ALMA Observations of the Gravitational Lens SDP.9

We present long-baseline ALMA observations of the strong gravitational lens H-ATLAS J090740.0−004200 (SDP.9), which consists of an elliptical galaxy at $z_L=0.6129$ lensing a background submillimeter galaxy into two extended arcs. The data include Band 6 continuum observations, as well as CO J = 6−5 molecular line observations, from which we measure an updated source redshift of $z_S=1.5747$. The image morphology in the ALMA data is different from that of the Hubble Space Telescope (HST) data, indicating a spatial offset between the stellar, gas, and dust component of the source galaxy. We model the lens as an elliptical power law density profile with external shear using a combination of archival HST data and conjugate points identified in the ALMA data. Our best model has an Einstein radius of ${\theta }_E=0.66\pm 0.01$ and a slightly steeper than isothermal mass profile slope. We search for the central image of the lens, which can be used constrain the inner mass distribution of the lens galaxy including the central supermassive black hole, but do not detect it in the integrated CO image at a 3σ rms level of 0.0471 Jy km s⁻¹.