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

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.Comment: 29 pages including 28 figures. matches the published version (PASJ 2011 Dec. 25 issue). The E-open option was chosen for this article, i.e., the official version available from PASJ site (http://pasj.asj.or.jp/v63/n6/630613/630613-frame.html) without restrictio

Spectra from Forming Region of the First Galaxies : The Effect of Aspherical Deceleration

Ly$\alpha$ line emission from the Loeb-Rybicki (LR) halo, which is the expanding HI IGM (intergalactic medium) around the first star clusters and the ionized interstellar medium, is investigated by solving a radiative transfer problem. While the initial scattering optical depth is $\sim 10^5$ for the Ly$\alpha$ photons, most of the Ly$\alpha$ photons can escape when the cumulative frequency-shift due to the expansion of the HI IGM becomes significantly large. The current paper improves upon previous treatments of the scattering processes and the opacity for the Ly$\alpha$ transfer. Confirming the previous results of the LR halo, we investigate the effect of the aspherical expansion of the IGM. The asphericity is hypothesized to follow the initial stage of the gravitational deceleration to form the large scale filamentary structure of the Universe. According to our results, the effect of the asphericity lets the peak wavelength of the line profile shift to longer wavelengths and the FWHM of the profile become wider than those of the spherically expanding model. To detect these features is meaningful if we are interested in the initial evolution of the large scale structure, since they reflect the dynamical properties of the IGM at that time. Furthermore, given the recent discovery of the high redshift cosmological reionization, we briefly comment on the effects of the redshift and the cosmological parameters on the line profile.Comment: 18 pages, 8 figures, accepted for publication in the Astrophysical Journa