Banner credit: ESA/Hubble, NASA, M. Kornmesser
Elusive AGN
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Active Galactic Nuclei (AGN) are the phase of black holes where they are actively accreting their surrounding material. Black holes and their host galaxies are thought to be fundamentally linked, but the details of this link have been hard to determine. One of the long standing problem is that identifying all the AGN in a population of galaxies is not trivial. Different AGN present different signatures and in an ideal case, we would look for AGN using very deep X-ray, optical, mid-infrared, and radio data simultaneously. This is illustrated in the image (right) of the mid-infrared colors (where a color is a ratio of two discrete wavelength observations) of AGN, where some AGN (orange triangles) have too much dust in the way of their black holes for us to see them in X-rays.
Obtaining all these observations for the same field is expensive and is subject to the limitations of current facilities. Particularly, the wavelength coverage of the Spitzer Space Telescope, an amazing infrared telescope, was only able to give us an incomplete picture of very dust obscured AGN. JWST/MIRI will greatly expand our ability to find these elusive AGN. |
Adapted from Donley et al. 2012 and Caputi et al. 2014: The distribution of AGN and galaxies in mid-infrared color space (circles) . AGN can be identified via their X-ray luminosity (red, green, orange circles) and/or their infrared properties (in the areas marked by the dashed and solid lines). Some (orange triangles) show up in the infrared only and not the X-ray due to dust obscuration.
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AGN in faint radio galaxies
The intrinsic X-ray luminosity as a function of radio luminosity at 6 GHz. AGN are identified via a number of indicators. The combination of ultra-deep X-ray and radio allows us to push the X-ray data deeper by looking for excess X-ray over that provided by stellar processes, identifying new AGN (light purple diamonds). A flat radio spectral slope provides another useful indicator (hatched points). Alberts et al. 2020, submitted.
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The Hubble Ultra Deep Field and surrounding area is a small patch of the sky that contains some of the best and deepest data available, including uniquely deep X-ray imaging. Recent ultra-deep, high resolution radio surveys (PI W. Rujopakarn) over the same region allows us to combine multiple indicators of AGN activity (image left) including high X-ray luminosity, hard X-ray spectra, MIR colors, optical-MIR SED fitting, and radio indicators, including the radio spectral slope.
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Using these indicators, we find AGN in fully half of our radio sample, showing that AGN persist in faint radio populations, though the radio emission itself is mostly supplied by star formation rather than the AGN itself. This study provides as complete a census of AGN as currently possible with photometric datasets and allows us to make predictors for JWST/MIRI, which will further characterize these known AGN and find new, heavily obscured AGN. For further reading, see Alberts et al. 2020.
Finding AGN with JWST/MIRI
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MIRI Colors
The JWST/MIRI photometric filters are uniquely situated to sample different parts of a galaxy's spectrum which can reveal AGN activity: hot dust from around the accreting black hole can peak over the emission from stars at 3-5μm while emission from small grains around forming stars (polycyclic aromatic hydrocarbons or PAHs) at longer wavelengths reveal the host galaxy. The improved coverage of these features allows us to construct a color space that is similar, but much more powerful, than the current Spitzer color space. With just a few MIRI observations, AGN over a range of obscuration and luminosity will be identified by JWST cosmic noon (z~1-3), where AGN and star formation activity are at their peak. For an expanded discussion on the optimal MIRI color spaces for AGN identification, go here or check out Kirkpatrick, Alberts, et al. 2017.
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JWST/MIRI color space at z~1 optimized to identify AGN and composites (where the AGN and host galaxy emission are comparable in strength). Color space optimized for z~1.5 and z~2 have also been identified. From Kirkpatrick, Alberts, et al. 2017.
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Exploiting the Full MIRI Photometric Coverage
The increased coverage of the MIRI photometric bands allows something that was not possible with Spitzer: continuous coverage of the rest 3-5μm region up to z~2.5, with flanking bands. In this wavelength range, even the most heavily obscured AGN will outshine its host galaxy, which has a minimum in its emission in this region. This can be seen in the righthand image where an AGN (NGC 4945; orange circles) peaks out over the galaxy (blue), even though this AGN is attenuated in the X-ray by a factor of 30-40x. The JWST/MIRI filters at the bottom of the image cover this vital region and will pick up on AGN down to Seyfert levels and low Eddington ratios. For a broader discussion of heavily obscured AGN and MIRI's ability to find them, check out the discussion in Alberts et al. 2020.
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A galaxy spectrum (blue line) compared to the WISE photometry of a nearby heavily obscured AGN (NGC 4945; orange circles) redshifted to z=1.5. The JWST/MIRI filters (5.6-25.5μm) are shown for reference. The gray shaded region shows where there is a minimum in stellar emission, where even a heavily obscured AGN will reveal itself.
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