Detection of Pah and Far-infrared Emission from the Cosmic Eye: Probing the Dust and Star Formation of Lyman Break Galaxies

We report the results of a Spitzer infrared study of the Cosmic Eye, a strongly lensed, L∗UV Lyman Break Galaxy (LBG) at z = 3.074. We obtained Spitzer IRS spectroscopy as well as MIPS 24 and 70 μm photometry. The Eye is detected with high significance at both 24 and 70 μm and, when including a flux limit at 3.5 mm, we estimate an infrared luminosity of LIR = 8.3 +4.7 −4.4 × 10 11 L⊙ assuming a magnification of 28±3. This LIR is eight times lower than that predicted from the rest-frame UV properties assuming a Calzetti reddening law. This has also been observed in other young LBGs, and indicates that the dust reddening law may be steeper in these galaxies. The mid-IR spectrum shows strong PAH emission at 6.2 and 7.7 μm, with equivalent widths near the maximum values observed in star-forming galaxies at any redshift. The LPAH-to-LIR ratio lies close to the relation measured in local starbursts. Therefore, LPAH or LMIR may be used to estimate LIR and thus, star formation rate, of LBGs, whose fluxes at longer wavelengths are typically below current confusion limits. We also report the highest redshift detection of the 3.3 μm PAH emission feature. The PAH ratio, L6.2/L3.3 = 5.1 ± 2.7, and the PAH-to-LIR ratio, L3.3/LIR = 8.5 ± 4.7 × 10 , are both in agreement with measurements in local starbursts and ULIRGs, suggesting that this line may serve as a good proxy for LPAH or LIR at z > 3 with the James Webb Space Telescope. Subject headings: galaxies: high-redshift, galaxies: individual (J213512.73-010143, Cosmic Eye), galaxies: starburst, infrared: galaxies