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 Lyα emission around z≳3 QSOs, we performed a systematic study of QSO circumgalactic Lyα emission at z≈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 LνLL=1031.1−32.3 erg s−1 Hz−1. We found that 14 out of 16 QSOs are associated with Lyα nebulae with projected linear-sizes larger than 50 physical kpc (pkpc). Among them, four nebulae have enormous Lyα emission with the Lyα surface brightness SBLyα >10−17 erg s−1 cm−2 arcsec−2extending beyond the field of view of KCWI. Our KCWI observations reveal that most z≈2 QSO nebulae have a more irregular morphology compared to those at z≳3. In turn, we measure that the circularly-averaged surface brightness (SB) at z≈2 is 0.4 dex fainter than the redshift-corrected, median SB at z≳3. The Lyα SB profile (SBLyα) of QSOs at z≈2 can be described by a on the >100 kpc scale, power law of SBLyα,z≈2.3 =3.7×10−17×(r/40)−1.8 erg s−1 cm−2 arcsec−2, with the slope similar to that at z≳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≈2