VLA observations of ultraluminous iras galaxies: Active nuclei or starbursts?

We employed the Very Large Array (VLA) of the National Radio Astronomy Observatory 5 in C configuration to map 39 ultraluminous IRAS galaxies at λ = 6 cm and 20 cm, at resolutions of ≈4″ and ≈15″, respectively. All sources were detected at both frequencies. The sources were selected from the flux-limited redshift survey of Strauss et al. and comprise nearly half of those sources with far-infrared luminosity ≥4.3 × 10¹¹ L_⊙ (for H₀ = 75 km s^-1 Mpc^-1) in the northern sky. Twenty-four of these sources were later remapped with the VLA in A configuration at λ = 6 cm with a resolution of ≈0″.5. The fluxes of our sources range from 1.2 to 187 mJy at 6 cm, with the majority of the sources fainter than 20 mJy, and from 4 to 150 mJy at 20 cm, with the majority of the sources fainter than 50 mJy. Most of the sources have radio spectral indices indicative of synchrotron emission (α ≈ -0.65). There is one source, however, that shows an inverted spectrum with α = +2.1; observations at higher frequencies show that the spectrum peaks between 5 and 8 GHz, as high as any of the "gigahertz peaked spectrum" sources studied by O'Dea et al. We discuss the implications of this source for observations of fluctuations in the cosmic microwave background. Eighteen of the sources are resolved at 4″ resolution. The extended sources fall into several categories: two show multiple unresolved components, another four are doubles with at least one resolved component, 14 show extended emission which could arise from a disk, and two show arcsecond-long jets. We find that our data fit the tight correlation found by Helou et al. between far-infrared and microwave luminosity; this correlation extends to the highest infrared luminosities. We find also that the correlation is weaker if only the extended or the nuclear components of the radio luminosity are used. Therefore, we argue that the far-infrared emission in the majority of these higher luminosity galaxies arises from the same mechanism as the lower luminosity FIR sources, which is believed to be star formation, rather than nonthermal activity in the nucleus. Moreover, the star formation is not confined to the extended disk in these sources but is important in the nucleus as well.