Deep Extragalactic VIsible Legacy Survey (DEVILS): the size–mass relation of galaxies and their components in HST-COSMOS over the last 8 billion years

We present the evolution of the size–mass relation since z=1 in the COSMOS region of the Deep Extragalactic VIsible Legacy Survey (DEVILS). We combine structural decomposition measurements with stellar mass estimates from fitting spectral energy distributions to multiwavelength photometry. We implement a novel technique to fit 2D light profiles to repeated observations, removing the requirement to co-add images, which maximises the effective signal-to-noise ratio and avoids issues arising when averaging point spread functions. The sample is then separated into distinct morphological classifications, which reveals that the size–mass relation of disc-dominated galaxies shows an overall flattened slope with very little redshift evolution over 0.3 < z < 1.0. In contrast, spheroid-dominated morphologies show a much steeper relation and are generally more compact at a given stellar mass. The size–mass relations of bulge and disc components are also examined revealing that diffuse bulges occupy a similar region to disc structures, in stark contrast to the size–mass relation of compact bulges. Furthermore, the size–mass relation of discs becomes steeper in the presence of a compact bulge, whereas the relation for discs hosting a diffuse bulge is identical to that of pure-discs. The lack of evolution in disc-dominated galaxies (i.e. R_eff ∝ (1+z)^-0.13±0.02) is inherent to their self-similar assembly. In contrast, the size–mass relation of spheroid-dominated morphologies is rapidly evolving despite minimal growth in the individual compact bulge components, with average sizes increasing at a pace of R_eff ∝ (1 + z)^-3.0±0.2 and a slope that flattens with time as d log ₁₀(R_eff /d log ₁₀(M_✬) ∝ (1 + z)^2.8±0.2.