APEX at the QSO MUSEUM: molecular gas reservoirs associated with $z\sim$3 quasars and their link to the extended Ly$\alpha$ emission

Cool gas (T$\sim$10${}^4$~K) traced by hydrogen Ly$\alpha$ emission is now routinely detected around $z\sim 3$ 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 CI emission lines for 9 quasars from the QSO MUSEUM survey which have similar UV luminosities, but very diverse Ly$\alpha$ nebulae. These observations ($⟨\mathrm{r}\mathrm{m}\mathrm{s}⟩=2.6$~mJy in 300~km~s${}^{-1}$) detected three CO(6-5) lines with 3.4$\leq I_{\rm CO(6-5)} \leq$5.1Jykms$^{-1}$,620$\leq$FWHM$\leq$707kms$^{-1}$, and three CI lines with 2.3$\le I_{[\mathrm{C}\mathrm{I}](2-1)}\le$15.7~Jy~km~s${}^{-1}$, 329$\le$FWHM$\le$943~km~s${}^{-1}$. For the CO and [CI] detected sources, we constrain the molecular gas reservoirs to be $\rm M_{H_{2}} = (0.4-6.9) \times 10^{11} M_{\odot}$,whilethenon-detectionsimply$\rm M_{H_{2}} < 1.1\times 10^{11} M_{\odot}$\mathrm{.}WecompareourobservationswiththeextendedLy$\alpha$ properties to understand the link between the cool and the molecular gas phases. We find large velocity shifts between the bulk of Ly$\alpha$ and the molecular gas systemic redshift in five sources (from $\sim$-400 to $\sim +$1200~km~s${}^{-1}$). The sources with the largest shifts have the largest Ly$\alpha$ 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 ($I_{[\mathrm{C}\mathrm{I}](2-1)}=15.7\pm 3.7\mathrm{J}\mathrm{y}\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}$) and the widest (FWHM$\sim$900~km~s${}^{-1}$) lines are detected for the smallest and dimmest Ly$\alpha$ nebulae. From this, we speculate that host galaxy obscuration can play an important role in reducing the ionizing and Ly$\alpha$ photons able to escape to halo scales, and/or that these systems are hosted by more massive halos.