4.1. Host Galaxies
The Hubble Space Telescope (HST) has provided critical new information on the nature of the host galaxies in which quasars reside and about the nature of quasar environments. The excellent image quality of the repaired telescope gives the best combination of angular resolution and light gathering power yet applied to quasars. Here I report on two papers, which of course build on previous ground-based work.
Bahcall et al. (1997) presented results with the Wide-Field Camera of HST for 20 luminous quasars with z < 0.3. For the host galaxies, they found that 2 were as bright as the brightest cluster galaxies, 10 were like normal elliptical galaxies, 3 were normal spirals, 3 were complex, interacting systems, and in 2 cases there was faint nebulosity surrounding the quasar. For the radio-quiet quasars, 7 occurred in elliptical galaxies and 3 in spirals. For the 6 radio-loud quasars, 3 to 5 of them were in elliptical galaxies. On average, the host galaxies were 2.2 magnitudes brighter than normal field galaxies. In 8 cases, they detected companion galaxies within a projected distance of 10 kpc from the quasar nucleus. The interactions, presence of companions, and higher density of galaxies seen around quasars suggest that interactions are important to quasar activity.
Boyce et al. (1998) used HST in a complementary study of 14 low-redshift quasars. They find that 9 occur in elliptical galaxies (all 6 of the radio-loud quasars and 3 radio-quiet objects); 2 radio-quiet quasars are in disk galaxies, and the other 3, which are radio-quiet, ultraluminous IR objects, occur in violently interacting systems. The average luminosity of the quasar host galaxies is 0.8 magnitudes brighter than L*, while the radio-loud objects are 0.7 magnitudes brighter than the radio-quiet ones.
It is evident, as Bahcall et al. point out, that the hosts and environments of quasars are complex, and that the previous ideas about radio-quiet quasars residing in spiral galaxies and radio-loud quasars in ellipticals may not hold up. However, it is perhaps more important to realize that the HST observations provide powerful support for the concept of quasars residing at the centers of galaxies and that galaxy interactions play an important role in quasar activity.