The ALFALFA Search for (Almost) Dark Galaxies across the HI Mass Function

TL;DR

The ALFALFA survey is a sensitive, high-resolution HI survey that aims to catalog local HI-rich galaxies, revealing that truly dark, HI-rich galaxies are extremely rare, and providing valuable data for galaxy evolution studies.

Contribution

This paper presents the initial results of the ALFALFA survey, highlighting its improved capabilities and the first evidence that optically dark, HI-rich galaxies are very uncommon.

Findings

ALFALFA detects HI masses from 10^6 to 10^10.8 solar masses.

Less than 1% of high HI mass sources lack optical counterparts.

The survey suggests a negligible population of dark, HI-rich galaxies.

Abstract

The Arecibo Legacy Fast ALFA (ALFALFA) survey is a second generation blind extragalactic HI survey currently in progess which is exploiting Arecibo's superior sensitivity, angular resolution and digital technology to derive a census of the local HI universe over a cosmologically significant volume. As of the time of this meeting, some 4500 good quality extragalactic HI line sources have been identified in about 15% of the final survey area. ALFALFA is detecting HI masses as low as 10**6 solar masses and as large as 10**10.8 solar masses with positional accuracies typically better than 20", allowing immediate identification of the most probable optical counterparts. Only 3% of all extragalactic HI sources and less than 1% of detections with HI mass 10**9.5 solar masses cannot be identified with a stellar component. Because ALFALFA is far from complete, the discussion here focuses on limitations of past surveys that ALFALFA will overcome because of its greater volume, sensitivity and reduced susceptibility to source confusion and on a sampling of illustrative preliminary results. First ALFALFA results already suggest, in agreement with previous studies, that there does not appear to be a cosmologically significant population of optically dark but HI rich galaxies. ALFALFA promises a wealthy dataset for the exploration of many issues in near-field cosmology and galaxy evolution studies, setting the stage for their extension to higher redshifts in the future with the Square Kilometer Array (SKA).