A search for HeH+ and CH in a high-redshift QSO
  We performed a search for the HeH+ J=1-0 line (\nu_{rest} = 2010.183873 GHz)
and simultaneously for the CH ^2\Pi_{3/2}(F_2)J=3/2 - ^2\Pi_{1/2}(F_2)J=1/2
lines (\nu_{rest}\approx 2006.8 and 2010.8 GHz) toward one of the
highest-redshift quasars known, SDSS J114816.64+525150.3 (z= 6.4189). No
clearly visible line was detected after obtaining an rms noise level of ~ 0.4
mK (~ 3 mJy) in 16 MHz (18 km s^{-1}) channels. At a level of 2.9sigma,
however, there is a tentative emission feature shifted by about 100 km s^{-1}
from the expected frequency of the HeH+ line. This shift is well within the
width of the line profiles for CO and C+. The putative feature is about four
times narrower than the previously detected CO and C+ lines. The difference in
velocities as well as in the line widths could be explained by quite different
conditions required for formation and excitation of HeH+ with respect to CO and
C+. The HeH+ emission, if real, could probably arise in the dense ionized gas
of this QSO. The velocity integrated flux in this tentative feature is 0.62 +/-
0.21 Jy km s^{-1} which corresponds to a total luminosity of L(HeH+)\approx 7.1
x 10^8 L_{sun}. As long as there is no independent confirmation, these values
should be considered rather as upper limits.
