Friday, 29 February 2008

Line-of-sight CIV and MgII absorber distributions

from Wild et al.,

These plots show the linear number densities of narrow CIV (left) and
MgII (right) seen in SDSS QSO spectra. On average an excess of
absorbers is seen near QSOs with a high-velocity tail in CIV, and the
authors address the question of whether these are due to galaxy
clustering (i.e. gas in the halos of galaxies near the QSOs) or
intrinsic to the AGN by attempting to model these distributions.
Three components are included in their model: galaxy clustering,
peculiar velocities, and the radius within which the QSO photoionizes
the absorbers.

In both plots, the dashed blue line shows the predicted number of
absorbers assuming only galaxy clustering (with photoionization radii
of 0.18 Mpc and 0.8 Mpc for CIV and MgII respectively). While
provides a good fit to MgII, it cannot predict the high-velocity tail
of CIV absorbers. The solid black line in the left-hand plot shows
the predictions from their full model, with the clustering component
from this model shown as the red line. The spike of CIV absorbers
seen near v=0 thus appears to consist of about 55% external galaxies
and 45% intrinsic quasar absorbers, while the high-velocity tail is
entirely due to intrinsic absorption. Also, it's notable that the
ionization radius for MgII (0.8 Mpc) is much larger than typical QSO
host halos, implying that the quasars also destroy MgII in external

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