Mid-Infrared Spectral Diagnosis of Submillimeter Galaxies

March 2008 • 2008ApJ...675.1171P

Authors • Pope, Alexandra • Chary, Ranga-Ram • Alexander, David M. • Armus, Lee • Dickinson, Mark • Elbaz, David • Frayer, David • Scott, Douglas • Teplitz, Harry

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.

Links

• PREPRINT http://arxiv.org/abs/0711.1553
• NED https://ned.ipac.caltech.edu/uri/NED::InRefcode/2008ApJ...675.1171P
• ELECTR https://doi.org/10.1086%2F527030
• SIMBAD http://simbad.u-strasbg.fr/simbo.pl?bibcode=2008ApJ...675.1171P
• PDF https://stacks.iop.org/0004-637X/675/1171/pdf
• DATA https://irsa.ipac.caltech.edu/bibdata/2008/P/2008ApJ...675.1171P.html
• SPIRES http://inspirehep.net/search?p=find+eprint+arXiv:0711.1553