Chemodynamics of barred galaxies in cosmological simulations: On the Milky Way's quiescent merger history and in-situ bulge

TL;DR

This study uses cosmological simulations to show that the Milky Way's quiescent merger history has led to an in-situ bulge and that bars significantly influence disc chemodynamics, with implications for understanding galaxy evolution.

Contribution

It demonstrates that Milky Way-like galaxies with quiescent histories produce bulges with chemodynamical properties similar to the MW, highlighting the role of bars and merger history.

Findings

Milky Way analogues have quiescent merger histories since z~3.5.

Ex-situ stars in bulges are less than 1%.

Bar-induced resonances create metal-rich ridges in the disc.

Abstract

We explore the chemodynamical properties of a sample of barred galaxies in the Auriga magneto-hydrodynamical cosmological zoom-in simulations, which form boxy/peanut (b/p) bulges, and compare these to the Milky Way (MW). We show that the Auriga galaxies which best reproduce the chemodynamical properties of stellar populations in the MW bulge have quiescent merger histories since redshift $z\sim3.5$: their last major merger occurs at $t_{\rm lookback}>12\,\rm Gyrs$, while subsequent mergers have a stellar mass ratio of $\leq$1:20, suggesting an upper limit of a few percent for the mass ratio of the recently proposed Gaia Sausage/Enceladus merger. These Auriga MW-analogues have a negligible fraction of ex-situ stars in the b/p region ($<1\%$), with flattened, thick disc-like metal-poor stellar populations. The average fraction of ex-situ stars in the central regions of all Auriga galaxies with b/p's is 3% -- significantly lower than in those which do not host a b/p or a bar. While the central regions of these barred galaxies contain the oldest populations, they also have stars younger than 5Gyrs (>30%) and exhibit X-shaped age and abundance distributions. Examining the discs in our sample, we find that in some cases a star-forming ring forms around the bar, which alters the metallicity of the inner regions of the galaxy. Further out in the disc, bar-induced resonances lead to metal-rich ridges in the $V_{\phi}-r$ plane -- the longest of which is due to the Outer Lindblad Resonance. Our results suggest the Milky Way has an uncommonly quiet merger history, which leads to an essentially in-situ bulge, and highlight the significant effects the bar can have on the surrounding disc.