The Extragalactic Lens VLBI Imaging Survey (ELVIS). I. A Search for the Central Image in the Gravitational Lens PMN J1838−3427

The Extragalactic Lens VLBI Imaging Survey (ELVIS) searches for centralimages of lensed radio quasars, in order to measure the central densityprofiles of distant galaxies. Here we present sensitive multi-epoch Very LongBaseline Array (VLBA) observations of PMN J1838-3427 at 8 GHz, with a 1 sigmanoise level of 38 microJy beam^(-1). Based on the absence of a central image ofthe background source at this level, we explore the possibilities for thecentral matter distribution in the lens galaxy. A power-law density profile,rho ~ r^(-gamma), must have gamma > 1.93. Thus the density profile is close toan isothermal profile (gamma = 2) or steeper. The upper limit on anyconstant-density core in an otherwise isothermal profile is ~< 5 parsecs. Wealso derive the constraints on models in which the density profile isisothermal on kiloparsec scales, but is allowed to have a different power lawin the central ~100 parsecs. If the lens galaxy harbors a supermassive blackhole, the galaxy profile is allowed to be shallower, but for the expected blackhole mass the galaxy profile must still be close to isothermal or steeper.Comment: accepted to ApJ. 8 pages, 6 figures, 3 tables, uses \emulateap