Cold mode gas accretion on two galaxy groups at z ∼ 2

We present Keck Cosmic Web Imager (KCWI) integral field spectroscopy (IFS) observations of rest-frame UV emission lines Ly alpha, CIV lambda lambda 1548 angstrom, 1550 angstrom, and HeII 1640 angstrom observed in the circumgalactic medium (CGM) of two z = 2 radio-loud quasar host galaxies. We detect extended emission on 80-90 kpc scale in Ly alpha in both systems with CIV and HeII emission also detected out to 30-50 kpc. All emission lines show kinematics with a blue and redshifted gradient pattern consistent with velocities seen in massive dark matter haloes and similar to kinematic patterns of inflowing gas seen in hydrodynamical simulations. Using the kinematics of both resolved Ly alpha emission and absorption, we can confirm that both kinematic structures are associated with accretion. Combining the KCWI data with molecular gas observations with Atacama Large Millimeter/submillimeter Array (ALMA) and high-spatial resolution of ionized gas with Keck OSIRIS, we find that both quasar host galaxies reside in proto-group environments at z = 2. We estimate 1-6 x 10(10)M(circle dot) of warm-ionized gas within 30-50 kpc from the quasar that is likely accreting on to the galaxy group. We estimate inflow rates of 60-200 M-circle dot yr(-1), within an order of magnitude of the outflow rates in these systems. In the 4C 09.17 system, we detect narrow gas streams associated with satellite galaxies, potentially reminiscent of ram-pressure stripping seen in local galaxy groups and clusters. We find that the quasar host galaxies reside in dynamically complex environments, with ongoing mergers, gas accretion, ISM stripping, and outflows likely playing an important role in shaping the assembly and evolution of massive galaxies at cosmic noon.

Automated summary

We present Keck Cosmic Web Imager (KCWI) observations of the circumgalactic medium (CGM) of two radio-loud quasar host galaxies at z = 2. We detect extended emission on 80-90 kpc scale in Ly alpha with CIV and HeII also detected out to 30-50 kpc. Kinematics of the emission lines are consistent with velocities seen in dark matter haloes and inflowing gas in hydrodynamical simulations. Combining the data with molecular gas observations and high-spatial resolution imaging, we find that the quasar host galaxies reside in proto-group environments and are undergoing mergers, gas accretion, and outflows, shaping the assembly and evolution of massive galaxies at cosmic noon.