2011MNRAS.415L..78Z

We performed a search simultaneously for the HeH⁺ J = 1–0 line (ν_rest= 2010.183 873 GHz) and for the CH ²Π_3/2(F₂)J = 3/2–²Π_1/2(F₂)J = 1/2 lines (ν_rest ≈ 2006.8 and 2010.8 GHz) towards 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) channels. At a level of 2.9σ, however, there is a tentative emission feature shifted by about 100 km/s 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 linewidths could be explained by quite different conditions required for the 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 quasi-stellar object. The velocity-integrated flux in this tentative feature is 0.62 ± 0.21 Jy km/s which corresponds to a total luminosity of L(HeH⁺) ≈ 7.1 {x} 10⁸ L_☉. As long as there is no independent confirmation, these values should be considered rather as upper limits.