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) 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 the University of Turku. 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 through a number of automated surveys such as the LAMOST Survey in China, SDSS in USA, the Gaia-ESO Survey in Chile, and the AEGIS survey in Australia. The surveys 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 its 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 4243 SDSS BHB stars (Xue et al. 2011), 6036 SDSS/SEGUE K giants (Xue et al., 2014) and 13377 LAMOST K giants (Bird et al., 2020), with the most distant stars of these samples reaching past 100 kpc. After the spacecraft Gaia's successful launch in 2013, it maintained 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. 2020, Constraints on the assembly history of the Milky Way's smooth, diffuse stellar halo from the metallicity-dependent, radially-dominated velocity anisotropy profiles probed with K giants and BHB stars using LAMOST, SDSS/SEGUE, and Gaia and Bird et al. 2019, Anisotropy of the Milky Way's stellar halo using K giants from LAMOST and Gaia (click the titles to read more), we use halo K-giant and BHB stars with line-of-sight velocities from LAMOST and SDSS 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 20 kpc, with an amplitude that depends on the metallicity of the stars, dropping from around 0.9 for -1.7<[Fe/H]<-1 (for the bulk of the stars) to around 0.6 for the lowest metallicities ([Fe/H]<-1.8). After 20 kpc, the profile for more metal rich stars -1.7<[Fe/H]<-1 gently falls to have a stronger tangential component (the radial component nevertheless always dominates), but the profile for more metal poor stars [Fe/H]<-1.7 remains constant with distance.

We are currently using this halo K giant and BHB samples to estimate the mass profile of our Galaxy using the spherical Jeans equation. The anisotropy profile from Bird et al. 2020 we know with high precision and we are investigating how to use its dependency on metallicity as a leverage to margenalize over the less precise density profile. 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.

Figures from Bird et al. 2020. Velocity anisotropy profile for halo K giant stars (left) and blue horizontal branch stars (right), divided into different metallicity ranges.


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).

Figure: Left panel is M87. Right panel is Cen A.


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.

Figure: Lines trace the orbits of simulated globular clusters around a Milky Way-type galaxy.







Created and designed by Sarah Ann Bird.
Last update: (June 13, 2020) (el trece de junio 2020)