Why does the Milky Way have a bar?

Abstract

ABSTRACT There is no doubt that the Milky Way is a barred galaxy; however, factors that establish its prominent morphology remain largely elusive. In this work, we attempt to constrain the history of the MW by tracing the present-day parameters and evolution of a set of MW and M31 analogues from the TNG50 simulations. We find that the strength of bars at $z=0$ correlates well not only with the stellar mass build-up but also, more crucially, with the time of onset of stellar discs. Discs of strongly barred galaxies form early ($z \gtrsim 2-3$), compared to weakly and non-barred galaxies ($z \approx 1-1.5$). Although we are cautious to draw ultimate conclusions about the governing factor of discs formation due to the complexity and correlations between different phenomena, the observed morphological diversity can be tentatively explained by a substantial variation in the gas angular momentum around proto-galaxies already at $z\approx 3-5$; in such a way, early discs formed from gas with larger angular momentum. By comparing the formation time-scales of discs of barred galaxies in the TNG50 sample, we infer that the MW has a strong bar ($0.35\lt A_2\lt 0.6$), and that its stellar disc started to dominate over the spheroidal component already at $z \approx 2$, with a mass of $\approx 1 \pm 0.5 \times 10^{10} M_\odot$. We conclude that the presence of a strong bar in the MW is a natural manifestation of the early formation of the stellar disc, which made possible bursty but highly efficient star formation at high redshift.