Feeding the Dead: Neutral Gas Inflow in a Long-quenched Ancient Massive Galaxy at z ∼ 2.7 Observed with JWST/NIRSpec

Abstract We report the spectroscopic detection of neutral gas inflow into a massive ( M * ≃ 4 × 10 10 M ⊙ ) quiescent galaxy observed at z spec = 2.6576 with the James Webb Space Telescope (JWST). From the redshifted absorption of the Na I doublet at λλ 5890, 5896, we estimate an inflow velocity v = 27 8 − 79 + 79 km s −1 and a column density log ( N NaI / cm 2 ) = 13.0 2 − 0.03 + 0.03 . We derive the inflowing mass of the gas M in = 1 . 6 − 0.1 + 0.1 × 1 0 8 M ⊙ and rate M ̇ in = 1 9 − 7 + 6 M ⊙ yr − 1 . The presence of several surrounding galaxies suggests that the galaxy may be accreting gas from nearby companions. However, we cannot confirm this with current data, and the intergalactic medium or cosmic filaments are also viable sources of the inflowing gas. Despite the ongoing inflow, the galaxy remains quiescent, with an upper limit to the star formation rate of 0.2 M ⊙ yr −1 . Moreover, its star formation history suggests that the galaxy has remained quiescent during the past ∼1 Gyr, with half of its stars formed by redshift z 50 = 1 1 − 3 + 18 . We discuss that the inflow is not massive, dense, or long-lived enough to ignite significant star formation (SF), or it is fueling low-level active galactic nucleus activity instead. This is direct evidence that quiescent galaxies can accrete cold gas after their quenching while keeping their SF subdued. Follow-up observations with JWST and the Atacama Large Millimeter/submillimeter Array will be needed to constraint the nature of the inflowing gas.