GPPU Seminar

Date

10:30-12:00, July 27, 2026

Place

(hybrid) Room 745, Science Complex B (H-03), Zoom registration map

Abstract

1) The buildup of dense galactic bulge 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 JWST near-infrared imaging, combined with 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 50 most massive galaxies per redshift bin at constant number density of ~ 3.2 x 10^-5 cMpc^-3. To robustly constrain central stellar populations, we separate active galactic nuclei (AGN) components affecting the photometry using multi-band morphological decomposition and SED analysis. We find that the most massive galaxies exhibits evolutionary trend of rapid bulge formation at z ~ 4 (1.5 Gyr after the Big Bang), during which the median central 1 kpc stellar mass increases 7 times over ~ 400 Myr. The majority of X-ray detected AGN (67% ± 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 bulge formation. Following the bulge formation, central specific star formation rates (sSFR) decline ~ 40 times over ~ 700 Myr at z ~ 3 (2.1 Gyr after the Big Bang) while remaining elevated galaxy-wide, signaling the onset of inside-out quenching. Despite this central suppression, 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 star formation quenching and a transition toward enhanced black-hole to stellar growth ratio.

2) Stars form from cold gas, making star formation one of the fundamental processes that drive the evolution of galaxies. To continue forming stars over billions of years, galaxies must continuously acquire fresh supplies of cold gas from the surrounding cosmic web. This suggests that a galaxy's evolution is influenced not only by its own internal processes but also by the environment in which it lives. Understanding this connection is particularly important during the peak of cosmic star formation, around 3 billion years after the Big Bang (redshift z ≈ 2), when galaxies were building up most of their stellar mass. In this study, we use deep observations from the James Webb Space Telescope (JWST) to identify galaxy overdensities, which probe the ancestors of today's galaxy clusters, across multiple regions of the sky. We then investigate how both the large-scale environment and its local sureoundings affect galaxy evolution. Our results show that both environmental scales play important roles in regulating star formation and the quenching of galaxies, with their influence becoming particularly significant around z ≈ 2, providing new insights into how galaxies transition from actively forming stars to becoming quiescent.

Point

GSP 1