Joanna S. Bridge


GitHub Profile

Research Interests


Research Interests

Ever since Partridge and Peebles (1967) postulated that the hydrogen emission line of Lyman alpha (Lyα) would hold one of they keys to studying high-redshift galaxies, the question of what emission lines can tell us about the properties and evolution of galaxies has driven much of our research. I present in my thesis a survey of emission lines across cosmic history. Just as galaxies evolve with redshift, so does how we can leverage emission line information from these galaxies.

One project I’m doing uses resolved Lyα emission in nearby galaxies to do pixel-by-pixel photometry to study emission on very small galactic scales and to characterize Lyα photon scattering. Using the Lyman-Alpha Reference Sample, a set of 14 starbursting galaxies with redshifts of 0.02 < z < 0.2 observed with the Hubble Space Telescope Solar Blind Channel on the Advanced Camera for Surveys, I have begun to examine how Lyα photons scatter. Since Lyα is a resonant line, the photons are absorbed and re-emitted by the neutral hydrogen in galaxies until they finally escape or are absorbed by dust. Understanding how this random walk process affects where and how much Lyα we observe is very important, particularly for us to be able to use Lyα as a signpost of galaxy evolution at high redshifts. By comparison, the Hα transition, which is optically thin and therefore where we observe H-alpha photons are likely where they were emitted originally, is much easier to trace. I have developed a method for comparing the Hα and Lyα distributions in the LARS galaxies to determine a characteristic scattering length for Lyα photons. This is an important step in understanding exactly how much a Lyα photon scatters before escaping, and is important for understand the overall escape fraction of Lyα, both a low and high redshifts.

Shifting to higher redshifts of z ∼ 0.5, we can no longer use emission lines to study resolved images of the emission from galaxies. Instead, I have examined how we can study global galaxy properties using the emission lines, and how we can relate that to galaxy size and morphology. For this study, I used a sample of 284 [O II] emitting galaxies from the Hobby Eberly Telescope Dark Energy Survey (HETDEX) Pilot Survey. Using the forbidden transition of [O II] as a star-formation rate indicator, we found the these generally galaxies corroborate the main sequence of star forming galaxies. We also confirmed, however, that the [O II] is very sensitive to metallicity and abundance. Using image cutouts from COSMOS and GOODS-N, we found that the star formation in these galaxies was generally not due to galaxy mergers, but more likely the result in situ cold gas accretion.

Galaxy emission activity becomes more complicated as we approach cosmic noon (z ∼ 2), when star formation was at its height. To address this issue, I looked at how we can use both Balmer and oxygen emission lines to help disentangle star formation activity from emission caused by active galactic nuclei. I have done this by simulating z ∼ 2 galaxies with Hβ at 4863 Å and [O III]λ5007 emission and performing mock HST grism observations with the infrared G141 grism on the Wide Field Camera 3 (WFC3). The benefit of observing these galaxies using a grism is that we are able to gather spatial information about where the emission is coming from, whether it is nuclear and from more extended regions. By developing a method for separating these emission regions in our observations, we provide a way to find obscure or low-mass active galactic nuclei (AGN) that are undetectable by other methods such as X-ray or radio observations.

Finally, I’m also looking at the prospects for using emission lines to probe the first galaxies at z > 6 and how we can leverage new technology to catch them in the act of formation. I am a member of the ongoing HST WFC3/G102 grism program CANDELS Lyman-alpha Emission at Reionization (CLEAR). As part of this work, I have examined the completeness of the survey, simulating Lyα emission at these high redshifts. This is a necessary step in being able to leverage the ∼ 90 Lyα emitters that will be found to tell us not only about galaxy formation at high redshift but also about the luminosity function of Lyα emitters during this cosmic era.

Emission lines are one of the most important avenues through which we study galaxy evolution. My research as a graduate student has given me solid preparation to pursue a similar redshift evolution focused on the area of Lyα emission in galaxies, from the local universe, through cosmic noon, and to z > 6 galaxies.