Teja Teppala
University of Missouri
This guest post was written by Teja Teppala, a Ph.D. candidate in the Department of Physics and Astronomy at the University of Missouri. He earned a Master’s in Physics from the University of Hyderabad, India. He also earned a Master’s in Physics and a Graduate Certificate in Public Engagement from the University of Missouri. His research focuses on constraining the timescales of bursty star formation in low-mass galaxies beyond the local Universe.
Title: Cosmic Reionization in the JWST Era: Back to AGNs?
Authors: Piero Madau, Emanuele Giallongo, Andrea Grazian, and Francesco Haardt
First Authors’ Institution: Department of Astronomy & Astrophysics, University of California – Santa Cruz
Status: Accepted to ApJ [open access]
Cosmic Reionization: a primer
The Universe as we know it started with a Big Bang. It expanded rapidly, bringing the temperature down, but it was still too hot for neutral atoms to form. The Universe remained in a state of ionized, opaque plasma for about 370,000 years until it cooled sufficiently to let cosmic hydrogen recombine into neutral atoms. At this point, the Universe became transparent, dark, and devoid of any light sources such as stars except for the cosmic microwave background (CMB). These “Dark Ages” persisted up until 1 billion years after the Big Bang, when hydrogen in the Universe began to reionize. This “era of reionization” is integral to the evolution of the Universe, as it is closely tied to how the first stars and galaxies formed and how they shaped the large-scale structure of the Universe. But a question remains: what was the trigger for cosmic reionization? In this paper, the authors evaluate the scenario where Active Galactic Nuclei (AGNs) could have possibly driven the cosmic reionization process, leveraging data from the James Webb Space Telescope (JWST).
Young galaxies vs AGNs
Conventionally, massive stars in young galaxies were thought to provide the photons required to ionize hydrogen. The photons, known as Lyman continuum photons (LyC), have energies above the Lyman limit (> 13.6 eV). However, only a few star-forming galaxies that leak LyC photons are known to exist in the early Universe, leaving much of the necessary ionizing radiation unaccounted for. The authors approach this problem through AGNs. AGNs are accreting black holes at the centers of galaxies that release an enormous amount of energy (~ up to 100 billion times the luminosity of the Sun) across the electromagnetic spectrum. Recently, deep surveys from the JWST have discovered large numbers of moderate luminosity AGNs at high redshifts. AGNs typically leak ≳ 80% of LyC radiation into the surrounding intergalactic medium (IGM), making them a substantial candidate for being the reionization source.
Modeling the emissivity of AGNs
There aren’t many non-active star-forming galaxies that are known to be LyC leakers – in fact, many of the LyC-emitting galaxies in the local Universe are observed to host accretion-powered sources, most likely AGNs. The authors modeled the energy emitted by AGN using data from previous surveys: Figure 1 shows the emissivities of AGNs and host galaxies above the Lyman limit (> 13.6 eV) across different redshifts. The solid curve shows the functional form of the AGN emissivity model.
Predicting reionization histories
The reionization driven by this AGN emissivity can be modeled analytically by using an ordinary differential equation that relates the ionized volume fraction Q with the rate of LyC injection, gas densities, and time taken for recombination. The authors used two AGN emissivity models to derive the reionization histories for hydrogen (H I) and ionized helium (He II): Model I is flatter and includes substantial absorption from the gas and stars surrounding the nuclei, while Model II is steeper with negligible internal absorption. Figure 2 shows the fiducial reionization histories: the ionized volume fractions of hydrogen (QHII) and helium (QHeIII) are depicted in blue and orange respectively. Hydrogen in the intergalactic medium is fully reionized when QHII = 1, while helium is fully doubly reionized when QHeIII = 1.
In these models, 99% of hydrogen is reionized by redshift z ≃ 5.3 – 5.5 (roughly 1.1 billion years after the Big Bang), while helium gets doubly reionized by redshift z ≃ 2.8 – 3.0 (about a billion years later), which is consistent with observations of absorption lines arising from the Lyman-alpha transition at this epoch. As a sanity check, the authors have also checked the consistency of their models with the observed CMB anisotropy data and potential contributions to the observed X-ray background (XRB) and found the AGN-dominated scenario doesn’t violate the observations. Figure 3 shows the consistency of these models with observations from a variety of probes and techniques: The solid and dot-dashed curves denote the percentage of ionized hydrogen for Model I and Model II respectively. Adding a small contribution from the host galaxy in Model I can push the complete reionization to earlier times, as depicted by the dashed line.
Are AGNs the sole drivers of cosmic reionization? TBD
With a plethora of data from the early Universe being observed using JWST, the authors urge that their results should promote further investigation into AGN-dominated scenarios for cosmic reionization. However, they note that their results are based on assumptions and therefore only meant to be suggestive, since there is no definite picture of the AGN fraction and internal absorption properties in the early Universe. If proven otherwise, the cosmic reionization models may require reverting to the conventional young galaxies-dominated scenario.
Astrobite edited by: Katherine Lee
Featured image credit: J. A. Biretta et. al., Hubble Heritage Team (STScI/AURA), NASA
Discover more from astrobites
Subscribe to get the latest posts to your email.
