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Most of the visible matter in the universe is contained in galaxies, which are agglomerations of several billions of stars held together by their mutual gravity. Galaxies come in a great range of shapes and sizes. The most common types are gas-rich spirals, such as our own Milky Way, or elliptical galaxies that exhibit much less rotation and contain much less interstellar material. Under normal conditions stars die and are reborn at a slow, steady rate while they move in stable orbits around the galaxy's center. In an important minority of galaxies, however, a collision or some other factor causes the galaxy to become anomalously bright for a limited period. At the IfA we study both normal and pathological galaxies in an effort to understand their origins and evolution.
Bob Joseph is interested in the underlying processes responsible for the infrared emission from normal spiral galaxies. Using the Infrared Space Observatory (ISO) he and recent PhD graduate George Bendo have produced the ISO Atlas of Bright Spiral Galaxies (Joseph was a Co-Investigator on this space mission), with data spanning 1-850 microns. These data show that star formation is correlated galaxy morphology, and the star formation rate is proportional to the molecular gas mass in the galaxy. Using 2.5 micron spectroscopy they are now investigating the properties and history of nuclear star formation in normal galaxies.
Given the large diameters of galaxies and their relatively close separation in space, it is inevitable that galaxies sometimes collide with each other. Gas falling toward the nuclei of colliding galaxies can trigger vast bursts of new star formation. These "starburst" nuclei can shine with the brilliance of a trillion Suns, mainly at infrared wavelengths. In extreme cases an AGN or "active galactic nucleus" may be formed. In an AGN, energy is released as matter spirals toward a massive black hole at the nucleus of one or both of the galaxies. Part of the energy is emitted as electromagnetic radiation that can be directly detected by telescopes, part goes into heating and accelerating the gas that surrounds the black hole, and part may be focused into jets that eject radio-emitting plasma clouds vast distances into intergalactic space. Quasars, the most powerful objects in the known universe, are examples of AGNs. Several astronomers at UH are trying to understand how AGNs are produced and why some galaxies contain starbursts and some contain AGNs.
Bob Joseph has been studying the astrophysics consequences of interactions and mergers in spiral galaxies for the past 20 years. With collaborators he showed that interactions induce rapid bursts of star formation and that mergers produce the highest luminosity starbursts. He has adduced evidence that the initial mass function of interaction-induced starbursts is truncated at both the low-mass and high-mass ends. Joseph and graduate student Barry Rothberg are currently working on a thorough study of whether spiral-spiral mergers are the origins of elliptical galaxies using the largest sample of candidate objects ever assembled. Using infrared imaging obtained with the University of Hawaii 2.2m they compare the distribution of stars in mergers with those in elliptical galaxies. This is used in conjunction with measurements of the distribution of stellar motion extracted from spectroscopic observations taken with Keck 10m. Together, these important diagnostics show that spiral mergers not only produce galaxies that will eventually look like ellipticals, but will produce galaxies with the the same physical and dynamical properties as elliptical galaxies.
Josh Barnes simulates galaxy collisions using a computer to compress 100 million years of star motions into a few minutes. He calculates the gravitational forces between the stars and dark matter in the colliding galaxies, and also the much more complicated interactions between the colliding gas clouds in the spaces between the stars. By matching his theoretical calculations with observations of real colliding galaxies, he works out how the collision started and can predict what the end result will be. The example shown here is a simulation of the collision that produced the "Mice" galaxies (NGC 4676)
Several IfA astronomers are using telescopes on Mauna Kea to investigate the connection between galaxy collisions, starbursts, and AGNs. Alan Stockton estimates the ages of the newly formed stars from visible-wavelength spectra taken with the Keck Telescope. David Sanders measures how gas accumulates around the nuclei of the galaxies by observing submillimeter-wavelength carbon monoxide emission lines with the Caltech Submillimeter Observatory and the James Clark Maxwell Telescope. Galaxies forming stars at high rate, or "starbursts" emit more strongly in the mid-infrared (from 5 to 20 microns) than the quiescent ones because they tend to contain more dust, and because part of their dust is strongly heated by the young giant stars they contain. Numerous star bursting galaxies were detected in the surveys performed at 15 microns by the Infrared Space Observatory, and their space density Hervé Aussel is currently working on the interpretation of the results of these surveys, and compares the population detected in the mid-infrared to the one observed in the sub-millimeter. Another tell-tale sign of an AGN is its variable radio emission: graduate student Peter Capak is measuring redshifts and properties of a sample of variable radio sources to see if they are consistent with AGNs of different types. Part of the reason there is so much interest in AGNs is simply that because they are so bright, they can be detected at very great distances. Much of what we know about the early universe is therefore based on the study of AGNs at high redshift. Alan Stockton and Ken Chambersare both trying to understand the processes taking place in very distant AGNs, but they differ in their approaches. Dr. Stockton measures the colors and spectra of distant galaxies to deduce what mixture of stars the galaxies contain. The nature of the mixture of stars can be used to estimate the time since the galaxy was formed. Dr. Chambers' approach is to measure the amount of polarization in the light from radio galaxies. Polarization occurs when light from an AGN is scattered by dust or free electrons surrounding the nucleus; its detection in a distant galaxy can reveal the presence of an AGN that would otherwise be hidden from us by interstellar dust.