2019ApJS..245...23C

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. 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_νLL_=10^31.1–32.3 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. Among them, four nebulae have enormous Lyα emission with a Lyα surface brightness (SB) SB_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 ≃ 2 QSO nebulae have a more irregular morphology compared to those at z >= 3. In turn, we measure that the circularly averaged SB at z ≃ 2 is 0.4 dex fainter than the redshift-corrected median SB at z >= 3. The Lyα SB profile (SB_Lyα) of QSOs at z ≃ 2 can be described by a power law of SB_Lyα,z~2.3_=3.7×10^–17×(r/40)^–1.8 erg s^–1 cm^–2 arcsec^–2, with a 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.