Newly Developed Model for Reionization of the Universe is Based on Galaxy's Dust Content

Reionization as illustrated by data from the Hubble and Chandra space telescopes. CREDIT: NASA/CXC/M.WEISS.

An assistant professor at the University of California, Riverside and collaborators have published two papers that demonstrate why the universe has sufficient energy to become transparent.

The research headed by Naveen Reddy, an assistant professor in the Department of Physics and Astronomy at UC Riverside, highlights the first quantitative study of how the volume of gas existing within galaxies scales with the volume of interstellar dust.

This study points out that the gas in galaxies resembles a “picket fence,” where a few parts of the galaxy have little gas and are directly visible. The other parts consist of huge volumes of gas and are effectively opaque to ionizing radiation.

The findings are featured in The Astrophysical Journal.

The hydrogen ionization process is considered to be significant because of its effects on how galaxies grow and develop. The concept of examining the contribution of various astrophysical sources, like black holes or stars, to the budget of ionizing radiation is extremely important.

Several studies propose that faint galaxies are responsible for supplying adequate radiation to ionize the gas in the early history of the universe. There is also anecdotal evidence that the quantity of ionizing radiation capable of escaping from galaxies depends on the quantity of hydrogen existing within the galaxies.

The research team created a model that has the potential to predict the quantity of ionizing radiation escaping from galaxies based on straightforward measurements on how dusty or red their spectra appear to be.

With direct measurements of the ionizing escape fraction, the model could be used to limit the intrinsic production rate of ionizing photons to almost two billion years after the Big Bang.

All of these practical applications of the model will play a crucial role in understanding the escaping of radiation during the cosmic “dark ages,” a topic that is destined to gain popularity with the development of 30-meter telescopes, which will enable research that currently seems impractical, and the James Webb Space Telescope, NASA’s upcoming orbiting observatory and the successor to the Hubble Space Telescope.

The study ties back to some 400,000 years following the Big Bang, when the universe penetrated into the cosmic “dark ages,” where stars and galaxies had yet to develop amongst the dark matter, helium and hydrogen.

After almost hundred million years, the universe made an entry into the “Epoch of Reionization,” where the dark matter’s gravitational effects enabled helium and hydrogen to blend into galaxies and stars. A huge volume of ultraviolet radiation (photons) was discharged, resulting in stripping of the electrons from the nearby neutral environments, a process referred to as “cosmic reionization.”

Reionization - the point at which the hydrogen present in the Universe became ionized - has become a significant research area in astrophysics. Ionization enabled the Universe to become transparent to these photons, permitting the discharge of light from sources to freely pass through the cosmos.

The data for this work was obtained through the low resolution imaging spectrograph on the W.M. Keck Observatory.

The collaborators of this work are Charles Steidel (Caltech), Max Pettini (University of Cambridge), Milan Bogosavljevic (Astronomical Observatory, Belgrade) and Alice Shapley (UCLA).

The papers are “The Connection Between Reddening, Gas Covering Fraction, and the Escape of Ionizing Radiation at High Redshift” and “Spectroscopic Measurements of the Far-Ultraviolet Dust Attenuation Curve at z~3.”

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