Paschen-α emission in the gravitationally lensed galaxy SMM

We report the detection of the Paα 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(Paα) = (2.05 0.33) × 10⁴² erg s^-1, corrected for gravitational lensing. Comparing the Hα and Paα luminosities, we derive a nebular extinction, A(V) = 3.6 0.4mag. The dust-corrected luminosity, L(Paα) = (2.57 0.43) × 10⁴² erg s^-1, corresponds to an ionization rate, Q ₀ = (1.6 0.3) × 10⁵⁵ γs^-1. The instantaneous SFR is ψ = 171 28 M _ȯ yr^-1, assuming a Salpeter-like initial mass function from 0.1 to 100 M _ȯyr^-1. The total IR luminosity derived using 70, 450, and 850μm data is L_IR = (5-10) × 10¹¹ L _⊙, corrected for gravitational lensing. This corresponds to ψ = 90-180 M _⊙ yr^-1, where the upper range is consistent with that derived from the Paα luminosity. While the L(8 μm)/L(Paα) ratio is consistent with the extrapolated relation observed in local galaxies and star-forming regions, the rest-frame 24μm luminosity is significantly lower with respect to local galaxies of comparable Paα luminosity. Thus, SMM J163554.2+661225 arguably lacks a warmer dust component (T_D ∼ 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, L _IR 10¹⁰ L _⊙, but "scaled up" by a factor of 10-100.