Some 400,000 years after the Big Bang the Universe entered the cosmic “dark ages”, where galaxies and stars had yet to form amongst the dark matter, hydrogen and helium present. 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 has become a major area of current research in astrophysics, which marks the point at which the hydrogen in the Universe became ionized. Ionization made the Universe transparent to these photons, allowing the release of light from sources to travel mostly freely through the cosmos.
The ionization of hydrogen is important because of its effects on how galaxies grow and evolve. A particular area of interest is in assessing the contribution of different astrophysical sources (i.e., 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 study lead by Prof. Naveen Reddy [UC Riverside] marks the first quantitative study of how the gas content scales with the amount of interstellar dust. This analysis shows that the gas in galaxies is like a “picket fence”, where some parts of the galaxy have little gas and are directly visible, whereas other parts have lots of gas and are effectively opaque to ionizing radiation. In this framework, the research team has developed a powerful model that can be used to predict the amount of escaping ionizing radiation from galaxies based on straightforward measurements of 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 2 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 the >30m telescopes and the James Webb Space Telescope.
The data for this research was acquired through the Low Resolution Imaging Spectrograph on the W.M. Keck Observatory.
Reionization as illustrated by data from the Hubble and Chandra space telescopes. Credit: NASA/CXC/M.Weiss.