A Survey for Low-Redshift High Column Density QSO Absorption Line Systems

Details A Survey for Low-Redshift High Column Density QSO Absorption Line Systems A Survey for Low-Redshift High Column Density QSO Absorption Line Systems

Aug 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001noao.prop..103t&link_type=abstract

NOAO Proposal ID #2001B-0103

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HI 21 cm emission-line observations are used to study the distribution of neutral gas in the local Universe. Most of this HI appears to be associated with luminous spirals. However, at redshifts as low as z ~ 0.1, this method becomes ineffective due to radio receiver and sensitivity limitations. A far more effective way to study HI at low-z (z ~ 0.1-1.0) is to observe the damped Ly(alpha) absorption lines that large HI columns produce in the spectra of background QSOs, but a detailed study would require an unrealistically large number of UV spectra. Fortunately, during the course of an HST QSO absorption-line survey, we uncovered an empirical correlation which allows us to make an optical spectroscopic search for large columns of low-z neutral gas (e.g. N_HI > 10^20 atoms cm^-2). The project requires low-to-moderate S/N spectra of ~ 500 QSOs, but the result will have a very significant scientific impact. With our results we can study the incidence of large columns of HI on the sky at redshifts 0.23 < z < 0.93 and make direct comparisons with the z=0 results from 21 cm observations and the z> 1.65 results from ground- based QSO damped Ly(alpha) absorption-line surveys. These results will then set the stage for future determinations of: (1) the cosmological mass density of the neutral gas, (2) its column density distribution and (3) the luminosity function of associated galaxies, all in the redshift interval 0.23< z< 0.93 which covers ~ 30% of the age of the Universe. So far we have completed ~ 22% of our program.

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