Outflows from star-forming galaxies in the early universe

Abstract

In this thesis, I present three studies centered on galactic outflows driven by star formation in the early universe, and the systems that play host to them.

Observations of outflowing molecular gas from star-forming galaxies are crucial to understanding how outflows may regulate the growth of galaxies' stellar mass, as molecular gas provides the fuel for star formation. In the first part of this thesis, I investigate signatures of outflows in molecular gas from dusty star-forming galaxies in the early universe (z > 4). I re-visit observations of hydroxyl (OH) absorption lines from five such galaxies that a previous study identified as exhibiting evidence for molecular outflows. I show that the spectral features that were used to diagnose outflows are not statistically significant. Moreover, I show that these features suffer from inherent pitfalls that make them unreliable indicators of outflows. I then conduct a more thorough search for outflow signatures, utilising the full spatio-kinematic information as seen in spatially-resolved channel maps. I find spatio-kinematic patterns consistent with outflows in all five sources inspected, including one not identified in the prior study, thus providing more robust evidence of ubiquitous molecular outflows in such galaxies.

Due to limited spatial resolution when observing distant objects, searches for outflows in the early universe are often forced to rely on spectral signatures alone. For this reason, Lyman-α emission has become a valuable indicator of outflows due to its resonant nature, which couples the emergent line profile to the kinematics of the gas in and around galaxies. In my second study, I present a spectral analysis of 339 gravitationally-lensed star-forming galaxies in the early universe that exhibit Lyman-α emission. The lensing magnification provides a boost in signal-to-noise ratio, facilitating the detection of weak spectral signatures that I use, in conjunction with the Lyman-α profiles, to diagnose clumpy, multiphase outflows from these objects. I also search for relationships between the Lyman-α spectral profiles and other spectral features, finding evidence that sources with double-peaked Lyman-α profiles are associated with HII regions with more intense ionization fields yet slower outflows, while those with single peaks are associated with more gas-rich environments and faster outflows. I compare these findings with idealised models that reproduce the Lyman-α profiles using expanding shells of partially neutral gas.

In the final study, I investigate the star formation history of one of the most highly gravitationally-magnified sources from the second study. A previous study found this object to have a compact size < 100pc, and a stellar population with a young age of < 10Myr. I perform spatially-resolved stellar population modelling of the source, using images from the James Webb Space Telescope. I present, for the first time, evidence that this source also harbors an older underlying stellar population (age ≲ 1Gyr). I argue that the morphology is consistent with a picture in which the old stellar population extends to a larger radius than the young. I consider a scenario in which metal-enriched gas ejected into the circumgalactic medium by the first generation of stars is accreted to fuel the present star formation.