MIR Spectroscopy of Heavily Obscured IRAS Galaxies

the iras bright galaxy sample contains several moderateluminosity galaxies exhibiting the similar iras spectra to those of cool ultraluminous infrared galaxies such as arp 220 (e.g., ngc 2623). namely, their infrared luminosities are less than 10^12 l(sun), and their iras spectra are nearly blackbody with color temperatures of 6080 k (see figure). these moderateluminosity blackbodylike infrared galaxies (mbigs) should be the early, dustenshrouded stages in the evolution of seyfert/starburst galaxies. this is because most of mbigs are oh megamasers/absorbers, mergers, and/or show the enhanced ratios of the infrared luminosity to the moleculargas mass. these are the properties characteristic of ultraluminous infrared galaxies, which are considered as the initial stages of quasars. therefore, the studies of mbigs will be essential to the complete understanding of seyfert/starburst phenomena, as in the case of ultraluminous infrared galaxies being crucial to the study of quasars. however, the dominant energy sources of mbigs are unknown. hence we propose the photos observation of mbigs. the main purpose is the measurement of the pah emission feature at 7.7 micron and the silicate absorption feature at 9.7 micron so as to determine the dominant energy sources (agns or ob stars). the pah features are useful in estimating the contribution from ob stars to the farinfrared luminosity. this is because (1) the pah features are excited by uv photons from ob stars, (2) the ratio of the number of the photons emitted in a certain pah feature to the total number of absorbed uv/optical photons is constant, and (3) pahs are destroyed by xrays from agns. the 7.7 micron feature is stronger by a factor of 310 than the pah features observable from the ground. moreover, the reddening effect is very small at 7.7 micron. on the other hand, the depth of the silicate absorption feature is a measure of interste truncated [truncated!, Please see actual data for full text]