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We present Spitzer infrared (IR) photometry and spectroscopy of the lensedLyman 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) LBGto date. Broadband photometry with IRAC (3-10 micron), IRS (16 micron), andMIPS (24, 70 & 160 micron) was obtained as well as IRS spectroscopy spanning5.5-35 microns. A fit of stellar population models to the optical/near-IR/IRACphotometry gives a young age (~9 Myr), forming stars at ~98 M_sun/yr, with atotal stellar mass of ~10^9 M_sun formed thus far. The existence of an oldstellar population with twice the stellar mass can not be ruled out. IRspectral energy distribution fits to the 24 and 70 micron photometry, as wellas previously obtained submm/mm, data give an intrinsic IR luminosity L_IR =1-2 x10^11 L_sun and a star formation rate, SFR ~20-40 M_sun/yr. The UV derivedstar formation rate (SFR) is ~3-5 times higher than the SFR determined usingL_IR or L_Halpha because the red UV spectral slope is significantly overpredicting the level of dust extinction. This suggests that the assumedCalzetti starburst obscuration law may not be valid for young LBGs. We detectstrong line emission from Polycyclic Aromatic Hydrocarbons (PAHs) at 6.2, 7.7,and 8.6 microns. The line ratios are consistent with ratios observed in bothlocal and high redshift starbursts. Both the PAH and rest-frame 8 micronluminosities predict the total L_IR based on previously measured relations instarbursts. Finally, we do not detect the 3.3 micron PAH feature. This ismarginally inconsistent with some PAH emission models, but still consistentwith PAH ratios measured in many local star-forming galaxies.

Spitzer Observations of the z=2.73 Lensed Lyman Break Galaxy, MS1512-cB58