2008ApJ...689...59S

We present Spitzer infrared (IR) photometry and spectroscopy of the lensed Lyman break galaxy (LBG) MS 1512-cB58 at z=2.73. The large (factor ∼30) magnification allows for the most detailed IR study of an L^*_UV(z=3) LBG to date. Broadband photometry with IRAC (3-10 µm), IRS (16 µm), and MIPS (24, 70, and 160 µm) was obtained, as well as IRS spectroscopy spanning 5.5-35 µm. A fit of stellar population models to the optical/near-IR/IRAC photometry gives a young age (∼9 Myr), forming stars at ∼98 M_☉/yr, with a total stellar mass of ∼10⁹ M_☉ formed thus far. The existence of an old stellar population with twice the stellar mass cannot be ruled out. IR spectral energy distribution fits to the 24 and 70 µm photometry, as well as previously obtained submillimeter/millimeter data give an intrinsic IR luminosity L_IR=1-2x10¹¹ L_☉ and a star formation rate (SFR) ∼20-40 M_☉/yr. The ultraviolet (UV) derived SFR is ∼3-5 times higher than the SFR determined using L_IR or L_Hαbecause the red UV spectral slope is significantly overpredicting the level of dust extinction. This may suggest that the assumed Calzetti starburst obscuration law is not valid in young LBGs. We detect strong line emission from polycyclic aromatic hyrdrocarbons (PAHs) at 6.2, 7.7, and 8.6 µm. The line ratios are consistent with ratios observed in both local and high-redshift starbursts. Both the PAH and rest-frame 8 µm luminosities predict the total L_IR based on previously measured relations in starbursts. Finally, we do not detect the 3.3 µm PAH feature. This is marginally inconsistent with some PAH emission models, but still consistent with PAH ratios measured in many local star-forming galaxies.