MIDIS

Context. Thanks to decades of observations using the Hubble Space Telescope (HST), the structure of galaxies at redshift z>2 has been widely studied in the rest-frame ultraviolet regime, which traces recent star formation from young stellar populations. However, we still have little information about the spatial distribution of the older, more evolved stellar populations, constrained by the rest-frame infrared portion of the galaxies’ spectral energy distribution. Aims. We present the morphological characterization of a sample of 49 massive galaxies (log(M⋆/M⊙)>9) at redshift 3<z<5. These galaxies are observed as part of the MIRI Deep Imaging Survey (MIDIS), the guaranteed time observations program with the MIRI instrument on board the James Webb Space Telescope (JWST). Deep MIRI 5.6 μm imaging (28.64 mag 5σ depth) allows us to characterize the rest-frame near-infrared structure of galaxies beyond cosmic noon, at higher redshifts than possible with NIRCam, tracing their older and dust-insensitive stellar populations. Methods. We derived the nonparametric morphology of galaxies, focusing on the Gini, M20, concentration, asymmetry, and deviation statistics. Furthermore, we modeled the light distribution of galaxies with a single Sérsic component and derived their parametric morphology (i.e., effective radius and Sérsic index). Results. We find that at z>3 massive galaxies show a smooth distribution of their rest-infrared light, strongly supporting the increasing number of regular disk galaxies already in place at early epochs. These results are further reinforced by the analysis of JWST/NIRCam data at 4.4 μm. On the contrary, the ultraviolet structure obtained from HST/WFC3 and JWST/NIRCam observations at ∼1.5 μm is generally more irregular, catching the most recent episodes of star formation. Importantly, we find a segregation of morphologies across cosmic time, where galaxies at redshift z>3.75 show later-type morphologies compared to z∼3 galaxies. These findings suggest a transition phase in galaxy assembly and central mass build-up, which takes place already at z∼3−4. Conclusions. The combined analysis of NIRCam and MIRI imaging datasets allows us to prove that the rest-frame near-infrared morphology of massive galaxies at cosmic noon is typical of compact disk galaxies with a smooth mass distribution.