PROBING THE STAR FORMATION HISTORY AND INITIAL MASS FUNCTION OF THE z ∼ 2.5 LENSED GALAXY SMM J163554.2+661225 WITH HERSCHEL

We present the analysis of Herschel Spectral and Photometric Imaging Receiver far-infrared (FIR) observations of the z = 2.515 lensed galaxy SMM J163554.2+661225. Combining new 250, 350, and 500 mu m observations with existing data, we make an improved fit to the FIR spectral energy distribution of this galaxy. We find a total infrared (IR) luminosity of L(8-1000 mu m) = 6.9 +/- 0.6 x 10(11) L-circle dot, a factor of three more precise over previous L-IR estimates for this galaxy, and one of the most accurate measurements for any galaxy at these redshifts. This FIR luminosity implies an unlensed star formation rate (SFR) for this galaxy of 119 +/- 10 M-circle dot yr(-1), which is a factor of 1.9 +/- 0.35 lower than the SFR derived from the nebular Pa alpha emission line (a 2.5 sigma discrepancy). Both SFR indicators assume an identical Salpeter initial mass function (IMF) with slope Gamma = 2.35 over a mass range of 0.1-100 M-circle dot; thus this discrepancy suggests that more ionizing photons may be necessary to account for the higher Pa alpha-derived SFR. We examine a number of scenarios and find that the observations can be explained with a varying star formation history (SFH) due to an increasing SFR, paired with a slight flattening of the IMF. If the SFR is constant in time, then larger changes need to be made to the IMF by either increasing the upper mass cutoff to similar to 200 M-circle dot, or a flattening of the IMF slope to 1.9 +/- 0.15, or a combination of the two. These scenarios result in up to double the number of stars with masses above 20 M-circle dot, which produce the requisite increase in ionizing photons over a Salpeter IMF with a constant SFH.