Biomedical subjects

LL Cowie

Publications and source records attributed to LL Cowie.

4 recordsLinked to original sources

PubMed2000

Resolving the extragalactic hard X-ray background

The origin of the hard (2-10 keV) X-ray background has been a mystery for over 35 years. Most of the soft X-ray background has been resolved into individual sources (mainly quasars), but these sources do not have the spectral energy distribution required to match the spectrum of the X-ray background as a whole. Here we report the results of a deep survey, using the Chandra satellite, in which the detected hard X-ray sources account for at least 75 per cent of the hard X-ray background. The mean X-ray spectral energy distribution of these sources is in good agreement with that of the background. Moreover, most of those hard X-ray sources are associated unambiguously with either the nuclei of otherwise normal bright galaxies, or with optically faint sources. The latter could be active nuclei in dust-enshrouded galaxies or a population of quasars at extremely high redshift.

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PubMed2000

Caltech Faint Galaxy Redshift Survey. XI. The Merger Rate to Redshift 1 from Kinematic Pairs.

The rate of mass accumulation due to galaxy merging depends on the mass, density, and velocity distribution of galaxies in the near neighborhood of a host galaxy. The fractional luminosity in kinematic pairs combines all of these effects in a single estimator that is relatively insensitive to population evolution. Here we use a k-corrected and evolution-compensated volume-limited sample having an R-band absolute magnitude of Mk,eR</=-19.8+5logh mag drawing about 300 redshifts from the Caltech Faint Galaxy Redshift Survey and 3000 from the Canadian Network for Observational Cosmology field galaxy survey to measure the rate and redshift evolution of merging. The combined sample has an approximately constant comoving number and luminosity density from redshift 0.1 to 1.1 (OmegaM=0.2, OmegaLambda=0.8); hence, any merger evolution will be dominated by correlation and velocity evolution, not density evolution. We identify kinematic pairs with projected separations less than either 50 or 100 h-1 kpc and rest-frame velocity differences of less than 1000 km s-1. The fractional luminosity in pairs is modeled as fL&parl0;Deltav,rp,Mk,er&parr0;&parl0;1+z&parr0;mL, where &sqbl0;fL,mL&sqbr0; are &sqbl0;0.14+/-0.07,0+/-1.4&sqbr0; and &sqbl0;0.37+/-0.7,0.1+/-0.5&sqbr0; for rp</=50 and 100 h-1 kpc, respectively (OmegaM=0.2, OmegaLambda=0.8). The value of mL is about 0.6 larger if Lambda=0. To convert these redshift-space statistics to a merger rate, we use the data to derive a conversion factor to a physical space pair density, a merger probability, and a mean in-spiral time. The resulting mass accretion rate per galaxy (M1,M2>/=0.2M*) is 0.02+/-0.01&parl0;1+z&parr0;0.1+/-0.5M* Gyr-1. Present-day high-luminosity galaxies therefore have accreted approximately 0.15M* of their mass over the approximately 7 Gyr to redshift 1. Since merging is likely only weakly dependent on the host mass, the fractional effect, deltaM&solm0;M approximately 0.15M*&solm0;M, is dramatic for lower mass galaxies but is, on the average, effectively perturbative for galaxies above 1M*.

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PubMed1999

Optically Faint Microjansky Radio Sources.

We report on the identifications of radio sources from our survey of the Hubble Deep Field (HDF) and the Small Selected Area 13 fields, both of which comprise the deepest radio surveys to date at 1.4 and 8.5 GHz, respectively. About 80% of the microjansky radio sources are associated with moderate-redshift starburst galaxies or active galactic nuclei within the I-magnitude range of 17-24 with a median of I=22 mag. Thirty-one (20%) of the radio sources are (1) fainter than I>25 mag, with two objects in the HDF IAB>28.5, (2) often identified with very red objects I-K>4, and (3) not significantly different in radio properties than the brighter objects. We suggest that most of these objects are associated with heavily obscured starburst galaxies with redshifts between 1 and 3. However, other mechanisms are discussed and cannot be ruled out with the present observations.

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PubMed1999

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 Low Resolution Imaging Spectrometer (LRIS) and the High-Resolution Echelle Spectrometer (HIRES) on the Keck telescopes. Lower resolution spectrophotometrically calibrated observations made with LRIS over the wavelength region 4500-9600 Å 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 DA 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 (Jnu>>4x10-23 ergs cm-2 s-1 Hz-1 sr-1 at z approximately 4.72) to produce a solution which is consistent with even minimum nucleosynthesis estimates of the baryon density. We also report the presence of an Mg ii absorption system of extremely high equivalent width [Wlambda,rest&parl0;2796&parr0;=1.73 Å] at z=2.304.

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