HUBBLE-SPACE-TELESCOPE IMAGES OF A SAMPLE OF 20 NEARBY LUMINOUS QUASARS

Observations with the Wide-Field Camera of the Hubble Space Telescope (HST) are presented for a representative sample of 20 intrinsically lu minous quasars with redshifts smaller than 0.30. These observations sh ow that luminous quasars occur in diverse environments that include el lipticals as bright as the brightest cluster galaxies (two), apparentl y normal ellipticals (10), apparently normal spirals with H II regions (three), complex systems of gravitationally interacting components (t hree), and faint surrounding nebulosity (two). The quasar host galaxie s are centered on the quasar to the accuracy of our measurements, 400 pc. There are more radio-quiet quasars in galaxies that appear to be e llipticals (seven) than in spiral hosts (three), contrary to expectati ons. However, three, and possibly five, of the six radio-loud quasars have detectable elliptical hosts, in agreement with expectations. The luminous quasars studied in this paper occur preferentially in luminou s galaxies. The average absolute magnitude of the hosts is 2.2 mag bri ghter than expected for a field galaxy luminosity function. The superb optical characteristics of the repaired HST make possible the detecti on of close galactic companions; we detect eight companion galaxies wi thin projected distances of 10 kpc from quasar nuclei. The presence of very close companions, the images of current gravitational interactio ns, and the higher density of galaxies around the quasars suggest that gravitational interactions play an important role in triggering the q uasar phenomenon.