| Abstract: | Soft X-ray photoelectric absorption of high-z quasars has been known for more
than a decade, but has no unambiguous astro-physical context. We construct the
largest sample to date of more than 50 high redshift quasars (z > 0.5) selected
from the XMM-Newton archive based on high photon count criteria. We will present
results for the measured optical depth ô, at 0.5 keV for our sample. We aim to
find which physical parameters of the quasars drive their observed absorption,
e.g., redshift, radio luminosity, radio loudness, or the X-ray luminosity. We
also compare the absorption behavior with redshift with the pattern expected if
the diffuse intergalactic medium (IGM) is responsible for the observed
absorption. We find that although the z > 2 quasar opacity is consistent with
IGM absorption, absorption of intermediate z (0.5 < z < 2) quasars is for the
most part too low for this scenario. Generally, z < 2 and radio quiet objects
show no significant absorption, and provide only upper limits for the optical
depth. Thus, we use censored statistics methods to seek correlations between
the measured parameters. We find only a weak correlation between ô and z, and
even a weaker correlation between ô and radio luminosity, which leads to the
conclusion that although a simple IGM origin for the quasar absorption is
unlikely, optical depth does seem to increase with redshift as ô ~ (1+z)^2.4. |
