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Milky Way and Other Galaxies
In cosmology and physics, there is now compelling evidence that only 4 percent of the energy density in the universe can be seen directly. Of the remainder, about 22 percent is thought to be composed of dark matter, a non-baryonic material of unknown character whose presence can be inferred only from its gravitational effect on visible matter. SIM Lite will probe the nature of dark matter and its role in the formation of the Milky Way and other galaxies (S. Majewski, E. Shaya; SIM book Chapters 4 and 7).
Recent reports are listed below, along with the originally accepted key projects and recent community science studies.
Researchers inspired to see what SIM light can do on their own selection of targets can use the SIM Lite Time and Performance Estimator (TaPE) now to explore the performance of SIM Lite.
Key Projects
Taking the Measure of the Milky Way
A study of the motion of stars in our galaxy to determine the forces that cause the motion to understand better the distribution of matter in the Milky Way.
Dr. Steven R. Majewski
University of Virginia, Charlottesville
Key Project information at:
http://www.astro.virginia.edu/~rjp0i/takingmeasure/index.shtml
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 Summary Full proposal (PDF) SIM book Theme II |
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Anchoring the Population II Distances and Ages of Globular Clusters
This program will make observations to determine the ages and distances of globular clusters which are needed to determine the age of the universe.
Dr. Brian C. Chaboyer
Dartmouth College
Hanover, N.H.
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| Summary Full proposal (PDF) SIM book Theme III |
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Dynamical Observations of Galaxies
By determining the precise distances and motion of nearby galaxies, this scientific program will study the formation of the local group of galaxies.
Dr. Edward J. Shaya
University of Maryland
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| Summary Full proposal (PDF) SIM book Theme II |
COMMUNITY SIM SCIENCE STUDIES
Determining the Nature of Dark Matter Using Proper Motions of Stars in the Milky
Way Satellites
Manoj Kaplinghat (UC Irvine)
Dwarf spheroidal (dSph) satellite galaxies in the Local Group provide ideal laboratories for deciphering the nature of dark matter and testing theories of hierarchical structure formation on small scales. Theoretically, their status as the most dark-matter-dominated galaxies in the Universe enables the determination of their dark matter density structure without the intrinsic uncertainties usually associated with baryonic mass contributions. Observationally, their proximity allows for detailed studies of their dark matter density structure via proper motion studies with SIM Lite. Moreover, the intrinsically high phase-space densities of these small galaxies make them ideal candidates for constraining the particle properties of dark matter. We propose to develop methods to use proper motion measurements to constrain fundamental properties of the dark matter particle. In the standard model, the dark matter is a cold thermal relic and is born with a high primordial phase-space density that allows the dark matter to collapse into halos with very steep density cusps in their centers. Observations of dark-matter-dominated galaxies suggest that dark matter halos may have shallow density slopes in their centers, which is more suggestive of “warm” dark matter models. However, there are several potential systematic problems with interpreting these observations associated with uncertain baryon physics. We propose to develop methods to constrain the central densities of dwarf spheroidal galaxies using proper motion observations. While line-of-sight motions alone are unable to place constraints on the log-slope, proper motions will provide a definitive measurement of the log slope and a direct way to connect the dynamic properties of stars in local dwarf galaxies to the microphysical properties of dark matter. We will identify the best Milky Way satellite candidates for this purpose and develop the theoretical machinery necessary to connect measured log-slopes to constraints on the primordial phase-space density of dark matter and to the microphysical properties of the dark matter particle.
Using Rotational Parallax to Estimate 1 Percent Luminosity-Independent
Distances to Nearby Galaxies
Robert Olling (U. Maryland)
SIM LIte can provide data with high enough quality to determine luminosity-independent distances to the nearest spiral galaxies by employing the rotational parallax (RP) technique. Since proper motion is defined as velocity over distance, the distance follows from proper motion and radial velocity observations. An accuracy of around 1 percent is possible for M31 and M33 using about 200 stars per galaxy. Due to its large random internal motions, the Large Magellanic Cloud (LMC) is not a SIM Lite target for RP. A 1 percent error is ~8 times better than the systematic error on H0 attained by the Hubble Space Telescope, the Wilkinson Microwave Anisotropy Probe (WMAP), and extragalactic water masers. In our review of methods that can potentially yield extragalactic distances at the 1 percent level, we find that the RP method is the most accurate distance indicator because: 1) it is a 100 percent geometric method (e.g., eclipsing binaries also rely on astrophysics to derive distances), and 2) it samples a large part of the stellar disk so that non-axisymmetric motions can be determined accurately (e.g., in contrast to nuclear water masers that sample just three lines of sight). It has been shown that knowledge of the Hubble constant to better than 1 percent is crucial for constraining the equation of state of dark energy, in combination with Planck data. Accurate RP distances facilitate detailed comparisons between almost all standard candles between various zero-points (MW, M31, M33, the LMC, and NGC4258). Successful cross-checks are crucial if we are to believe galaxy distances (and H0) at the 1 percent level. The RP technique may be complicated by noncircular motions that could be due to, for example, spiral structure. However, the initial analyses suggest that these effects can be diagnosed and remedied. Because SIM Lite will provide five-dimensional phase space information, the RP galaxies (along with the Milky Way) will be the galaxy-dynamics laboratories for decades to come. Currently, there is no RP SIM Lite project, while the Key Project on proper motions of nearby galaxies will observe just a few stars per galaxy. We propose extensive theoretical analyses of the RP technique, focused on noncircular motions. In addition, we will investigate observational aspects such as the trades between the number of stars, the accuracy per star, and mission time. We will also specify the requirements of a locally defined astrometric grid.
Project Runaway: Calibrating the Spectroscopic Distance Scale Using
Runaway O and Wolf-Rayet Stars
William Hartkopf (USNO)
Massive stars play an essential role in enriching the interstellar medium with material later recycled into stars and planets. However, the most basic information about these stars — their masses — is not welldetermined. Major reasons for this include high multiplicity rates, crowded fields, and interstellar extinction, all leading to poorly known distances. Runaway O stars can provide a potential “clean sample” of single O stars that reduces some of these problems, allowing much more accurate calibration of the spectroscopic distance scale. We propose to verify the runaway nature of the current tentative list of these objects and to augment that list from current catalogs of O stars and Wolf-Rayet stars, using new proper motion information from the upcoming Third USNO CCD Astrograph Catalog (UCAC3). The new runaway sample will provide an observing list for SIM Lite parallax determinations of these important objects. SIM Lite has distinct advantages over Gaia in its ability to provide these new parallaxes.
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