The bulk of the stellar growth over cosmic time is dominated by IR-luminous galaxies at cosmic noon (z = 1-2), many of which harbor a hidden active galactic nucleus (AGN). We use state-of-the-art infrared color diagnostics, combining Spitzer and Herschel observations, to separate dust-obscured AGNs from dusty star-forming galaxies (SFGs) in the CANDELS and COSMOS surveys. We calculate 24 μm counts of SFGs, AGN/star-forming "Composites, " and AGNs. AGNs and Composites dominate the counts above 0.8 mJy at 24 μm, and Composites form at least 25% of an IR sample even to faint detection limits. We develop methods to use the Mid-Infrared Instrument (MIRI) on JWST to identify dust-obscured AGNs and Composite galaxies from z ∼ 1-2. With the sensitivity and spacing of MIRI filters, we will detect > 4 times as many AGN hosts as with Spitzer/IRAC criteria. Any star formation rates based on the 7.7 μm PAH feature (likely to be applied to MIRI photometry) must be corrected for the contribution of the AGN, or the star formation rate will be overestimated by ∼ 35% for cases where the AGN provides half the IR luminosity and ∼ 50% when the AGN accounts for 90% of the luminosity. Finally, we demonstrate that our MIRI color technique can select AGNs with an Eddington ratio of λEdd ∼ 0.01 and will identify AGN hosts with a higher specific star formation rate than X-ray techniques alone. JWST/MIRI will enable critical steps forward in identifying and understanding dust-obscured AGNs and the link to their host galaxies.
Bibliographical noteFunding Information:
A.K. thanks Sandy Faber and Belinda Wilkes for helpful conversations. A.K. gratefully acknowledges support from the YCAA Prize Postdoctoral Fellowship. A.P. and A.S. acknowledge NASA ADAP13-0054 and NSF AAG grants AST-1312418 and AST-1313206.
© 2017. The American Astronomical Society. All rights reserved.
- galaxies: active
- galaxies: photometry
- galaxies: star formation