Finding AGN in the Distant Universe

Full description of color selection in Kirkpatrick et al. (2017)

Observations with the Mid-Infrared Instrument (MIRI) on JWST will transform our understanding of the dust–attenuated phases of star formation in galaxies and AGN at z = 1-3, when such objects were most active. The majority of this star formation and black hole growth was obscured by dust and re-radiated in the thermal IR, much more than at present. The mid-IR emission can be characterized by polycyclic aromatic hydrocarbons (PAHs), which are small grains of dust that lie near star-forming regions. The mid-IR can also be characterized by hot dust from an AGN. The figure below (credit: Casey Papovich) shows simulated MIRI images using galaxy IR luminosities and redshifts estimated from HST and Spitzer. JWST observations will easily detect IR emission from galaxies and obscured AGN a factor 10 fainter than possible with previous IR surveyors.

Color selection is a powerful technique for identifying likely AGNs, star-forming galaxies, and AGN/star-forming composites. I have identified the optimal MIRI filter combinations for separating galaxies into these three classes at z=1-3, using simulated photometry from galaxies at this redshift range. As many JWST/MIRI observations will be carried out in fields with available photometric redshifts, or in parallel with NIRcam and NIRspec observations, I include redshift information in our color diagnostics to improve reliability and completeness. These color diagnostics hinge on tracing the strength of the 6.2 µm and 7.7 µn;m PAH features, which are diminished in AGN and composites, and the stellar minimum at 3-5 µm, which will be outshone by the torus in AGN.

z ~ 1

This diagnostic uses the 7.7 µm, 10 µm, 15 µm, and 18 µm filters to cover the PAH features in galaxies from z=0.75-1.25. The figure shows the combination of the filters applied to simulated galaxies. I have divided the diagnostic into AGN, composite, and star-forming galaxy regions (black lines), based on completeness and reliability. Overplotted are the contours of all the synthetic galaxies classified as SFGs (blue lines), Composites (purple lines), and AGNs (maroon lines) to allow easier viewing of where each category predominantly lies. In the bottom right corner, I show where the photometry filters fall on an SFG (black), Composite (blue), and AGN (red) template at z=1.
The boundaries of each region are circles, with AGN lying inside the inner circle, SFGs lying outside the outer circle, and Composites lying in between.

inner: (log S15 / S7.7 - 0.40)2 + (log S18 / S10 - 0.38)2 = 0.252
outer: (log S15 / S7.7 - 0.35)2 + (log S18 / S10 - 0.45)2 = 0.652

The colors can also be combined to estimate f(AGN)MIR, or the fraction of mid-IR luminosity (5 - 15 µm) due to the torus emission from an AGN. f(AGN)MIR is crucial for removing the AGN contribution to LIR if LIR is being used to calculate star formation rates.

f(AGN)MIR = -0.97 (log S15 / S7.7) - 0.10 (log S18 / S10) + 1.29

z ~ 1.5

This diagnostic uses the 10 µm, 12.8 µm, 18 µm, and 21 µn;m filters to cover the PAH features in galaxies from z=1.25-1.75. The figure shows the combination of the filters applied to simulated galaxies. In the bottom right corner, I show where the photometry filters fall on an SFG (black), Composite (blue), and AGN (red) template at z=1.5.
The boundaries of each region are circles (black lines), with AGN lying inside the inner circle, SFGs lying outside the outer circle, and Composites lying in between.

inner: (log S21 / S10 - 0.49)2 + (log S18 / S12.8 - 0.18)2 = 0.212
outer: (log S21 / S10 - 0.60)2 + (log S18 / S12.8 - 0.03)2 = 0.652

The colors can also be combined to estimate f(AGN)MIR.

f(AGN)MIR = -0.56 (log S21 / S10) - 0.85 (log S18 / S12.8) + 1.29

z ~ 1.5

This diagnostic uses the 10 µm, 15 µm, 18 µm, and 21 µn;m filters to cover the PAH features in galaxies from z=1.75-2.25. The figure shows the combination of the filters applied to simulated galaxies. In the bottom right corner, I show where the photometry filters fall on an SFG (black), Composite (blue), and AGN (red) template at z=1.5.
The boundaries of each region are circles (black lines), with AGN lying inside the inner circle, SFGs lying outside the outer circle, and Composites lying in between.

inner: (log S18 / S10 - 0.43)2 + (log S21 / S15 - 0.18)2 = 0.182
outer: (log S18 / S10 - 0.50)2 + (log S21 / S15 - 0.12)2 = 0.522

The colors can also be combined to estimate f(AGN)MIR.

f(AGN)MIR = -0.55 (log S18 / S10) - 1.01 (log S21 / S15) + 1.25