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

Motivated by the recent discovery of the near-ubiquity of Ly α emission around z  ≳ 3 QSOs, we performed a systematic study of QSO circumgalactic Ly α 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 Ly α nebulae with projected linear sizes larger than 50 physical kpc. Among them, four nebulae have enormous Ly α emission with a Ly α 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 Ly α SB profile (SB Ly α ) 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.