Mapping the Milky Way Outer Halo with SEGUE and BOSS

Case Western Reserve University, Cleveland OH

Investigators

Abstract

This work analyzes the large-scale morphology and constituents of the Galactic halo by finding, characterizing, and mapping the red giant stars and blue horizontal branch (BHB) stars in the Sloan Digital Sky Survey (SDSS) region. The analysis uses as many as 12,000 K giant stars from the Sloan Extension for Galactic Understanding and Exploration surveys I and II (SEGUE I+II) including a few hundred luminous K giants with larger distances up to about 150 kpc. The halo structure is mapped in detail with field stars rather than globular clusters or streams. The structure of elliptical galaxies has been studied with field stars, but this has not been done much for the Milky Way. Other studies using globular clusters and dwarf spheroidal (dSph) galaxies are also important, but the use of field stars gives a less biased picture of the Milky Way halo. These stars make up most of the mass of the Milky Way halo, and similarly of the halos of other galaxies. Here the large number of test particles (12,000 stars) should provide a sound statistical base. Radial velocity measurements of these stars are used to group the stars into lumps and streams. More K giant stars will be added from the Baryon Oscillation Spectroscopic Survey (BOSS). Data for K giants from the SEGUE Stellar Parameter Pipeline (SSPP) also provide chemical information for these stars such as metallicity (the relative iron to hydrogen ratios), or the ratios of alpha elements (e.g., oxygen, silicon, calcium) relative to iron. Other follow-up spectroscopic observations are done to obtain the required abundance information if spectra from the surveys do not provide reliable data. The signal-to-noise ratio of BOSS or SEGUE-II spectra might be insufficient to yield accurate distances, and it becomes necessary to follow up with telescope time to target those stars individually. Combining the dynamical and chemical information leads to a better understanding of the Galactic halo components, and how the halo assembled. Comparisons to other galaxies such as M31 can provide further insights about galaxy formation. Once the density distribution of the outer halo is mapped, the BHB and giant stars are used as tracers to study the properties and history of the outer halo. This can be compared to properties of the inner halo, which is suspected to have evolved differently than the outer halo. One important measurement is the mass determination of the Milky Way out to these large radii from the Galactic center. Broader Impact: The PI has been involved with the calibration of the data pipelines for the SDSS spectra for several years and is an expert at analyzing and understanding these data. These data have a large impact in the astronomical community. The work here supports a graduate student and two undergraduate students. Case astronomy is partnering with the Fisk/Vanderbilt Masters Bridge Program for minorities, and the principal investigator is actively involved in this partnership. The PI has an ongoing commitment to undergraduate teaching, public outreach, and mentoring minorities and women in scientific research.