PASCHEN-alpha EMISSION IN THE GRAVITATIONALLY LENSED GALAXY SMM J163554.2+661225
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We report the detection of the Pa 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(Pa) = (2.05 0.33) 1042 erg s-1, corrected for gravitational lensing. Comparing the H and Pa luminosities, we derive a nebular extinction, A(V) = 3.6 0.4mag. The dust-corrected luminosity, L(Pa) = (2.57 0.43) 1042 erg s-1, corresponds to an ionization rate, Q 0 = (1.6 0.3) 1055 s-1. The instantaneous SFR is = 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 850m data is LIR = (5-10) 1011 L , corrected for gravitational lensing. This corresponds to = 90-180 M yr-1, where the upper range is consistent with that derived from the Pa luminosity. While the L(8 m)/L(Pa) ratio is consistent with the extrapolated relation observed in local galaxies and star-forming regions, the rest-frame 24m luminosity is significantly lower with respect to local galaxies of comparable Pa 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, L IR 1010 L , but "scaled up" by a factor of 10-100. 2009. The American Astronomical Society. All rights reserved.