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tokens: with high stellar mass surface densities (see next section). Figure 5. Comparison of circularized half-light radii at ALMA/870 μ m (red circles) with those at HST / H 160 band (cyan squares) for our sample of 12 galaxies with size measurements at 870 μ m. Gray circles indicate the rest-optical sizes for star-forming galaxies around the main sequence at z = 1.9–2.7, which are drawn from the 3D- HST survey. A dashed line shows their fitting function, . Table 2 Galaxy Properties for 12 SFGs with 870 μ m Size Measurements Notes. a Sérsic indices and half-light radii at 1.6 μ m. We do not use U4-16795 because the best-fit Sérsic index reaches the constrained limit of n = 8. b Half-light radii at 870 μ m. c Stellar mass surface density within a central 1 kpc calculated in stellar mass maps. d SFR surface density within a central 1 kpc calculated from the best-fit exponential models at 870 μ m and total SFRs. e Bulge formation timescales to reach the stellar mass surface density of log(Σ M bulge / M ⊙ kpc −2 ) = 10 (Equation ( 3 )). f Gas depletion timescales by star formation and outflows (Equation ( 4 )). In the analysis of size measurements, we do not include two massive star-forming galaxies with log( M * / M ⊙ ) > 11 in the parent sample of galaxies identified by the narrowband survey. One is not observed with ALMA, and the other one (U4-36568) is not detected in the high-resolution map (Figure 1 ). Given the high completeness of 86% (12/14) in the stellar mass range, our results are not significantly affected by the sample selection. Therefore, we find massive galaxies to commonly form stars in the extremely compact central region as at least 64% (9/14) have small 870 μ m sizes of R 1/2,870 μ m 1.5 kpc. This result is in excellent agreement with similar and independent evidence coming from an ALMA/870 μ m study of six massive star-forming galaxies at z ∼ 2 (Barro et al. 2016 ). Barro et al. ( 2016 ) find that the mean half-SFR radius is ∼30% smaller than the mean half-mass radius. The main difference between our work and Barro et al. ( 2016 ) is that they preselect only optically compact star-forming galaxies while our study almost completely selects main-sequence galaxies. 4. BULGE FORMATION IN EXTENDED, ROTATING DISKS Given that our ALMA sample is already massive, , they are likely to soon thereafter quench the active star formation and to be observed as quiescent galaxies in the local universe. Quiescent galaxies are always smaller than star-forming galaxies at any redshift and any stellar mass and have a cusp profile ( n > 2) unlike star-forming galaxies with exponential disks (Wuyts et al. 2011b ; van der Wel et al. 2014b ). A spatial distribution of stars within galaxies would not be changed unless a violent process like major mergers happens. However, centrally concentrated star formation reduces the half-light or half-stellar-mass radii of galaxies, and their Sérsic index would increase by central bulge formation. We quantitatively