The Arecibo Galaxy Environment Survey: precursor observations of the NGC 628 group

The Arecibo Galaxy Environment Survey (AGES) is one of several H I surveys utilizing the new Arecibo L-band Feed Array (ALFA) fitted to the 305-m radio telescope at Arecibo.(a) The survey is specifically designed to investigate various galactic environments to higher sensitivity, higher velocity resolution and higher spatial resolution than previous fully sampled, 21-cm multibeam surveys. The emphasis is on making detailed observations of nearby objects although the large system bandwidth (100 MHz) will allow us to quantify the H I properties over a large instantaneous velocity range. In this paper, we describe the survey and its goals and present the results from the precursor observations of a 5 x 1 -deg(2) region containing the nearby (similar to 10 Mpc) NGC 628 group. We have detected all the group galaxies in the region including the low-mass (M-HI similar to 10(7) M-.) dwarf, dw0137+1541. The fluxes and velocities for these galaxies compare well with previously published data. There is no intragroup neutral gas detected down to a limiting column density of 2 x 10(18) cm(-2). In addition to the group galaxies we have detected 22 galaxies beyond the NGC 628 group, nine of which are previously uncatalogued. We present the Hi data for these objects and also SuperCOSMOS images for possible optical galaxies that might be associated with the H I signal. We have used V/V-max analysis to model how many galaxies beyond 1000 km s(-1) should be detected and compare this with our results. The predicted number of detectable galaxies varies depending on the H I mass function (HIMF) used in the analysis. Unfortunately the precursor survey area is too small to determine whether this is saying anything fundamental about the HIMF or simply highlighting the effect of low number statistics. This is just one of many questions that will be addressed by the complete AGES survey.