A search for HeH+ and CH in a high-redshift QSO

Abstract

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.