Revealing the complex nature of the strong gravitationally lensed system H-ATLAS J090311.6+003906 using ALMA

TL;DR

This study uses ALMA imaging to analyze a high-redshift gravitational lens system, revealing a complex, clumpy dust distribution, a rotating gas disk, and signs of a merger-induced starburst in an infrared-luminous galaxy.

Contribution

It provides detailed kinematic and morphological modeling of the lensed system, highlighting the galaxy's dynamic state and star formation activity at high redshift.

Findings

Clumpy dust distribution on 200 pc scales.

Smooth, rotating gas disk with 320 km/s velocity.

High star formation rate of 470 solar masses per year.

Abstract

We have modelled Atacama Large Millimeter/sub-millimeter Array (ALMA) long baseline imaging of the strong gravitational lens system H-ATLAS J090311.6+003906 (SDP.81). We have reconstructed the distribution of band 6 and 7 continuum emission in the z=3.042 source and we have determined its kinematic properties by reconstructing CO(5-4) and CO(8-7) line emission in bands 4 and 6. The continuum imaging reveals a highly non-uniform distribution of dust with clumps on scales of 200 pc. In contrast, the CO line emission shows a relatively smooth, disk-like velocity field which is well fit by a rotating disk model with an inclination angle of 40+/-5 degrees and an asymptotic rotation velocity of 320 km/s. The inferred dynamical mass within 1.5kpc is (3.5+/-0.5)x10^{10} M_sol which is comparable to the total molecular gas masses of (2.7+/-0.5)x10^{10} M_sol and (3.5+/-0.6)x10^{10} M_sol from the dust continuum emission and CO emission respectively. Our new reconstruction of the lensed HST near-infrared emission shows two objects which appear to be interacting, with the rotating disk of gas and dust revealed by ALMA distinctly offset from the near-infrared emission. The clumpy nature of the dust and a low value of the Toomre parameter of Q=0.3 suggest that the disk is in a state of collapse. We estimate a star formation rate in the disk of 470+/-80 M_sol/yr with an efficiency 65 times greater than typical low-redshift galaxies. Our findings add to the growing body of evidence that the most infra-red luminous, dust obscured galaxies in the high redshift Universe represent a population of merger induced starbursts.