Research
The launch of the James Webb Space Telescope (JWST) on December 22, 2021 marks a paradigm-shifting new phase in the field of observational astronomy. One of the primary science drivers of the JWST mission is the study of early galaxy formation. I am a member of 12 JWST programs totaling over 2000 hours of observing time that together will help us understand the earliest phases of our cosmic origin story.
My research program is centered around galaxy formation and evolution: understanding how the Universe evolved from its uniform state shortly after the Big Bang to the diversity of galaxies we see today. I use cutting-edge observational techniques to determine the structural and kinematic evolution of galaxies and how star formation is regulated. These two fundamental processes in turn provide the strong constraints on theoretical models of galaxy formation. I am excited to understand the physical processes that drive the emergence of galactic structure and regulation of star formation in the early Universe using the James Webb Space Telescope (JWST) on the observational side and magnetohydrodynamical cosmological simulations on the theoretical side. Some of my research initiatives are described below. I’m looking for excellent, kind, graduate students who are excited to understand the formation of the first galaxies.
Impossibly Massive Galaxies
We recently published a paper in Nature on 6 impossibly massive galaxy candidates we found in new James Webb data. (60 Minutes, Science Friday Colorado, Press Release, Colorado Public Radio). These candidate galaxies may contain as many stars as the modern day Milky Way, except they’re at the dawn of time. The early universe should not have had time to organize itself that quickly, It’s bananas. Lambda-CDM has been a cornerstone of modern astrophysics for more than two decades. While flipping through the first JWST images, I noticed a number of galaxies that were extremely red, surprisingly bright, and not detected with HST (the Hubble Space Telescope). Fitting their redshifts and stellar masses, we realized that these galaxies were so massive at such early times that they are difficult or impossible to form in our standard LCDM cosmology (Labbe, van Dokkum, Nelson et al. Nature). Specifically, the masses of these galaxies exceed the limit set by the number of available baryons in the most massive dark matter halos; there are no dark matter halos massive enough to form these galaxies even if all available baryons were converted into stars.
James Webb Space Telescope Observing Programs
Jades
JADES
Discovering and characterizing the first galaxies to form in the early Universe is one of the prime reasons for building a large, cold telescope in space, JWST. I am a member of the JWST Advanced Deep Extragalactic Survey (JADES), an 800 hour joint program of the NIRCam and NIRSpec Guaranteed Time Observations teams likely to shape the course of high redshift investigations for the 2020s. This program will study the evolution of galaxies from z~2-14: luminosity functions beyond the current redshift frontier, the build up of stellar mass, the evolution of galaxy structure, chemical enrichment, and the discovery of the first quenched galaxies in the first few billion years of cosmic time. {Astro2020 Science White Paper}
FRESCO
As a result of the high spatial resolution and slitless nature of spectroscopy with JWST’s NIRCam grism, it has a few key advantages over the use of a traditional spectrograph. First, it provides spectra for every object in the field of view; because no preselection for the placement of slits is required, it dramatically expands the potential discovery space. Second, instead of a single unresolved spectrum for each object, it yields spatially resolved maps of line emission in addition to velocity fields. The JWST Cycle 1 FRESCO program is using this capability to conduct the “First Reionization Epoch Spectroscopically COmplete” Survey (who doesn’t love a tortured acronym) which will provide spectra for >300 galaxies in the cosmic dark ages (z~7-9) and 1200 in the epoch of reionization (z~5-6.5). Having pioneered the method, I am the leader in the measurement and interpretation of resolved properties of ionized gas from space-based slitless spectroscopy (Nelson et al. 2012). With emission line maps from FRESCO, we will be able to determine how galaxies grew during the epoch of reionization at z>6, the first measurements of their kind. With much smaller separations between galaxies at these early cosmic times, do galaxies build through merger-induced central starbursts or inside-out as a result of accretion of gas from the cosmic web?
Ultra-deep Nirspec and nirCam ObserVations before the Epoch of Reionization to find the first galaxies deep into the dark ages. (public release)
Walking the Plank to Spatially Resolved Stellar Populations with Pirate
New observations and technology help drive the galaxy evolution field at a fast past, continually stretching the horizons of our cosmic perspective. A new leap in technology is happening now with the James Webb Space Telescope, and not too far down the road the 30-meter era of ground-based astronomy. With this next generation, a wealth of higher spatial resolution imaging is becoming available for galaxies spanning from the rest-frame ultraviolet to near-infrared over most of cosmic time. JWST will invest thousands of hours imaging extragalactic fields with the goal of understanding the structural evolution of galaxies. But the exquisite imaging that JWST provides is not enough on its own, we also require a resolved stellar population synthesis framework to turn the measured fluxes into physically meaningful tracers of galaxy evolution; namely, maps of stellar mass and star formation. We are at a critical junction, we do not yet have the proper tools to leverage the full potential of this data. I am the PI of Pirate, a next generation tool under development that will enable us to do this. Pirate is a sophisticated forward-modeling tool for combining multi-wavelength JWST data spanning over an order of magnitude in resolution from 0.03 to > 0.5” into a cohesive physical model while preserving spatial complexity. This tool is being built on the existing Prospector Bayesian Inference framework to be Prospector-Resolved (Prospector-R, pronounced “Prospector-arrrhh”) and is hence known as Pirate. The fitting is sped up by a factor of up to 1000 owing to a neural net emulator (Parrot). In addition to allowing us to place new constraints on how galaxies grow and quench, as a forward looking tool that will be publicly released, Pirate will enable a wealth of science by the community for years to come.