The Kennicutt-Schmidt Relation in Extremely Metal-Poor Dwarf Galaxies
  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 HI data occupy the same region as dwarfs, and extend the relation for
low surface brightness galaxies. Considering the HI 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$_{\rm gas}$). Low SFE$_{\rm gas}$ in XMPs may
be the result of the metal-poor nature of the HI gas. Alternatively, the HI
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$_{\rm gas}$, as may galaxy winds, which can expel
most of the gas into the intergalactic medium. Hence, on global scales, XMPs
could be HI-dominated, high specific SFR ($\gtrsim $ 10$^{-10}$ yr$^{-1}$), low
SFE$_{\rm gas}$ ($\lesssim$ 10$^{-9}$ yr$^{-1}$) systems, in which the total HI
mass is likely not a good predictor of the total H$_2$ mass nor of the SFR.
