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 elemental abundances. My advisor is
Mariska Kriek.
Outside of astronomy, I enjoy knitting, cooking, 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).
Contact
Email: cheng@strw.leidenuniv.nl
Address: Leiden Observatory
Gorlaeus Building
Einsteinweg 55, BW3.29
NL-2333 CC Leiden
The Netherlands
