Evolution of Density Profiles in High-z Galaxies: Compaction and
  Quenching Inside-Out
  Using cosmological simulations, we address the interplay between structure
and star formation in high-redshift galaxies via the evolution of surface
density profiles. Our sample consists of 26 galaxies evolving in the redshift
range $z=7-1$, spanning the stellar mass range $(0.2-6.4)\times 10^{10}M_\odot$
at $z=2$. We recover the main trends by stacking the profiles in accordance to
their evolution phases. Following a wet compaction event that typically occurs
when the stellar mass is $\sim10^{9.5}~M_{\odot}$ at $z\sim2-4$, the gas
develops a cusp inside the effective radius, associated with a peak in
star-formation rate (SFR). The SFR peak and the associated feedback, in the
absence of further gas inflow to the centre, marks the onset of gas depletion
from the central 1 kpc, leading to quenching of the central SFR. An extended,
star-forming ring that forms by fresh gas during the central quenching process
shows as a rising specific SFR (sSFR) profile, which is interpreted as
inside-out quenching. Before quenching, the stellar density profile grows
self-similarly, maintaining its log-log shape because the sSFR is similar at
all radii. During the quenching process, the stellar density saturates to a
constant value, especially in the inner 1 kpc. The stellar mass and SFR
profiles deduced from observations show very similar shapes, consistent with
the scenario of wet compaction leading to inside-out quenching and the
subsequent saturation of a dense stellar core. We predict a cuspy gas profile
during the blue nugget phase, and a gas-depleted core, sometimes surrounded by
a ring, in the post-blue nugget phase.
