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 ≈ 15°42′. 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 ρH i[z = 0.125] = (1.0 ± 0.3)ρ0, where ρ0 is the H i density at zero redshift.