Biomedical subjects

A Songaila

Publications and source records attributed to A Songaila.

6 recordsLinked to original sources

PubMed2001

Resolving the structure of ionized helium in the intergalactic medium with the far ultraviolet spectroscopic explorer.

The neutral hydrogen (H I) and ionized helium (He II) absorption in the spectra of quasars are unique probes of structure in the early universe. We present Far-Ultraviolet Spectroscopic Explorer observations of the line of sight to the quasar HE2347-4342 in the 1000 to 1187 angstrom band at a resolving power of 15,000. We resolve the He II Lyman alpha (Lyalpha) absorption as a discrete forest of absorption lines in the redshift range 2.3 to 2.7. About 50 percent of these features have H I counterparts with column densities N(H I) > 10(12.3) per square centimeter that account for most of the observed opacity in He II Lyalpha. The He II to H I column density ratio ranges from 1 to >1000, with an average of approximately 80. Ratios of <100 are consistent with photoionization of the absorbing gas by a hard ionizing spectrum resulting from the integrated light of quasars, but ratios of >100 in many locations indicate additional contributions from starburst galaxies or heavily filtered quasar radiation. The presence of He II Lyalpha absorbers with no H I counterparts indicates that structure is present even in low-density regions, consistent with theoretical predictions of structure formation through gravitational instability.

Journal Article↗

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*.

Journal Article↗

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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PubMed1998

Heavy-element enrichment in low-density regions of the intergalactic medium.

Models for the composition of the diffuse intergalactic medium predict that low-density intergalactic gas at high redshift should be very poor in heavy elements. This is because locations of early star formation (and thus of heavy-element synthesis) and of gas delivery from such stars are located preferentially within higher-density regions of the intergalactic gas. Here we present a method for analysing carbon and oxygen absorption lines in quasar spectra that allows us to probe the heavy-element abundances at a redshift of three within low-density regions of intergalactic gas. We find that the ratio of triply ionized carbon to neutral hydrogen is roughly constant over a wide range of densities, and that, even as the density approaches zero, the ratio remains high. This unexpected enrichment of low-density gas in heavy elements suggests that early generations of small galaxies might be much more efficient at ejecting heavy elements into the intergalactic medium than has previously been thought.

Astronomical Phenomena↗

PubMed1997

A high deuterium abundance in the early Universe.

Intergalactic gas clouds at high redshifts have element abundances that are close to primordial. The ratio of deuterium to hydrogen (D/H) within such clouds-which is determined from absorption lines in the spectra of more distant quasars that lie along the same line of sight-provides the best estimate of the density of baryons (omegaB) in the Universe. Previous estimates of D/H in the early Universe have yielded values that differ by about an order of magnitude, with the lower values implying a high density of baryons that may be difficult to reconcile with both estimates of the primordial abundances of other light elements (especially 4He) and the known number of light neutrinos. The accuracy of such D/H determinations is heavily dependent on the inferred column density of neutral hydrogen in the absorbing clouds. Here we report an independent measurement of the neutral hydrogen column density in the cloud towards the quasar Q1937 - 1009, for which one of the low D/H values was derived. Our measurement requires a substantial revision to the D/H value reported previously; we obtain a lower limit of D/H > 4 x 10(-5) for this cloud, which implies omegaB < 0.016 for a Hubble constant of 100 km s(-1) Mpc(-1). This reduced upper limit for the baryon density relieves any conflict with standard Big Bang nucleosynthesis.

Deuterium↗

PubMed1993

Faint galaxy surveys.

Various K band galaxy surveys have now established 2.2-m galaxy counts from K = 10 to K = 23. The K band counts rise slightly faster than a Euclidean slope to K = 17, at which point they turn over; beyond this magnitude, galaxies also become much bluer. Spectroscopic samples are available between K = 10 and 20 and show that the conventional distance laws hold rather precisely out to a redshift of about 0.6. Beyond this, galaxies appear fainter than expected. The results appear to favor rapid merging at modest galactic redshifts.

Journal Article↗