Here are some of the highlights from my 1st first-author paper which was focused on studying the formation times, and building blocks, of progenitors to Milky Way-like galaxies in the cosmological zoom-in baryonic simulations from the FIRE project:
By looking into when a galaxy's stellar mass growth transitions from being primarily merger-supported, to self-supported (in-situ growth), and by looking into when the most massive galaxy in a system becomes the most dominant component, we can define that the progenitor to the main galaxy "formed".
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With these definitions, galaxies like the Milky Way formed by z ~ 3.4 (11.9 Gyr ago); see top right figure. When a galaxy is able to form more than half of its own stars than it gains from mergers, or when the curves cross the horizontal dotted line at 0.5, the progenitor "forms".
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Galaxies in paired systems (similar to the Milky Way and Andromeda pair) form earlier than isolated galaxies, implying environment may play a role in galaxy formation. In the top right figure, paired galaxies are represented as dotted curves. They cross a fraction of 0.5 before the isolated galaxies (in the solid curves) and hence form earlier!
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Nearly 100 dwarf galaxies helped "build" the main galaxy, about 4 - 5 times as many dwarf galaxies as there are at present-day. The bottom right figure shows how many galaxies, above a given mass, merge into the virial halo of the galaxy, as a function of redshift (time).
