PASCHEN-α EMISSION IN THE GRAVITATIONALLY LENSED GALAXY SMM J163554.2+661225
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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 LIR = (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 (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.
author list (cited authors)
Papovich, C., Rudnick, G., Rigby, J. R., Willmer, C., Smith, J., Finkelstein, S. L., Egami, E., & Rieke, M.