Abstract

Boxy/peanut (b/p) or X-shaped bulges have been extensively explored with theory and numerical simulations of isolated galaxies. However, it is only recently that advances in hydrodynamical cosmological simulations have made it possible to explore b/p bulges in a cosmological setting, with much remaining to be understood about their formation and evolution. By using the Auriga magnetohydrodynamical cosmological zoom-in simulations, we characterize the structural parameters of b/p bulges and how they form and evolve throughout cosmic history. We develop a method for estimating the b/p strength that allows us to identify the formation time and size of these structures. We find that b/p bulges in Auriga form between ∼1.1 and 1.6 Gyr after bar formation, following a ‘buckling’ episode; some galaxies undergo multiple bucklings and events of b/p growth, with some b/p structures ‘dissolving’ between buckling events. We find that at z = 0, the b/p bulges have an extent of almost half the bar length. Finally, we analyse the evolution of the b/p fraction over redshift, finding that at z = 0, two-thirds of galaxies host a bar, and of these, 45 per cent have a b/p. This b/p fraction is within the observed range at z = 0, although on the low end as compared to some observational studies. The b/p fraction decreases to 20 per cent at z = 0.5, and falls to zero at z ∼ 1; this is in line with the observed trend of declining b/p fraction with redshift. We discuss possible culprits for the apparent mismatch in b/p occurrence between observations and cosmological simulations, what causes them to form (or not) in these simulations, and what this might reveal about models of galaxy formation and evolution. © 2025 The Author(s).