This work addresses the formation and structure of the Milky Way galaxy with particular focus on the galactic halo. The distribution, kinematics, and chemical composition of halo stars can be used to back-track the formation history of the halo which formed by mergers of small galaxies and star associations. The tidal interaction of the Milky Way galaxy with dwarf galaxies or stellar clusters also led to tidally stripped stars which are now traceable as tidal streams in the galactic halo. This characterizes the galactic gravitational potential from stellar motions and the distribution of dark matter.
New tidal streams are searched for and known streams better characterized using photometric and spectroscopic data from the Sloan Digital Sky Survey (SDSS, SDSS II, & SDSS III) and methodologies developed in previous work by the investigators. The results from this study will lead to three dimensional density model of the Milky Way halo, including its smooth component, streams and other substructure, over the 12,600 square degrees of high Galactic latitude sky imaged by the SDSS.
The observations are used to build a realistic spatial model of the density distribution of stars in spheroid globular cluster and dwarf galaxy tidal debris streams in the Milky Way?s halo. When combined with radial velocities, these data will better constrain the gravitational potential of the Milky Way. This in turn helps to determine the density structure of the dark matter by matching orbits and N-body simulations to multiple tidal debris streams simultaneously. It is an advantage to consider more than one tidal debris stream at a time and that more realistic potential models are used. Including the distribution properties and density of stars along the tidal debris stream rather than only using their average position and velocity is another advantage for models that aim to constrain the gravitational halo potential and the mass distribution in our galaxy.
Two graduate students, and 3 undergraduate student researchers will participate in this project which will help to train the next generation of scientists. The principal investigator has a strong education and outreach program which continues as part of this work. In collaboration with the Dudley Observatory, the Greater Capital Region Teacher Center, and the Summer@Rensselaer program office, twenty-five K-12 teachers will able to take classes to bring astronomy in the class room. This projects also supports the MilkyWay@home web site run by the principal investigator, which directly involves a volunteer computing community of about 40,000 people from 166 countries, who contribute hundreds of TeraFLOPS of computing power to Milky Way research.
This grant resulted in seven refereed journal articles. One paper reports the discovery of the Pisces Stellar Stream, which is a "stream" of stars that were stripped by tidal forces from a dwarf galaxy that fell into the Milky Way and are now spread across the outer parts of our galaxy. A second paper characterizes the Cetus Polar Stream, which is another tidal stream of stars pulled from a dwarf galaxy as it merged with the Milky Way. A third and fourth paper show that the older populations of stars in the Milky Way all have "turnoff" stars with similar intrinsic brightnesses. This means that we can use them to statistically determine the density distribution of stars in the Milky Way, using a technique we call "statistical photometric parallax," which was developed with this grant. A fifth paper uses statistical photometric parallax to determine the density of stars along a tidal stream that is often called the "leading tidal tail of the Sagittarius dwarf galaxy." It is pointed out that it is possible this tidal stream is not properly identified; there is a second, fainter tidal tail that could instead be the leading tidal tail. A sixth paper explores the "Monoceros Ring," which is an apparent ring of stars in the disk of the Milky Way, more than twice as far from the Galactic center as the Sun is. We conclude that this ring is probably not the same as the thick disk of the Milky Way, though we cannot determine that conclusively. The seventh paper presents a census of the 12,060 bluest spectra in the Sloan Digital Sky Survey. We identify 7458 DA white dwarfs, 1145 DB white dwarfs, 273 rarer white dwarfs (including carbon, DZ, DQ, and magnetic), 294 subdwarf O stars, 648 subdwarf B stars, 679 blue horizontal branch stars, 1026 blue stragglers, 13 cataclysmic variables, 129 white dwarf - M dwarf binaries, 36 objects with spectra similar to DO white dwarfs, 179 QSOs, and 10 galaxies. The grant supported the operation and algorithm development for MilkyWay@home, which is a 0.5 PetaFLOPS volunteer computing platform that utilizes, at any given time, the volunteered computer power from tens of thousands of people all over the world. Since 2007, a total of over 180,000 people all over the world have volunteered their computers. This server compares data with a model, and optimizes the parameters in the model to best fit the data. We have been using this computing resource to determine the spatial density distribution of stars in the outer parts (spheroid) of the Milky Way, including the Sagittarius dwarf tidal stream, which comprises a significant fraction of the observed stars. We are developing an application that will simulate the tidal disruption of dwarf galaxies in the Milky Way, and compare the simulation results with the actual, observed, distribution of stars. In the long term, we plan to use this platform to find the density distribution of dark matter in the Mliky Way. Since most of the mass in the Milky Way is dark matter, dark matter is the primary source of the tidal forces that rip stars from dwarf galaxies as the fall into and are merged with the Milky Way. We will therefore be able to tune the model parameters for the density distribution of dark matter to best match the observed distribution of stars in tidal streams. The grant supported the research for one PhD thesis, six masters degrees, and three students who are working towards a PhD thesis. In addition, it supported 22 undergraduate research students, who were learning to do research for the first time. One high school student was mentored, who was selected as one of 40 finalists in the Intel Science Talent Search 2013. Three of the journal articles produced by this grant had graduate students as first authors. Three had undergraduate students as the first author. The seventh was led by the high school student. Many of the other students were co-authors on published papers or presented posters at meetings of the American Astronomical Society. One of the undergraduate students won the Rensselaer School of Science Class of 1002 Research Prize for his paper. The grant also supported four teacher training programs for K-12 science teachers, mostly (but not all) in New York state. About 20 teachers attended each of these week-long summer programs that taught astronomy content, hands-on activities, and pedagogy in the context of the NYS Science Standards. These were co-sponsored by Summer@Rensselaer and the Dudley Observatory, with additional funding from the NYS-funded Greater Capital Region Teacher Center, and the NASA/NY Space Grant.
