APEX at the QSO MUSEUM: molecular gas reservoirs associated with z ∼ 3 quasars and their link to the extended Ly α emission

Cool gas (T∼104 K) traced by hydrogen Ly α emission is now routinely detected around z ∼ 3 quasars, but little is known about their molecular gas reservoirs. Here, we present an APEX spectroscopic survey of the CO(6-5), CO(7-6), and [C i](2-1) emission lines for nine quasars from the QSO MUSEUM survey, which have similar UV luminosities, but very diverse Ly α nebulae. These observations ($\lesssim ngle ~\rm rms~\rangle =2.6$ mJy in 300 km s−1) detected three CO(6-5) lines with 3.4≤ICO(6-5) ≤5.1 Jy km s−1, 620≤FWHM≤707 km s−1, and three [C i](2-1) lines with 2.3$\le I_{\rm [C{\small I}](2-1)} \le$15.7 Jy km s−1, 329≤FWHM≤943 km s−1. For the CO and [C i] detected sources, we constrain the molecular gas reservoirs to be $\rm M_{H_{2}} = (0.4-6.9) \times 10^{11}\, M_{\odot }$, while the non-detections imply $\rm M_{H_{2}} \lt 1.1\times 10^{11}\, M_{\odot }$. We compare our observations with the extended Ly α properties to understand the link between the cool and the molecular gas phases. We find large velocity shifts between the bulk of Ly α and the molecular gas systemic redshift in five sources (from ∼−400 to ∼+1200 km s−1). The sources with the largest shifts have the largest Ly α line widths in the sample, suggesting more turbulent gas conditions and/or large-scale inflows/outflows around these quasars. We also find that the brightest ($I_{\rm [C{\small I}](2-1)}=15.7\pm 3.7~\rm Jy~km~s^{-1}$) and the widest (FWHM∼900 km s−1) lines are detected for the smallest and dimmest Ly α nebulae. From this, we speculate that host galaxy obscuration can play an important role in reducing the ionizing and Ly α photons able to escape to halo scales, and/or that these systems are hosted by more massive haloes.