Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Zhiyuan Ji, Christina C. Williams, Sandro Tacchella, Katherine A. Suess, William M. Baker
| Summary:
We present a spatially resolved study of stellar populations in 6 galaxies
with stellar masses $M_*sim10^{10}M_odot$ at $zsim3.7$ using 14-filter
JWST/NIRCam imaging from the JADES and JEMS surveys. The 6 galaxies are
visually selected to have clumpy substructures with distinct colors over
rest-frame $3600-4100r{A}$, including a red, dominant stellar core that is
close to their stellar-light centroids. With 23-filter photometry from HST to
JWST, we measure the stellar-population properties of individual structural
components via SED fitting using Prospector. We find that the central stellar
cores are $gtrsim2$ times more massive than the Toomre mass, indicating they
may not form via single in-situ fragmentation. The stellar cores have stellar
ages of $0.4-0.7$ Gyr that are similar to the timescale of clump inward
migration due to dynamical friction, suggesting that they likely instead formed
through the coalescence of giant stellar clumps. While they have not yet
quenched, the 6 galaxies are below the star-forming main sequence by $0.2-0.7$
dex. Within each galaxy, we find that the specific star formation rate is lower
in the central stellar core, and the stellar-mass surface density of the core
is already similar to quenched galaxies of the same masses and redshifts.
Meanwhile, the stellar ages of the cores are either comparable to or younger
than the extended, smooth parts of the galaxies. Our findings are consistent
with model predictions of the gas-rich compaction scenario for the buildup of
galaxies’ central regions at high redshifts. We are likely witnessing the
coeval formation of dense central cores, along with the onset of galaxy-wide
quenching at $z>3$.
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