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When astronomers see far into space they look back in time. Alan Dressler and colleagues use Magellan and the Hubble Space Telescope to study galaxy evolution—how galaxy structures and shapes change, the pace and character of star birth, and how large galaxies form from earlier, smaller systems. Dressler and staff member Gus Oemler were principals in a project known as “the Morphs,” which focused on the evolution of spiral galaxies over the last 7 billion years. The group found that star formation was more common in galaxies 5 billion years ago than it is today, and that much of this star formation occurred in starbursts, in contrast to the more steady star formation seen in today's galaxies.
The Morphs collaboration addressed basic questions about the origins of the different galaxy types—elliptical, S0, and spiral. Generally round elliptical galaxies are the oldest and formed from violent galaxy mergers 2 to 3 billion years after the Big Bang; their star formation ceased long ago. Spiral galaxies, however, continue to birth new stars within their star-and gas-filled spiral arms. S0 galaxies are intermediate, with structures like spirals, but void of gas and new stars like ellipticals. The prevalence of S0 galaxies in rich galaxy clusters suggests that spirals are transformed into S0s through some process related to high galactic density, or the cluster’s strong central core of galaxies and hot gas. Morphs found that the interaction of spirals, as they fall into the rich cluster, is a principal transformation mechanism.
The work continues in a new collaboration, The IMACS Cluster Building Survey, of Dressler, Oemler, Mike Gladders (Chicago) and Bianca Poggianti (Padova, Italy). The ICBS follows the evolution, in stellar population and structure, of a large sample galaxies in the outskirts of distant clusters, where spiral galaxies first encounter the high-density environment. The ICBS team uses the Inamori Magellan Areal Camera and Spectrograph (IMACS) on the Baade telescope, which was built by a 10-person team led by Dressler. With its ½-degree field, IMACS is one of the world's most powerful, and certainly the most versatile spectrograph on any large telescope. It can obtain spectra of hundreds and even thousands of faint galaxies simultaneously -- this enables new studies that were previously intractable. The team is assembling the spectra of approximately 10,000 galaxies over 7 rich clusters at 4 to 7 billion years ago to trace the transformation of spirals into S0 galaxies, and to understand why powerful starbursts are involved.
Dressler and Pat McCarthy are also collaborating with Crystal Martin (UC Santa Barbara) to search for the first generation of starforming galaxies, the smaller ones that became today's typical galaxies. A key question is whether these young galaxies were sufficiently abundant to ionize the universe a billion years after its birth, a process that is observed but not yet explained. Again, the exceptionally wide field of IMACS and its recently-enhanced sensitivity are key in this challenging search.
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