Learning how the Milky Way was Assembled through Measurements of Positions, Velocities and Elements in Stars in the Halo

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

The Milky Way Galaxy is our home in the universe. The Milky Way is our most important reference for understanding the hundreds of billions of galaxies in the universe around us. It is the only galaxy where we can observe individual stars, like our sun, in detail. However, there remain fundamental unanswered questions about the Milky Way and the structure we see today. We still do not know the mass of our Galaxy to within a factor of two. The most accepted model for how galaxies form is that galaxies grow by merging with smaller satellite galaxies. The investigators will check this model by optical observations of ancient stars in the outer parts of the Milky Way, called the Halo. Using optical spectra, the investigators will measure the abundance of elements in these stars, which can tell us about the original, smaller, galaxies in which those old stars were born. When combined with other observations of these stars, the investigators will be able to answer basic questions about the history of our own Milky Way galaxy. The deep spectroscopy of stars in the galactic halo add line-of-sight velocity, chemical abundance information and improved distance estimates to the proper motions for these stars. The investigators' observations will determine the three components of the velocity, three components of each star's position in the Galaxy and a critical seventh dimension elemental abundances of the star. The spectroscopy and proper motions will be measured for main sequence stars as distant as 90 kpc. In total, this program will obtain spectra for 450 main sequence halo stars over 11 different lines of sight in the halo. These data contain unique information about the growth of the Milky Way's stellar halo and accretion of satellites, which will be used to reconstruct the mass accretion history of our Galaxy. Furthermore, the full phase-space information will be used to measure the total mass of the Milky Way; an important, yet poorly constrained, astrophysical quantity. The transverse velocity information from the investigator's proper motion data will be used to measure the velocity anisotropy of distant halo stars. The velocity anisotropy is a major source of systematic uncertainty in our knowledge of the total mass of the Milky Way. The investigators will search the multi-dimensional data for substructure in the distant halo, especially substructure from low luminosity satellites. The tidal remnants of these dwarf galaxies have been hidden from previous searches. Measuring the number of low-luminosity halo progenitors will test models for the connection between dark matter sub-halos and dwarf satellite galaxies. This is an important consistency test of the predictions of cosmological galaxy formation models. From the three dimensions of velocity, the investigators will measure the orbits of the progenitor systems that were accreted into the halo. From the metallicities, the luminosities of the progenitors can be estimated. The abundances of the alpha-elements will be measured for the brighter stars in the sample and used to estimate the accretion times of the satellites. The distributions of orbits, accretion times and luminosities of the progenitors defines the accretion history of our Galaxy's halo. Their outreach plan includes bringing in high school students via the UC Santa Cruz Science Internship Program (SIP). High school SIP interns work with their mentors on research projects, and past interns have presented papers at Astronomical Society meetings and have been co-authors on refereed publications. The two graduate students will mentor the students in collaboration with the investigators. This will give the graduate students experience in non-classroom teaching. Graduate students will develop a Python-based research methods module for high school students that draws from the research program. The investigators will work to increase the participation of underrepresented minorities in STEM fields through their participation in programs like SIP, Research Methods, the California State Summer School for Mathematics and Science and the Project for Inmate Education in the Santa Cruz county jail.