Wide Area Extragalactic HI Mapping: An ALFA pilot project

A Search for Low Mass HI Clouds in the Local Hubble Volume: ALFALFA Sprouts!

We have begun a spectral line mapping program at Arecibo that is designed to detect HI clouds down to a mass limit of 106.5 solar masses in the very local universe. While the principal objective of this program is to probe the faint end of the HI mass function, this project will also allow for a comparison of the quality of baselines derived from drift mapping and on-the-fly (OTF) mapping, a question that has significant ramifications for future ALFA extragalactic HI survey stategies. Furthermore, this pilot project will be useful in developing the software and analysis tools necessary to tackle the massive amounts of data that will be produced by future projects with ALFA. The first 100 hours of observations were made on January 22-27 and April 1-14, 2003. This allocation amounts to one third of the time that simulations predict will be needed to derive a statistically meaningful result.

CDM numerical simulations predict the existence of a large number of low mass halos, well in excess of the faint tail of the galaxy luminosity function. This prediction has prompted the reevaluation of the idea that some, relatively compact, high velocity HI clouds may be the "missing" population of low-mass halos in the Local Group. Large blind HI surveys (with few systems detected with MHI< 108 solar masses, and almost none with MHI < 107 solar masses) disagree in the estimate of the space density of objects with masses of MHI ~107 solar masses by an order of magnitude. The current survey is aimed at resolving this discrepancy.

The program takes advantage of the fact that the ratio of low mass to high mass halos in numerical simulations appears to be the same in high and low density regions. Square-degree sized fields are mapped, centered on known galaxy groups for which primary distance indicators give distances between 3 and 10 Mpc. Maps of this size will inevitably include a large number of continuum sources (as can be seen in the map of the NGC 2683 region shown in the Figure below), which provide a challenge in the data reduction process.

The strategy for observing mode varies as a function of the target group's distance. The more distant groups required 12 (or more) seconds of integration per beam in order to achieve the required HI mass limit. Therefore, observations of those areas were made in drift mode. Groups at nearer distances were observed using OTF mapping, with a slew rate set so as to reach the mass limit (3 seconds per beam in the fastest case). Preliminary results suggest that short integration time OTF mapping, in which the telescope is driven at a rate up to half the maximum slew speed, results in data quality which is indistinguishable from data that was taken in drift mode. That is, the channel-to-channel rms is the same in both cases, and no long period oscillations are introduced by the fast slew rate. Preliminary quantitative results were presented by Kristine Spekkens at the Extragalactic ALFA workshop held at Arecibo in March (see: http://alfa.naic.edu/extragal/meeting1/minutes/ALFALFAsprouts.html).

These observations represent just the first step in a larger HI mapping project, with more surveys planned for use with ALFA, with the goal of achieving survey coverage across the entire (Arecibo) sky. On-going work involves experimentation with the best methods to perform bandpass calibration and subtraction, continuum cleaning, beam characterization and deconvolution, noise characterization, and HI signal detection and extraction.