First Light And Reionisation Epoch Simulations (FLARES): XVI. Size evolution of massive dusty galaxies at cosmic dawn from the ultraviolet to infrared

We used the First Light And Reionisation Epoch Simulations (FLARES) suite to study the evolution of the rest-frame ultraviolet (UV) and far-infrared (FIR) sizes for a statistical sample of massive (≳10⁹ M_·) high-redshift galaxies (z' [5, 10]). The galaxies were post-processed using the SKIRT radiative transfer code to self-consistently obtain the full spectral energy distribution (SED) and surface brightness distribution. We created mock observations of the galaxies for the Near Infrared Camera (NIRCam) to study the rest-frame UV (1500 A) morphology. We also generated mock rest-frame FIR (50 μm) photometry and mock ALMA 158 μm (0.012; - 0.03 and 0.3 angular resolution) observations to study the dust-continuum sizes. We find the effect of dust on observed sizes is reduced with a rising wavelength from the UV to optical (0.6 times the UV at 0.4 μm), with no evolution in FIR sizes. Observed sizes vary within 0.4 - 1.2 times the intrinsic sizes at different signal-to-noise ratios (S/N =5 - 20) across redshifts. The effects of the point spread function (PSF) and noise makes bright structures prominent, whereas fainter regions blend with noise, leading to an underestimation (by a factor of 0.4 - 0.8) of sizes at S/N =5. At S/N =15 - 20, the underestimation reduces (factor of 0.6 - 0.9) at z' =' 5 - 8, but due to the PSF, at z' =' 9 - 10, bright cores are dominant, resulting in an overestimation (factor of 1.0- 1.2) of sizes. For ALMA, low (0.3) resolution sizes are affected by noise that tends to behave as extended emission. The size evolution in UV is in overall agreement with current observational samples and other simulations. This work is one of the first to analyse the panchromatic sizes of a statistically significant sample of simulated high-redshift galaxies. These results supplement a growing body of research highlighting the importance of conducting equivalent comparisons between observed galaxies and their simulated counterparts in the early Universe.