The MOSDEF Survey: Metallicity dependence of the PAH emission at High Redshift: Implications for 24 micron-inferred IR luminosities and star formation rates at z~2

Irene Shivaei, Naveen Reddy, Alice Shapley, Brian Siana, Mariska Kriek, Bahram Mobasher, Alison Coil, William Freeman, Ryan Sanders, Sedona Price, Mojegan Azadi

Published 2016-09-15Version 1

We present results on the variation of Polycyclic Aromatic Hydrocarbon (PAH) emission at 7.7 micron 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$\leq z\leq$2.61 from the MOSFIRE Deep Evolution Field (MOSDEF) survey to infer metallicities and ionization states. Spitzer/MIPS 24 micron observations are used to derive rest-frame 7.7 micron luminosities (L(7.7)) and far-IR data from Herschel/PACS 100 and 160 micron to measure total IR luminosities (L(IR)). We find significant trends between the ratio of L(7.7) to L(IR) (and to dust-corrected SFR) and both metallicity and [OIII]/[OII] (O$_{32}$) emission-line ratio. The latter is an empirical proxy for the ionization parameter. These trends indicate a paucity of PAH molecules in low metallicity environments with harder and more intense radiation fields. Additionally, L(7.7)/L(IR) is significantly lower in the youngest quartile in our sample (ages of ~ 400 Myr) compared to older galaxies, which may be a result of the delayed production of PAHs by AGB stars. The relative strength of L(7.7) to L(IR) is also lower by a factor of ~2 for galaxies with masses M$_*<10^{10} M_{\odot}$, compared to the more massive ones. We demonstrate that commonly-used conversions of L(7.7) (or 24 micron flux density; f(24)) to L(IR) underestimate the IR luminosity by more than a factor of 2 at M$_*$~$10^{9.6-10.0} M_{\odot}$. Consequently, the SFR-M$_*$ relation has a shallower slope than previously derived from studies that neglected the metallicity dependence of the 24 micron-to-IR conversion factor. Our results suggest a higher IR luminosity density at z~2 than previously measured, which corresponds to a ~ 30% increase in the SFR density.