PKS 1830-211: A Face-On Spiral Galaxy Lens

摘要

We present new Hubble Space Telescope images of the gravitational lens PKS1830-211, which allow us to characterize the lens galaxy and update thedetermination of the Hubble constant from this system. The I-band image showsthat the lens galaxy is a face-on spiral galaxy with clearly delineated spiralarms. The southwestern image of the background quasar passes through one of thespiral arms, explaining the previous detections of large quantities ofmolecular gas and dust in front of this image. The lens galaxy photometry isconsistent with the Tully-Fisher relation, suggesting the lens galaxy is atypical spiral galaxy for its redshift. The lens galaxy position, which was themain source of uncertainty in previous attempts to determine H_0, is now knownprecisely. Given the current time delay measurement and assuming the lensgalaxy has an isothermal mass distribution, we compute H_0 = 44 +/- 9 km/s/Mpcfor an Omega_m = 0.3 flat cosmological model. We describe some possiblesystematic errors and how to reduce them. We also discuss the possibilityraised by Courbin et al. (2002), that what we have identified as a single lensgalaxy is actually a foreground star and two separate galaxies.