Properties of low mass dwarf galaxies in the ALFALFA survey

ALFALFA is a large blind extragalactic HI survey currently underway at the Arecibo Observatory. Using a preliminary catalog, we study the properties of dwarf galaxies identified in this survey. First, we present the signal extraction tool that was written for the purposes of the survey to detect automatically faint signals. The algorithm is based on the cross-correlation of templates with the data in Fourier space. With templates built out of a combination of Hermite functions, it allows for the fast and reliable detection of galaxies. Survey simulations are used to study the completeness and reliability of the survey catalogs and to help in establishing the observing strategy. Using the signal extractor on data covering a region of 135 deg2 between 22h and 3h of right ascension and in a 2° strip centered at +27° of declination, a sample of 488 galaxies is extracted. The properties of the galaxies in this catalog are discussed. We find that five of the six galaxies detected with MHI < 107.5 M⊙ are satellites of either the NGC672/IC1727 nearby galaxy pair or their neighboring dwarf irregular galaxy NGC784. The data include a slice through the Pisces-Perseus foreground void, a large nearby underdensity of galaxies, but we report no detections within the void, where our catalog is complete for systems with HI masses of 108 M⊙ . Gas-rich, optically-dark galaxies do not seem to constitute an important void population, and therefore do not suffice at producing a viable solution to the void phenomenon. By studying the distribution of a larger sample of 3200 ALFALFA galaxies, we show evidence that the galaxies in the lower density environments are on average more massive. Finally, the preliminary ALFALFA catalog was used to identify a sample of 25 low HI-mass nearby low surface brightness dwarf galaxies for which we obtained optical spectra with the Palomar 5m telescope. These galaxies tend to be metal-poor, dark-matter dominated, with moderate to very large gas mass fractions. They follow the metallicity-luminosity relation derived from other samples of low-luminosity galaxies. Assuming that these galaxies are "closed-box" systems, their effective oxygen yields are consistent with models for chemical enrichment that have a variable initial mass function. No dependency of metallicity on environment was found in the present sample.