A Study in Lyman-alpha

Open Access
Hagen, Alex Richard
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
November 17, 2017
Committee Members:
  • Robin Bruce Ciardullo, Dissertation Advisor
  • Robin Bruce Ciardullo, Committee Chair
  • Caryl Ann Gronwall, Committee Member
  • Sarah Elizabeth Shandera, Committee Member
  • Donald P Schneider, Outside Member
  • Astronomy
  • Astrophysics
This thesis concentrates on the physical and morphological properties of galaxies via image analysis and spectral energy distribution fitting. My research primarily focuses on Lyman-α emitting galaxies (LAEs) at z ∼ 2, but to place the investigations in context, comparison samples of galaxies are also used. These galaxies present the opportunity to study formation and evolution of low-mass galaxies at this redshift, which is extremely difficult using continuum selection methods. Chapter 1 gives an introduction to LAEs and puts them in the context of galaxy formation and evolution. Chapter 2 was originally published as Hagen et al. (2014), which studied the physical properties of LAEs at 1.9 ≤ z ≤ 3.5 from the HETDEX Pilot Survey (Adams et al. 2011; Blanc et al. 2011). This paper found that LAEs span almost a three dex range in stellar mass and could be simply drawn for the star-forming galaxy mass function. We also found that, while most LAEs are dust poor, there were some objects that weren’t. The Lyman-α photons seem to experience the same dust opacity as the UV continuum, suggesting that in these galaxies Lyman-α does not undergo significant scattering before escaping. This result, when matched with radiative transfer simulations, will help to constrain how Lyman-α escapes galaxies. Chapter 3, originally published as Hagen et al. (2016), describes research that studies how Lyman-α emitters are drawn from star-forming galaxies. This work used galaxies identified via their optical emission lines (oELGs) as a comparison sample (Zeimann et al. 2015a). These oELGs have a mass range similar to LAEs and are an excellent comparison sample to LAEs. Previous comparison samples, such as LBGs, are two dex more massive than LAEs and thus do not represent galaxies in the same evolutionary state as LAEs. We compared ten physical and morphological properties between Lyman-α and non-Lyman-α emitting galaxies and found no statistically significant differences between these two populations. This null result is actually very exciting, as this suggests that LAEs – such as the 106 LAEs soon to be found by HETDEX – can be used as unbiased tracers of the star-forming galaxy population. Chapter 4, originally Hagen et al. (2017), examines the rest-frame UV and rest- frame optical sizes of LAEs and oELGs from Zeimann et al. (2015a). We found that these morphological properties are indistinguishable between the samples, and that the galaxies are smaller in the rest-frame optical compared to the rest-frame UV. This suggests that low-mass galaxies are undergoing inside-out galaxy formation, which some papers had thought was only in higher mass galaxies. Chapter 5 is a proof-of-concept to show that LAEs detected from HETDEX commissioning observations can be cross-matched to Hubble Space Telescope counter- parts, which then can be used to perform significant research into galaxy formation and evolution. This chapter will continue to be developed into a journal article. Chapter 6 summarizes the findings of this work and discusses future research opportunities with HETDEX.