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The universe harbors billions of galaxies, each containing dark matter, gas, and millions to billions of stars. How these galaxies assembled from primordial matter and evolved is a central issue in current astrophysics. François Schweizer studies galaxy assembly and evolution through observations of nearby galaxies, focusing on how collisions and mergers affect the galaxies' properties. His research is contributing to a growing awareness that such events are a dominant process in shaping galaxies and determining their stellar and gaseous contents.
The spectacular phenomena observed when nearby galaxies collide and merge yield valuable clues about processes that occurred much more frequently in the younger, distant universe. When two gas-rich galaxies collide, their pervasive interstellar gas gets compressed, clumps into dense clouds, and fuels the sudden birth of billions of new stars and thousands of star clusters.
Some of the newborn clusters are so dense that they survive the merger intact and orbit in the remnant galaxy for billions of years as "globular clusters." Schweizer and collaborators use the Hubble Space Telescope to search for young globulars and study their formation and evolution. Follow-up spectroscopy with Magellan and other ground-based telescopes then yields the clusters' velocities, chemical compositions, and ages.
Schweizer is also collaborating with astronomers using NASA's Chandra X-ray Observatory to study point sources and the hot interstellar gas in merger galaxies. The team's observations seem to connect the X-ray point sources to black holes being dynamically ejected from the newborn star clusters. The data yield direct measurements of the chemical enrichment of the gas by exploding supernovae.
Understanding the detailed processes of galaxy formation and evolution is part of understanding our own origin. Whereas galaxies once appeared to be "island universes," we can now see them collide, assemble hierarchically, and form stars that cook up the heavy elements of which planets, and we ourselves, are made.
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