2025ApJ...994L..53Z

The cold gas in the circumgalactic medium (CGM) and its interaction with galaxies remain poorly understood. Strong Mg II (λλ2796, 2803) absorptions seen in background quasar spectra reveal large reservoirs of neutral hydrogen, potentially serving as progenitors of star-forming galaxies at high redshifts. In this study, we search for galaxies in the vicinity of very strong Mg II absorbers (rest-frame equivalent width W_r > 2 Å) with high kinematic velocities (>500 km s^–1) at 2.0 < z < 6.0. Observations were conducted with the Very Large Telescope/MUSE, JWST/NIRCam, and the Atacama Large Millimeter/submillimeter Array to detect Lyα and nebular emission lines and dust continuum emission. We identify two Lyα emitters associated with a strong Mg II absorber pair, separated by ∼1000 km s^–1 at z ∼ 4.87, in the vicinity of quasar J1306+0356. We observe relative differences in metallicity, dust content, and ionization states in this ultralarge absorption pair system, indicating potential metal and dust transfer within the system. For another strong Mg II absorber at z = 2.5662 (W_r = 2.638 ± 0.124 Å), toward a second quasar J0305-3150, we detect a dusty star-forming galaxy at a projected distance of D = 38 kpc. This galaxy exhibits prominent He I, [S III], and Paschen γ lines, along with significant dust continuum. It has a star formation rate of ∼121 ± 33 M_⊙ yr^–1 and likely harbors a rotating disk. These findings tentatively suggest that cold gas at high redshifts fuels disk formation and participates in metal and dust transfer within overdense CGM regions.