  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 (PAHs) 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, Fnu ~ nu^-2, likely representing a < 32% AGN
contribution 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 9.7um than local ULIRGs. We also find
stronger 6.2um PAH emission in SMGs (relative to the 7.7um 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 local ULIRGs
or nuclear starbursts, 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.
