Distance to main progenitor at formation time, r_birth, as function of cosmic formation time, t_birth, for all star particles (black dots) bound to the example galaxy TNG50-714463 (see Fig. 2). Star particles born within the main progenitor (i.e. r_birth < r_glx) are classified as in situ; those with r_birth > r_glx (above the threshold radius shown as a red horizontal dashed line) are defined as ex situ, or ‘accreted’. The example galaxy accreted two satellites at t ≈ 3.7 Gyr and t ≈ 7.5 Gyr (indicated with black arrows). Stars born in these satellites contribute log(M_acc/M_⊙) = 7.68 to the host galaxy, for a total accreted-mass fraction of f_acc = 0.053. Stars in the outer stellar halo at z = 0 (r > r_glx, labelled ‘oSH’) are highlighted with empty squares: blue if they are in situ, and red if they are accreted (see Fig. 2). The grey shaded region denotes distances smaller than time-evolving, force-softening length for stars and dark matter, ϵ_DM, *.

Fig. 2.

Edge-on projection of simulated dwarf galaxy (TNG50-714463, stellar and virial masses are shown in the legend, left panel), split into its in situ (centre panel) and accreted (right panel) stellar components. Our definition of galaxy radius, r_glx = 0.25 r₂₀₀ = 24.25 kpc, is indicated by the cyan circles, separating the main galaxy from its outer stellar halo. This example has an accreted mass comparable to the median value for galaxies of comparable mass (f_acc = 0.053). Brighter colours indicate a higher number density of star particles, while starred symbols show outer halo stars coloured according to their origin (blue for in situ; red for accreted). Images generated with PySPHViewer (Benítez-Llambay 2017).