The nature of dark matter (DM) remains one of the leading unsolved problems in cosmology today, and absent any direct detection of the particles themselves, the structure of nearby dwarf galaxies remains one of the more promising avenues for learning about the temperature and self-interaction properties of DM. 

Modern ideas regarding how galaxies form in the Universe involve some form of "hierarchical" process by which small systems progressively merge into ever larger systems like our Milky Way Galaxy. A key element of this paradigm regards how mergers proceed, which, in turn, relies on understanding the nature of the DM needed to drive the formation of galaxies within the observed lifetime of the Universe. Dwarf galaxies are the smallest, simplest entities with DM that also contain visible baryonic material in the form of stars and/or gas. As such, they---and particularly their dark halos---represent our best local analogs of the building blocks that initiated the hierarchical process. The broad goal of this project is to use empirical techniques---primarily kinematic and chemical  observations of stars in nearby dwarf galaxies---to determine key properties of DM halos in these systems in a largely model-independent way.

The goals of this project included (1) learning how DM is distributed within individual galaxies by measuring the central mass densities and core properties of DM halos, (2) exploring how the spatial evolution of the baryonic matter may have altered the DM components in galaxies, (3) determining if DM halos are truly scale free as the major model predicts, (4) mapping the extents of DM halos in dwarfs and understanding how their dark and visible components have merged into the greater halo of our Galaxy, and (5) laying the groundwork to reconstruct the properties of the primordial population of dwarfs that contributed to the assembly of our Galaxy. This project builds on decades of NSF-sponsored instrument building and research grants to aid our understanding of the nature of DM.

In order to accomplish these goals, we used large telescopes in Arizona and Chile to observe the stellar populations in dwarf galaxies.  Our major product is a catalog, built from these observations, that lists velocity, temperature, density and chemical composition that we have measured for each of more than 16,000 stars in almost 40 different galaxies,  Much of these data were obtained using M2FS, an instrument largely funded via an NSF/MRI grant;  an example of the instrumentation set up using M2FS is provided in the accompanying figure to this report.  The catalog contains repeat measurements for more than 3000 stars, letting us identify binary star systems as those whose velocities change over time.  One exciting result is the confirmation of six newly-discovered star clusters orbiting within the Milky Way; these are among the first star clusters to be discovered via the common motions of their stars.  Using the catalog, we have also identified nearly 50 stars that reside at extremely large distances from the centers of their host galaxies, letting us examine the strengths of competing forces due to the dark matter within the dwarf galaxy and external Milky Way.  We have also detected many examples of binary stars within these dwarf galaxies, from which we will be able to obtain more precise inferences of the dark matter content within these systems.

Along the way, the project has supported the research of graduate students, postdoctoral scholars, and astronomers from many other institutions.   While the pandemic made it hard for students to actually travel to and directly use the observatory facilities central to the research supported by this grant, they had direct access to, and were lead authors on, several papers related to the core science accomplishments of this grant.  Hundreds of other undergraduate students at various institutions related to this grant (Univ. of Michigan, Carnegie Mellon University, Univ. of Arizona) have experienced live virtual tours of the Magellan telescopes conducted for them by various project co-Is of this grant.

Last Modified: 05/20/2024
Modified by: Mario L Mateo