Drs Mateo and Olszewski will measure the motions of stars within dwarf galaxies, using a spectrograph at the Magellan telescope built by Dr. Mateo. These least-luminous of the galaxies appear to contain far more mass than is present as stars: we observe those stars to be moving so rapidly that they would escape from the galaxy if not for the additional gravitational pull of unseen dark matter. By careful studies of stellar motion, the group will improve measurements of the total mass present in nearby dwarf galaxies, and test whether they are being pulled apart by gravitational forces from neighbor galaxies. In more distant galaxies, they will measure motions of the surrounding globular star clusters to estimate the mass of the galaxy and hence of its dark matter. The group will collaborate with statisticians to analyze and interpret their results.
The results of this work will test theories of galaxy formation, many of which predict that a galaxy must exceed a minimum mass in order to continue making stars. Students will be trained as they participate in the research. Both Dr Mateo and Dr. Olszewski will present public lectures on their work in their local communities. Through a program at U. Arizona, Dr. Olszewski will continue his work with elementary-school teachers, with star parties and visits to the Steward Observatory telescopes.
The goals of this project were to learn more about the nature and distribution of the dark matter in and surrounding galaxies. At this time, no dark matter particle has ever been captured in the lab, so our understanding of dark matter comes from astronomical measurements of its gravitational effects. The main focus of this grant was to use measurements of the velocities of stars in the simplest of all galaxies, the dwarf spheroidals, to measure the distribution of the unseen dark matter. The motions of the stars come from the sum of the gravity of all of the stars in these galaxies and from the gravity of the dark matter. In these galaxies the dark matter is the dominant form, the stars barely contribute gravitationally, and these galaxies have almost no gas. We find that the distribution of dark matter in the central regions is not what is expected from the most successful theory of structure formation, cold dark matter. We find that all of the dwarf galaxies are embedded in similar dark matter halos. We find evidence for an end (largest extent) to the dark matter halos of dwarf galaxies, thus allowing a measurement of the total mass of these objects. We find that the properties of the dark matter halos of the dwarfs smoothly connect to the properties of dark matter distribution in larger galaxies. We have shown that the outermost regions of the Large Magellanic Cloud (LMC) are unlike those that were expected, namely, that there is a smooth disk to unprecedented distances. The implications of such a smooth disk are that the Large Magellanic Cloud is, at this moment, relatively undisturbed. This could not be the case in the old paradigm of the LMC orbiting the Milky Way for most of the past 15 billion years. We therefore conclude that others' measurements that imply that the LMC is entering the gravitational influence of the Milky Way for the first time in the recent past are correct. Surprisingly, the outermost regions of the LMC produced a set of stars 8 billion years ago. We have not yet connected this global set of stars to any major event. We also see that the regions between the two Magellanic Clouds, recently thought to be composed of newly expelled gas and young stars formed in place, do have intermediate-age stars in and near the regions of densest gas, but not everywhere. This is consistent with new models of formation of structure in the Magellanic Clouds. We have found that the "ordinary" globular cluster NGC 1851 is quite extraordinary. It has a halo of stars out to distances of hundreds of parsec, making it most probably the core of a destroyed dwarf galaxy, and a poster child for the fact that many stars in the Milky Way's halo came from destroyed smaller galaxies. We have investigated the structure of current-day dwarf galaxies by imaging. There is less evidence for tidal distortion than was expected, although some of the nearest ultra-faint dwarfs do show extension towards the center of the Milky Way, as if they have been modified by the proximity to the Milky Way. This research extensively used an instrument built with other NSF funds awarded to one of the investigators, and was the impetus for a new instrument also built with NSF funds, that was/is available to the entire community of the Magellan Consortium, and well as to the entire astronomical community through collaborations and through cooperative agreements with the NSF (such as the former TSIP program). Students have been trained and supported through the Michigan side of this grant, and one of the unfunded major collaborators recently received a faculty position. General scientific knowledge is disseminated by the PI through two Facebook sites, and prior to that in this grant period, with direct contact with a small number of teachers. The published data catalogs have been used, in published analyses, by a large number of independent groups, attested to the broader impact of this grant.
