Re-examining the case for neutral gas near the redshift 7 quasar ULAS
J1120+0641
(2015)cite arxiv:1505.06880Comment: 8 pages, 6 figures MNRAS submitted.Abstract
Signs of damping wing absorption attenuating the Lyman-$\alpha$ emission line
of the first known $z 7$ quasar, ULAS J1120+0641, recently provided
exciting evidence of a significantly neutral IGM. This long-awaited signature
of reionization was inferred, in part, from a deficit of flux in the quasar's
Lyman-$\alpha$ emission line based on predictions from a composite of
lower-redshift quasars. The composite sample was chosen based on its C IV
emission line properties; however, as the original study by Mortlock et al.
noted, the composite contained a slight velocity offset in C IV compared to
ULAS J1120+0641. Here we test whether this offset may be related to the
predicted strength of the Lyman-$\alpha$ emission line. We confirm the
significant ($10$ per cent at r.m.s.) scatter in Lyman-$\alpha$ flux for
quasars of a given C IV velocity and equivalent width found by Mortlock et al.
We further find that among lower-redshift objects chosen to more closely match
the C IV properties of ULAS J1120+0641, its Lyman-$\alpha$ emission falls
within the observed distribution of fluxes. Among lower-redshift quasars chosen
to more closely match in C IV velocity and equivalent width, we find that ULAS
J1120+0641 falls within the observed distribution of Lyman-$\alpha$ emission
line strengths. This suggests that damping wing absorption may not be present,
potentially weakening the case for neutral gas around this object. Larger
samples of z$>$7 quasars may therefore be needed to establish a clearer picture
of the IGM neutral fraction at these redshifts.
Description
[1505.06880] Re-examining the case for neutral gas near the redshift 7 quasar ULAS J1120+0641