Mid-infrared spectroscopy of candidate AGN-dominated submillimeter
  galaxies
  Spitzer spectroscopy has revealed that ~80% of submm galaxies (SMGs) are
starburst (SB) dominated in the mid-infrared. Here we focus on the remaining
~20% that show signs of harboring powerful active galactic nuclei (AGN). We
have obtained Spitzer-IRS spectroscopy of a sample of eight SMGs which are
candidates for harboring powerful AGN on the basis of IRAC color-selection
(S8/S4.5>2; i.e. likely power-law mid-infrared SEDs). SMGs with an AGN
dominating (>50%) their mid-infrared emission could represent `missing link'
sources in an evolutionary sequence involving a major merger. First of all, we
detect PAH features in all of the SMGs, indicating redshifts from 2.5-3.4,
demonstrating the power of the mid-infrared to determine redshifts for these
optically faint dusty galaxies. Secondly, we see signs of both star-formation
(from the PAH features) and AGN activity (from continuum emission) in our
sample: 62% of the sample are AGN-dominated in the mid-infrared with a median
AGN content of 56%, compared with <30% on average for typical SMGs, revealing
that our IRAC color selection has successfully singled out sources with
proportionately more AGN emission than typical SB-dominated SMGs. However, we
find that only about 10% of these AGN dominate the bolometric emission of the
SMG when the results are extrapolated to longer infrared wavelengths, implying
that AGN are not a significant power source to the SMG population overall, even
when there is evidence in the mid-infrared for substantial AGN activity. When
existing samples of mid-infrared AGN-dominated SMGs are considered, we find
that S8/S4.5>1.65 works well at selecting mid-infrared energetically dominant
AGN in SMGs, implying a duty cycle of ~15% if all SMGs go through a subsequent
mid-infrared AGN-dominated phase in the proposed evolutionary sequence.
