We present new Hubble Space Telescope images of the gravitational lens PKS 1830–211 , which allow us to characterize the lens galaxy and update the determination of the Hubble constant ( H _ { 0 } ) from this system .
The I -band image shows that the lens galaxy is a face-on spiral galaxy with clearly delineated spiral arms .
The southwestern image of the background quasar passes through one of the spiral arms , explaining the previous detections of large quantities of molecular gas and dust in front of this image .
The lens galaxy photometry is consistent with the Tully-Fisher relation , suggesting the lens galaxy is a typical spiral galaxy for its redshift .
The lens galaxy position , which was the main source of uncertainty in previous attempts to determine H _ { 0 } , is now known precisely .
Given the current time delay measurement and assuming the lens galaxy has an isothermal mass distribution , we compute H _ { 0 } = 44 \pm 9 km s ^ { -1 } Mpc ^ { -1 } for an \Omega _ { m } = 0.3 flat cosmological model .
We describe some possible systematic errors and how to reduce them .
We also discuss the possibility raised by Courbin et al .
( 2002 ) , that what we have identified as a single lens galaxy is actually a foreground star and two separate galaxies .