Dust-Embedded AGN in Unusually Warm IRAS Galaxies

Extract

The IRAS Point Source Catalog contains only 61 sources identified as galaxies whose energy distribution peaks at 60 mμ. The scarcity of such galaxies has prompted a search for possible common properties. This sample of ‘60 mμ peakers’, 21 of which are previously identified galaxies, partially overlaps with that of warm IRAS galaxies studied by de Grijp et al. (1987) and contains similar percentages of Seyfert (65%) and starburst galaxies on the one hand, and of strong and weak radio sources on the other hand. A remarkable characteristic is, however, that about half of the 60 mμ peakers seem to be early-type galaxies. The fact that such galaxies are rarely IRAS sources and, if so, have FIR energy distributions peaking at 100 mμ similar to those of spirals, implies that we are sampling active or nuclear starburst early-type galaxies with a very large success rate. The observational data accumulated so far further show that:

1. (i) objects with smaller FIR to near-IR flux ratios have redder J-K colors and warmer 60 to 25 mμ colors, i.e., an infared spectrum dominated by warmer dust and/or a nonthermal source (Figs. 1a,b);

2. (ii) out of 32 objects with radio data, the 5 compact radio sources with luminosities intermediate between those af radio-quiet and radio-loud AGN have among the warmest 60 to 25 mμ colors (Fig. 2). Such warm FIR colors are not a common characteristic of radio galaxies and quasars (Golombek et al. 1987, Neugebauer et al. 1986).

3. (iii) the 60 mμ luminosities range from 10⁹ to 10¹² L₀, and are largest for Mkn 231, 2306+0505 (Hill et al. 1987) and 2046+1925 (Frogel et al. 1988). The latter 2 objects, along with 0052-7054 (Frogel and Elias 1987) which also belongs to our sample, are Seyfert 2 galaxies with evidence for the presence of a dust-obsured broad line region.