch optical photometry for 201 quasars, taken from the FIRST Bright Quasar Survey, which span a wide range in radio loudness. Comparison with the magnitudes of these objects on the POSS-I plates provides by far the largest sample of long-term variability amplitudes for radio-selected quasars yet produced. We find the quasars to be more variable in the blue than in the red band, consistent with work on optically selected samples. The previously noted trend of decreasing variability with increasing optical luminosity applies only to radio-quiet objects. Furthermore, we do not confirm a rise in variability amplitude with redshift, nor do we see any dependence on radio flux or luminosity. The variability over a radio-optical flux ratio range spanning a factor of 60,000 from radio-quiet to extreme radio-loud objects is largely constant, although there is a suggestion of greater variability in the extreme radio-loud objects. We demonstrate the importance of Malmquist bias in variability studies, and develop a procedure to correct for the bias in order to reveal the underlying variability properties of the sample.

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arXiv2000

The Evolution and Space Density of Damped Lyman-alpha Galaxies

The results of a new spectroscopic survey of 66 $z \simgt 4$ quasars for Damped Lyman-alpha absorption systems are presented. The search led to the discovery of 30 new DLA candidates which are analysed in order to compute the comoving mass density of neutral gas in a non-zero lambda Universe. The possible sources of uncertainty are discussed and the implications of our results for the theories of galaxy formation and evolution are emphasized. A subsequent paper will present details of the calculations summarised here and a more extensive explanation of the consequences of our observations for the understanding of the nature of DLAs.

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arXiv2000

Infrared observations of gravitational lensing in Abell 2219 with CIRSI

We present the first detection of a gravitational depletion signal at near-infrared wavelengths, based on deep panoramic images of the cluster Abell 2219 (z=0.22) taken with the Cambridge Infrared Survey Instrument (CIRSI) at the prime focus of the 4.2m William Herschel Telescope. Infrared studies of gravitational depletion offer a number of advantages over similar techniques applied at optical wavelengths, and can provide reliable total masses for intermediate redshift clusters. Using the maximum likelihood technique developed by Schneider, King & Erben (1999), we detect the gravitational depletion at the 3 sigma confidence level. By modeling the mass distribution as a singular isothermal sphere and ignoring uncertainty in the unlensed number counts, we find an Einstein radius of 13.7 +3.9/-4.2 arcsec (66% confidence limit). This corresponds to a projected velocity dispersion of approximately 800 km/s, in agreement with constraints from strongly-lensed features. For a Navarro, Frenk and White mass model, the radial dependence observed indicates a best-fitting halo scale length of 125/h kpc}. We investigate the uncertainties arising from the observed fluctuations in the unlensed number counts, and show that clustering is the dominant source of error. We extend the maximum likelihood method to include the effect of incompleteness, and discuss the prospects of further systematic studies of lensing in the near-infrared band.

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arXiv2000

The INT Wide Field Imaging Survey(WFS)

The 2.5m Isaac Newton Telescope(INT) is currently being used to carry out a major multi-colour, multi-epoch, CCD based wide field survey over an area of 100 square degrees. The survey parameters have been chosen to maximise scientific return over a wide range of scientific areas and to complement other surveys being carried out elsewhere. Unique aspects of the survey is that it concentrates on regions of sky that are easily accessible from telescopes in both Northern and Southern terrestrial hemispheres and that it the first public survey to use filters similar to that being used by the Sloan Digital Sky Survey. A major aim of the the INT Wide Field Survey program is to bridge the gap between the all-sky photographic 2 and 3 band surveys such as the Palomar and UK Schmidt sky surveys and the ultra-deep keyhole surveys such as the Hubble Deep Field.

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arXiv1999

Galaxies at z>5: The View from Hawaii

We review the properties of z>5 galaxies studied with HST and with the Keck telescopes, and discuss the detectability of Lyman alpha emission-line galaxies out to z~6.5 based on these data and ongoing narrowband imaging surveys. The brightest sources may show (R-Z) color breaks, although the high sky background at Z (lambda_{eff} ~ 9200 Ang), makes such observations challenging for typical faint sources. Keck LRIS observations of the z=5 SDSS quasar and z>5 galaxies observed with HST in the HDF show that the strength of the Lyman break is evolving more slowly than extrapolations from models at z~3 would predict.

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arXiv1999

Limits on the Gunn-Peterson Effect at z = 5

We report new limits on the Gunn-Peterson effect at a redshift near 5 using spectroscopic observations of the z = 5 Sloan Digital Sky Survey quasar, J033829.31+002156.3, made with the LRIS and HIRES spectrographs on the Keck telescopes. Lower resolution spectrophotometrically calibrated observations made with LRIS over the wavelength region 4500-9600 angstroms were used to obtain a continuum shape and to flux calibrate much higher resolution (R = 36,000) observations made with HIRES. The LRIS data show an Oke D_A index of 0.75. Portions of the HIRES spectrum return to near the extrapolated continuum level. Including both statistical and systematic errors we place an upper limit of tau = 0.1 on the regions of minimum opacity. We argue that, even if this opacity arises in underdense regions of the intergalactic gas, we require a high value of the metagalactic ionizing flux at these redshifts ($J_ν \gg 4 \times 10^{-23}~{\rm ergs cm^{-2} s^{-1} Hz^{-1} sr^{-1}}$ at $z\sim4.72$) to produce a solution which is consistent with even minimum nucleosynthesis estimates of the baryon density. We also report the presence of a Mg II absorption system of extremely high equivalent width ($W_{λ,rest}(2796)=1.73$ Å) at z=2.304.

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arXiv1999

Submillimetre observations of luminous z>4 radio-quiet quasars and the contribution of AGN to the submm source population

We present sensitive 850 micron SCUBA photometry of a statistically-complete sample of six of the most luminous, z>4 radio-quiet quasars, reaching noise levels comparable with the deep blank sky surveys. One quasar (BR2237-0607; z=4.55) is detected at 850micron with a flux of 5.0mJy (4.5sigma), whilst a second (BR0019-1522; z=4.52) has a detection at the 2 sigma level. When combined with our previous millimetre measurements of z>4 quasars, we find that there is a large range (5--10) in far infrared (FIR) luminosity at fixed UV luminosity, and that the typical quasar has a L_FIR and mass of cool (50K) dust similar to that of the archetyepal low redshift (z=0.018) ultraluminous IRAS} galaxy Arp220. If one assumes a fiducial FIR luminosity of 5x10^12 Lsun for for all quasars with M_B<-23, we find that around 15 per cent of the sources in the SCUBA deep surveys could be classical broad-lined radio-quiet AGN. Thus if one considers the observed ratio of Seyfert II to Seyfert I galaxies at low redshift and any contribution from totally optically obscured AGN, a significant fraction of the SCUBA source population will harbour AGN and hence the inferred star formation rates from submm fluxes may be overestimated if the active nuclei are bolometrically dominant or the IMF is top heavy.

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