Enduring Stellar Growth of Giant Galaxies Fuelled by the Intracluster Medium

by Prof. Jeremy Lim

University of Hong Kong
at Astrophysics and Cosmology Seminar

Wed, 24 May 2023, 11:10
Sacta-Rashi Building for Physics (54), room 207


The largest and most massive galaxies in the universe reside at or near the centres of galaxy clusters, immersed together with hundreds of other galaxies in hot X-ray emitting gas comprising the intracluster medium (ICM). The manner by which these Brightest Cluster Galaxies (BCGs) attained their remarkable stellar dimensions, setting them apart from the general population of elliptical galaxies, challenges our understanding of galaxy assembly. Here, we show that star formation in BCGs is strictly dependent on the physical state of their host clusters, restricted to but ubiquitous among those judging from their ICM morphology as closest to being dynamically relaxed. The ICM of these clusters also display the steepest increase in density along with the steepest decrease in temperature inward, and consequently have the highest central thermal pressures and the lowest central entropies (hence shortest cooling times); by contrast, the ICM of the most strongly disturbed clusters show little change if not an inward rise in both temperature and entropy. These results clearly implicate cooling of the ICM for fuelling star formation at cluster centres, and mergers with other clusters – the only mechanism able to impact the physical states of clusters on a global scale – for altering the ICM thermodynamics such as to halt cooling and hence star formation. Because all the massive clusters in our study have likely undergone mergers, the ubiquity of star-forming BCGs among closely-relaxed clusters implies that, following a merger, cooling resumes to fuel star formation once the ICM returns closely to thermodynamic equilibrium. Persistent star formation at cluster centres fuelled by ICM cooling, temporarily interrupted only by cluster mergers when the disturbances generated reach the cluster core, provides a hitherto much underrated if not widely overlooked pathway for the indefinite stellar growth of BCGs.

Created on 21-05-2023 by Zitrin, Adi (zitrin)
Updaded on 21-05-2023 by Zitrin, Adi (zitrin)