I am interested in a variety of topics related to galaxy formation and evolution within the first billion years of cosmic time, and how these early galaxies drove the process of cosmic reionization. I am particularly interested in (1) understanding the stellar populations and ionizing properties of these early galaxies, and (2) using the Lyα emission line to probe the timeline and topology of reionization.
Star Formation Histories of Galaxies over Cosmic Time
Understanding the cosmic star formation history is a key component of understanding both galaxy formation and evolution in the early Universe and how these objects drove the process of cosmic reionization. While we cannot directly observe the full history of a galaxy's formation and evolution, the properties it has at the time we observe it provides indirect insights into its past, providing us with independent constraints on galaxies at very early times (which are very complementary to direct constraints on galaxies being directly observed at z > 12).
In a series of papers over the course of my thesis work with Professor Dan Stark, I have investigated the star formation histories (SFHs) of galaxies in the first billion years of cosmic time over a large range of redshifts and luminosities.
Probing Reionization with Lyα Emission from Galaxies
Lyα emission is highly susceptible to attenuation by neutral hydrogen, so can be used to infer information about the quantity and distribution of neutral hydrogen in the intergalactic medium (IGM) during reionization. When compared to the expected intrinsic Lyα emission that escapes a galaxy, the amount of Lyα that transmits through the reionizing IGM provides insights into the volume-averaged fraction of neutral hydrogen in the IGM at a given time. The details of the line shape and the spatial distribution of galaxies that emit Lyα provides insights into the topology of the reionization process.
As part of my thesis work, I have searched for an overdensity of galaxies near two of the earliest known Lyα emitting galaxies at a redshift of z=8.7. If such an overdensity exists, it may be expected to trace the presence of a large ionized bubble
As an undergraduate, I also worked at the Center for Astrophysics | Harvard & Smithsonian with Dr. Charlotte Mason to improve a physical model of Lyα transmission through the IGM, to be used to infer the timeline of cosmic reionization from observations of Lyα emission from galaxies. Specifically, I implemented scatter in the relation between galaxy UV luminosity and dark matter halo mass and inferred the volumetric neutral fraction at z~7 with the improved model.
The Hydrogen Epoch of Reionization Array
The neutral hydrogen in the IGM provides a different method to study reionization. Neutral hydrogen emits radiation with a wavelength of 21 centimeters during its spin-flip transition, and this spectral line is a promising probe of the EoR. However, actually detecting the line requires overcoming significant observational challenges. These effects can be environmental (e.g. bright radio foregrounds from galactic and extragalactic sources, radio frequency interference, and atmospheric attenuation), or instrumental (e.g. calibration errors).
The Hydrogen Epoch of Reionization Array (HERA) is a radio interferometer currently under construction in South Africa designed to probe the large-scale structure during reionization via power spectral measurements of 21 cm line. I used HERA commissioning data to examine the effects of radio frequency interference and calibration systematics on measurements of the 21 cm power spectrum from the EoR, with the eventual goal of understanding the impacts of these systematics on astrophysical parameter estimation from 21 cm EoR measurements.
The Radio Halo of NGC 4565
I imaged the nearby edge-on galaxy NGC 4565 with data from the Low Frequency Array (LOFAR) at 144 MHz from the LOFAR Two-meter Sky Survey. Using this low frequency image, I created vertical intensity and spectral index profiles and characterized the cosmic ray transport mechanism in the galaxy as being primarily diffusive, rather than advective. This has implications for galactic outflows and the transfer of energy and heavy elements between the interstellar, circumgalactic, and intergalactic media.
See the paper here: Heesen et al. (2019)
Modelling Far-Infrared Line Emission from Galaxies
I contributed code to the SImulator of GAlaxy Millimetre/submillimetre Emission (SÍGAME) module, a code to post-process the results of cosmological galaxy formation simulations to extract the far-infrared line emission of galaxies. I primarily added analysis capabilities, including calculating radial profiles of a given far-infrared line and maps of line emission ratios (e.g., [CII]158μm/[NII]205μm). I also worked towards validation efforts, compiling observations of far-infrared lines at both high and low redshift to compare to simulation results.