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

Abstract

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](2-1) emission lines for 9 quasars from the QSO MUSEUM survey which have similar UV luminosities, but very diverse Ly$\alpha$ nebulae. These observations ($⟨\mathrm{rms}⟩=2.6$~mJy in 300~km~s${}^{-1}$) detected three CO(6-5) lines with 3.4$\le I_{\mathrm{CO}(6-5)}\le$5.1~Jy~km~s${}^{-1}$, 620$\le$FWHM$\le$707~km~s${}^{-1}$, and three [CI](2-1) lines with 2.3$\le I_{[\mathrm{CI}](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 ${\mathrm{M}}_{{\mathrm{H}}_2}=(0.4-6.9)\times 10^{11}{\mathrm{M}}_{\odot }$, while the non-detections imply ${\mathrm{M}}_{{\mathrm{H}}_2}<1.1\times 10^{11}{\mathrm{M}}_{\odot }$. We compare our observations with the extended Ly$\alpha$ 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 ($I_{[\mathrm{CI}](2-1)}=15.7\pm 3.7\mathrm{Jy}\mathrm{km}{\mathrm{s}}^{-1}$) 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.

Accepted on January 3, MNRAS. 25 pages, 17 figures

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