Brightest Galaxies Evolve: Structure and Star Formation at Cosmic Noon.

Analysis of Brightest Group Galaxies using deep infrared imaging reveals quiescent galaxies are more compact and evolve faster than star-forming counterparts, with effective radii changing as $r_e \propto (1+z)^{-0.2}$ and $-0.3$ respectively. Star formation surface density increases with redshift, and quenching correlates with bulge dominance.

Understanding how massive galaxies assemble and cease star formation remains a central challenge in modern cosmology. Recent observations indicate that galaxy morphology and star formation history are intimately linked, yet disentangling the relative contributions of internal processes and environmental influences requires detailed study of galaxies across cosmic time. A collaborative team, comprising Ghassem Gozaliasl, Lilan Yang, Jeyhan S. Kartaltepe, and a further 28 researchers, have undertaken a comprehensive investigation into the structural evolution of Brightest Group Galaxies (BGGs) at a redshift of 3.7, utilising data from the James Webb Space Telescope’s COSMOS-Web survey. Their findings, detailed in the article “COSMOS Web: Morphological quenching and size-mass evolution of brightest group galaxies from z = 3.7”, reveal a clear correlation between galaxy structure, star formation rate, and the quenching process, providing new insights into the baryonic assembly of these central galaxies within group-scale halos.

Brightest Group Galaxies Reveal Evolutionary Divergence Across Cosmic Time

Brightest group galaxies (BGGs) – the most luminous galaxies within their respective groups – serve as pivotal nodes in the cosmic web and indicators of large-scale structure formation. Recent research utilising deep imaging data from the James Webb Space Telescope’s COSMOS-Web survey has detailed the structural evolution of these galaxies from redshifts of approximately 0 to 6, corresponding to a period spanning much of cosmic history.

The study establishes a clear distinction in size between star-forming and quiescent BGGs, demonstrating fundamentally different evolutionary pathways. Quiescent galaxies – those no longer actively forming stars – consistently exhibit smaller effective radii (the radius containing half of the galaxy’s light) than their star-forming counterparts. This suggests a correlation between the cessation of star formation and a corresponding decrease in galactic size. Researchers precisely measured stellar masses, star formation rates and structural parameters, enabling a detailed comparative analysis.

High-resolution imaging and multi-wavelength photometry revealed that quiescent galaxies display steeper size-mass slopes. This indicates a more compact morphology compared to star-forming galaxies of similar mass. Effective radii evolve with redshift, decreasing over time, with star-forming galaxies expanding at a greater rate than quiescent galaxies.

A significant finding concerns the evolution of star formation surface density – the rate of star formation per unit area – with redshift. This parameter increases as the universe ages, particularly for more massive BGGs, suggesting that star formation becomes increasingly concentrated within these galaxies over time. This concentration may be driven by gas accretion – the inflow of gas fuelling star formation – or internal processes within the galaxy.

The research identifies a structural transition coinciding with the quenching process – the cessation of star formation. Quiescent BGGs are overwhelmingly bulge-dominated, with over 80% exhibiting this morphology. This indicates that the morphological transformation towards a more centrally concentrated structure occurs alongside the suppression of star formation.

These results underscore the co-evolution of structure and star formation activity, establishing BGGs as valuable testbeds for understanding the assembly and morphological transformation of central galaxies within group-scale dark matter halos. Future work should focus on disentangling the relative contributions of galaxy mergers, gas accretion, and feedback mechanisms (such as energy released by supermassive black holes) in shaping the observed structural differences. Detailed modelling of the galaxy population, incorporating these processes, will be crucial for advancing our knowledge of galaxy formation and evolution.