Joel Bregman

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Joel Bregman

H.D. Curtis Professor of Astronomy

University of Michigan Department of Astronomy
1085 S. University
Ann Arbor, MI 48109

Office Location(s): 301E West Hall
Phone: 734.764.2667
Fax: 734.763.6317
View Curriculum Vitae

  • About

    Joel Bregman applies his expertise in X-ray observation and theory to account for the “missing baryons” (normal matter) in galaxies. Based on the accepted ratio of normal to dark matter, galaxies seem to be missing some 70-95% of their baryonic material. Bregman and his students have been working to determine whether the missing material might reside in the halos of hot gas that surround galaxies, as many astronomers have proposed. Using a variety of techniques to measure these hard-to-detect halos, they’ve demonstrated that the missing baryons cannot be accounted for in the hot gas halo, for example, around the Milky Way.

    Notable Results

    Bregman proposes a model of galaxy formation that may explain why the missing baryons are not in galaxies’ hot halos. While galaxies themselves seem to be missing baryons, clusters of galaxies contain the expected amounts. To explain this, many theorists assumed that galaxies’ normal matter was formed then later spewed into the intergalactic/intra-cluster space. Bregman believes the normal matter never fell into the galaxies at all. Instead, he proposes a model of galaxy formation led by an early population of high-mass stars. When these stars exploded, he argues, they propelled hot gas beyond the influence of their resident dwarf galaxies and produced most of the metals we see today. His idea is a significant change that aims to better align with observations.

    Key Interests

    High-energy astrophysics; gaseous component of the universe; intermediate-mass black holes; elliptical galaxies; and globular clusters.


    BS, SUNY/Stony Brook; PhD, UC/Santa Cruz. Postdoc, Columbia University; Assistant Professor, NYU; Scientist, National Radio Astronomy Observatory.


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    Biblography (PDF)