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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

  • Author(s): Shivaei, I
  • Reddy, NA
  • Shapley, AE
  • Siana, B
  • Kriek, M
  • Mobasher, B
  • Coil, AL
  • Freeman, WR
  • Sanders, RL
  • Price, SH
  • Azadi, M
  • Zick, T
  • et al.

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© 2017. The American Astronomical Society. All rights reserved. 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.

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