Infrared Spectral Energy Distribution of Galaxies in the AKARI All Sky Survey: Correlations with Galaxy Properties, and Their Physical Origin

Abstract

Abstract We have studied the properties of more than 1600 low-redshift galaxies by utilizing high-quality infrared flux measurements of the AKARI All-Sky Survey and physical quantities based on optical and 21-cm observations. Our goal is to understand the physics determining the infrared spectral energy distribution (SED). The ratio of the total infrared luminosity, $L_{\rm TIR}$, to the star-formation rate (SFR) is tightly correlated by a power-law to specific SFR (SSFR), and $L_{\rm TIR}$ is a good SFR indicator only for galaxies with the largest SSFR. We discovered a tight linear correlation for normal galaxies between the radiation field strength of dust heating, estimated by infrared SED fits ($U_h$), and that of galactic-scale infrared emission ($U_{\rm TIR} \propto L_{\rm TIR}/R^2$), where $R$ is the optical size of a galaxy. The dispersion of $U_h$ along this relation is $\sim $0.3 dex, corresponding to $\sim $13% dispersion in the dust temperature. This scaling and the $U_h/U_{\rm TIR}$ ratio can be explained physically by a thin layer of heating sources embedded in a thicker, optically-thick dust screen. The data also indicate that the heated fraction of the total dust mass is anti-correlated to the dust column density, supporting this interpretation. In the large $U_{\rm TIR}$ limit, the data of circumnuclear starbursts indicate the existence of an upper limit on $U_h$, corresponding to the maximum SFR per gas mass of $\sim $10 Gyr$^{-1}$. We find that the number of galaxies sharply drops when they become optically thin against dust-heating radiation, suggesting that a feedback process to galaxy formation (likely by the photoelectric heating) is working when dust-heating radiation is not self-shielded on a galactic scale. Implications are discussed for the $M_{{\rm H{ I}}}$-size relation, the Kennicutt-Schmidt relation, and galaxy formation in the cosmological context.