New Model For Universe’s Reionization Unveiled

MAUNAKEA, Hawaii — Two papers published by an assistant professor at the University of California, Riverside and several collaborators explain why the Universe has enough energy to become transparent. The study used data from the W. M. Keck Observatory Maunakea, Hawaii and was led by Naveen Reddy an assistant professor in the Department of Physics and Astronomy at UC Riverside. This is the first quantitative study of how the gas content within galaxies scales with the amount of interstellar dust. It shows that the gas in galaxies is like a “picket fence”: some parts of the galaxy have little gas and are directly visible, while other parts have lots of gas and are effectively opaque to ionizing radiation. The findings were published today in the Astrophysical Journal.

The ionization of hydrogen is important because of its effects on how galaxies grow and evolve. A particular area of interest is assessing the contribution of different astrophysical sources, such as stars or black holes, to the budget of ionizing radiation.

Most studies suggest that faint galaxies are responsible for providing enough radiation to ionize the gas in the early history of the Universe. Moreover, there is anecdotal evidence that the amount of ionizing radiation that is able to escape from galaxies depends on the amount of hydrogen within the galaxies themselves.

The research team led by Reddy, gathered the data using Keck Observatory’s LRIS instrument. The scientists then developed a model that can be used to predict the amount of escaping ionizing radiation from galaxies based on straightforward measurements on how “red,” or dusty, their spectra appear to be.

Alternatively, with direct measurements of the ionizing escape fraction, their model may be used to constrain the intrinsic production rate of ionizing photons at around two billion years after the Big Bang.

These practical applications of the model will be central to the interpretation of escaping radiation during the cosmic “dark ages,” a topic that is bound to flourish with the coming of 30-meter telescopes, which will allow for research unfeasible today, and the James Webb Space Telescope, NASA’s next orbiting observatory and the successor to the Hubble Space Telescope.

The research ties back to some 400,000 years after the Big Bang, when the Universe entered the cosmic “dark ages,” where galaxies and stars had yet to form amongst the dark matter, hydrogen and helium.

A few hundred million years later, the Universe entered the “Epoch of Reionization,” where the gravitational effects of dark matter helped hydrogen and helium coalesce into stars and galaxies. A great amount of ultraviolet radiation (photons) was released, stripping electrons from surrounding neutral environments, a process known as “cosmic reionization.”

Reionization, which marks the point at which the hydrogen in the Universe became ionized, has become a major area of current research in astrophysics. Ionization made the Universe transparent to these photons, allowing the release of light from sources to travel mostly freely through the cosmos.

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes near the summit of Maunakea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and world-leading laser guide star adaptive optics systems.

The Low Resolution Imaging Spectrometer (LRIS) is a very versatile visible-wavelength imaging and spectroscopy instrument commissioned in 1993 and operating at the Cassegrain focus of the Keck I telescope. Since it has been commissioned it has seen two major upgrades to further enhance its capabilities: addition of a second, blue arm optimized for shorter wavelengths of light; and the installation of detectors that are much more sensitive at the longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This large range of wavelength coverage, combined with the instrument’s high sensitivity, allows the study of everything from comets (which have interesting features in the ultraviolet part of the spectrum), to the blue light from star formation, to the red light of very distant objects. LRIS also records the spectra of up to 50 objects simultaneously, especially useful for studies of clusters of galaxies in the most distant reaches, and earliest times, of the Universe.

Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.