Star formation and its efficiency
S. Boissier

An important properties of star formation on galactic scale is the
empirical fact that the star formation rate density is related in
a simple way to the gas density (Schmidt Law). During the last years,
a number of progresses have been made in this field (e.g. GALEX view
of  extended star formation, THINGS team analysis on local scales).
The presentation reviews some of these results, especially those
presented at the conference: "SFR@50, filling the cosmos with stars"
(at the occasion of the 50th birthday of the Schmidt Law).
I will also show some result concerning star formation at low density
(in dwarfs, XUV and LSB galaxies). In this regime, important (opened)
questions concern the H-alpha to UV ratio (due to IMF or time
effect ?) and the presence or not of a threshold. Finally, I will show
with simple toy-models that the eventual presence of a threshold,
and the relation between the various Schmidt laws (local, radial,
global) can be investigated with simple toy models showing the role
of spiral arms (or other structures) enhancing locally the density.