Learning from a Flashlight in the Sky

While many of us are dazzled by the spectacular examples of galaxies each one with its own 10-100 billion stars, some of us choose instead to study the regions between the galaxies.

There is a fair amount of hydrogen gas in this “intergalactic medium,” yet this gas typically is too faint to see directly in images. We are able to study this dim gas only by looking at how it affects the light coming from bright objects in the background called quasars.

There are a great many bright quasars, or galaxies with extremely bright nuclei, in the universe. These quasars each produce a tremendous amount of light much like the welcome sight of a flashlight on an otherwise dark and deserted hiking trail.

Indeed if you were to look at the flashlight of a distant hiker during an evening walk, you may see the flashlight seem to ‘flicker’ not because the battery was running out but rather as a result of obstructions along the way such as tree branches. By conducting such an experiment you may be able to discern how dense the foliage is between yourself and the other hiker. Similarly, by studying the amount and placement of artificial ‘flickers’ in the quasar light caused by the intervening hydrogen gas, we can put together the properties of that elusive environment.

What we find is that hydrogen in the intergalactic medium near the Milky Way is completely ionized, which means that the electrons have been stripped from the protons by UV light. So, if you are wondering if a powerful sunblock should be in order should you ever find yourself in between galaxies that the answer is yes (although there would likely be many other, more pressing concerns like the lack of water)!

From such studies of the absorption of light towards quasars we also learn that at that at an earlier time in cosmic history, the intergalactic medium was neutral. During that time each proton was still attached to an electron.

An interesting question to ask is which event or events happened in the universe to cause the intergalactic medium to suffer a deluge of UV light which ionized all the hydrogen atoms? While the current cause of this “reionization” is still unknown, one hypothesis is that faint, small and fairly weak galaxies in the early universe were the culprits. Although the UV light production of any one 'dwarf' galaxy is small, they win owing to their steady production rate and large numbers.

Dr. Brenda Frye

About Dr. Brenda Frye

Brenda L. Frye is an observational cosmologist at the Department of Astronomy/Steward Observatory, University of Arizona. She earned her Ph. D. in Astrophysics from the University of California at Berkeley, assisted by a National Science Foundation Graduate Research Fellowship.

Her thesis work involved measuring the concentration of the total mass of visible plus dark matter in the fields of massive galaxy clusters, a program requiring the use of some of the largest telescopes in the world.

Moving a mile from her Ph. D. institution, she assumed a postdoctoral position with the Supernova Cosmology Project at Lawrence Berkeley National Laboratory under the direction of Professor Saul Permutter.

She then treked across the country to take a National Science Foundation Astronomy and Astrophysics Postdoctoral Fellowship and a Princeton Council on Sciences and Technology Fellowship both at Princeton University.

Moving further east, she became a Lecturer in Physics at Dublin City University in Dublin, Ireland, where a number of European collaborations were formed.

From there she crossed back across the pond to the west coast of the U. S. to become a tenure-track Assistant Professor of Physics at the University of San Francisco.

Her travels have now landed her at her Alma Mater in Tucson, where she teaches and does research. The aims of her research continue to be to use gravitational telescopes in space as 'lenses' to study the properties of dark matter and those of distant galaxies back to when the universe was <900 million years old.

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Learning from a Flashlight in the Sky — 1 Comment

  1. Given the quasar flashlights shining into these vast intergalactic regions, will the Tyndall effect provide an additional mechanism beyond the stated hydrogen absorption and emission mechanisms for quantifying hydrogen aggregations contributing to observed dark matter. Do the extremely cold conditions (~1×10^-16 K) exist for these hydrogen aggregations (potentially kilometers in size)?

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