Observations of host galaxies

The earliest discovered quasars, in the 1960s, were so bright that their optical images showed no signs of the host galaxies. This resulted in the names quasistellar objects (QSOs) and quasars and caused a lot of confusion and even some unusual explanations and models. Interestingly, the much earlier discovery of the first Seyfert galaxies, by Seyfert in 1943, raised no such questions. The luminosity of the central sources in these galaxies was about 2 orders of magnitude below the luminosity of the first discovered quasars, and the galaxy was clearly seen in all cases.

The host galaxies of the earlier quasars were soon discovered by ground-based telescopes in sites with good seeing conditions. Faint nebulosities were discovered in all objects with redshift less than about 0.5, where conditions allowed such detection. The launch of the Hubble Space Telescope (HST), in 1990, resulted in superb resolution observations and the extension of such studies to redshifts 2 and 3. Some of the information was detailed enough to enable a systematic study of the morphology, color, and even stellar population of the hosts. Such information is now available for numerous low-redshift AGNs where the study is made easier because of the lower luminosity of low-z AGNs and the vast improvement in the performance of adaptive optics (AO) systems on giant ground-based telescopes. Similar observations of type-II AGNs, where the optical–UV radiation of the central source is completely obscured, allowed us to extend such studies to more sources and to higher redshift. Figure 8.1 shows several examples of the hosts of luminous type-I AGNs observed with the HST. Several such hosts show clear signs of distortion and interaction.