Quasar brightness lower bound, real or my error?

For this plot of quasar Mag vs z

https://inspirehep.net/record/120804...dist1_dr10.png


There is a lower bound for quasar Mag that looks artificial. At
z=0.4 the lower limit is about M = -23. I tried to check if this
is due to a lower limit for sdss scope via using CalcTool for
apparent to absolute magnitude calculator
http://www.calctool.org/CALC/phys/as...star_magnitude

apparent Mag limit for sdss is 22 from their site, which yields
absolute mag limit at z=0.4 ~4e9 LYr from Ned Wrights calculator.

Entering those values, 22 apparent and 4e9 Lyr distance yields a
limit of -18.44 Mag.

This is 5 Mag brighter than the lower cut off in the plot,

Meaning?????

Quasars have a sharp lower brightness bound that gets brighter with
increasing z????

What am I doing wrong?

rt

[[Mod. note -- Could it be that you're seeing Malmquist bias?
(https://en.wikipedia.org/wiki/Malmquist_bias) Without doing the
numbers I can't see whether this is what you're seeing here, but
it would result in just such an observational-selection "sharp
lower brightness bound that gets brighter with increasing z".
-- jt]]