Spitzer, Fls Survey: Far-infrared Spectral Energy Distribution

We summarize the advantages and significance of observations in the infrared, the physical mechanisms of infrared emission in galaxies, and the possible infrared contributions from active galactic nucleus (AGN). We also review the observational and theoretical results of galactic spectral energy distribution (SED) and the corresponding optical spectral classification.We also briefly introduce Spitzer Space Telescope and the Sloan Digital Sky Survey (SDSS). Basing on the Spitzer xFLS (the extragalactic field of the Spitzer First Look Survey) and SDSS surveys, we obtained 315 extra-galactic sources with both infrared flux and optical spectra. We constructed the mid-to-far infrared SEDs of each sample source after measuring the five bands mid-infrared flux (3.6,4.5,5.8,8.0,24μm) and matching the two far-infrared flux (70 and 160μm) from Frayer et al.'s catalog. Meanwhile, we also classified all this sources according to their optical spectra.For the classified sample galaxies and QSOs, we did model fittings to the individual SEDs. We studied the dust properties of different spectral types, the results show that:(1) The near-to-mid infrared SEDs of QSOs car be decribed by power-law, which can be represented by the AGN template NGC5506.(2) The mid infrared SEDs of absorption galaxies are dominated by an old stellar continuum with little dust contribution. They can be fitted well by a 13Gyr stellar population synthesis model from GRASIL.(3) HII galaxies, LINERs, mixture type galaxies, even Seyfert 2 galaxies can be well fitted by a 13Gyr old stellar continuum plusing one parameter(a) dust model of Dale et al.(2001). The a values can characterize the star formation activities of galaxies. The larger a values of sample galaxies indicate that all these galaxies tend to be quiescent, and statistics indicate that HII galaxies are relatively more active than the other twos (LINERs and mixture types).(4)The a distributions of different morphologies of SINGS galaxies (Dale et al. 2005) show that the activity of galaxies may also relates with either Hubble morphologies or bars.On average, the galaxies with different spectral types present the different infrared properties. However we can not pick out the spectral type only from its individual mid-to-far SEDs. Confined by the 7σ limit of 70 or 160μm flux, there would exist a bias for this sample. To avoid this, a larger and more complete sample is needed in the future analysis.