Formation and evolution of boxy/peanut bulges in the Auriga cosmological simulations

López, PD, Fragkoudi, F, Cora, SA, Scannapieco, C, Pakmor, R, Grand, RJJ, Gómez, F and Marinacci, F (2025) Formation and evolution of boxy/peanut bulges in the Auriga cosmological simulations. Monthly Notices of the Royal Astronomical Society, 540 (3). pp. 2031-2048. ISSN 0035-8711

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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 magneto-hydrodynamical cosmological zoom-in simulations, we characterise 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 $\sim 1.1-1.6\, \rm Gyr$ 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, 45percnt 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 20percnt 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.

Item Type:	Article
Uncontrolled Keywords:	5101 Astronomical Sciences; 51 Physical Sciences; 0201 Astronomical and Space Sciences; Astronomy & Astrophysics; 5101 Astronomical sciences; 5107 Particle and high energy physics; 5109 Space sciences
Subjects:	Q Science > QB Astronomy Q Science > QC Physics
Divisions:	Astrophysics Research Institute
Publisher:	Oxford University Press (OUP)
Date of acceptance:	19 May 2025
Date of first compliant Open Access:	9 June 2025
Date Deposited:	09 Jun 2025 12:12
Last Modified:	09 Jun 2025 12:15
DOI or ID number:	10.1093/mnras/staf818
URI:	https://researchonline.ljmu.ac.uk/id/eprint/26555

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