Mid-infrared spectral diagnosis of submillimeter galaxies

Abstract

We present deep mid-IR spectroscopy with Spitzer of 13 SMGs in the GOODS-N field.We find strong PAH emission in all of our targets, which allows us to measure mid-IR spectroscopic redshifts and place constraints on the contribution from star formation and AGN activity to the mid-IR emission. In the high-S/N composite spectrum, we find that the hot dust continuum from an AGN contributes at most 30% of the mid-IR luminosity. Individually, only 2/13 SMGs have continuum emission dominating the mid-IR luminosity; one of these SMGs, C1, remains undetected in the deep X-ray images but shows a steeply rising continuum in the mid-IR indicative of a Compton-thick AGN. We find that the mid-IR properties of SMGs are distinct from those of 24 μm–selected ULIRGs at z~2; the former are predominantly dominated by star formation, while the latter are a more heterogeneous sample with many showing significant AGN activity.We fit the IRS spectrum and the mid-IR to radio photometry of SMGs with template SEDs to determine the best estimate of the total IR luminosity from star formation. While many SMGs contain an AGN as evinced by their X-ray properties, our multiwavelength analysis shows that the total IR luminosity, L_(IR), in SMGs is dominated by star formation.We find that high-redshift SMGs lie on the relation between L_(IR) and L_(PAH,6.2) (or L_(PAH,7.7) or L_(PAH,11.3))that has been established for local starburst galaxies. This suggests that PAH luminosity can be used as a proxy for the SFR in SMGs. SMGs are consistent with being a short-lived cool phase in a massive merger where the AGN does not appear to have become strong enough to heat the dust and dominate the mid- or far-IR emission.