tokens: law may be more appropriate in these cases. Figure 9. A two parameter estimate of accreted mass fractions . The distribution of stellar mass fractions and 2D power-law slopes for the mass-matched sample of TNG100 galaxies, colour coded by the fraction of stellar mass beyond 2 half-mass radii. The full TNG100 parent sample is shown in grey points. Smaller symbols indicate galaxies for which a single power law was not an acceptable assumption, included here only for completeness (approximately ∼37 per cent of the samples require a more complex fit; see Section 6.2 for details). In general, shallower slopes correspond to higher accreted fractions, albeit with significant scatter. At fixed power-law slope, galaxies with higher stellar mass fractions in the outskirts have higher total accreted fractions. Figs 8 and 9 also raise the question of why these observations seem to be ‘missing’ stellar mass beyond 20 kpc (or, phrased differently, why TNG100 galaxies appear to be more massive and extended than DNGS galaxies at fixed stellar mass). No galaxy in DNGS has a stellar halo mass fraction above 10 per cent (when measured outside of 20 kpc), whereas the TNG100 galaxies in Fig. 9 can have up to 40 per cent of their stellar mass beyond this radius. One possible explanation is that DNGS galaxies may have low total accreted mass fractions relative to their mass-matched counterparts in TNG100. It is worth pausing to point out, however, that our capacity to ask (and answer) this question depends heavily on the validity of three assumptions: The parameters we chose to observationally estimate the accreted mass fraction (stellar halo mass fractions and power-law slopes) are able to provide an appropriate description of the outer profiles of galaxies – and, further, that these profiles contain sufficient information to characterize the assembly histories of galaxies. TNG100 reproduces observed properties of galaxies accurately enough that we should expect agreement in stellar halo properties. DNGS represents an unbiased sample of Milky Way-mass spiral galaxies. We will delve more deeply into these assumptions in Section 8 . First, however, we will proceed in Section 7 as though each of these are true, and attempt to carve out a more complete physical understanding of galaxies with low accreted mass fractions. 7 INSIGHTS FROM STELLAR PARTICLES We have seen from Figs 6 – 9 that DNGS galaxies appear to have low accretion fractions relative to their mass-matched TNG100 galaxies. The next logical question, then, is: ‘What are the assembly histories of (TNG100) galaxies with low accretion fractions?’ If, for example, this subset of galaxies shares any additional aspects of their assembly histories, we can potentially gain physical insight into our observations. In this section, we explore several different properties of our mass-matched TNG100 galaxies, combining information from the density profiles and individual stellar particles to characterize their past
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