We study the Kennicutt-Schmidt relation between average star formation rate
and average cold gas surface density in the Hi dominated ISM of nearby spiral
and dwarf irregular galaxies. We divide the galaxies into grid cells varying
from sub-kpc to tens of kpc in size. Grid-cell measurements of low SFRs using
H-alpha emission can be biased and scatter may be introduced because of
non-uniform sampling of the IMF or because of stochastically varying star
formation. In order to alleviate these issues, we use far-ultraviolet emission
to trace SFR, and we sum up the fluxes from different bins with the same gas
surface density to calculate the average ΣSFR at a given value of
Σgas. We study the resulting Kennicutt-Schmidt relation in 400 pc, 1
kpc and 10 kpc scale grids in nearby massive spirals and in 400 pc scale grids
in nearby faint dwarf irregulars. We find a relation with a power law slope of
1.5 in the HI-dominated regions for both kinds of galaxies. The relation is
offset towards longer gas consumption timescales compared to the molecular
hydrogen dominated centres of spirals, but the offset is an order-of-magnitude
less than that quoted by earlier studies. Our results lead to the surprising
conclusion that conversion of gas to stars is independent of metallicity in the
HI dominated regions of star-forming galaxies. Our observed relations are
better fit by a model of star formation based on thermal and hydrostatic
equilibrium in the ISM, in which feedback driven turbulence sets the thermal
pressure.Comment: 11 pages, 7 figures, 5 tables. Accepted for publication in MNRAS Main
Journal. For the definitive version visit http://mnras.oxfordjournals.org
