JADES and BlackTHUNDER : rest-frame Balmer-line absorption and the local environment in a Little Red Dot at z = 5

We present a ‘Little Red Dot’ (LRD) broad-line active galactic nucleus (AGN) at z = 5 . 077, observed with NIRSpec/MSA (micro-shutter assembly) and NIRSpec/IFU (integral-field unit) by the JADES ( JWST Advanced Deep Extragalactic Survey) and BlackTHUNDER (Black holes in THe early Universe aNd their DensE surRoundings) surveys. Combining spatially resolved and high-resolution spectroscopy, we characterize its central engine, host, and environment. H α has multiple components, including two broad Gaussians, yielding a black-hole mass log ( M•/M ) = 7 . 65, while [O III ] λ5007 gives a galaxy dynamical mass log ( Mdyn / M) = 9 . 1, suggesting an overmassive black hole relative to the host galaxy. The target is immersed in a 7-kpc wide pool of ionized gas and has three neighbours: a satellite galaxy, a possible satellite/gas cloud, and a tentatively detected spatially detached outflow. H α shows strong, rest-frame absorption, deeper than the continuum, ruling out a stellar origin. The velocity and velocity dispersion are vabs = −13 km s−1 and σabs = 120 km s−1 . There is tentative evidence (2.6 σ) of temporal variability in the equivalent width of the H α absorber over two rest-frame months, suggesting a highly dynamic nucleus. Notably, while the H α absorber is clearly visible and even dominant in the high-resolution G 395 H observations, it is not detected in the medium-resolution G 395 M data of the same epoch. This implies that the current incidence rate of absorbers in LRDs – and especially of rest-frame absorbers –may be severely underestimated, because most LRDs rely on lower resolution spectroscopy. The high incidence rate of rest-frame absorbers in LRDs may indicate a configuration that is either intrinsically stationary, such as a rotating disc, or that exhibits time-averaged stability, such as an oscillatory ‘breathing mode’ accretion with cyclic expansion and contraction of the gas around the supermassive black hole.