Paschen-α Emission in the Gravitationally Lensed Galaxy SMM J163554.2+661225

We report the detection of the Paalpha emission line in the z = 2.515 galaxy SMM J163554.2+661225 using Spitzer spectroscopy. SMM J163554.2+661225 is a submillimeter-selected infrared-luminous galaxy maintaining a high star formation rate (SFR), with no evidence of an active galactic nucleus 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. We measure a line luminosity, L(Paalpha) = (2.05 +/- 0.33) X 1042 erg s-1, corrected for gravitational lensing. Comparing the Halpha and Paalpha luminosities, we derive a nebular extinction, A(V) = 3.6 +/- 0.4 mag. The dust-corrected luminosity, L(Paalpha) = (2.57 +/- 0.43) X 1042 erg s-1, corresponds to an ionization rate, Q 0 = (1.6 +/- 0.3) X 1055 gamma s-1. The instantaneous SFR is psi = 171 +/- 28 M yr-1, assuming a Salpeter-like initial mass function from 0.1 to 100 M yr-1. The total IR luminosity derived using 70, 450, and 850 mum data is LIR = (5-10) X 1011 L, corrected for gravitational lensing. This corresponds to psi = 90-180 M yr-1, where the upper range is consistent with that derived from the Paalpha luminosity. While the L(8 mum)/L(Paalpha) ratio is consistent with the extrapolated relation observed in local galaxies and star-forming regions, the rest-frame 24 mum luminosity is significantly lower with respect to local galaxies of comparable Paalpha luminosity. Thus, SMM J163554.2+661225 arguably lacks a warmer dust component (TD ~ 70 K), which is associated with deeply embedded star formation, and which contrasts with local galaxies with comparable SFRs. Rather, the starburst in SMM J163554.2+661225 is consistent with star-forming local galaxies with intrinsic luminosities, LIR 1010 L, but 'scaled up' by a factor of ~10-100.