Star Formation Seen with Akari

Figure 3.3 represents the far-IR observation of M101 with Akari (Suzuki et al.2007). Using 4 bands of Akari FIS and 3 bands of ISO, the SED of this galaxy is fitted with two components of dust emissions with temperatures $T_w=55^{+9}_{-25}{\rm K}$ and $T_c=18^{+4}_{-9}{\rm K}$. Spiral galaxies (not classified with either AGN or starburst galaxy) have a far-IR temperature with 20K-50K as a whole from Revised Shapley-Ames galaxy survey by IRAS De Jong et al 1984). As shown in Fig.3.4 (a), cold component well correlates with that of gas distribution, while warm one has a relation to star formation regions. Fig.3.4(b) plots the warm-to-cold ratio of intensities, which seems to show the distribution of star formation efficiency $\equiv$ star formation rate/gas mass. Some peaks in this plot correspond to famous HII regions. Center of the galaxy and the end of the galactic bar indicate peaks in this map. Since barred galaxies have larger star formation rate than spiral galaxies without bars, the central peaks are regarded as a region where gas is accumulated by the effect of a bar potential. Equations (eq:Kennicutt-correlation-1) and (3.7) indicates that the star formation efficiency

$$\Sigma_{\rm SFR}/\Sigma_{\rm gas}\propto \Sigma_{\rm gas}^{0.3-0.4},$$ (3.8)

M101 galaxy do not seem to be explained by a simple law as the star formation efficiency is an increasing function of gas density.