The Incidence of Damped LY alpha Systems in the Redshift Interval 0 < Z < 4

Details The Incidence of Damped LY alpha Systems in the Redshift Interval 0 < Z < 4 The Incidence of Damped LY alpha Systems in the Redshift Interval 0 < Z < 4

Aug 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...449..488r&link_type=abstract

Astrophysical Journal v.449, p.488

Astronomy and Astrophysics

Astronomy

50

Galaxies: Evolution, Galaxies: Quasars: Absorption Lines

Scientific paper

The determination of the incidence and column density distribution of damped Lyα systems from QSO absorption-line surveys in the redshift interval 0 < z < 1.65 is crucial for constraining the evolution of neutral gas in galaxies. This redshift interval corresponds to a time interval of ˜62% of the age of the universe for q0 = 0 (77% for q0 = 0.5). However, because damped Lyα systems are relatively rare in spectra, a sensitive UV spectroscopic survey would consume considerable telescope time. We have therefore undertaken a new approach that takes advantage of the fact that all damped Lyα systems studied so far are found to have corresponding Mg II metal-line absorption. For redshifts z < 1.65 we have used available UV data to study the Lyα absorption line corresponding to 43 Mg II systems. After accounting for possible biases, we find that at a mean redshift of = 0.8 the number of damped Lyα absorption lines with neutral hydrogen column density N(H I) ≥ 2 × 1020 cm-2 per unit redshift is nDLyα(z = 0.8) ≤ 0.12. A similar analysis based on studying the Lyα line in Lyman-limit systems is consistent with this result.
This limit on the observed number density at z ≍ 0.8 is consistent with the trend at higher redshift as well as the number density at the current epoch inferred from H I cross sections of nearby disk galaxies. The result that n(z) in the redshift interval 0 < z < 1.65 conforms to a trend established by three different observing techniques over three redshift regimes provides evidence that this trend actually traces the evolution of neutral gas in galaxies. The data at z > 1.65 combined with the constraint from nearby galaxies at redshift z = 0 result in a parameterization that is valid from redshift z ≍ 4 to the present epoch: nDLyα(z) = (0.015±0.004) (1 + z)2.27±0.25. The power-law index of 2.27±0.25 indicates that damped Lyα absorbers undergo evolution, either in size or in number density, from a redshift of about 4 to the present epoch.

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