1608 + 656: A GRAVITATIONALLY LENSED POSTSTARBURST RADIO GALAXY

摘要

The gravitational lens system 1608 + 656 displays four flat-spectrum, pointlike components that are the images of the unresolved core of a double-;obed radio source. The lensing mass is a galaxy at z = 0.630. New spectra of this system enable us to determine a conclusive redshift of 1.394 for the lensed object. The spectra show prominent high-order Balmer absorption lines and Mg Ⅱ absorption. These lines, and the absence of [O Ⅱ] emission, indicate that this is a poststarburst or E + A galaxy. It is unique among lensed objects in not being a quasar and among E + A galaxies in having the highest known redshift. Even allowing for lens magnification, the lensed object is a very luminous galaxy, with an absolute magnitude, M(r) = -22.8 mag. The deconvolved infrared image indicates that the galaxy may be slightly resolved. The radio luminosity density of the lobes is L_(1.4) = 5.78 x 10~(25) W Hz~(-1), which puts the source on the boundary between FR Ⅰ and FR Ⅱ radio galaxies. Together with the redshift for the lens and a satisfactory mass model, the determination of the lensed object redshift makes this system an excellent candidate for measuring H_0.