PRIMER: JWST/MIRI reveals the evolution of star-forming structures in galaxies at <i>z</i> ≤ 2.5

Context. The stellar structures of star-forming galaxies (SFGs) undergo significant size growth during their mass assembly and must pass through a compaction phase as they evolve into quiescent galaxies (QGs). The mechanisms behind this structural evolution remain, however, poorly understood. Aims. We study the morphology of the star-forming components in SFGs to reveal the mechanisms that drive the structural evolution of their stellar components. Methods. We used high-resolution observations at 18 μm from the Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST) taken as part of the Public Release IMaging for Extragalactic Research (PRIMER) survey to measure the morphology of star-forming components in 665 SFGs at 0 &lt; z &lt; 2.5 and with M * ≳ 10 9.5 M ⊙ . We fit single Sérsic models to get the mid-infrared (MIR) structural parameters of these galaxies. The rest-frame optical morphology was taken from the literature and the effects of radial color gradients (due to dust or stellar aging) were corrected to obtain the intrinsic structural parameters for the stellar components of these galaxies. Results. The stellar and star-forming components of most SFGs (66%) have extended disk-like structures (Sérsic index, n MIR ∼ 0.7 and n optical ∼ 1; flat axis ratio distribution; hereafter called extended-extended galaxies) that are well aligned with each other and of the same size. Similar to the stellar components, the star-forming components of these galaxies follow a mass–size relation, with a slope of 0.12, and the normalization of this relation increases by ∼0.23 dex from z ∼ 2.5 to 0.5. At the highest masses ( M * ≳ 7 × 10 10 M ⊙ ), the optical Sérsic index of these SFGs increases to n optical ∼ 2.5, suggesting the presence of a dominant stellar bulge. Because their star-forming components remain in a disk-like structure, these bulges cannot have formed by secular in situ growth. We also observe a second population of galaxies lying below the MIR mass–size relation, with compact star-forming components embedded in extended stellar components. These galaxies are rare (15%; called extended-compact galaxies) but become more dominant at high masses (∼30% at M * &gt; 3 × 10 10 M ⊙ ). The star-forming components of these galaxies are compact, concentrated ( n MIR &gt; 1), and slightly spheroidal ( b / a &gt; 0.5), suggesting that this compaction phase can build dense stellar bulges in situ. We identified a third population of galaxies with both compact stellar and star-forming components (19%; called compact-compact galaxies). The density and structure of their stellar cores ( n optical ∼ 1.5; b / a ∼ 0.8) resemble those of QGs and are compatible with them being the descendants of extended-compact galaxies. Conclusions. The structural evolution of the stellar components of SFGs is mainly dominated by an inside-out secular growth. However, this secular growth might be interrupted by compaction phases triggered by either internal or external mechanisms, which build dominant central stellar bulges as those of QGs.