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 M_sun 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, the centers and outer parts of
the galaxies built up at roughly the same rate between z~2.5 and z~1. We
therefore conclude that a "standard" model for the formation of spiral
galaxies, with the bulge assembling first and the disk building around it, is
probably not correct. Instead, bulges (and black holes) likely formed in
lockstep with disks, through bar instabilities, clump 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.
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