DEEP 21 cm H I OBSERVATIONS AT z ∼ 0.1: THE PRECURSOR TO THE ARECIBO ULTRA DEEP SURVEY

The 'ALFA Ultra Deep Survey' (AUDS) is an ongoing 21 cm spectral survey with the Arecibo 305 m telescope. AUDS will be the most sensitive blind survey undertaken with Arecibo's 300 MHz Mock spectrometer. The survey searches for 21 cm H I line emission at redshifts between 0 and 0.16. The main goals of the survey are to investigate the H I content and probe the evolution of H I gas within that redshift region. In this paper, we report on a set of precursor observations with a total integration time of 53 hr. The survey detected a total of eighteen 21 cm emission lines at redshifts between 0.07 and 0.15 in a region centered around α2000 ∼ 0h, δ ∼ 15042'. The rate of detection is consistent with the one expected from the local H I mass function. The derived relative H I density at the median redshift of the survey is ρHI[z = 0.125] = (1.0 ± 0.3)ρ0, where ρ0 is the H I density at zero redshift.