Abstract

The Luminous Polycyclic Aromatic Hydrocarbon Emission Features: Applications to High Redshift Galaxies and Active Galactic Nuclei

Heath Shipley (Tufts University)

Heath V. Shipley^1,2, Casey J. Papovich², George H. Rieke³

¹Department of Physics & Astronomy, Tufts University, 574 Boston Avenue Suites 304, Medford, MA 02155, USA

²George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, and Department of Physics & Astronomy, Texas A & M University, College Station, TX 77843-4242

³Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85719, USA

With greater sensitivity than any previous IR mission, the James Webb Space Telescope (JWST) will be a powerful tool in galaxy evolution studies out to z ≤ 2 by utilizing the polycyclic aromatic hydrocarbon (PAH) emission features that accurately trace star-formation in galaxies. We provide a new robust star-formation rate (SFR) calibration using the luminosity emitted from the PAHs at 6.2μm, 7.7μm and 11.3μm. The PAH features emit strongly in the mid-infrared (mid-IR; 5—25μm) mitigating dust extinction, containing on average 5—10% of the total IR luminosity in galaxies. We use a sample of 105 star-forming galaxies covering a range of total IR luminosity, L_IR = L(8—1000μm) = 10⁹ - 10¹² L_☉ and redshift 0 < z < 0.4, with mid-IR spectroscopy from the Spitzer Infrared Spectrograph (IRS), and data covering other SFR indicators (Hα emission and rest-frame 24μm continuum emission). The PAH luminosity correlates linearly with the SFR as measured by the Hα luminosity (corrected for attenuation using the mono-chromatic rest-frame 24μm emission), with a tight scatter of <0.15 dex. The scatter is comparable to that between SFRs derived from the Paα and dust-corrected Hα emission lines. The PAH SFR calibration shows a dependency on galaxy gas-phase metallicity, where the PAH luminosity of galaxies with Z ≤ 0.7 Z_☉ departs from the linear SFR relationship. We calibrate a correction to the PAH SFRs using a simple empirical model that depends linearly on metallicity. We present a case study in advance of JWST, which will be capable of measuring SFRs (from 8μm rest-frame photometry, i.e. PAHs) in distant galaxies (z ≤ 2) with JWST/MIRI to SFRs as low as ∼10 M_☉yr^-1, because the PAH features are so bright. We use Spitzer/IRS observations of PAH features in lensed star-forming galaxies at 1 < z < 3 to demonstrate the utility of the PAHs to derive SFRs that agree with those available from Paα. To utilize JWST, a simple method would be to image at 7.7μm, 12.8μm, 18μm and 21μm (or a subset). This would allow PAH emitting galaxies and AGNs (that may contribute too much at rest-frame 8μm) to be identified to give valid SFR estimates. This new SFR indicator will be useful for probing the peak of the SFR density in the universe (1 < z < 3) and for studying the co-evolution of star-formation and supermassive blackhole accretion contemporaneously in a galaxy.

Mode of presentation: poster