Sarah A. Bird's Web Page


Welcome


Thanks for visiting my web page! I am an Aliyun Fellow doing research in astrophysics at National Astronomical Observatories, Chinese Academy of Sciences (NAOC), China. LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope, www.lamost.org, www.lamost.us) is located outside of Beijing and is dedicated to surveying an expected 10 million stars in the Milky Way over five years (2012-2016). As part of my fellowship at NAOC, I am studying the halo stars from the LAMOST survey and combining the data with the recent second data release from the Gaia satellite. In addition to science with LAMOST and Gaia, I also focus on the stellar halo of nearby galaxies. See below for a further discussion of my research. My PhD thesis is entitled "Halos of Galaxies" and was completed in 2014 at Tuorla Observatory, University of Turku, Finland. I am from the United States of America and began studying physics in Missouri. This web page began during my studentship at University of Wyoming's summer Research Experience for Undergraduates. Thanks Wyoming for the introduction to HTML!

Contact me
Curriculum Vitae - pdf, includes my contact information, education, astrophysics job experience, activities, and more.
Publications, as listed on ADS (SAO/NASA Astrophysical Data System) and as a pdf.
Dynamical and Chemical Structure of Galactic Halos, my Ph.D. thesis accessible as a pdf through Doria, the multi-institutional repository maintained by the National Library of Finland. A paper copy of the thesis can also be purchased at the University of Turku's online bookstore.
College Papers that I wrote while attending the University of Missouri-Columbia (2003-2007).
Wyoming REU
AAS Meetings, learn about the meeting, view my poster contribution from research during the REU program, and check out my additional contributions.
Acknowledgments


Research

1. My main set of projects currently are to study the halo stars of the Milky Way. Huge increases in the numbers of known halo stars have been taking place in the last few years as a number of automated surveys have completed, and much larger increases still are expected in the next few years from surveys such as the LAMOST Survey in China, the Gaia-ESO Survey in Chile, and the AEGIS survey in Australia. The surveys will produce spectra for several million stars and are a huge step forward relative to previous work before the turn of the century. LAMOST, now after it's initial five year survey, continues to observe and is adding several thousands of stars within 20 kpc to the current stellar halo catalogs, such as the 6036 SEGUE K giants (Xue et al., 2014) and 7600 LAMOST K giants (Bird et al., 2018a). After the spacecraft Gaia's successful launch in 2013, it maintains a five year mission which includes measuring the proper motions of millions of halo stars. With the large sample of line-of-sight velocities from the recent spectroscopic surveys combined with the proper motions from Gaia, the 3D kinematics of the Milky Way halo are now available. Before Gaia 3D kinematics were only directly measured within the inner halo (<10 kpc) or for only a very small number of halo objects past 15 kpc. The velocity dispersion profile and mass estimate of our Galaxy depend on accurate measurements of the 3D velocities.

In Bird et al. 2018a, Anisotropy of the Milky Way's stellar halo using K giants from LAMOST and Gaia (click the title to read), we use over 7000 halo K-giant stars with line-of-sight velocities from LAMOST and proper motions from Gaia to reduce the uncertainty in the Milky Way mass estimates by making the most precise measurement of the 3D velocity dispersion and anisotropy profiles possible to date. The anisotropy remains remarkably unchanged with Galactocentric radius from approximately 5 to 25 kpc, with an amplitude that depends on the metallicity of the stars, dropping from around 0.9 for -1.8<[Fe/H]<-1.3 (for the bulk of the stars) to around 0.6 for the lowest metallicities ([Fe/H]<-1.8). After 30 kpc, the profile gently falls to have a stronger tangential component, but the radial component nevertheless always dominates.

