Evolution of the Cool Gas in the Circumgalactic Medium of Massive Halos: A Keck Cosmic Web Imager Survey of Lyα Emission around QSOs at z ≈ 2

Motivated by the recent discovery of the near-ubiquity of Ly alpha emission around z greater than or similar to 3 QSOs, we performed a systematic study of QSO circumgalactic Lya emission at z approximate to 2 utilizing the unique capability of the Keck Cosmic Web Imager (KCWI)-a new wide-field, blue sensitive integral field spectrograph. In this paper, we present KCWI observations of a sample of 16 ultraluminous Type I QSOs at z = 2.1-2.3 with ionizing luminosities of L-nu LL = 10(31.1-32.3) erg s(-1) Hz(-1). We found that 14 out of 16 QSOs are associated with Ly alpha nebulae with projected linear sizes larger than 50 physical kpc. Among them, four nebulae have enormous Ly alpha emission with a Ly alpha surface brightness (SB)(Ly alpha) > 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 approximate to 2 QSO nebulae have a more irregular morphology compared to those at z greater than or similar to 3. In turn, we measure that the circularly averaged SB at z approximate to 2 is 0.4 dex fainter than the redshift-corrected median SB at z greater than or similar to 3. The Lya SB profile (SBLy alpha) of QSOs at z approximate to 2 can be described by a power law of SBLy alpha,Z approximate to 2.3 = 3.7 x 10(-17) x (r/40)(-1.8) erg s(-1) cm(-2) arcsec(-2), with a slope similar to that at z greater than or similar to 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 approximate to 2.