Host Galaxies

When they were first discovered, quasars appeared to be point sources like stars. Eventually, careful observation showed jets and faint nebulosity associated with them. With modern advances in observation, it is now possible to resolve in some cases a galaxy where the quasar appears to reside. These are called host galaxies. The observation of host galaxies for quasars suggests strongly that they are a natural property of at least some galaxies for some portion of their lives. The following images show Hubble Space Telescope images of quasars and their host galaxies. These images not only show host galaxies, but indicate that in some cases the quasar is involved in collisions and mergers of galaxies (click on "Show Labels" to annotate).

These six images (a)-(f) have been interpreted in the following way (you may find it useful to click on the above image to launch it in a separate resizable window to make comparison with the following points easier):

(a) This image of quasar PG 0052+251 at a distance of 1.4 billion light years appears to show a quasar in the center of an undisturbed spiral galaxy.

(b) Quasar IRAS04505-2958, 3 billion light years from Earth, appears to be fueled by the collision of two galaxies. The quasar is at the center of the image (the bright object at the top is a foreground star). It is thought that the bright material at the bottom is part of a colliding galaxy and the rest of it lies in front of the quasar, surrounded by bright star-forming regions.

(c) Quasar 0316-346, 2.2 billion light years away, appears to have captured a "tidal tail" (above the bright central region), probably from a collision with another galaxy not in the image.

(d) Quasar PHL 909 is 1.5 billion light years from Earth and appears to reside in a normal-looking elliptical galaxy.

(e) Quasar PG 1012+008, at a distance of 1.6 billion light years, is at the center of the image. It appears to be merging with a bright galaxy that lies just below it in the image, about 31,000 light years from the quasar. The swirls of gas and dust around the pair suggest that they are interacting strongly. The compact galaxy to the left of the quasar may also be beginning to merge with the quasar.

(f) The quasar in the center is IRAS13218+0552, 2 billion light years from Earth. The image suggests that it resides in the center of merging galaxies which may have orbited each other several times before merging, leaving the loops of glowing gas around the quasar. The elongated diagonal brighter region near the center may be the cores of the two merging galaxies.

The four images on the right support the theory that will be advanced later that quasars and other active galaxies are powered by supermassive black holes in the cores of galaxies. According to this idea, most galaxies (including our own) have such black holes, but they only become quasars when the galaxies are disturbed in some way that causes mass to pour into the black hole. However, the observation from the two images on the left that quasars appear to be able to exist in the centers of both normal elliptical and spiral galaxies without significantly disturbing the rest of the galaxy suggests that there may be other mechanisms to turn on the quasar. As in most advances in observation, the data raise new questions as they answer old ones!