H I and star formation in the most metal-deficient galaxies

Abstract

We present Giant Metrewave Radio Telescope (GMRT) observations for three (viz., DDO 68, SDSS J2104-0035 and UGC 772) of the six most metal-deficient actively star-forming galaxies known. Although there is a debate as to whether these galaxies are undergoing their first episode of star formation or not, they are 'young' in the sense that their interstellar medium is chemically unevolved. In this regard, they are the nearest equivalents of young galaxies in the early Universe. All three galaxies, that we have observed, have irregular H I morphologies and kinematics, which we interpret as either due to tidal interaction with neighbouring galaxies, or the consequences of a recent merger. The remaining three of the six most metal-deficient galaxies are also known to have highly disturbed H I distributions and are interacting. It is interesting because these galaxies were chosen solely on the basis of their metallicity and not for any particular signs of interaction. In this sense (i.e. their gas has not yet had time to settle into a regular disc), one could regard these extremely metal deficient (XMD) galaxies as 'young'. The current star-formation episode is likely to have been triggered by interaction/merger. It is also possible that the tidal interaction has lead to enhanced mixing with metal-poor gas in outer disc, and hence to a low gas-phase metallicity in the central star-forming regions. We also try to determine the threshold gas density for star-formation in our sample of galaxies, and find that in general these galaxies do not show a one-to-one correspondence between regions of high H I column density and regions with current star formation. However, to the extent that one can define a threshold density, its value (~10²¹ atoms cm⁻²) is similar to that in galaxies with much higher metallicity. The highest column densities that we detect in regions far outside star-forming regions (i.e. a lower limit to the star-formation threshold) are ~2 × 10²¹ atoms cm⁻².