Mid-Infrared Spectroscopy of Submillimeter Galaxies: Extended Star Formation in Massive High-redshift Galaxies

Abstract

We used the Spitzer Infrared Spectrograph (IRS) to study the mid-IR properties and investigate the energetics of 24 Submillimeter Galaxies (SMGs). This is the largest sample of SMGs observed with Spitzer IRS and covers the full extent of properties of the radio-identified population of SMGs in the redshift range of z ~ 0.65-3.2. We observe broad emission features from polycyclic aromatic hydrocarbons in more than 80% of our sample. We find that the median mid-IR spectrum is well described by a starburst component with an additional power law, F_ν ~ ν^(–2), likely representing a <32% contribution from an active galactic nucleus to the bolometric luminosity. Our results thus confirm that starburst activity dominates the bolometric luminosity in SMGs. We find that SMGs show weaker silicate absorption at ~9.7 μm than local ULIRGs. We also find stronger 6.2-μm PAH emission in SMGs (relative to the 7.7 μm PAH feature) than in local nuclear starbursts, which may be attributed to lower extinction by ice along the line of sight to SMGs. This suggests that the continuum and PAH emitting regions of SMGs are less obscured than in local starbursts and similarly luminous low-redshift ULIRGs. We interpret these results as evidence for a more extended distribution of cool and warm dust in SMGs compared to the more compact emitting regions in local ULIRGs and starbursts. Together these results suggest that SMGs are not simple high-redshift analogs of nuclear starbursts or local ULIRGs, but instead they appear to have star formation which resembles that seen in less-extreme star-forming environments at z~0 -suggesting their intense activity is distributed across a far larger region than the ~1 kpc nuclear bursts in local ULIRGs.