Extragalactic Surveys

The will have a major impact on unbiased surveys in . 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 line circumvent these optical selection biases. Such searches would make it possible to construct an unbiased mass function for the local universe and to probe the evolution of galaxies and the formation of large scale structure in the range 0 < z < 0.8. Three examples of such surveys are:

• An All Sky survey in 0 < z < 0.2: the good surface brightness sensitivity of the VLA D 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, at z = 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 < z < 0.2 (RFI permitting).
• The evolution of gas at 0.2 < z < 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 < z < 0.8, and to sample a volume of . In a 100-hr integration more than thousand galaxies will be detected with the upgraded VLA.

While Arecibo is more sensitive for directed studies, and the GBT about as sensitive, the VLA's large field of view relative to both single dishes makes it many times faster than either for unbiased surveys. At higher redshifts ( 0.06) the higher angular resolution and better interference rejection of the VLA are also major advantages.

The technical requirements include:

• Robust total power measurements.
• Wide-band spectral line capability.
• RFI excision and extending the 1.4 GHz band down to 800(?) MHz, for the high-redshift survey.