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Evolution of the Cool Gas in the Circumgalactic Medium (CGM) of Massive Halos -- A Keck Cosmic Web Imager (KCWI) Survey of Ly$α$ Emission around QSOs at $z\approx2$

Motivated by the recent discovery of the near-ubiquity of Ly$α$ emission around $z \gtrsim 3$ QSOs, we performed a systematic study of QSO circumgalactic Ly$α$ emission at $z\approx2$, utilizing the unique capability of the Keck Cosmic Web Imager (KCWI) -- a new wide-field, blue sensitive integral-field spectrograph (IFU). In this paper, we present KCWI observations on a sample of 16 ultraluminous Type-I QSOs at $z=2.1-2.3$ with ionizing luminosities of $L_{\rm{ν_{\rm{LL}}}}=10^{31.1-32.3}$ erg s$^{-1}$ Hz$^{-1}$. We found that 14 out of 16 QSOs are associated with Ly$α$ nebulae with projected linear-sizes larger than 50 physical kpc (pkpc). Among them, four nebulae have enormous Ly$α$ emission with the Ly$α$ surface brightness $SB_{\rm{Lyα}} >10^{-17}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ on the $>100$ kpc scale, extending beyond the field of view of KCWI. Our KCWI observations reveal that most $z\approx2$ QSO nebulae have a more irregular morphology compared to those at $z\gtrsim3$. In turn, we measure that the circularly-averaged surface brightness (SB) at $z\approx2$ is 0.4 dex fainter than the redshift-corrected, median SB at $z\gtrsim 3$. The Ly$α$ SB profile (SB$_{\rm{Lyα}}$) of QSOs at $z\approx 2$ can be described by a power law of SB$_{\rm{Lyα},z\approx2.3}= 3.7\times10^{-17}\times(r/40)^{-1.8}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$, with the slope similar to that at $z\gtrsim3$. The observed lower redshift-corrected, circularly-averaged SB may be mainly due to the lower covering factor of cool gas clouds in massive halos at $z\approx2$.