Research
The launch of the $10 billion James Webb Space Telescope (JWST) on December 22, 2021 marked 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 six Cycle 1 programs that together will help us understand the earliest phases of our cosmic origin story.
In a little more detail, 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 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, students and postdocs who are excited to understand the formation of the first galaxies.
James Webb Space Telescope Programs
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}
image credit: https://www.sciencemag.org/news/2016/02/building-james-webb-biggest-boldest-riskiest-space-telescope
Forming Massive Galaxy Cores
A cauldron of star birth in the center of a young galaxy. This illustration reveals the celestial fireworks deep inside the crowded core of a developing galaxy, as seen from a hypothetical planetary system. The sky is ablaze with the glow from nebulae, fledgling star clusters, and stars exploding as supernovae. The rapidly forming core may eventually become the heart of a mammoth galaxy similar to one of the giant elliptical galaxies seen today. Credit: NASA, Z. Levay, and G. Bacon (Space Telescope Science Institute).
Since galaxies grow inside-out (see below), most massive galaxies are thought to have formed their dense stellar cores at early cosmic epochs. However, this formation phase has been notoriously difficult to observe. Undertaking a large program with MOSFIRE and NIRSPEC on the Keck telescopes we uncovered one of these massive galaxy cores in it's turbulent formation phase in the early universe. This result was published in Nature (Nelson et al. 2014, arXiv) with accompanying press releases (NASA, Yale, Keck, ESA) and articles (The Washington Post, Christian Science Monitor, and TIME – plus a pretty hilarious one in The Week). We also showed that the population of progenitors likely followed a simple inside-out growth trajectory (van Dokkum, Nelson et al 2015). In the future, I hope to use the ability to map Hα, Hβ, and IR emission at high spatial resolution with JWST to fully uncover the core formation phase of massive galaxies.
Star formation builds galaxies inside-out
{Nelson et al. 2012} A galaxy’s Hα emission gives us information on where it is growing via star formation at the epoch of observation; its near-infrared (NIR) continuum emission tells us where it grew in the past. By mapping the distribution of Hα and NIR continuum emission in galaxies at 0.7 < z < 1.5, I showed that the Hα has a larger half-light radius. Taken at face value, this means that during this epoch when a third of the total star formation in the history of the Universe took place, most galaxies are increasing their radii due to star formation, assembling from the inside-out.
Looking ahead, to fully understand this size evolution in its physical context, requires determining whether the integrated size growth we observed in van Dokkum et al. 2013 and van Dokkum, Nelson et al. 2015 is explained by this star formation or if the Universe requires other processes to dissipate angular momentum.
Star formation is spatially coherent
As part of my thesis at Yale, I pioneered a new method to map the emergence of galactic structure using the Wide Field Camera 3 grism on the Hubble Space Telescope. By combining information on the spatial distribution of ionized gas from the 3D-HST survey and dynamics from Keck spectroscopy, I shed new light on where star formation occurs in galaxies.
{Nelson et al. 2016b} The star forming main sequence describes the derivative (star formation rate) - integral (stellar mass) pair serving as an organizing principle for galaxy growth. Key to understanding the physical drivers of the star forming main sequence is where star formation is located in normal star forming galaxies, as well as where it is enhanced in galaxies above the main sequence and where it is suppressed in galaxies below the main sequence. With the first systematic census of star formation across the SFR-M∗ plane at z ∼ 1, I found that star formation is spatially coherent: star formation is enhanced at all radii in galaxies above the main sequence and suppressed at all radii in galaxies below it. This provides strong constraints on the physical processes driving the enhancement and suppression of star formation in models of galaxy formation.
Most galaxies form most of their stars in disks
{Nelson et al. 20013} I marshaled a number of lines of evidence to buttress this claim. Stacking Hα maps of galaxies homogenized by size, I showed that the ionized gas displays nearly exponential surface brightness profiles. By comparing with expectations from model axis ratio distributions, I determined that the geometries of the ionized gas distributions were consistent with disks under different viewing angles. Conducting a multi-night follow-up campaign with the NIRSPEC spectrograph on the Keck telescope, I showed that a representative sample of normal star-forming galaxies all exhibited rotation-dominated kinematics. These lines of evidence converged to show that the Hα emission we observe originates predominantly from the exponential disks of galaxies. This result thus implies that most star formation occurs in galactic disks, at least in the regimes in which Hα is relatively unobscured and can be taken as a proxy for star formation (but see below).
Dust and In-Situ Bulge Building
{Nelson et al. 2016a} Probably the single greatest uncertainty in the interpretation of the maps and dynamics of Hα emission as star formation is radial gradients in dust extinction within galaxies. Using the WFC3 grism to map the distribution of Hα and Hβ emission, I made the first spatially resolved measurements of the Balmer decrement at z > 1. Systematic measurements of the Balmer decrement provide stringent constraints on the radial gradients in dust attenuating emission form star formation in galaxies. This allows us to make fully dust-corrected measurements of the radial distribution of star formation. With this information I showed that dust is unimportant at all radii in galaxies with M ∗ < 10 10 M ⊙ . Galaxies with M* > 10 10 M ⊙, on the other hand, have significant attenuation toward their centers. This has significant implications for how galaxies grow: low mass galaxies form most of their stars in exponential disks while higher mass galaxies have significant fraction of their star formation in their centers. This means that bulge growth can occur at least in part due to in situ star formation.
Millimeter Mapping at z~1
The formation of bulges in galaxies has remained something of a puzzle, due in part to a lack of spatial resolution at the epoch during which bulge formation was underway. Recently, however, this situation changed: the increased sensitivity and resolution of a new generation of publicly accessible mm/submm interferometers are allowing us to resolve the dust-obscured star formation and molecular gas kinematics of galaxy centers while they were forming. As PI, I led a program using the Northern Extended Millimeter Array (NOEMA) millimeter interferometer to complete the first dust continuum mapping of an Andromeda-like galaxy progenitor during the epoch of bulge growth (Nelson et al. 2019).
