A SPectroscopic Survey of Biased Halos in the Reionization Era (ASPIRE): JWST Discovers an Overdensity around a Metal Absorption-selected Galaxy at z ∼ 5.5

Abstract The launch of JWST opens a new window for studying the connection between metal-line absorbers and galaxies at the end of the Epoch of Reionization. Previous studies have detected absorber–galaxy pairs in limited quantities through ground-based observations. To enhance our understanding of the relationship between absorbers and their host galaxies at z > 5, we utilized the NIRCam wide-field slitless spectroscopy to search for absorber-associated galaxies by detecting their rest-frame optical emission lines (e.g., [O III ] + H β ). We report the discovery of a Mg ii -associated galaxy at z = 5.428 using data from the JWST ASPIRE program. The Mg ii absorber is detected on the spectrum of quasar J0305–3150 with a rest-frame equivalent width of 0.74 Å. The associated galaxy has an [O III ] luminosity of 10 42.5 erg s −1 with an impact parameter of 24.9 pkpc. The joint Hubble Space Telescope–JWST spectral energy distribution (SED) implies a stellar mass and star formation rate of M * ≈ 10 8.8 M ⊙ , star-formation rate ≈ 10 M ⊙ yr −1 . Its [O III ] equivalent width and stellar mass are typical of [O III ] emitters at this redshift. Furthermore, connecting the outflow starting time to the SED-derived stellar age, the outflow velocity of this galaxy is ∼300 km s −1 , consistent with theoretical expectations. We identified six additional [O III ] emitters with impact parameters of up to ∼300 pkpc at similar redshifts (∣ dv ∣ < 1000 km s −1 ). The observed number is consistent with that in cosmological simulations. This pilot study suggests that systematically investigating the absorber–galaxy connection within the ASPIRE program will provide insights into the metal-enrichment history in the early Universe.