Through a Lens Darkly : Evidence for Dusty

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 at universe with a large value of the cosmological constant. We look for one signature of dust, namely reddening, by examining optical-infrared colors of gravitationally lensed images of quasars. We nd that the lensed systems identiied in radio and infrared searches have redder optical-IR colors than optically selected ones. This could be due to a bias against selecting reddened (hence extincted) quasars in the optical surveys, or due to the diierences in the intrinsic colors of optical and radio quasars. Comparison of the radio-selected lensed and unlensed quasars shows that the lensed ones have redder colors. We therefore conclude that at least part of the color diierence between the two lens samples is due to dust. From the color diierence between lensed and unlensed radio quasars (and assuming Galactic extinction law) we can reconcile a large cosmological constant ((= 0:9) with the number of lensed systems observed in ux 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 innationary 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.