Radio continuum properties of luminous infrared galaxies

Identifying the presence of an AGN in the radio^⋆,⋆⋆

¹ Department of Physics, University of Crete, 71003 Heraklion, Greece
e-mail: eleniv@physics.uoc.gr
² Institute for Astronomy, Astrophysics, Space Applications & Remote Sensing, National Observatory of Athens, 15236 Athens, Greece
³ Chercheur Associé, Observatoire de Paris, 75014 Paris, France
⁴ Infrared Processing & Analysis Center, MS 100-22, California Institute of Technology, Pasadena, CA, USA
⁵ Nucleo de Astronomia de la Facultad de Ingenieria, Universidad Diego Portales, Av. Ejercito Libertador 441, Santiago, Chile
⁶ Department of Astronomy, University of Virginia, Charlottesville, VA, USA

Received: 3 May 2014
Accepted: 11 August 2014

Abstract

Context. Luminous infrared galaxies (LIRGs) are systems enshrouded in dust, which absorbs most of their optical/UV emission and radiates it again 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.

Aims. 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, containing 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 (AGN).

Methods. 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 S_ν and frequency ν, S_ν ~ ν^{− α}, where α is the radio spectral index. By studying the spatial variations in α, 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 equivalent width of the 6.2 μm PAH feature) and optical (BPT diagram) AGN diagnostics.

Results. We find that 21 out of the 46 objects in our sample (~45%) are radio-AGN, 9 out of the 46 (~20%) are classified as starbursts (SB) based on the radio analysis, and 16 (~35%) are AGN/SB. After comparing to other AGN diagnostics we find 3 objects out of the 46 (~7%) 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.