SINGG: Search for Ionization in Neutral-Gas Galaxies

To understand galaxies, we must understand how and when the stars in them formed. By looking to more and more distant galaxies, we can try to chart the evolution of the global cosmic star formation rate as a function of time. However, we must do this carefully, making sure that we have good measurements of the global star formation rate in the nearby universe, and making sure that the samples we select for such studies are truly representative of the universe as a whole.

These are difficult requirements because galaxies that are undergoing intense star formation tend to be easier to find than ones that are not as active, or ones in which the activity is spread out over a large area. To date nearly all samples of galaxies selected for study have been selected on the basis of their optical and infrared radiation, and are henced biased in favor of the brighter galaxies.

The Parkes telescope has recently undertaken an ambitious survey to find sources of hydrogen 21-cm line radiation across the whole southern sky. Galaxies the HI Parkes All Sky Survey (HIPASS) are thus identified on the basis of their neutral gas content, not their stellar content.

The Survey for Ionization in Neutral-Gas Galaxies (SINGG) is an NOAO survey program to measure the emission from ionized hydrogen in a subset of the galaxies found by HIPASS. The survey will examine 500 HIPASS galaxies chosen to uniformly cover the HI mass function. Each of these will be imaged in H-alpha and the stellar continuum. The H-alpha emission line, at rest wavelength 6563 Angstroms, traces the ionized medium (HII), which in turn traces high-mass, recently-formed stars that ionize this gas. Since a gaseous medium is a prerequisite for star formation, this survey will uniformly sample all galaxies that could form stars and hence will provide a relatively unbiased view of the local star formation demographics. The scientific goals of SINGG are:

  • 1. to estimate how many stars are forming per year per unit volume of the local universe and compare this result to other measures of the global star formation rate;
  • 2. provide a census of star formation morphologies in the local universe;
  • 3. study properties of the HII region luminosity function for the full range of gas rich galaxies;
  • 4. determine the distribution of star formation rates as a function of various optical and HI properties (e.g. luminosity, HI mass, velocity dispersion; and
  • 5. estimate the epoch when the star formation in the local universe will exhaust its gas supply.

    The SINGG database will provide a reference H-alpha survey of the local universe, and a rich resource for research into the star formation process in galaxies.