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

TL;DR

This study analyzes infrared data from over 1600 low-redshift galaxies to understand the physical mechanisms shaping their infrared spectral energy distributions and how these relate to galaxy properties and star formation activity.

Contribution

It reveals new correlations between dust heating radiation, infrared luminosity, and galaxy size, and proposes a physical model explaining the dust heating processes in galaxies.

Findings

L_TIR/SFR correlates with specific SFR.

U_h tightly correlates with U_TIR with 0.3 dex dispersion.

Maximum SFR per gas mass is around 10 Gyr^{-1}."

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_TIR, to the star-formation rate (SFR) is tightly correlated by a power-law to specific SFR (SSFR), and L_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_TIR ~ L_TIR/R^2), where R is the optical size of a galaxy. The dispersion of U_h along this relation is 0.3 dex, corresponding to 13% dispersion in the dust temperature. This scaling and the U_h/U_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_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 ~ 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_HI-size relation, the Kennicutt-Schmidt relation, and galaxy formation in the cosmological context.