Interplay of Compaction, Quenching, and Black Hole Growth in the Most Massive Galaxies since z ∼ 5: Insights from JWST and Chandra Data

The buildup of dense stellar cores is expected to mark an important transition in the star formation and black hole growth of massive galaxies. Using spatially resolved spectral energy distribution (SED) fitting of James Webb Space Telescope near-infrared imaging, combined with a stacking analysis of Chandra X-ray data, we trace stellar mass buildup and average black hole accretion in the most massive galaxies at z < 5, selecting the 50 most massive galaxies per redshift bin at constant number density of ∼4.4 × 10 −5 cMpc −3 . To robustly constrain central stellar populations, we separate active galactic nucleus (AGN) components affecting the photometry using multiband morphological decomposition and SED analysis. We find that the sample selected with constant number density exhibits an evolutionary trend of rapid central compaction at z ∼ 4, during which the median central 1 kpc stellar mass increases by ∼0.60 dex over ∼400 Myr. The majority of X-ray-detected AGN (63% ± 12%) are hosted by galaxies undergoing the compaction, while we find neither individually detected X-ray sources nor a significant stacked X-ray signal at z > 4, indicating that substantial average black hole growth emerges primarily during, rather than before, the compaction. Following the compaction, central specific star formation rates (sSFRs) decline by ∼1.24 dex over ∼700 Myr at z ∼ 3 while remaining elevated galaxy-wide, signaling the onset of inside-out quenching. Despite this central suppression, the specific black hole accretion rate remains coupled to the total sSFR. Our results suggest that dense core formation in the most massive galaxies marks the onset of inside-out quenching and a transition toward an enhanced black hole to stellar growth ratio.