Most AGN are Obscured at the highest redshifts but almost of the AGN we Observe are Unobscured …
The very energy that makes these sources detectable at high-z via emission from the broad-line region overwhelms the star-formation signatures within their host galaxies. Unobscured AGN are predicted to be a small fraction of the total early Universe AGN population. According to theory and observations over 99% of AGN are expected to be heavily obscured by their host-galaxies by z>7, thus the over-abundance discovered in the first year of JWST science may be significantly larger than we think.
Finding the Highest Redshift Obscured AGN
My research program is dedicated to finding the most obscured AGN in the Universe to study the interplay between dusty, massive galaxy growth and potential AGN triggering and impact. Using a multitude of wavelengths, I hunt for these elusive sources in bands where AGN emission is less affected by attenuation (i.e radio, MIR, X-ray), and place these sources in the context of their host-galaxy properties using deep JWST and sub-mm data-sets.
Using a wealth of multi-wavelength information, I identified a powerful, radio-loud (RL), growing supermassive black hole (SMBH) with significant spectral steepening of the radio SED . In conjunction with ALMA, deep ground-based observations, ancillary space-based data, and the unprecedented resolution and sensitivity of JWST, we find no evidence of AGN contribution to the UV/optical/NIR data and thus infer heavy amounts of obscuration. Using the lessons learned from this source, I’ve developed a selection method for finding more of these sources within the COSMOS-Web data-set, and for the first time, we are on the cusp of a less biased view of powerful AGN in the Epoch of Re-Ionization. Check out https://arxiv.org/abs/2308.12823 to learn more!
There are event obscured AGN lurking in samples at lower redshifts
X-ray background population synthesis models of AGN predict a large fraction that must grow behind dense, obscuring screens of gas and dust. Deep X-ray surveys are thought to have provided the most complete and unbiased samples of AGN, but there is strong observational evidence that a portion of the population of obscured AGN is being missed. In Lambrides+20, I used a sample of AGN derived from the deepest X-ray survey to date, Chandra 7Ms GOODS-South Survey, to investigate the nature of low flux X-ray sources. Using sources that would be classified as weak AGN or even galaxies, we find a surprising fraction with radio, infrared and optical counterparts that indicate moderate to powerful AGN classification. I found the predicted column densities is on average an order of magnitude higher than the calculated column densities via X-ray data alone. When we compare the estimation of the obscured AGN space density with and without these objects, we find a difference of 40\% in the lowest X-ray luminosity regime probed by our sample.
Erini Lambrides 