Affiliation: CEA Paris Saclay

Contribution: Poster

Title: JWST/MIRI reveals the evolution of star-forming structures in z<2.5 galaxies.

Abstract: 
It has been observed that the disks of star-forming galaxies (SFGs) undergo significant size growth during their mass assembly, while finally, as SFGs evolve into quiescent galaxies (QGs), their stellar components seem to go through a compaction phase. Unfortunately, the mechanisms behind this structural evolution remain poorly understood. This lack of understanding stems mainly from the fact that, until recently, very little was known about the distribution of ongoing star formation. Although UV/optical or far-infrared observations have provided information on this subject, these studies have been hampered either by the dust attenuation or by small-sample statistics. In this talk, I will present new constraints on the star formation distribution of a large (~700) and mass-complete (>10^9 Msun) sample of SFGs up to z=2.5, obtained through the PRIMER (Public Release IMaging for Extragalactic Research) 18um JWST/MIRI observation. I will show that the stellar and star-forming components of most SFGs (~70%) have extended disk-like structures (n_SF~0.7 and n_stars~1; flat axis ratio distribution) that are well aligned with each other and are generally of the same size. Like their stellar disks, the star-forming disks of these galaxies also follow a mass-size relation whose normalization increases by a factor of about two from z~2.5 to 0.5. However, at high masses (>10^10.5 Msun), the optical Sérsic index of these SFGs increases up to 2.5, suggesting the presence of a dominant bulge. Since their star-forming components still remain disk-like, these bulges cannot have formed by secular in-situ disk growth but rather by a more violent process like minor megers. Apart from this population of SFGs with extended stellar and star-forming disks, I will present that the growth of compact stellar bulges in massive galaxies could also be associated with the emergence of a population of SFGs with compact star-forming components embedded in extended stellar disks. These galaxies are globally rare (15%), but become more dominant at high masses (~30% at >10^10.5 Msun). Their star-forming components are compact, concentrated (n_SF>1), and slightly spheroidal (b/a>0.5), suggesting that this violent compaction phase can build dense stellar bulges in situ. Finally, I will show that if this compaction phase is not followed by a rejuvenation of the star-forming disk, it will lead to the formation of a third population of SFGs (15%) with both compact stellar and star-forming components. The density and stellar core structure (n_stars~1.5 and b/a~0.8) of these galaxies resemble those of QGs, suggesting that they could be the missing link between SFGs and QGs. Overall, these results reveal a structural evolution of the stellar components in SFGs that is mainly dominated by secular inside-out growth, which can however be interrupted by compaction phase(s) triggered by either internal or external mechanisms, building stellar bulges that might further cease star formation.

This contribution can be found at the Poster Hall.