The central energy source of 70micron-selected galaxies: Starburst or
  AGN?
  We present the first AGN census in a sample of 61 galaxies selected at
  70microns, 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 (3.6-160micron) data, as well as optical
spectroscopy and photometry from the Deep Extragalactic Evolutionary Probe 2
(DEEP2) survey for the Extended Groth Strip (EGS) field. We investigate
spectral line diagnostics ([OIII]/Hbeta and [NeIII]/[OII] ratios, Hdelta Balmer
absorption line equivalent widths and the strength of the 4000Ang break), X-ray
luminosities and spectral energy distributions (SEDs). We find that the
70micron sources are undergoing starburst episodes and are therefore
characterised 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 starbursts (SBs, 10^10<L_IR<10^11 L_solar), Luminous InfraRed
Galaxies (LIRGs, 10^11<L_IR<10^12 L_solar) and UltraLuminous InfraRed Galaxies
(ULIRGs,10^12<L_IR<10^13 L_solar), 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 70micron-selected
galaxies are primarily powered by star-formation.
