FIGGS: faint irregular galaxies GMRT survey-overview, observations and first results

Abstract

The Faint Irregular Galaxies GMRT Survey (FIGGS) is a Giant Metrewave Radio Telescope (GMRT) based HI imaging survey of a systematically selected sample of extremely faint nearby dwarf irregular galaxies. The primary goal of FIGGS is to provide a comprehensive and statistically robust characterization of the neutral interstellar medium properties of faint, gas-rich dwarf galaxies. The FIGGS galaxies represent the extremely low mass end of the dwarf irregular galaxies population, with a median M_B^~−13.0 and median HI mass of ^~3 × 10⁷ M_⊙, extending the baseline in mass and luminosity space for a comparative study of galaxy properties. The H i data are supplemented with observations at other wavelengths. In addition, distances accurate to ˜10 per cent are available for most of the sample galaxies. This paper gives an introduction to FIGGS, describes the GMRT observations and presents the first results from the HI observations. From the FIGGS data, we confirm the trend of increasing HI to optical diameter ratio with decreasing optical luminosity; the median ratio of D_HI/D_Ho for the FIGGS sample is 2.4. Further, on comparing our data with aperture synthesis surveys of bright spirals, we find at best marginal evidence for a decrease in average surface density with decreasing HI mass. To a good approximation, the discs of gas-rich galaxies, ranging over three orders of magnitude in HI mass, can be described as being drawn from a family with fixed HI average surface density.