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

TL;DR

This study uses Spitzer infrared observations to analyze the dust, PAH emission, and star formation in the high-redshift Lyman Break Galaxy Cosmic Eye, revealing insights into dust properties and star formation indicators at z>3.

Contribution

First detection of PAH features and infrared emission in the Cosmic Eye, providing new methods to estimate star formation rates in distant galaxies.

Findings

Infrared luminosity is lower than UV-based predictions, indicating steeper dust reddening laws.

Strong PAH emission detected at 6.2 and 7.7 microns, with ratios similar to local starbursts.

Highest redshift detection of the 3.3 micron PAH feature, useful for future high-z studies.

Abstract

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 micron photometry. The Eye is detected with high significance at both 24 and 70 microns and, when including a flux limit at 3.5 mm, we estimate an infrared luminosity of L_IR = 8.3 (+4.7-4.4) x10^11 L_sun assuming a magnification of 28+-3. This L_IR 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 microns, 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 micron PAH emission feature. The PAH ratio, L_6.2/L_3.3=5.1+- 2.7, and the PAH-to-L_IR ratio, L_3.3/L_IR = 8.5 +- 4.7 x10^-4, are both in agreement with measurements in local starbursts and 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.