We present Spitzer infrared ( IR ) photometry and spectroscopy of the lensed Lyman break galaxy ( LBG ) , MS1512-cB58 at z = 2.73 .
The large ( factor \sim 30 ) magnification allows for the most detailed infrared study of an L ^ { * } _ { UV } ( z = 3 ) LBG to date .
Broadband photometry with IRAC ( 3-10 \mu m ) , IRS ( 16 \mu m ) , and MIPS ( 24 , 70 & 160 \mu m ) was obtained as well as IRS spectroscopy spanning 5.5-35 \mu m. A fit of stellar population models to the optical/near-IR/IRAC photometry gives a young age ( \sim 9 Myr ) , forrming stars at \sim 98 M _ { \odot } yr ^ { -1 } , with a total stellar mass of \sim 10 ^ { 9 } M _ { \odot } 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 \mu m photometry , as well as previously obtained submm/mm , data give an intrinsic IR luminosity L _ { IR } = 1 - 2 \times 10 ^ { 11 } L _ { \odot } and a star formation rate , SFR \sim 20 - 40 M _ { \odot } yr ^ { -1 } .
The UV derived star formation rate ( SFR ) is \sim 3 - 5 times higher than the SFR determined using L _ { IR } or L _ { H \alpha } 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 \mu m. The line ratios are consistent with ratios observed in both local and high redshift starbursts .
Both the PAH and rest-frame 8 \mu m luminosities predict the total L _ { IR } based on previously measured relations in starbursts .
Finally , we do not detect the 3.3 \mu m PAH feature .
This is marginally inconsistent with some PAH emission models , but still consistent with PAH ratios measured in many local star-forming galaxies .