High-resolution ALMA observations of SDP.81. II. Molecular clump properties of a lensed submillimeter galaxy at z = 3.042

Abstract

We present spatially resolved properties of molecular gas and dust in a gravitationally lensed submillimeter galaxy H-ATLAS J090311.6+003906 (SDP.81) at z = 3.042 revealed by the Atacama Large Millimeter/submillimeter Array (ALMA). We identified 14 molecular clumps in the CO(5-4) line data. The surface density of molecular gas (ΣH2) and star-formation rate (ΣSFR) of the clumps are more than three orders of magnitude higher than those found in local spiral galaxies. The clumps are placed in the "burst" sequence in the ΣH2-ΣSFR plane, suggesting that z∼3 molecular clumps follow the star-formation law derived for local starburst galaxies. With our gravitational lens model, the positions in the source plane are derived for the molecular clumps, dust clumps, and stellar components identified in the Hubble Space Telescope image. The molecular and dust clumps are confined within a ∼ 2 kpc region, while the spatial extent of the stellar components is as large as ∼ 6 kpc and offset toward the west. The molecular clumps have a systematic velocity gradient in the north-south direction, which may indicate a rotating gas disk. One possible scenario is that the components of molecular gas, dust, and stars are distributed in a several-kpc-scale rotating disk, and the stellar emission is heavily obscured by dust in the central star-forming region. Alternatively, SDP.81 can be explained by a merging system, where dusty starbursts occur in the region where the two galaxies collide, surrounded by tidal features traced in the stellar components.