2025ApJ...986..190S

We report the small-scale spatial variation in cool (T ∼ 10⁴ K) Mg II absorption detected in the circumgalactic medium (CGM) of a star-forming galaxy at z ≈ 0.8. The CGM of this galaxy is probed by a spatially extended bright background gravitationally lensed arc at z = 2.76. The background arc continuously samples the CGM of the foreground galaxy at a range of impact parameters between 54 and 66 kpc. The Mg II absorption strengths vary by more than a factor of 2 within these ranges. A power-law fit to the fractional variation of absorption strengths yields a coherence length of 5.8 kpc within this range of impact parameters. This suggests a high degree of spatial coherence in the CGM of this galaxy. The host galaxy is driving a strong galactic outflow with a mean outflow velocity ≈ -179 km s^–1 and mass outflow rate ${\dot{M}}_{\mathrm{out}}\,\geqslant \,6{4}_{-27}^{+31}$M_⊙ yr^–1 traced by blueshifted Mg II and Fe II absorption lines. The galaxy itself has a spatially extended emission halo with a maximum spatial extent of ≈33 kpc traced by [O II], [O III], and Hβ emission lines. The extended emission halo shows kinematic signatures of corotating halo gas with solar metallicity. Taken together, these observations suggest evidence of a baryon cycle that is recycling the outflowing gas to form the next generation of stars.