tokens: TNG100 parent galaxy sample selection . Red dots show the positions of central disc galaxies in TNG100 with 10 10 ≤ M stell /M ⊙ ≤ 10 11 . Orange dots point to satellite disc galaxies in the same mass range; to avoid Virgo-like clusters, we require that satellite galaxies have no more than one massive () neighbour within 1 Mpc. Galaxies rejected on the basis of these environmental and morphological criteria are shown in black, for reference. In the background, the spatial distribution of all galaxies in TNG100 is shown in blue. In addition to this parent sample, we also defined smaller samples of stellar mass-matched TNG100 galaxies for each DNGS galaxy. This time we applied a more observer-friendly definition of stellar mass – specifically the integral of the stellar mass surface density profile. We denote this ‘observed’ stellar mass as M stell, obs . When calculating TNG100 stellar masses in this way, we only integrated the density profiles down to a floor of kpc −2 , as this corresponds to the typical depth of the DNGS observations (see Section 5.1 for more details). The physical radius corresponding to this surface density threshold varies between ∼20 and 70 kpc for the DNGS sample, although we note that this is an extrapolation (from a last observed point of kpc −2 ) in two of the eight galaxies. TNG100 galaxies exhibit an even larger variation in physical extent under this definition, ranging from ∼40 to 140 kpc. We can infer from these numbers that TNG100 galaxies are more extended than DNGS galaxies, and we will return to this idea in Sections 6 and 8.2 . As a consequence of the shape of the stellar mass function, if we were to simply select all TNG100 galaxies that fall within the observational errors (see Table 1 ), each mass-matched sample would remain strongly weighted towards low-mass galaxies by number. To alleviate this issue, we instead used rejection sampling (Press et al. 1992 ) to obtain a normal distribution of TNG100 galaxy stellar masses for each DNGS galaxy. We calculated the probability P ( M stell, obs ) of drawing each TNG100 stellar mass from a Gaussian distribution with a mean and standard deviation set by M stell, DNGS and , respectively, and accepted the first 50 TNG100 galaxies with P ( M stell, obs ) less than a random number drawn from the same distribution. We emphasize that, in addition to the definition of the mass-matched samples, all comparisons between observations and simulations were done using quantities measured from TNG100 galaxies (e.g. surface density profiles or stellar halo mass fractions) in a manner identical to the DNGS sample. When exploring parameter space that contains unobservable properties, however, we will occasionally use the ‘true’ stellar mass instead. In the following sections we will explicitly distinguish between the parent and mass-matched samples, and between ‘true’ and ‘observed’ stellar masses where applicable. 3 FROM STELLAR PARTICLES TO IMAGES The first step towards
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