The central energy source of 70 μm-selected galaxies: starburst or AGN?

We present the first active galactic nuclei (AGN) census in a sample of 61 galaxies selected at 70 mu m, a wavelength which should strongly favour the detection of star-forming systems. For the purpose of this study, we take advantage of deep Chandra X-ray and Spitzer infrared (3.6-160 mu m) data, as well as optical spectroscopy and photometry from the Deep Extragalactic Evolutionary Probe 2 (DEEP 2) survey for the Extended Groth Strip (EGS) field, as part of the All-Wavelength Extended Groth Strip International Survey. We investigate spectral line diagnostics ([O iii]/H beta and [Ne iii]/[O ii] ratios, H delta Balmer absorption line equivalent widths and the strength of the 4000 A break), X-ray luminosities and spectral energy distributions (SEDs). We find that the 70-mu m sources are undergoing starburst (SB) episodes and are therefore characterized by a predominance of young stars. In addition, 13 per cent of the sources show AGN signatures and hence potentially host an AGN. When the sample is split into SBs (1010 < L(IR) < 1011 L(circle dot)), luminous infrared Galaxies (LIRGs; 1011 < L(IR) < 1012 L(circle dot)) and ultraluminous infrared Galaxies (ULIRGs; 1012 < L(IR) < 1013 L(circle dot)), the AGN fraction becomes 0, 11 and 23 per cent, respectively, showing an increase with total infrared luminosity. However, by examining the sources' panchromatic SEDs, we conclude that although the AGN is energetically important in 1 out of 61 objects all 70-mu m-selected galaxies are primarily powered by star formation. When compared to a sample of DEEP 2 galaxies in the same redshift range and with similar optical colours, we find that the 70-mu m population is characterized by younger stellar ages and a higher AGN incidence, indicating that strongly star-forming populations might be a key in studying the relationship between black hole and stellar growth.