Title: Understanding the Formation of an Unexpected Population of Red and Compact Massive Galaxies at z>~6 Uncovered by JWST

Speaker: Dr. Pedro Cataldi  (Institute of Astronomy and Space Physics, University of Buenos Aires, Argentina)

Time: 10:30am, January 10, 2025

Location: 5-516, PMO Xianlin Campus

Abstract: JWST data analysis has shown an unexpected population of red galaxies and high masses of M* > 10^10Msun  with extremely small effective radii at z≳6. Considering the surprising results of the high redshift size-mass / luminosity relation consistently found both in observation and various simulations at high redshift, this work aims to understand the mechanisms that produce these unexpected trends.  This study investigates the physical transition of galaxy gas and stellar components during early cosmic epochs at redshift z≳5, using the FirstLight (FL) cosmological simulation suite. This suite comprises 300 high-resolution zoom-ins and accurately depicts the reionization of the large-scale intergalactic medium while resolving galaxy properties. We find that the half-mast stellar radius (rhm,star) for simulated galaxies shows a positive relation with stellar mass at M∗ < 109Msun, followed by a general trend of decrease afterwards. After galaxies reach a benchmark stellar mass, M_peak ≈ 10^9Msun (or M_200 ≈ 10^11Msun), a reduction in galaxy size is predominantly observed, during which galaxies undergo a phase of rapid compaction and gas depletion. Early Universe simulated galaxies transitioned to a rapid and bursty SFR phase with the following formation of a subpopulation of compact massive galaxies. We identify the compaction of high stellar masses galaxies as a consequence of both, the phase state of the primordial gas at early cosmic epochs (with low metallicity, cold cooling streams, and extremely low cooling rates) and the deepens of the gravity well potential, which favours the infalling of star-forming gas toward the galaxy centre.