Detection of an Extended Ly$\alpha$ Halo around a $\textit{z}=6.64$ Broad Absorption Line Quasar with the Keck Cosmic Web Imager

Abstract

We present the first results from a program searching for extended Ly$\alpha$ halos around high redshift ($ z \gtrsim 6.5$) quasars using the red channel of the Keck Cosmic Web Imager (KCWI). Our observations reveal a Ly$\alpha$ halo extending to $\simeq11$ pkpc around the $z=6.64$ broad absorption line quasar J0910$-$0414. The Ly$\alpha$ velocity field displays a rotation-like gradient, and the gas velocity dispersion is consistent with gravitationally dominated motion ($\sigma_{\mathrm{Ly\alpha}}<300$ km s$^{-1}$). Comparison with the $[\mathrm{C\;II}]$ kinematics of the host galaxy core from ALMA observations shows that the Ly$\alpha$-emitting gas extends over a much larger region, shows distinct kinematics, and has a smaller velocity dispersion ($\sigma_{\mathrm{Ly\alpha}} \simeq 0.6\sigma_{\mathrm{[C\;II]}}$). The Ly$\alpha$ spectral region of the quasar is largely obscured by a deep $\mathrm{N\;V}$ absorption trough, and as a result, roughly $55\%$ of the total Ly$\alpha$ flux is from the extended halo. These observations demonstrate the potential of KCWI for probing the cool gas reservoir that fuels the growth of quasars and their hosts in the epoch of reionization.