Spitzer Observations of the Z = 2.73 Lensed Lyman Break Galaxy, Ms1512-cb58

We present Spitzer infrared (IR) photometry and spectroscopy of the lensed Lyman break galaxy (LBG), MS1512-cB58 at z = 2.73. The large (factor ∼ 30) magnification allows for the most detailed infrared study of an L∗UV (z = 3) LBG to date. Broadband photometry with IRAC (3-10 μm), IRS (16 μm), and MIPS (24, 70 & 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), forrming stars at ∼ 98 M⊙ yr , with a total stellar mass of ∼ 10 M⊙ formed thus far. The existence of an old stellar population with twice the stellar mass can not be ruled out. IR spectral energy distribution fits to the 24 and 70 μm photometry, as well as previously obtained submm/mm, data give an intrinsic IR luminosity LIR = 1− 2× 10 11 L⊙ and a star formation rate, SFR ∼ 20− 40 M⊙ yr . The UV derived star formation rate (SFR) is ∼ 3−5 times higher than the SFR determined using LIR or LHα because the red UV spectral slope is significantly over predicting the level of dust extinction. This suggests that the assumed Calzetti starburst obscuration law may not be valid for young LBGs. We detect strong line emission from Polycyclic Aromatic Hydrocarbons (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 LIR 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. Subject headings: galaxies: high-redshift, galaxies: individual (MS1512-cB58), infrared: galaxies