Eric Bell

Eric Bell studies the physics of galaxy formation and growth using large survey datasets in concert with rich simulation datasets. His current areas of interest are in understanding the effects that galaxy mergers have on the disks, bulges and black holes of galaxies, how these mergers affect their satellite galaxies, and the processes that cause star formation to slow or stop completely in galaxies. Bell is also deeply involved in work to use institutional data to advance equity in our courses and majors, and to improve climate at the University of Michigan. 

Notable Results

Bell's main current research interest is to learn how to decode galactic merger histories from present-day galaxies, and using that information to learn about what mergers do to galaxies and their satellites. Using resolved-star data for the outskirts of nearby galaxies and theoretical galaxy simulations in concert, Bell and his collaborators developed methods for estimating when and with what a galaxy has previously merged. By applying this to our nearby neighbor, the Andromeda Galaxy, they learned that Andromeda has been undergoing a minor merger for the last few billion (!) years with a massive galaxy of which M32 is the left-over core. This merger delivered satellites, thickened Andromeda's disk and triggered a starburst. Bell and his collaborators are applying this method to other galaxy groups, and have discovered that the number of satellites that a Milky Way galaxy has scales with the mass of its most massive previous merger, a result which strongly suggests that many (or most) satellites are delivered to galaxies like the Milky Way in group mergers. 

Bell provided some of the first direct observational support for the evolution of non star-forming galaxies. These galaxies were widely thought to have been formed in the early Universe, aging passively to the present day. Theorists challenged this view, proposing that such galaxies should be formed continually through galaxy mergers and perhaps other processes. Using a combination of HST and COMBO-17 data, Bell showed that the number of these early-type, non star-forming galaxies increases with time, at a rate that approximately fits the rate of galaxy mergers. But why do these galaxies stop forming stars? Subsequent work has shown that these non star-forming galaxies distinguish themselves from those that continue to form stars by virtue of their central density, or - in work led by former graduate student Terrazas - when one has access to it, the relative prominence of a galaxy's supermassive black hole. These observational signatures are expected in models where the ejection of material from the environs of a supermassive black hole heats or ejects gas throughout the entire halo of a galaxy, and is a strong observational indication that AGN feedback is the key physical process that slows or shuts off star formation in galaxies. 

Publications

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