The MOSDEF Survey: Metallicity Dependence of PAH Emission at High Redshift and Implications for 24 µm Inferred IR Luminosities and Star Formation Rates at z ∼ 2

Abstract

We present results on the variation of 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission in galaxies spanning a wide range in metallicity at z ∼ 2. For this analysis, we use rest-frame optical spectra of 476 galaxies at 1.37 ≤ z ≤ 2.61 from the MOSFIRE Deep Evolution Field (MOSDEF) survey to infer metallicities and ionization states. Spitzer/MIPS 24 μm and Herschel/PACS 100 and 160 μm observations are used to derive rest-frame 7.7 μm luminosities ({L}_7.7) and total IR luminosities ({L}_{IR}), respectively. We find significant trends between the ratio of {L}_7.7 to {L}_{IR} (and to dust-corrected star formation rate [SFR]) and both metallicity and [O III]/[O II] ({{{O}}}₃₂) emission line ratio. The latter is an empirical proxy for the ionization parameter. These trends indicate a paucity of PAH emission in low-metallicity environments with harder and more intense radiation fields. Additionally, {L}_7.7/{L}_{IR} is significantly lower in the youngest quartile of our sample (ages of ≲500 Myr) compared to older galaxies, which may be a result of the delayed production of PAHs by AGB stars. The relative strength of {L}_7.7 to {L}_{IR} is also lower by a factor of ∼2 for galaxies with masses {M}_* < {10}¹⁰ {M}_⊙ , compared to the more massive ones. We demonstrate that commonly used conversions of {L}_7.7 (or 24 μm flux density, f ₂₄) to {L}_{IR} underestimate the IR luminosity by more than a factor of 2 at {M}_* ∼ {10}^9.6{--10.0} {M}_⊙ . We adopt a mass-dependent conversion of {L}_7.7 to {L}_{IR} with {L}_7.7/{L}_{IR} = 0.09 and 0.22 for {M}_* ≤slant {10}¹⁰ and > {10}¹⁰ {M}_⊙ , respectively. Based on the new scaling, the SFR-M _* relation has a shallower slope than previously derived. Our results also suggest a higher IR luminosity density at z ∼ 2 than previously measured, corresponding to a ∼30% increase in the SFR density.