The Distribution and Cosmic Density of Relativistic Iron Lines in Active
  Galactic Nuclei
  X-ray observations of several active galactic nuclei show prominent iron
K-shell fluorescence lines that are sculpted due to special and general
relativistic effects. These observations are important because they probe the
space-time geometry close to distant black holes. However, the intrinsic
distribution of Fe line strengths in the cosmos has never been determined. This
uncertainty has contributed to the controversy surrounding the relativistic
interpretation of the emission feature. Now, by making use of the latest
multi-wavelength data, we show theoretical predictions of the cosmic density of
relativistic Fe lines as a function of their equivalent width and line flux. We
are able to show unequivocally that the most common relativistic iron lines in
the universe will be produced by neutral iron fluorescence in Seyfert galaxies
and have equivalent widths < 100 eV. Thus, the handful of very intense lines
that have been discovered are just the bright end of a distribution of line
strengths. In addition to validating the current observations, the predicted
distributions can be used for planning future surveys of relativistic Fe lines.
Finally, the predicted sky density of equivalent widths indicate that the X-ray
source in AGNs can not, on average, lie on the axis of the black hole.
