About Me

I am a PhD candidate at Leiden Observatory, the astronomy department at Leiden University. I work on galaxy formation and evolution, with a particular interest in chemical abundances. My advisor is Mariska Kriek.

Outside of astronomy, I enjoy playing quidditch (pictured above), cooking, knitting, sewing, exercising, and singing! I am originally from Markham, Ontario, but I have also lived in Arcata, California, Toronto, Ontario, and Leiden, The Netherlands.

Education:

  • PhD in Astronomy at Leiden Observatory: 2022 - Present, supervised by Mariska Kriek.
    Thesis: Unravelling the formation histories of distant quiescent galaxies using ultra-deep spectroscopy.
  • MSc in Physics at the University of Waterloo and the Waterloo Centre for Astrophysics: 2020 - 2022, supervised by Michael Balogh.
    Thesis: Testing the extremes of initial mass function variability using compact stellar systems.
  • Hon. BSc in Astronomy and Physics at the University of Toronto: 2016 - 2020, supervised by Jo Bovy.
    Thesis: Testing the chemical homogeneity of chemically-tagged dissolved birth clusters.

Research Interests: I am broadly interested in galaxy formation and evolution. In general, I use spectroscopy to reveal the early stages of galaxy evolution and star formation, using the archaeological record contained in the chemistry of stellar populations. I enjoy mixing observational techniques with statistical and computational methods.

Publications: I publish under the name "Chloe M. Cheng". Here is an ADS link to my astronomy publications. If you are interested, I also have some co-authored publications in the fields of neuroscience and nuclear physics! Here is a link to my Google Scholar page.

Research

I currently study quiescent galaxies at intermediate-to-high redshifts. Previously, however, I have worked on low-redshift stellar systems as well as the Milky Way. In all of my work, I use spectroscopy to examine the stellar populations of different regions. Below is a summary of each of my major projects. (Image credit: NASA)

Unravelling the formation histories of distant quiescent galaxies using ultra-deep spectroscopy
Fundamental insights into the chemical enrichment, assembly, and star formation histories of galaxies across cosmic time can be gained by examining the fossil records encoded in their stellar populations. Previously, Beverage et al. (2021) and Beverage et al. (in prep). measured the integrated chemical abundance patterns in a large sample of massive early-type galaxies (ETGs) at redshifts between 0.6 and 0.75 from the LEGA-C survey. From these results, we have learned fascinating new details about chemical enrichment regulation, star formation timescales, and galaxy quenching. However, the detailed star formation and assembly histories of galaxies are imprinted in the spatial distributions of their chemical abundances. We are currently working on measuring spatially-resolved stellar population parameters in the LEGA-C ETGs by employing the alf models over different apertures. alf is a suite of full-spectrum stellar population synthesis models that simultaneously fit for ages, metallicities, and detailed chemical abundances.

In the near future, we will apply the methods that we develop with the LEGA-C data to our approved Cycle 1 JWST/NIRSpec program. We will obtain deep rest-frame optical spectra of quiescent and star-forming galaxies, extending this analysis to higher redshifts (between 1.0 and 2.5). We will additionally be able to combine this with data from the Heavy Metal survey, which contains ultra-deep spectra of 20 quiescent galaxies at redshifts 1.3 to 2.3. Together, these three datasets will provide a deeper understanding of the formation histories of quiescent galaxies over ~5 Gyr of cosmic time.

Stay tuned for new results coming soon!

Testing the extremes of initial mass function variability using compact stellar systems
A fundamental concept underpinning star formation and galaxy evolution studies is the distribution of birth stellar masses in a galaxy, called the initial mass function (IMF). Traditionally, most studies have assumed that the IMF is "universal", because measurements made via the direct method of resolved star counts in the Milky Way and in nearby star-forming regions have found little-to-no variation. This assumption has been challenged by increasingly detailed, indirect measurements in diverse, extragalactic stellar populations. However, observations of variations in the IMF with environment are still debated, since there is still not a satisfactory theoretical framework to explain this.

