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We analyze the merger and assembly histories of Milky Way (MW) and Andromeda(M31)-like galaxies to quantify how, and how often, disk galaxies of this masscan survive recent major mergers (stellar mass ratio $\ge$ 1:4). For this, weuse the cosmological magneto-hydrodynamical simulation TNG50 and identify 198analog galaxies, selected based on their $z=0$ stellar mass ($10^{10.5-11.2}{\rm M_{\odot}}$), disky stellar morphology and local environment. Firstly,major mergers are common: 85 per cent (168) of MW/M31-like galaxies in TNG50have undergone at least one major merger across their lifetime. In fact, 31galaxies (16 per cent) have undergone a recent major merger, i.e. in the last 5Gyr. The gas available during the merger suffices to either induce starburstsat pericentric passages or to sustain prolonged star formation aftercoalescence: in roughly half of the cases, the pre-existing stellar disk isdestroyed because of the merger but reforms thanks to star formation. Moreover,higher merger mass ratios are more likely to destroy the stellar disks. Incomparison to those with more ancient massive mergers, MW/M31-like galaxieswith recent major mergers have, on average, somewhat thicker stellar disks,more massive and somewhat shallower stellar haloes, larger stellar ex-situ massfractions, but similarly massive kinematically-defined bulges. All this isqualitatively consistent with the different observed properties of the Galaxyand Andromeda and with the constraints on their most recent major mergers, 8-11and ~2 Gyr ago, respectively. According to contemporary cosmologicalsimulations, a recent quiet merger history is not a pre-requisite for obtaininga relatively-thin stellar disk at $z=0$.

The merger and assembly histories of Milky Way- and M31-like galaxies with TNG50: disk survival through mergers