BREAKDOWN OF KENNICUTT–SCHMIDT LAW AT GIANT MOLECULAR CLOUD SCALES IN M33

We have mapped the northern area (30′ × 20′) of a Local Group spiral galaxy M33 in 12CO(J = 1–0) line with the 45 m telescope at the Nobeyama Radio Observatory. Along with Hα and Spitzer 24 μm 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 (∼80 pc) 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 ΣSFR ≲ 10−10 to ∼10−6 M☉ yr−1 pc−2, whereas the values are mostly within 10–40 M☉ pc−2. The surface density of gas and that of SFR correlate well at an ∼1 kpc resolution, but the correlation becomes looser with higher resolution and breaks down at GMC scales. The scatter of the ΣSFR– 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.