These dark gaps could be caused by several different processes within reionization.
#QUASAR LIGHTS PATCH#
An over-dense patch of neutral gas causes a gap in the Lyman-α emission as nearly all of the flux from the quasar is absorbed. Emission lines directly from the quasar are marked by dashed colored lines. This optical thickness to high-energy photons results in a contiguous region of strong absorption in the spectrum that is known as a dark gap (see Figure 1).įigure 1: Example spectrum of a distant quasar with ionizing radiation emitted from the quasar intercepted by neutral gas along the line of sight. However, at redshifts before the end of the epoch of reionization, the primarily neutral gas in the way will be very opaque at this wavelength, as photons with wavelengths (energies) near Lyman-α struggle to pass through the neutral hydrogen, which is optically thick enough to suppress observed emission nearly completely. 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 typical quasar spectra, which are fairly well understood and have strong signals.Īs emission from a quasar 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 a wavelength determined by the redshift of the gas cloud. One such technique relies on observations of quasars (active supermassive black holes in the centers of galaxies and among the brightest objects in the universe) during the epoch of reionization.
#QUASAR LIGHTS SERIES#
Many of the techniques used to trace reionization, including those used in today’s article, involve the Lyman series transitions of hydrogen, especially the Lyman-α (n=2 to n=1) transition. 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). The ionizing radiation kicked out electrons from neutral hydrogen atoms until eventually most of the gas in the universe became ionized. Then, as the first stars, galaxies, and quasars began to form and started shining, these objects emitted high-energy photons that ionized the neutral gas around them. However, we do have the basics of the why down: before the epoch of reionization, the universe was mostly filled with neutral hydrogen gas. 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.
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. The who, what, when, and where of reionization are unresolved questions that have important implications for our understanding of the cosmos. Title: Long Dark Gaps in the Lyβ Forest at z<6: Evidence of Ultra Late Reionization from XQR-30 Spectraįirst Author’s Institution: University of California, Riverside We hope you enjoy this post from astrobites the original can be viewed at. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students.
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