Reconstructing the Assembly of Massive Galaxies. II. Galaxies Develop Massive and Dense Stellar Cores as They Evolve and Head Toward Quiescence at Cosmic Noon

TL;DR

This study uses advanced spectral energy distribution fitting to analyze the star formation histories of massive galaxies at cosmic noon, revealing how their structural evolution involves central mass buildup and quenching processes.

Contribution

It provides the first detailed empirical reconstruction of galaxy structural evolution linked to star formation history at high redshift, highlighting the role of central mass growth in quenching.

Findings

Compact SFGs experienced multiple star-formation episodes with early central assembly.

Extended QGs have star-formation histories similar to post-starburst galaxies.

Galaxies grow dense stellar cores as they evolve toward quiescence, starting from the center.

Abstract

We use the SED-fitting code Prospector to reconstruct the nonparametric star formation history (SFH) of massive ($\log M_*>10.3$) star-forming galaxies (SFGs) and quiescent galaxies (QGs) at redshift $z_{\rm{obs}}\sim2$ to investigate the joint evolution of star-formation activity and structural properties. We find significant correlations between the SFH of the galaxies and their morphology. Compared to extended SFGs, compact SFGs are more likely to have experienced multiple star-formation episodes, with the fractional mass formed during the older ($\ge1$ Gyr) episode being larger, suggesting that high-redshift SFGs assembled their central regions earlier and then kept growing in central mass as they become more compact. The SFH of compact QGs does not significantly differ from the average for this category, and shows an early burst followed by a gradual decline of the star formation rate. The SFH of extended QGs, however, is similar to that of post-starburst galaxies and their morphology is also frequently disturbed. Knowledge of the SFH also enables us to empirically reconstruct the structural evolution of individual galaxies. While the progenitor effect is clearly observed and accounted for in our analysis, it alone is insufficient to explain the observed structural evolution. We show that, as they evolve from star-forming phase to quiescence, galaxies grow massive dense stellar cores. Quenching begins at the center and then propagates outward to the rest of the structure. We discuss possible physical scenarios for the observed evolution and find that our empirical constraints are in good quantitative agreement with the model predictions from dissipative accretion of gas to the center followed by massive starbursts before final quiescence (wet compaction).