The Quadruple Gravitational Lens PG 1115+080: Time Delays and Models

Abstract

Optical photometry is presented for the quadruple gravitational lens\nPG1115+080. A preliminary reduction of data taken from November 1995 to June\n1996 gives component ``C'' leading component ``B'' by 23.7+/-3.4 days and\ncomponents ``A1'' and ``A2'' by 9.4 days. A range of models has been fit to the\nimage positions, none of which gives an adequate fit. The best fitting and most\nphysically plausible of these, taking the lensing galaxy and the associated\ngroup of galaxies to be singular isothermal spheres, gives a Hubble constant of\n42 km/s/Mpc for Omega=1, with an observational uncertainty of 14%, as computed\nfrom the B-C time delay measurement. Taking the lensing galaxy to have an\napproximately E5 isothermal mass distribution yields H0=64 km/sec/Mpc while\ntaking the galaxy to be a point mass gives H0=84 km/sec/Mpc. The former gives a\nparticularly bad fit to the position of the lensing galaxy, while the latter is\ninconsistent with measurements of nearby galaxy rotation curves. Constraints on\nthese and other possible models are expected to improve with planned HST\nobservations.\n