Evolution of the Cool Gas in the Circumgalactic Medium (CGM) of Massive Halos -- A Keck Cosmic Web Imager (KCWI) Survey of Ly$\hat{I}\pm$ Emission around QSOs at $z\approx 2$

Abstract

Motivated by the recent discovery of the near-ubiquity of Ly$\hat{I}\pm$ emission around $z\gtrsim 3$ QSOs, we performed a systematic study of QSO circumgalactic Ly$\hat{I}\pm$ emission at $z\approx 2$, 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 $L_{\hat{\mathrm{I}}{½}_{\mathrm{LL}}}=10^{31.1-32.3}$ erg s${}^{-1}$ Hz${}^{-1}$. We found that 14 out of 16 QSOs are associated with Ly$\hat{I}\pm$ nebulae with projected linear-sizes larger than 50 physical kpc (pkpc). Among them, four nebulae have enormous Ly$\hat{I}\pm$ emission with the Ly$\hat{I}\pm$ surface brightness $S{B}_{\mathrm{Ly}\hat{\mathrm{I}}\pm }>10^{-17}$ 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 $z\approx 2$ QSO nebulae have a more irregular morphology compared to those at $z\gtrsim 3$. In turn, we measure that the circularly-averaged surface brightness (SB) at $z\approx 2$ is 0.4 dex fainter than the redshift-corrected, median SB at $z\gtrsim 3$. The Ly$\hat{I}\pm$ SB profile (SB${}_{\mathrm{Ly}\hat{\mathrm{I}}\pm }$) of QSOs at $z\approx 2$ can be described by a power law of SB${}_{\mathrm{Ly}\hat{\mathrm{I}}\pm ,\mathrm{z}\approx 2.3}=3.7\times 10^{-17}\times (r/40{)}^{-1.8}$ erg s${}^{-1}$ cm${}^{-2}$ arcsec${}^{-2}$, with the slope similar to that at $z\gtrsim 3$. 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 $z\approx 2$.

23 pages, 15 figures, Accepted for Publications in ApJS