Radio Identifications of Extragalactic IRAS Sources

Abstract

Extragalactic sources detected at λ= 60 microns were selected from the IRAS Faint Source Catalog, Version 2 by the criterion S_60 microns_ >= S_12_ microns. They were identified by position coincidence with radio sources stronger than 25 mJy at 4.85 GHz in the 6.0 sr declination band 0^deg^ < δ < +75^deg^ (excluding the 0.05 sr region 12^h^40^m^< α < 14^h^40^m^, 0^deg^< +5^deg^) and with radio sources stronger than 80 mJy in the 3.4 sr area o^h^ <α < 2o^h^, -40^deg^ < δ < 0^deg^ (plus the region 12^h^40^m^ < α < 14^h^40^m^, 0^deg^<δ <+5^deg^). Fields containing new candidate identifications were mapped by the VLA at 4.86 GHz with about 15" FWHM resolution. Difficult cases were confirmed or rejected with the aid of accurate (σ ~ 1") radio and optical positions. The final sample of 354 identifications in {OMEGA} = 9.4 sr is reliable and large enough to contain statistically useful numbers of radio-loud FIR galaxies and quasars. The logarithmic FIR radio flux ratio parameter q can be used to distinguish radio sources powered by "starbursts" from those powered by "monsters." Starbursts and normal spiral galaxies in a λ = 60 micron flux-limited sample have a narrow (σ_q_ = 0.14 +/- 0.01) q distribution with mean <q> = 2.74 +/- 0.01, and none have "warm" FIR spectra [α(25 microns, 60 microns) < 1.5]. The absence of radio- quiet (but not completely silent) blazars indicates that nearly all blazars become optically thin at frequencies v<~100 GHz. Nonthermal sources with steep FIR/optical spectra and dusty-embedded sources visible only at FIR and radio wavelengths must be very rare.