As part of my thesis at Yale, I pioneered a new method to map the emergence of galactic structure by mapping star formation using the Wide Field Camera 3 grism on the Hubble Space Telescope. 


By combining information on the spatial distribution of ionized gas from HST and dynamics from Keck spectroscopy, I shed new light on where star formation occurs in galaxies.

1. 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. 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. 


2. Star Formation is Spatially Coherent (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-Mplane 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; in particular, apparently neither nuclear star bursts nor inside-out quenching are dominant mechanisms.