The Assembly of Milky Way-like Galaxies Since z~2.5

TL;DR

This study traces the assembly history of Milky Way-like galaxies since redshift 2.5, revealing significant early stellar mass buildup, central mass growth, and a combined bulge-disk formation process contrary to simple high-redshift bulge assembly models.

Contribution

It provides new insights into the simultaneous formation of bulges and disks in Milky Way-like galaxies through observational data up to z=2.5.

Findings

90% of stellar mass formed before z=1

Central 2 kpc mass increased by a factor of 3.2 from z=2.5 to 1

Bulge and disk formed in parallel, not sequentially

Abstract

Galaxies with the mass of the Milky Way dominate the stellar mass density of the Universe but it is uncertain how and when they were assembled. Here we study progenitors of these galaxies out to z=2.5, using data from the 3D-HST and CANDELS Treasury surveys. We find that galaxies with present-day stellar masses of log(M)~10.7 built ~90% of their stellar mass since z=2.5, with most of the star formation occurring before z=1. In marked contrast to the assembly history of massive elliptical galaxies, mass growth is not limited to large radii: the mass in the central 2 kpc of the galaxies increased by a factor of 3.2+-0.8 between z=2.5 and z=1. We therefore rule out simple models in which bulges were fully assembled at high redshift and disks gradually formed around them. Instead, bulges (and black holes) likely formed in lockstep with disks, through bar instabilities, migration, or other processes. We find that after z=1 the growth in the central regions gradually stopped and the disk continued to build, consistent with recent studies of the gas distributions in z~1 galaxies and the properties of many spiral galaxies today.