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

Irene Shivaei, Naveen A. Reddy, Alice E. Shapley, Brian Siana, Mariska Kriek, Bahram Mobasher, Alison L. Coil, William R. Freeman, Ryan L. Sanders, Sedona H. Price, Mojegan Azadi, Tom Zick

Research output: Contribution to journalArticlepeer-review

51 Scopus citations

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 (L7.7) and total IR luminosities (LIR), respectively. We find significant trends between the ratio of L7.7 to LIR (and to dust-corrected star formation rate [SFR]) and both metallicity and [O iii]/[O ii] (O32) 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 L7.7, 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 L7.7 to LIR is also lower by a factor of ∼2 for galaxies with masses M ∼ 109.6-10.0, compared to the more massive ones. We demonstrate that commonly used conversions of (or 24 μm flux density, f 24) to underestimate the IR luminosity by more than a factor of 2 at . We adopt a mass-dependent conversion of to with = 0.09 and 0.22 for M ≤ 1010 and > 1010M, 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.

Original languageEnglish
Article number157
JournalAstrophysical Journal
Volume837
Issue number2
DOIs
StatePublished - Mar 10 2017

Bibliographical note

Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved.

Keywords

  • ISM: molecules
  • galaxies: ISM
  • galaxies: general
  • galaxies: high-redshift
  • galaxies: star formation
  • infrared: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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