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 AGN at z = 5.077, observed with NIRSpec/MSA and NIRSpec/IFU by the JADES and BlackTHUNDER 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 (Mdyn/M⊙) = 7.65, while [O []iii] [λ][1=5007]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 EW 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 G395H observations, it is not detected in the medium-resolution G395M 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 SMBH.