Theses Doctoral

Rewinding the Milky Way in Time

Lu, Yuxi

Galactic Archaeology aims to understand the formation history of the Milky Way (MW). Observations from large spectroscopic and photometric surveys in the recent over the last decade have revolutionized this field. Many substructures and stellar populations have been discovered thanks to full sky surveys, such as Gaia, suggesting the MW is out of equilibrium. However, with numerous missions providing high-quality spectra and photometric time series for billions of stars, it has become increasingly difficult to interpret multidimensional data. One way to address the challenge of large data ensembles is to convey multidimensional information in a more compact way. This can be done by constructing a set of key summary statistics. In my thesis, I use photometric and abundance data to obtain the ages and birth radii of stars in the MW. These two physical quantities of stars, along with stellar abundances and kinematic measurements provide a ``Galactic timetable'' that marks the locations and times of occurrence of different events including mergers and enhancements in the star formation rate.

To infer stellar ages, I use gyrochronology, one of the only viable methods to age-date main-sequence (especially for low-mass K/M dwarfs) stars. This technique uses stellar rotation periods and temperature measurements as age indicators. Due to the complexity of magnetic fields in stars, no purely theoretical gyrochronology model currently exists. As a result, gyrochronology relies strongly on empirical calibrations to known stellar ages using other methods. However, none of the age-dating methods for single field stars are suitable for low-mass main sequence stars, as they are faint and their physical properties evolve slowly. To get ages for these stars, I apply the simple assumption that the velocity dispersion of stars increases over time and adopt an age--velocity--dispersion relation (AVR) to estimate average stellar ages, which we called gyro-kinematic ages, for groupings of stars with similar period, temperature, absolute G magnitude, and Rossby number values.

Since calculating gyro-kinematic ages requires a large number of stars with period and kinematic measurements, I measured rotation periods for K and M dwarfs using the Zwicky Transient Facility (ZTF). With conservative vetting criteria, I created the largest rotation period catalog (~ 40,000) for low-mass dwarf stars. By combining open cluster ages from literature and gyro-kinematic ages inferred from stars with 6-D kinematic from Gaia DR3 and rotation periods from Kepler and ZTF, I calibrated a fully empirical gyrochronology relation using Gaussian Processes. This approach is suitable for age-dating dwarf stars between 0.67 - 14 Gyr. Using this newly calibrated relation, I provide the community with the largest and most precise stellar ages for ~ 100,000 low-mass dwarf stars. This is the first time that the approach of gyrochronology has been used to date stars older than 4 Gyr. This sample can be used to study exoplanet evolution and the kinematic sub-structure in the solar neighborhood.

Stars move away from their birthplaces over time via a process known as radial migration, which blurs chemo-kinematic relations used for reconstructing the MW formation history. One of the ultimate goals of Galactic Archaeology, therefore, is to understand stars’ birth locations. In my thesis, I first tested the reliability and limitation of the only method \cite{Minchev2018} that is able to infer star by star birth radius. I do so by testing the underlying assumption --- the metallicity gradient is linear at all times --- using the cosmology zoomed-in simulation NIHAO-UHD.

This analysis concluded that for the MW, we can infer birth radii with an uncertainty of ~0.5 kpc if the metallicity gradient evolution is known and after the rotationally supported stellar disk has started to form. I then developed a method to recover the time evolution of the stellar birth metallicity gradient, d[Fe/H](R, t)/dR, through its inverse relation to the metallicity range as a function of age today. This allows me to place any star with age and metallicity measurements back to its birthplace, Rb. Applying this method to a high-precision large data set of MW disk subgiant stars, I find a steepening of the birth metallicity gradient from 11 to 8 Gyr ago, which coincides with the time of the last major merger, Gaia-Sausage-Enceladus (GSE). By dissecting the disk into mono-Rb populations, clumps in the low-[alpha/Fe] sequence appear, which are not seen in the total sample and coincide in time with known star-formation bursts.

Geographic Areas


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More About This Work

Academic Units
Thesis Advisors
Mac Low, Mordecai-Mark
Angus, Ruth
Ph.D., Columbia University
Published Here
September 27, 2023