Abstract
We present 0".2-resolution Atacama Large Millimeter/submillimeter Array
observations at 870 um for 25 Halpha-seleced star-forming galaxies (SFGs)
around the main-sequence at z=2.2-2.5. We detect significant 870 um continuum
emission in 16 (64%) of these SFGs. The high-resolution maps reveal that the
dust emission is mostly radiated from a single region close to the galaxy
center. Exploiting the visibility data taken over a wide $uv$ distance range,
we measure the half-light radii of the rest-frame far-infrared emission for the
best sample of 12 SFGs. We find nine galaxies to be associated with extremely
compact dust emission with R_1/2,870um<1.5 kpc, which is more than a factor
of 2 smaller than their rest-optical sizes, R_1/2,1.6um=3.2 kpc, and is
comparable with optical sizes of massive quiescent galaxies at similar
redshifts. As they have an exponential disk with Sersic index of n=1.2 in the
rest-optical, they are likely to be in the transition phase from extended disks
to compact spheroids. Given their high star formation rate surface densities
within the central 1 kpc of Sigma SFR1kpc=40 Msol/yr/kpc^2, the intense
circumnuclear starbursts can rapidly build up a central bulge with Sigma
M*1kpc>1e10 Msol/kpc^2 in several hundred Myr, i.e. by z~2. Moreover, ionized
gas kinematics reveal that they are rotation-supported with an angular momentum
as large as that of typical SFGs at z=1-3. Our results suggest bulges are
commonly formed in extended rotating disks by internal processes, not involving
major mergers.
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