Authors: Yongda Zhu, George D. Becker, Sarah E. I. Bosman, Laura C. Keating, Valentina D’Odorico, Rebecca L. Davies, Holly M. Christenson, Eduardo Bañados, Fuyan Bian, Manuela Bischetti, Huanqing Chen, Frederick B. Davies, Anna-Christina Eilers, Xiaohui Fan, Prakash Gaikwad, Bradley Greig, Martin G. Haehnelt, Girish Kulkarni, Samuel Lai, Andrea Pallottini, Yuxiang Qin, Emma Ryan-Weber, Fabian Walter, Feige Wang, Jinyi Yang
First Author’s Institution: Department of Physics & Astronomy, University of California, Riverside
Status: Accepted by ApJ [open access upon publication]
The who, what, when, and where of reionization are unresolved questions that have important implications for our understanding of the cosmos. This final phase transition of the Universe from neutral to ionized encompasses a variety of dramatic changes, as large scale structures formed and evolved, and the first stars and galaxies began to light up the Universe. While shining a light on the history of reionization would also help uncover the history of how objects in the Universe emerged and grew, this epoch is fundamentally dark.
However, we do have the basics of the why down: before the Epoch of Reionization (EoR), the Universe was mostly filled with neutral hydrogen gas. Then, as the first stars, galaxies, and quasars began to form and start shining, they emitted high energy photons which ionized the neutral gas around them. The ionizing radiation kicks out electrons from neutral hydrogen atoms, until eventually most of the gas in the Universe becomes ionized.
One key part of gaining a complete understanding of reionization is the when – precisely when did it begin and end, and how rapidly? There are a few main methods for probing these transition points, all of which suggest the process occurred early on, with a midpoint at roughly redshift z ~ 8 (600 million years after the Big Bang) and an endpoint somewhere around z ~ 5.5-6 (1 billion years after the Big Bang).
Absorbing it all
Many of the techniques used to trace reionization, including those in today’s paper, involve the Lyman series transitions of hydrogen, primarily the Lyman-α (n=2 to n=1) transition. One such technique relies on quasars (active supermassive black holes in the centers of galaxies, and among the brightest objects in the Universe) within the EoR. By observing distant quasars, we can understand the gas content in the Universe along that line-of-sight using the presence and absence of Lyman-α emission and absorption compared to quasar spectra, which are fairly well-understood and have strong signals. As this emission travels through material between the quasar and the observer, some emission gets intercepted by gas clouds along the way, which can absorb the emission and produce a Lyman-α absorption line at the gas cloud redshift. However, at redshifts before the end of the EoR, the primarily neutral gas in the way will be very opaque at this wavelength, as the scattering cross section of photons with wavelengths (energies) near Lyman-α with neutral hydrogen is very high, meaning the photons struggle to pass through the gas, which is optically thick enough to suppress observed emission nearly completely. This optical thickness to high energy photons results in a dark gap in the spectrum, or a contiguous region of strong absorption. (See Figure 1)
Filling in the gaps
The presence of these dark gaps could be caused by several different processes within reionization. For one, the ionizing background radiation itself could have some fluctuations – the authors explain that their results and other recent results disfavor this scenario as this doesn’t imply sufficient neutral gas at later times. Alternatively, the prevalence of so-called “islands” of neutral gas, like pockets of Lyman photon absorption, could be the cause. Lastly, reionization could simply end later in the history of the Universe, meaning more neutral gas is available to absorb high energy photons at lower redshifts.
These scenarios are difficult to disentangle with Lyman-alpha gaps alone. One solution, as presented in today’s paper, is to use a Lyman transition with a slightly shorter wavelength and a slightly lower optical depth to neutral gas. Today’s authors use this technique of tracing long dark gaps in quasar spectra but apply it to another Lyman transition, Lyman-β (n=3 to n=1). In order to study the dark Lyman-β gaps, they analyze spectra of a sample of EoR quasars at z > 5.5. Within each spectrum, they map out the dark gaps of Lyman-β absorption, the length of the gaps, and the redshift evolution of the gaps. As shown in Figure 2, one quasar spectrum in particular had a uniquely long dark gap down to z ~ 5.5. Within that line of sight, the authors found a low density region of galaxies, which supports the idea that highly opaque sightlines are associated with galaxy underdensities. This makes sense: in areas where there are fewer galaxies to ionize their surroundings, there is more remaining neutral gas.
The authors also emphasize the uniqueness of this study: the reionization scenarios are difficult to disentangle with Lyman-α gaps, as their signatures look similar. However, given Lyman-β’s lower optical depth, it is a more sensitive probe of neutral gas in the late IGM, and a useful tool to better understand the end of reionization. By further comparing their observations of dark gaps to expectations from cosmological simulations of these scenarios, the authors determine which reionization scenarios remain possible given their evidence.
Given the distinction enabled by Lyman-β data, the authors propose the best fit scenario for their dark gap sample is late reionization, with the EoR ending at z ~ 5.3. They demonstrate that rapid late reionization models, specifically with a fraction of neutral gas > 5% at z = 5.6, are consistent with the observations. Looking ahead, these dark Lyman-β gaps and future large samples of quasar spectra with gaps can help fill in the gaps in our knowledge of the timing of reionization.
Astrobite edited by Jana Steuer
Featured image credit: EarthSky (quasar image)