Through a lens darkly: Evidence for dusty gravitational lenses

Foreground galaxies that amplify the light from background quasars may also dim that light if the galaxies contain enough dust. Extinction by dust in lenses could hide the large number of lensed systems predicted for a flat universe with a large value of the cosmological constant Lambda. We look for one signature of dust, namely reddening, by examining optical-infrared colours of gravitationally lensed images of quasars. We find that the lensed systems identified in radio and infrared searches have redder optical-infrared colours than optically selected ones. This could be due to a bias against selecting reddened (hence dimmed) quasars in the optical surveys, or due to the differences in the intrinsic colours of optical and radio quasars, Comparison of the radio-selected lensed and unlensed quasars shows that the lensed ones have redder colours. We therefore conclude that at least part of the colour difference between the two lens samples is due to dust. From the colour difference between lensed and unlensed radio quasars (and assuming the Galactic extinction law) we can reconcile a large cosmological constant (Lambda = 0.9) with the number of lensed systems observed in flux-limited optical surveys. These results substantially weaken the strongest constraint on cosmological scenarios that invoke a non-zero cosmological constant to explain age discrepancy problems, satisfy predictions of inflationary models of the early Universe and play a role in large-scale structure formation models, They also raise the prospect of using gravitational lenses to study the interstellar medium in high-redshift galaxies.