A diversity of progenitors and histories for isolated spiral galaxies

TL;DR

This study uses 33 cosmological simulations to explore the diverse evolutionary paths and progenitor morphologies of Milky Way-like galaxies, revealing complex formation histories that influence their present-day structures.

Contribution

It provides a comprehensive analysis of galaxy evolution, highlighting the varied progenitor morphologies and the impact of merger and gas accretion histories on galaxy final types.

Findings

Progenitors of disk galaxies show diverse morphologies at z=1.

No strong correlation between z=1 and z=0 morphologies.

Low B/T galaxies have quiet merger and gas accretion histories.

Abstract

We analyze a suite of 33 cosmological simulations of the evolution of Milky Way-mass galaxies in low-density environments. Our sample spans a broad range of Hubble types at z=0, from nearly bulgeless disks to bulge-dominated galaxies. Despite the fact that a large fraction of the bulge is typically in place by z=1, we find no significant correlation between the morphology at z=1 and at z=0. The z=1 progenitors of disk galaxies span a range of morphologies, including smooth disks, unstable disks, interacting galaxies and bulge-dominated systems. By z=0.5, spiral arms and bars are largely in place and the progenitor morphology is correlated with the final morphology. We next focus on late-type galaxies with a bulge-to-total ratio B/T<0.3 at z=0. These show a correlation between B/T at z=0 and the mass ratio of the largest merger at z<2, as well as with the gas accretion rate at z>1. We find that the galaxies with the lowest B/T tend to have a quiet baryon input history, with no major mergers at z<2, and with a low and constant gas accretion rate that keeps a stable angular-momentum direction. More violent merger or gas accretion histories lead to galaxies with more prominent bulges. Most disk galaxies have a bulge Sersic index n<2. The galaxies with the highest bulge Sersic index tend to have histories of intense gas accretion and disk instability rather than active mergers.