Authors: Intae Jung, Casey Papovich, Steven L. Finkelstein, Raymond C. Simons, Vicente Estrada-Carpenter, Bren E. Backhaus, Nikko J. Cleri, Kristian Finlator, Mauro Giavalisco, Zhiyuan Ji, Jasleen Matharu, Ivelina Momcheva, Amber N. Straughn, Jonathan R. Trump
First Author’s Institution: Astrophysics Science Division, Goddard Space Flight Center
Status: First revision submitted to ApJ [open access upon publication]
Sometimes the things we don’t see can still give us insight. This strategy of getting clues from both detections and non-detections is common in astronomy, and the non-detections in today’s paper are used to better understand the process of reionization. Sometime during the first billion or so years of the Universe, a period of transition called the Epoch of Reionization (EoR) took place, as the first stars and galaxies formed and began emitting high energy light which ionized the then mostly neutral hydrogen gas filling the universe. Ionizing radiation can kick off electrons from neutral hydrogen atoms, and in the EoR this occurred enough to ionize the Universe’s gas nearly completely.
A whodunnit mystery
Early galaxies are a major source of ionizing photons and perhaps the main drivers of this ionization process; properties of early galaxies and how they evolved over the first billion years has great implications for processes within the EoR. However, understanding both how many photons are being produced, and then whether they manage to escape their galaxies and ionize the neutral gas around them, is highly dependent on the physical conditions of each galaxy, and therefore challenging to constrain and predict. These challenges lead to more challenges in determining precisely when and where reionization occurred, as well as what kinds of galaxies were primarily responsible.
Tracing the strength of emission from the Lyman-alpha (n=2 to n=1) transition of hydrogen from early galaxies can give us a sense of the where and who: what sorts of galaxies produce more of the ionizing photons and are they clustered together or spread out? This line of questioning corresponds to the spatial evolution of reionization. By tracking what fraction was ionized over time, the temporal evolution of reionization can also be constrained.
Today’s paper seeks to get at the whodunnit of reionization, focusing on galaxies in the EoR. More specifically, they aim to make distinctions between brighter and fainter galaxies, particularly within the ultraviolet (UV) range where photons are high enough energy to ionize hydrogen. By determining trends between a galaxy’s capacity to emit ionizing photons and the reionization near them, they can test the idea that UV-bright galaxies sit within highly ionized bubbles of gas, and that reionization is accelerated in these overdense regions of galaxies in the bubbles (illustrated in Figure 1).
Inequivalent equivalent widths
The paper seeks to answer one main question: is there any evolution of Lyman-alpha emission in EoR galaxies with respect to the UV brightness of those galaxies? To help answer this, they measure the strength of Lyman-alpha emission with a quantity called equivalent width as function of both redshift and the intrinsic UV brightness. In their sample, they had a few hundred galaxies with detailed spectroscopic observations, with this paper presenting new data from the Hubble Space Telescope. With these data, the team searched for any signal (continuum) or Lyman-alpha emission lines, and found no convincing Lyman-alpha emission or continuum detected galaxies within the spectra.
Still, these non-detections can help constrain the strength of Lyman-alpha emission coming from the galaxies. The logic is, they could (or even should) have detected something at the sensitivity of their observations if there is no redshift evolution of equivalent width before and after the near end of the EoR (redshift z ~ 6). This basically rules out the existence of strong Lyman-alpha emission (in other words, high equivalent widths) in this sample, which included more UV-faint galaxies than their detected sample from previous work.
By comparing the detected and non-detected sources and running some simulations of mock observations, the authors find some evidence for a different evolution of Lyman-alpha emission line strength between bright and faint galaxies through the EoR. Their analysis is consistent with a picture where reionization is spatially inhomogeneous with large ionized bubbles made by bright galaxies which have boosted Lyman-alpha transmission (Figure 1). They note that reionization is probably fairly complicated, with large spatial and temporal variations, and complex and variable processes. Nonetheless, while we can learn something from what we don’t see, the now operating JWST and other next generation telescopes will be sensitive to fainter distant galaxies, enabling detections and get a more clear picture of the EoR.
Astrobite edited by Evan Lewis
Featured image credit: DepositPhotos (bubbles) & NASA, ESA, J Lotz and the HFF Team at STSci (galaxy cluster)