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

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] + Hβ). 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.