Infrared spectral energy distribution of galaxies in the AKARI all sky survey: Correlations with galaxy properties, and their physical origin

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, LTIR, to the star-formation rate (SFR) is tightly correlated by a power-law to specific SFR (SSFR), and LTIR 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 (Uh), and that of galactic-scale infrared emission (U TIR ∝ LTIR=R2), where R is the optical size of a galaxy. The dispersion of Uh along this relation is ∼0.3 dex, corresponding to ∼13% dispersion in the dust temperature. This scaling and the Uh=UTIR 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 UTIR limit, the data of circumnuclear starbursts indicate the existence of an upper limit on Uh, corresponding to the maximum SFR per gas mass of ∼10Gyr-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 dustheating radiation is not self-shielded on a galactic scale. Implications are discussed for the MHI-size relation, the Kennicutt-Schmidt relation, and galaxy formation in the cosmological context. © 2011. Astronomical Society of Japan.