In Bird et al. 2018b (before the second data release of Gaia, we use line-of-sight velocities from over 10,000 LAMOST and SDSS/SEGUE halo K giant stars to measure the velocity dispersion, mass, and circular (rotational) velocity profiles out to approximately 200 kpc from the Galactic Center. We adopt a Navarro-Frenk-White dark halo density profile, estimate the viral radius and concentration parameter from the derived mass profile of the dark halo, and find a virial mass of M200=0.85 +/-0.05 (fitting of the data) +/-0.1 (systematic bias due to tracer density) +/-0.1 (systematic bias due to velocity anisotropy) +/-0.3 (systematic bias due to non-virial effects) x10**12 solar mass units.

The Milky Way does not necessarily have the smooth velocity profile of a virialized system but instead a profile displaying signs of recent disruption of satellites (Loebman et al., 2017). From line-of-sight velocity observations, Kafle, et al. (2012, The Astrophysical Journal, Volume 761, 17 pp.) calculated the broad motions of stars in the inner halo, where the stars are expected to be rather smoothly distributed. The observations have some strange things in them though -- sudden changes in the velocity distribution function of the stars -- which are not understood. We have performed simulations to test if a system of stars with the properties the observers are claiming can really move in the way they say and maintain the same density distribution -- i.e. remain stable in the long term. The resulting paper Fading Features Found in the Kinematics of the Far-Reaching Milky Way Stellar Halo (click the title to read), published in Monthly Notices of the Royal Astronomical Society, shows that the sudden change in the stellar motions smooths out in a relatively short time period.

We are currently using SDSS/SEGUE and Gaia to measure the anisotropy profile of blue horizontal branch halo stars. The profiles are similar to the K giants we have analyzed and also show dependency on metallicity. We plan to analyze stellar halo samples from cosmological simulations to find clues to what may cause such a dependency between velocity anisotropy and metallicity. The blue horizontal branch sample has distances with ten percent precision, which allow us to probe the anisotropy profile for fine structures with more detail than possible when using K giants. Using our precise anisotropy profile and the Jeans equations, we will make a more detailed profile for the mass of our Galaxy.

2. The second project is an observational study of the stars in the halos of two nearby giant galaxies. The first is the giant elliptical galaxy M87. I have used extremely deep exposures of the galaxy taken with the Hubble Space Telescope. We were able to detect individual ''red giant'' stars in this rather distant galaxy for the very first time, enabling us to measure the distance to the galaxy and gain an indication of the metal content (metallicity) of the stars. The distance we obtain compares very well with other distance measurements made by independent means, and the study has been published in the refereed international journal Astronomy and Astrophysics (The inner halo of M 87: a first direct view of the red-giant population, click the title to read). I have presently received observation time of almost 6 hours of another giant elliptical galaxy, Centaurus A (Cen A), from ESO's VLT. We have found a very low metallicity component of stars in the peculiar `outer halo' of Cen A of the type our collaborators have found in two other galaxies. These stars are potentially the very first formed in the galaxies, and hold very interesting clues to the cosmological conditions in which galaxies form. This work is in collaboration with Chris Flynn (Swinburne University of Technology, Australia), Bill Harris (McMaster University, Canada), and Mauri Valtonen (Tuorla Observatory). The paper Red giants in the outer halo of the elliptical galaxy NGC 5128/Centaurus A is published in Astronomy and Astrophysics (click the title to read).

3. A third study is of the so-called globular clusters in the Milky Way. These are gravitationally bound systems of hundreds of thousands of stars. I am making computer simulations of the effects on the orbits of globular clusters when a satellite galaxy merges with the much larger host galaxy. I have first developed a computer simulation in which the globular cluster system is initially stable in the gravitational field of the host galaxy. I then track the motions of the globular clusters as a small satellite merges with the host galaxy, and I examine the types of orbits the globulars have before and after the merger. Multiple mergers are also being studied, and we are also considering the effects that the merging satellite has on the globulars which are distributed in a halo as well as a disk in the host galaxy. The paper Distribution of Globular Clusters After a Late Merger (in progress) is in collaboration with Chris Flynn (Swinburne University) and Mauri Valtonen and Seppo Mikkola at Tuorla Observatory.






Created and designed by Sarah Ann Bird.
Last update: (February 3, 2019) (el tres de febrero 2019)