Complex Radio Spectral Energy Distributions in Luminous and Ultraluminous Infrared Galaxies

TL;DR

This study uses the Expanded Very Large Array to image radio emissions from local luminous and ultraluminous infrared galaxies across multiple frequencies, revealing spectral curvature and optical depth effects in their radio SEDs.

Contribution

It provides detailed measurements of radio spectral energy distributions in LIRGs and ULIRGs, confirming spectral curvature and high-frequency deficits with higher frequency data.

Findings

Radio spectral indices around -0.67 with 0.15 scatter.

Evidence of spectral curvature with flatter spectra near 1 GHz.

Confirmation of high-frequency radio emission deficits.

Abstract

We use the Expanded Very Large Array to image radio continuum emission from local luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) in 1 GHz windows centered at 4.7, 7.2, 29, and 36 GHz. This allows us to probe the integrated radio spectral energy distribution (SED) of the most energetic galaxies in the local universe. The 4-8 GHz flux densities agree well with previous measurements. They yield spectral indices \alpha \approx -0.67 (where F_\nu \propto \nu^\alpha) with \pm 0.15 (1\sigma) scatter, typical of nonthermal (synchrotron) emission from star-forming galaxies. The contrast of our 4-8 GHz data with literature 1.5 and 8.4 GHz flux densities gives further evidence for curvature of the radio SED of U/LIRGs. The SED appears flatter near \sim 1 GHz than near \sim 6 GHz, suggesting significant optical depth effects at the lower frequencies. The high frequency (28-37 GHz) flux densities are low compared to extrapolations from the 4-8 GHz data. We confirm and extend to higher frequency a previously observed deficit of high frequency radio emission for luminous starburst galaxies.