2007/11/20 by Alexandra Pope, Ranga‐Ram Chary, Ranga-Ram Chary +11 · 4 citations
Physics and Astronomy · #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Astrophysics and Star Formation Studies #Galaxies: Formation, Evolution, Phenomena #Galaxy #Infrared #Luminosity #Luminous infrared galaxy #Photometry (optics) #Physics #Redshift #Star formation #Stars #astro-ph
paper · pdf · doi:10.1086/527030
published as Astrophys.J.675:1171-1193,2008 · 59 pages, 13 figures, 5 tables. Accepted for publication in ApJ: November 19, 2007
arxiv created 2007/11/20 · openalex publication_date 2008/03/07 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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.