The IRAS Bright Galaxy Sample. III. 1-10 µm observations and coadded IRAS data for galaxies with L_(IR) ⩾ 10^(11) L⊙

Abstract

Galaxies from the IRAS Bright Galaxy Sample with infrared luminosities L_(IR) ⩾ 10^(11) L⊙ have been measured at 1.3, 1.6, 2.2, 3.7, and 10 µm. In addition, coadded IRAS measurements at all four IRAS bands have been obtained. It is found that an increase in the total infrared luminosity above L_(IR) ⩾ 10^(11) L⊙ is correlated with increased emission from hot dust with characteristic temperatures ~800 K contributing a substantial fraction of the 2.2 and 3.7 µm emission. This hot dust emission appears to "turn on" at luminosities of roughly 10^(11) L⊙. The far-infrared emission cannot be modeled with a single dust temperature, but requires a cold (T~30-50 K) component coupled with a warmer (T≳70 K) component. Although the relative contribution from the cold component decreases with increasing luminosity, the temperature of the warmer component is independent of luminosity. The f_v (12 µm)/f_v (25 µm) ratios for the galaxies in this sample are small compared with other extragalactic objects, indicating that the radiation at 12 and 25 µm is dominated by emission from large dust grains radiating at high temperatures, rather than PAHs. The spatial distribution of the 10 µm emission indicates a substantial extended component for most of the galaxies in this sample, implying that star-formation processes contribute significantly to the luminosities. However, one-third of the galaxies have exponential scale sizes characteristic of compact sources, and half of the galaxies have 10 µm emission consistent with a contribution of 50% or more from a central point source.

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