We present 0 ′′ .2-resolution Atacama Large Millimeter/submillimeter Array observations at 870 µm for 25 Hα-seleced star-forming galaxies around the main-sequence at z = 2.2 − 2.5. We detect significant 870 µm continuum emission in 16 (64%) of these galaxies. 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 log(M * /M ⊙ ) >11. We find nine galaxies to be associated with extremely compact dust emission with R 1/2,870µm < 1.5 kpc, which is more than a factor of 2 smaller than their rest-optical sizes, R 1/2,1.6µm =3.2 kpc, and is comparable with optical sizes of massive quiescent galaxies at similar redshifts. As they have an exponential disk with Sérsic index of n 1.6µm =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 ΣSFR 1kpc = 40 M ⊙ yr −1 kpc −2 , the intense circumnuclear starbursts can rapidly build up a central bulge with ΣM * ,1kpc > 10 10 M ⊙ 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 star-forming galaxies at z = 1 − 3. Our results suggest bulges are commonly formed in extended rotating disks by internal processes, not involving major mergers.

We present 0 2 resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations at 870 μm in a stellar mass-selected sample of 85 massive ( >  1kpc 10.5☉ , which is around the boundary between massive SFGs and QGs. These results suggest an outside-in transformation scenario in which a dense core is formed at the center of a more extended disk, likely via dissipative in-disk inflows. Synchronized observations at ALMA 870 μm and James Webb Space Telescope 3-4 μm will explicitly verify this scenario.

We present molecular gas reservoirs of eighteen galaxies associated with the XMMXCS J2215.9-1738 cluster at z = 1.46. From Band 7 and Band 3 data of the Atacama Large Millimeter/submillimeter Array (ALMA), we detect dust continuum emission at 870 µm and CO J = 2-1 emission line from 8 and 17 member galaxies respectively within a cluster-centric radius of R 200 . The molecular gas masses derived from the CO and/or dust continuum luminosities show that the fraction of molecular gas mass and the depletion time scale for the cluster galaxies are larger than expected from the scaling relations of molecular gas on stellar mass and offset from the main sequence of star-forming galaxies in general fields. The galaxies closer to the cluster center in terms of both projected position and accretion phase seem to show a larger deviation from the scaling relations. We speculate that the environment of galaxy cluster helps feed the gas through inflow to the member galaxies and also reduce the efficiency of star formation. The stacked Band 3 spectrum of 12 quiescent galaxies with M stellar ∼ 10 11 M within 0.5R 200 shows no detection of CO emission line, giving the upper limit of molecular gas mass and molecular gas fraction to be 10 10 M and 10%, respectively. Therefore, the massive galaxies in the cluster core quench the star formation activity while consuming most of the gas reservoirs.

In the redshift interval of 2 < z < 3, the physical conditions of the inter-stellar medium (ISM) in star-forming galaxies are likely to be different from those in the local Universe because of lower gaseous metallicities, higher gas fractions, and higher star formation activities. In fact, observations suggest that higher electron densities, higher ionization parameters, and harder UV radiation fields are common.In this paper, based on the spectra of Hα-selected star-forming galaxies at z = 2.5 taken with Multi-Object Spectrometer for InfraRed Exploration (MOSFIRE) on Keck-1 telescope, we measure electron densities (n e ) using the oxygen line ratio ( [Oii]λλ3726,3729), and investigate the relationships between the electron density of ionized gas and other physical properties. As a result, we find that the specific star formation rate (sSFR) and the surface density of SFR (Σ SFR ) are correlated with the electron density at z = 2.5 for the first time. The Σ SFR -n e relation is likely to be linked to the star formation law in Hii regions (where star formation activity is regulated by interstellar pressure). Moreover, we discuss the mode of star formation in those galaxies. The correlation between sSFR and Σ SFR suggests that highly star-forming galaxies (with high sSFR) tend to be characterized by higher surface densities of star formation (Σ SFR ) and thus higher n e values as well.

We present spatially resolved ALMA observations of the CO J = 3 − 2 emission line in two massive galaxies at z = 2.5 on the star-forming main sequence. Both galaxies have compact dusty star-forming cores with effective radii of R e = 1.3±0.1 kpc and R e = 1.2±0.1 kpc in the 870 µm continuum emission. The spatial extent of star-forming molecular gas is also compact with R e = 1.9 ± 0.4 kpc and R e = 2.3 ± 0.4 kpc, but more extended than the dust emission. Interpreting the observed position-velocity diagrams with dynamical models, we find the starburst cores to be rotation-dominated with the ratio of the maximum rotation velocity to the local velocity dispersion of. Given that the descendants of these massive galaxies in the local universe are likely ellipticals with v/σ nearly an order of magnitude lower, the rapidly rotating galaxies would lose significant net angular momentum in the intervening time. The comparisons among dynamical, stellar, gas, and dust mass suggest that the starburst COto-H 2 conversion factor of α CO = 0.8 M ⊙ (K km s −1 pc −2 ) −1 is appropriate in the spatially resolved cores. The dense cores are likely to be formed in extreme environments similar to the central regions of local ultraluminous infrared galaxies. Our work also demonstrates that a combination of mediumresolution CO and high-resolution dust continuum observations is a powerful tool for characterizing the dynamical state of molecular gas in distant galaxies.