2010A&A...521A..48S

We assess the relationships between the surface densities of the gas and star formation rate (SFR) within spiral arms of the nearby late-type spiral galaxies M81 and M101. By analyzing these relationships locally, we empirically derive a kiloparsec scale Kennicutt-Schmidt Law (Σ_SFR∝Σ^N_gas). Both M81 and M101 were observed with the Far-Infrared Surveyor (FIS) aboard AKARI in four far-infrared bands at 65, 90, 140, and 160µm. The spectral energy distributions of the whole galaxies show the presence of the cold dust component (T_C∼20K) in addition to the warm dust component (T_W∼60K). We deconvolved the cold and warm dust emission components spatially by making the best use of the multi-band photometric capability of the FIS. The cold and warm dust components show power-law correlations in various regions, which can be converted into the gas mass and the SFR, respectively. We find a power-law correlation between the gas and SFR surface densities with significant differences in the power-law index N between giant HII regions (N=1.0±0.5) and spiral arms (N=2.2±0.2) in M101. The power-law index for spiral arms in M81 is similar (N=1.9±0.4) to that of spiral arms in M101. The power-law index is not always constant within a galaxy. The difference can be attributed to the difference in the star formation processes on a kiloparsec scale. N≃2 seen in the spiral arms in M81 and M101 supports the scenario of star formation triggered by cloud-cloud collisions enhanced by a spiral density wave, while N≃1 derived in giant HII regions in M101 suggests the star formation induced by the Parker instability triggered by high-velocity HI gas infall. The present method can be applied to a large galaxy sample for which the AKARI All Sky Survey provides the same 4 far-infrared band data.