Radio continuum properties of luminous infrared galaxies. Identifying
  the presence of an AGN in the radio
  Luminous infrared galaxies are systems enshrouded in dust, which absorbs most
of their optical/UV emission and re-radiates it in the mid- and far-infrared.
Radio observations are largely unaffected by dust obscuration, enabling us to
study the central regions of LIRGs in an unbiased manner. The main goal of this
project is to examine how the radio properties of local LIRGs relate to their
infrared spectral characteristics. Here we present an analysis of the radio
continuum properties of a subset of the Great Observatories All-sky LIRG Survey
(GOALS), which consists of 202 nearby systems (z<0.088). Our radio sample
consists of 35 systems, or 46 individual galaxies, that were observed at both
1.49 and 8.44 GHz with the VLA with a resolution of about 1 arcsec (FWHM). The
aim of the project is to use the radio imagery to probe the central kpc of
these LIRGs in search of active galactic nuclei. We used the archival data at
1.49 and 8.44 GHz to create radio-spectral-index maps using the standard
relation between flux density Sv and frequency v, S~v^-a, where a is the radio
spectral index. By studying the spatial variations in a, we classified the
objects as radio-AGN, radio-SB, and AGN/SB (a mixture). We identified the
presence of an active nucleus using the radio morphology, deviations from the
radio/infrared correlation, and spatially resolved spectral index maps, and
then correlated this to the usual mid-infrared ([NeV]/[NeII] and [OIV]/[NeII]
line ratios and EQW of the 6.2 um PAH feature) and optical (BPT diagram) AGN
diagnostics. We find that 21 out of the 46 objects in our sample are radio-AGN,
9 are classified as starbursts (SB), and 16 are AGN/SB. After comparing to
other AGN diagnostics we find 3 objects out of the 46 that are identified as
AGN based on the radio analysis, but are not classified as such based on the
mid-infrared and optical AGN diagnostics presented in this study.
