THE KENNICUTT–SCHMIDT RELATION IN EXTREMELY METAL-POOR DWARF GALAXIES

Abstract

The Kennicutt–Schmidt (KS) relation between the gas mass and star formation rate (SFR) describes the star formation regulation in disk galaxies. It is a function of gas metallicity, but the low-metallicity regime of the KS diagram is poorly sampled. We have analyzed data for a representative set of extremely metal-poor galaxies (XMPs), as well as auxiliary data, and compared these to empirical and theoretical predictions. The majority of the XMPs possess high specific SFRs, similar to high-redshift star-forming galaxies. On the KS plot, the XMP H i  data occupy the same region as dwarfs and extend the relation for low surface brightness galaxies. Considering the H i  gas alone, a considerable fraction of the XMPs already fall off the KS law. Significant quantities of “dark” H 2 mass (i.e., not traced by CO) would imply that XMPs possess low star formation efficiencies (SFE gas ). Low SFE gas in XMPs may be the result of the metal-poor nature of the H i  gas. Alternatively, the H i  reservoir may be largely inert, the star formation being dominated by cosmological accretion. Time lags between gas accretion and star formation may also reduce the apparent SFE gas , as may galaxy winds, which can expel most of the gas into the intergalactic medium. Hence, on global scales, XMPs could be H i -dominated, high-specific-SFR (≳10 −10 yr −1 ), low-SFE gas (≲10 −9 yr −1 ) systems, in which the total H i  mass is likely not a good predictor of the total H 2 mass, nor of the SFR.