Complex radio spectral energy distributions in luminous and ultraluminous infrared galaxies

Abstract

We use the Expanded Very Large Array to image radio continuum emission from local luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) in 1GHz windows centered at 4.7, 7.2, 29, and 36GHz. This allows us to probe the integrated radio spectral energy distribution (SED) of the most energetic galaxies in the local universe. The 4-8GHz flux densities agree well with previous measurements. They yield spectral indices α -0.67 (where F ννα) with 0.15 (1σ) scatter, typical of nonthermal (synchrotron) emission from star-forming galaxies. The contrast of our 4-8GHz data with literature 1.5 and 8.4GHz flux densities gives further evidence for curvature of the radio SED of U/LIRGs. The SED appears flatter near 1GHz than near 6GHz, suggesting significant optical depth effects at lower frequencies. The high-frequency (28-37GHz) flux densities are low compared to extrapolations from the 4-8GHz data. We confirm and extend to higher frequency a previously observed deficit of high-frequency radio emission for luminous starburst galaxies. © 2011. The American Astronomical Society. All rights reserved.