The Cosmic Ultraviolet Baryon Survey (CUBS). VII. On the Warm-hot Circumgalactic Medium Probed by O VI and Ne VIII at 0.4 ≲ z ≲ 0.7

Abstract

This paper presents a newly established sample of 103 unique galaxies or galaxy groups at 0.4 ≲ z ≲ 0.7 from the Cosmic Ultraviolet Baryon Survey (CUBS) for studying the warm-hot circumgalactic medium (CGM) probed by both O VI and Ne VIII absorption. The galaxies and associated neighbors are identified at <1 physical Mpc from the sightlines toward 15 CUBS QSOs at z _QSO ≳ 0.8. A total of 30 galaxies or galaxy groups exhibit associated O VI λ λ 1031, 1037 doublet absorption within a line-of-sight velocity interval of ±250 km s^‑1, while the rest show no trace of O VI to a detection limit of logNOVI/cm‑2≈13.7 . Meanwhile, only five galaxies or galaxy groups exhibit the Ne VIII λ λ 770, 780 doublet absorption, down to a limiting column density of logNNeVIII/cm‑2≈14.0 . These O VI- and Ne VIII-bearing halos reside in different galaxy environments with stellar masses ranging from logMstar/M⊙≈8 to ≈11.5. The warm-hot CGM around galaxies of different stellar masses and star formation rates exhibits different spatial profiles and kinematics. In particular, star-forming galaxies with logMstar/M⊙≈9–11 show a significant concentration of metal-enriched warm-hot CGM within the virial radius, while massive quiescent galaxies exhibit flatter radial profiles of both column densities and covering fractions. In addition, the velocity dispersion of O VI absorption is broad with σ _υ > 40 km s^‑1 for galaxies of logMstar/M⊙>9 within the virial radius, suggesting a more dynamic warm-hot halo around these galaxies. Finally, the warm-hot CGM probed by O VI and Ne VIII is suggested to be the dominant phase in sub-L* galaxies with logMstar/M⊙≈9–10 based on their high ionization fractions in the CGM.