Paschen-alpha Emission in the Gravitationally Lensed Galaxy SMM
  J163554.2+661225
  We report the detection of the Paschen-alpha emission line in the z=2.515
galaxy SMM J163554.2+661225 using Spitzer spectroscopy. SMM J163554.2+661225 is
a sub-millimeter-selected infrared (IR)-luminous galaxy maintaining a high
star-formation rate (SFR), with no evidence of an AGN from optical or infrared
spectroscopy, nor X-ray emission. This galaxy is lensed gravitationally by the
cluster Abell 2218, making it accessible to Spitzer spectroscopy. Correcting
for nebular extinction derived from the H-alpha and Pa-alpha lines, the
dust-corrected luminosity is L(Pa-alpha) = (2.57+/-0.43) x 10^43 erg s^-1,
which corresponds to an ionization rate, Q = (1.6+/-0.3) x 10^55 photons s^-1.
The instantaneous SFR is 171+/-28 solar masses per year, assuming a
Salpeter-like initial mass function. The total IR luminosity derived using 70,
450, and 850 micron data is L(IR) = (5-10) x 10^11 solar luminosities,
corrected for gravitational lensing. This corresponds to a SFR=90-180 solar
masses per year, where the upper range is consistent with that derived from the
Paschen-alpha luminosity. While the L(8 micron) / L(Pa-alpha) ratio is
consistent with the extrapolated relation observed in local galaxies and
star-forming regions, the rest-frame 24 micron luminosity is significantly
lower with respect to local galaxies of comparable Paschen-alpha luminosity.
Thus, SMM J163554.2+661225 arguably lacks a warmer dust component (T ~ 70 K),
which is associated with deeply embedded star formation, and which contrasts
with local galaxies with comparable SFRs. Rather, the starburst is consistent
with star-forming local galaxies with intrinsic luminosities, L(IR) ~ 10^10
solar luminosities, but "scaled-up" by a factor of 10-100.
