Bright galaxies blowing bubbles

Bright galaxies blowing bubbles

Essay title: CLEAR: Boosted Ly╬▒ transmission of the intergalactic medium in UV light galaxies

Author: 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 can’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 years of the universe, a transition period called Epoch of Reionization (EoR) took place, when the first stars and galaxies formed and began to emit high-energy light that ionized that then mostly neutral hydrogen gas that fills the universe. Ionizing radiation can kick electrons off neutral hydrogen atoms, and in the EoR this happened enough to ionize the universe’s gas almost entirely.

A damn mystery

Early galaxies are an important source of ionizing photons and perhaps the main drivers of this ionization process; properties of early galaxies and how they evolved in the first billion years have major implications for processes within the EoR. However, understanding both how many photons are 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 exactly when and where reionization occurred, as well as what types of galaxies were primarily responsible.

Track the strength of emissions from Lyman alpha (n=2 to n=1) transition hydrogen from early galaxies can give us a sense of where and who: what types of galaxies produce more of the ionizing photons, and are they clustered or scattered? This question corresponds to the spatial evolution of reionization. By tracking which fraction was ionized over time, the temporal evolution of reionization can also be constrained.

Today’s paper aims to tackle the reionization whodunnit, focusing on galaxies in the EoR. More specifically, they aim to distinguish between brighter and fainter galaxies, especially in the ultraviolet (UV) region where photons have high enough energy to ionize hydrogen. By determining trends between a galaxy’s ability to emit ionizing photons and the reionization near them, they can test the idea that UV-bright galaxies sit within highly ionized gas bubbles and that reionization is accelerated in these overdense regions of galaxies within the bubbles (illustrated in Figure 1) .

Cartoon depiction of inhomogeneous processes in reionization.  On the right are earlier times with more neutral gas.  Faint galaxies have smaller bubbles of ionized gas around them and smaller clusters of galaxies than the brighter galaxies.  Lyman alpha photons can escape the larger bubbles.  On the left is the observer looking through the mostly ionized nearby (later times) universe.
Figure 1. Representation of the different processes during reionization, with the more UV-bright galaxies (larger symbols) sitting in larger ionized bubbles (black) in the neutral gas (white). The ionized bubbles create an environment for the Lyman alpha photons to escape and ionize the surrounding gas more easily. More UV-faint galaxies are likely in the galaxy overdensities in the bubbles but are too faint to be detected with the current data set. Picture 5 in the newspaper.

Unequal equivalent widths

The paper attempts to answer one main question: is there any evolution of Lyman-alpha emission in EoR galaxies with respect to the UV luminosity of these galaxies? To help answer this, they measure the strength of the Lyman-alpha emission with a quantity called corresponding width as a function of both redshift and the inherent UV brightness. In their sample, they had a few hundred galaxies with detailed spectroscopic observations, with this paper presenting new data from 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.

Nevertheless, these non-detections may help to constrain the strength of Lyman-alpha emission coming from the galaxies. The logic is that they could (or even should) have detected something with 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 essentially rules out the presence 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 undetected sources and running some simulations of mock observations, the authors find some evidence for a different evolution of the 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 that have enhanced Lyman-alpha transmission (Figure 1). They note that reionization is probably quite complicated, with large spatial and temporal variations, and complex and variable processes. Nevertheless, although we can learn something from what we do not see, it is now operative JWST and other next-generation telescopes will be sensitive to fainter distant galaxies, enabling discoveries and getting a clearer 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 clusters)

About Olivia Cooper

I’m a sophomore at UT Austin studying the dark early universe, specifically the formation and evolution of dusty star-forming galaxies. In my undergraduate degree at Smith College, I studied astrophysics and climate change communication. Besides doing science with beautiful pictures of distant galaxies, I also like to drive to the middle of nowhere to take beautiful pictures of our own galaxy!

#Bright #galaxies #blowing #bubbles

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