On the X-ray properties of sub-mm-selected galaxies

We present an analysis of the X-ray properties of 35 sub-mm galaxies (SMGs) in the Chandra Deep Field-North region. Using a sample of robust 850 mu m-selected galaxies, with sub-arcsecond positions from Spitzer and/or radio counterparts, we find 16 objects (45 +/- 8 per cent) with significant X-ray detections in the 2 Ms Chandra data. Six of these SMGs (similar to 17 +/- 6 per cent) have measured X-ray luminosities or upper limits consistent with those expected based on the far-infrared or radio-derived star formation rate, and hence with the X-rays coming solely from star-forming processes. Extrapolating observed X-ray/star formation rate relations to the luminosity of the SMGs, we find that the X-ray-derived star formation rates are typically in the range of 200-2000 M(circle dot) yr(-1). In another seven sources (20 +/- 7 per cent of the SMGs) a dominant active galactic nuclei (AGN) contribution to the X-ray emission is required, while in three more it is unclear whether the stellar process or accretion is responsible for the X-rays. Stacking of the X-ray-undetected SMGs reveals a highly significant (7 sigma) detection. Under the assumption that the stacked X-ray is due to star formation, this gives an average X-ray-derived star formation rate of around 150 M(circle dot) yr(-1). We deduce that the AGN fraction in SMGs based on X-ray observations is 20-29 +/- 7 per cent, which is towards the lower limit of previous estimates. Spectral analysis shows that in general, the SMGs are not heavily obscured in the X-ray but most of the SMGs classified as AGN show absorption with column densities in excess of 10(22) cm(-2). Of the secure AGN, the bolometric luminosity appears to be dominated by the AGN in only three cases. In around 85 per cent of the SMGs, the X-ray spectrum effectively rules out an AGN contribution that dominates the bolometric emission, even if the AGN is Compton thick. The evidence therefore suggests that intense star formation accounts for both the far-infrared and X-ray emission in most SMGs. We argue that, rather than having an especially high AGN fraction or duty cycle, SMGs have a high X-ray detection rate at very faint fluxes partly because of their high star formation rates and, in rarer cases, because the sub-mm emission is from an AGN.