An Optical Study of the Infrared/Radio Correlation of Galaxies

Kirsten Larson


We study the infrared/radio correlation as a function of optical galaxy type to understand the effects of active galactic nuclei on radio emission. Based on previous work, we hypothesized that galaxies with greater AGN activity will have greater radio emission than those without and will therefore lie further off of the infrared/radio correlation. In order to accomplish this, we cross correlate galaxies from data publicly available from the Sloan Digital Sky Survey (SDSS), Infrared Astronomical Satellite sky survey (IRAS), and the National Radio Astronomical Observatory Very Large Array Sky Survey (NVSS). These galaxies are classified by optical spectral type using the method determined by Kewley. We then investigate the infrared/radio correlation of these galaxies using various infrared bands. We find that all galaxies lie in approximately the same area on the infrared/radio diagram regardless of optical spectral type with only a few Seyfert and composite galaxies lying slightly above the correlation. We calculate the linear fit to the star-forming galaxies and obtain a slope of 0.884 � 0.006 using the 60 μm IR band, which is smaller than the one to one correlation found in the literature. We also calculate the linear fit and obtain a slope of 0.991 � 0.006 in the far-infrared (combined 60 μm and 100 μm IR bands), which is comparable to the one to one correlation found in literature. When comparing our SDSS selected data to 85 galaxies in the IRAS 1Jy ultraluminous infrared galaxy sample, we find that the RMS deviation from the star-forming linear fit in our SDSS selected sample is smaller than that of the ultraluminous galaxies. We conclude that the strength of radio emission in Seyfert and composite galaxies is not determined solely by the strength of their AGN. It is possible that the observed radio emission from galaxies with AGN is determined by a combination of orientation, black hole mass, and accretion rate, as well as the merger stage of a galaxy. This would explain the larger spread in radio luminosities observed in Seyfert galaxies. However more study is needed to determine this.