As matter accretes onto the central
supermassive black holes in active galactic nuclei (AGNs),
X-rays are emitted. We present a population synthesis model
that accounts for the summed X-ray emission from growing black
holes; modulo the efficiency of converting mass to X-rays,
this is effectively a record of the accreted mass. We need
this population synthesis model to reproduce observed
constraints from X-ray surveys: the X-ray number counts, the
observed fraction of Compton-thick AGNs [log (N H/cm-2) >
24], and the spectrum of the cosmic X-ray background (CXB),
after accounting for selection biases. Over the past decade,
X-ray surveys by XMM-Newton, Chandra, NuSTAR, and Swift-BAT
have provided greatly improved observational constraints. We
find that no existing X-ray luminosity function (XLF)
consistently reproduces all these observations. We take the
uncertainty in AGN spectra into account and use a neural
network to compute an XLF that fits all observed constraints,
including observed Compton-thick number counts and fractions.
This new population synthesis model suggests that,
intrinsically, 50% +/- 9% (56% +/-
7%) of all AGNs within z ≃ 0.1 (1.0) are Compton-thick.
In addition to this X-ray luminosity function, using the
unprecedented spectroscopic completeness of the BAT AGN
Spectroscopic Survey (BASS) data release 2, we have derived
the active black hole mass function (BHMF), and
Eddington-ratio distribution function (ERDF) for both
unobscured (Type 1) and obscured (Type 2) active galactic
nuclei (AGN). In addition to a straightforward 1/Vmax
approach, we also compute the intrinsic distributions,
accounting for sample truncation by employing a forward
modeling approach to recover the observed BHMF and ERDF. As
previous BHMFs and ERDFs have been robustly determined only
for samples of bright, broad-line (Type 1) AGNs and/or
quasars, ours is the first directly observationally
constrained BHMF and ERDF of Type 2 AGN. We find that after
accounting for all observational biases, the intrinsic ERDF of
Type 2 AGN is significantly skewed towards lower Eddington
ratios than the intrinsic ERDF of Type 1 AGN. This result
supports the radiation-regulated unification scenario, in
which radiation pressure dictates the geometry of the dusty
obscuring structure around an AGN. Calculating the ERDFs in
two separate mass bins, we verify that the derived shape is
consistent, validating the assumption that the ERDF is mass
independent.
About the Speaker:
Tonima Tasnim Ananna is a Postdoctoral
Research Associate in Professor Ryan Hickox's group at
Dartmouth College. She completed her Ph.D. at the Department
of Physics, Yale University, under the supervision of
Professor Claudia Megan Urry. Her research is primarily
focused on the growth of supermassive black holes in obscured
environments using innovative statistical and machine learning
techniques. Her research has been featured in Science News in
SN 10: 10 Scientists to Watch 2020.