A key limitation is that observations thus far have only probed metal-rich ETGs, which encompass a narrow region of mass-metallicity-density parameter space. In this work, we observe the integrated light spectra of diverse objects, including "compact" stellar systems (i.e. globular clusters and ultra-compact dwarf galaxies) and brightest cluster galaxies. We sample a wide range of metallicities (-1.7 < [Fe/H] < 0.01) and velocity dispersions (between 7.4 km/s and 275 km/s). We reduce high S/N Keck LRIS spectra. We measure the IMF by fitting the spectra with the alf models, which allow for IMF variations. This is a follow-up study to Villaume et al. (2017)

We show that compact stellar systems do not follow the same trends with physical parameters that have been found for ETGs. This is shown in the Figure above, where we plot the IMF mismatch parameter (ratio between the mass-to-light ratio for a fit allowing for IMF variation and a fit where we fix the IMF to the Milky Way value) as a function of stellar parameters. The objects in our sample are shown in colour and ETGs from the literature are shown in grey. The dashed line represents the value of the IMF mismatch parameter for a Kroupa (2001) Milky Way IMF. In particular, we find that previously established trends between metallicity and IMF variation may change in complex ways.

This work was published in Cheng et al. (2023).

Testing the chemical homogeneity of chemically-tagged dissolved birth clusters
Chemically tagging stars back to common formation sites in the Milky Way is crucial for understanding the chemical and dynamical history of the Galactic disc. In Price-Jones et al. (2020), 21 dissolved birth clusters were found in the APOGEE survey, by blindly chemically tagging an eight-dimensional chemical space using the Density-Based Spatial Clustering Applications with Noise algorithm (DBSCAN). In this work, we constrain the intrinsic abundance scatter in 17 of these groups. We do this by modeling the stellar spectra as a one-dimensional function of initial stellar mass, forward modeling the observed spectra, and comparing the data and the models using Approximate Bayesian Computation. We test the method on the well-studied open clusters M67, NGC 6819, and NGC 6791, using data from OCCAM (Donor et al. 2018). In general, we are able to strongly constrain the 15 elements that we examine. This is shown in the Figure above, which is asummary plot of the constraints on the chemical abundances for each element X in all of the clusters that we examine. The median constraint on each element across the chemically tagged birth clusters is shown in purple. The interquartile range is represented by the blue region. The elements used in Price-Jones et al. (2020) to tag the birth clusters are indicated by grey lines. The median constraint across the Milky Way open clusters from OCCAM are shown in green. This strengthens the statement that these groups of stars represent birth clusters.

This work was published in Cheng et al. (2021).

Community

Teaching: I have worked as a teaching assistant at the University of Waterloo from 2020 - 2022. I will begin working as a teaching assistant at Leiden University starting Winter 2023. Below are a list of courses that I have/will TA:

  • Leiden University:
    • Galaxies and Cosmology (Winter 2023)
  • University of Waterloo:
    • PHYS 375: Stars (Winter 2022)
    • PHYS 342: Electricity and Magnetism 2 (Fall 2021)
    • PHYS 112L: Physics 2 Laboratory (Winter 2021)
    • PHYS 121: Mechanics (Fall 2020)

Leadership & Outreach: I am always interested in getting involved in the astronomical community, and in particular I am actively looking for opportunities to contribute to Equity, Diversity, and Inclusion in astronomy. Here are some of my activities in this area:

  • Leiden Observatory Equity, Diversity, and Inclusion Committee. Member. Sept. 2022 - Present.
  • Leiden Observatory Social Committee. Member. April 2023 - Present.
  • Leiden Observatory Borrel Committee. Member. September 2023 - Present.
  • Seeing Stars Leiden. Volunteer. Sept. 2022.
  • Graduate Student Committee, Canadian Astronomical Society (CASCA). Social Media Coordinator and University of Waterloo Representative. Sept. 2021 - Aug. 2022.
  • Canadian Conference for Undergraduate Women in Physics. Volunteer. Jan. 2019.