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

Abstract

Abstract 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 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) 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 (SBLyα ) of QSOs at z ≈ 2 can be described by a power law of 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.