Extragalactic H I Surveys

The VLA Development Plan will have a major impact on unbiased surveys in H I. Our view of the large scale structure of the universe, with its large filaments, sheets, walls and voids, is based almost entirely on observations of high-luminosity galaxies, observed at optical wavelengths. Since nearly all investigations of the properties and spatial distribution of galaxies begin with optically (or IRAS) selected galaxy catalogs, any population of gas clouds with very low optical luminosity or surface brightness would largely have escaped detection. Direct searches in the H I line circumvent these optical selection biases. Such searches would make it possible to contruct an unbiased H I mass function for the local universe and to probe the evolution of galaxies and the formation of large scale structure in the range 0 0.8. Three examples of such surveys are:

• An All Sky H I survey in 0 0.1: the good surface brightness sensitivity of the VLA E configuration, combined with the large instantaneous velocity coverage of the new correlator will finally make such a survey feasible.

• The structure of a cluster of galaxies, combined with a pencil beam survey: at higher redshifts an entire cluster can be imaged within one VLA primary beam, e.g., at = 0.1, an Abell diameter is about . In parallel, a sensitive pencil beam survey can be carried out to sample the entire cone within this primary beam at 0 0.2 (RFI permitting).

• The evolution of gas at 0.2 0.8: a direct test of how the gas content of galaxies evolves with redshift, using the B configuration to survey a cone to as high a redshift as possible (again, RFI permitting). The large instantaneous velocity coverage of the new correlator will make it possible for a single pointing and LO setting to cover the range 0.2 0.8, and to sample a volume of . In a 100-hr integration more than thousand galaxies would be detected.

While Arecibo is more sensitive for directed studies, the VLA's large field of view makes it 25 times faster than Arecibo for unbiased H I surveys. At higher redshifts ( 0.06) the much higher angular resolution of the VLA is another major advantage.

The technical requirements are as in §1.3.6, with the addition that the 1.4 GHz band should be extended to cover frequencies down to 800 MHz for the high-redshift survey.