Breakdown of Kennicutt-Schmidt Law at GMC Scales in M33
  We have mapped the northern area (30'\times 20') of a local group spiral
galaxy M33 in 12CO(1-0) line with the 45-m telescope at the Nobeyama Radio
Observatory. Along with Halpha and Spitzer 24-micron data, we have investigated
the relationship between the surface density of molecular gas mass and that of
star formation rate (SFR) in an external galaxy (Kennicutt-Schmidt law) with
the highest spatial resolution (~80pc) to date, which is comparable to scales
of giant molecular clouds (GMCs). At positions where CO is significantly
detected, the SFR surface density exhibits a wide range of over four orders of
magnitude, from Sigma{SFR}<10^{-10} to ~10^{-6} M_solar yr^{-1}pc^{-2}, whereas
the Sigma_{H2} values are mostly within 10-40 M_solar pc^{-2}. The surface
density of gas and that of SFR correlate well at a ~1-kpc resolution, but the
correlation becomes looser with higher resolution and breaks down at GMC
scales. The scatter of the Sigma_{SFR}-Sigma_{H2} relationship in the ~80-pc
resolution results from the variety of star forming activity among GMCs, which
is attributed to the various evolutionary stages of GMCs and to the drift of
young clusters from their parent GMCs. This result shows that the
Kennicutt-Schmidt law is valid only in scales larger than that of GMCs, when we
average the spatial offset between GMCs and star forming regions, and their
various evolutionary stages.
