  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 massive galaxies with logM*>11. 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.
