Cosmic Evolution
Giant galaxies and quasars, with individual light outputs equivalent to many thousands of our Milky Way Galaxy, were dominant several billion years after the Big Bang when the universe was about a quarter its present age. Quasars are powered by the release of gravitational energy as gas falls into "supermassive" black holes at the centers of the host galaxies. These black holes are millions to billions of times the mass of our Sun. Gas funneling into the black holes through giant, spinning "accretion" disks contributes to their ongoing growth. Quasars that have exhausted their fuel supply and are no longer active have been recently detected in the local universe via the orbital motions of stars and gas near the galactic centers. The relationship between these black holes and the star formation taking place in their host galaxies is a key area of exploration.
By the present time, these dinosaurs of the universe have almost all died out, leaving a local universe that is filled with more numerous smaller galaxies and ``dead'' quasars. The reason for this "cosmic downsizing" is a major mystery, one which can only be solved by observing the time-history of galaxy and supermassive black hole evolution.
Many active quasars in the distant universe are hidden from optical view by gas and dust. However, after the launch in late 1999 of one of NASA's Great Observatories, the Chandra X-ray Observatory, it became possible to detect high-energy X-rays emitted in the supermassive black hole accretion process, even when the black holes are highly obscured.
UW-Madison researchers made important optical studies of the X-ray sources that were discovered to determine their distances and nature and to map the accretion history of supermassive black holes. A particularly profound implication of this work is that supermassive black holes are being assembled from the earliest times to the present, not just during the quasar era, as was previously thought.
Studies of local supermassive black holes find a remarkably tight relation between black hole mass and the random velocities of all the stars in the host galaxy, not just those that are directly influenced by the black hole's gravitational field. This indicates an intimate link between the assembly of a black hole and the formation of the stars in its host galaxy.
As with the energy output of quasars, galaxies in the distant past show the most vigorous star formation activity. Since many star forming galaxies are also surrounded by dust and gas, they cannot be seen easily at optical wavelengths. However, dust absorbs the radiation emitted by the stars and reradiates it at much longer wavelengths, in the far-infrared and submillimeter.
UW-Madison researchers helped make the first discovery in blank fields of giant, distant, dust-obscured galaxies at submillimeter wavelengths. Although the number density of these galaxies is low compared to that of optically detected sources, they contribute large amounts of light and cumulatively dominate the star formation at early times. However, just as with the quasars, these giant star formers have all but vanished at the present time. The current star formation, which is still substantial, is produced by much larger numbers of smaller galaxies with modest star formation rates.
We now need to tie the galaxy and quasar populations together to establish their overall properties and to learn how the star formation episodes are related to supermassive black hole growth. The resolution of single-dish submillimeter telescopes is poor, making it difficult to pinpoint the counterparts to the submillimeter sources at other wavelengths and learn about their properties. However, advances have been made by exploiting the well-known correlation between radio continuum luminosities and far-infrared luminosities, which allows one to take advantage of the subarcsecond resolution of radio interferometry to identify counterparts and hence measure distances.
With distances it is possible to map the dusty star-formation history of the universe. The overall star formation history is indeed found to parallel that of the accretion history. Recent submillimeter interferometry has enabled UW-Madison researchers to identify sources at even greater distances, beyond what can be done with current radio sensitivities. The future of this field lies with the remarkable Atacama Large Millimeter/submillimeter Array in Chile.
