Detection of Far-Infrared and Polycyclic Aromatic Hydrocarbon Emission from the Cosmic Eye: Probing the Dust and Star Formation of Lyman Break Galaxies

Abstract

We report the results of a Spitzer infrared (IR) study of the Cosmic Eye, a strongly lensed, L^*_(UV) Lyman break galaxy (LBG) at z = 3.074. We obtained Spitzer mid-IR 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 IR luminosity of L_(IR) = 8.3^(+4.7)_(–4.4) × 10^(11) L_⊙ assuming a magnification of 28± 3. This L_(IR) is eight times lower than that predicted from the rest-frame ultraviolet 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 polycyclic aromatic hydrocarbon (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 L_(PAH)-to-L_(IR) ratio lies close to the relation measured in local starbursts. Therefore, L_(PAH) or L_(MIR) may be used to estimate L_(IR), 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, L_(6.2)/L_(3.3) = 5.1 ± 2.7, and the PAH-to-LIR ratio, L_(3.3)/L_(IR) = 8.5 ± 4.7 × 10^(–4), are both in agreement with measurements in local starbursts and ultraluminous infrared galaxies (ULIRGs), suggesting that this line may serve as a good proxy for L_(PAH) or L_(IR) at z > 3 with the James Webb Space Telescope.

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