2024ApJ...963...28B

We report new observations of the cool diffuse gas around 29, 2.3 < z < 6.3 galaxies using deep JWST/NIRCam slitless grism spectroscopy around the sight line to the quasar J0100+2802. The galaxies span a stellar mass range of $7.1\leqslant {\rm{log}}{M}_{\ast }/{M}_{\odot }\leqslant 10.7$, and star formation rates (SFRs) of $-0.1\lt \mathrm{log}$ SFR/M_⊙ yr^–1 < 2.3. We find galaxies for seven Mg II absorption systems within 300 kpc of the quasar sight line. The Mg II radial absorption profile falls off sharply with radius, with most of the absorption extending out to 2-3R₂₀₀ of the host galaxies. Six out of seven Mg II absorption systems are detected around galaxies with ${\rm{log}}{M}_{\ast }/{M}_{\odot }\gt $ 9. The Mg II absorption kinematics are shifted from the systemic redshifts of host galaxies with a median absolute velocity ≈ 135 km s^–1 and standard deviation ≈ 85 km s^–1. The high kinematic offset and large radial separation (R > 1.3R₂₀₀), suggest that five out of the seven Mg II absorption systems are not gravitationally bound to their host galaxy. In contrast, most of the cool circumgalactic medium at z < 1 is gravitationally bound. The high incidence of unbound Mg II gas in this work suggests that toward the end of reionization, galaxy halos are in a state of remarkable disequilibrium, and are highly efficient in enriching the intergalactic medium. The two strongest Mg II absorption systems are detected at z ∼ 4.22 and 4.5, the former associated with a merging galaxy system and the latter associated with three kinematically close galaxies. Both of these galaxies reside in local galaxy overdensities, indicating the presence of cool Mg II absorption in two “protogroups” at z > 4.