How Galaxies Are Like Apricots

The common lore has it that in addition to hosting 10-100 billion stars, all galaxies are also covered in a massive invisible layer of dark matter. One cannot see this dark matter, nor is its composition known. Nevertheless, one can see the effects of the dark matter indirectly by the way they alter the motions of those myriad stars.

If one imagines each galaxy to be an apricot pit, then the dark matter is the orange fruit surrounding the pit. The stars in this analogy would all be situated inside the pit. If one find a way to separate the pit from the fruit by just a little bit then one could get important information concerning the properties of this mysterious dark matter.

There are no giant humans in space to bite into the fruit. Second best, if we could ram together two massive galaxies then we just might be able to see some of the fruit separated away from the pit.

This is a challenge to do in practice, as it requires knowing that the two galaxies have high motion towards each other across the sky (motion transverse to the line of sight). This measurement is generally infeasible given that the best way to do it is to wait a long time (millions of years), and hope to see that the two objects have moved a tiny amount in that time.

By contrast, it is easy to find galaxies with high motion in the radial direction towards or away from us. This is done by measuring their relative Doppler motion (the same technique used by the police to catch people speeding on the freeway).

In some special cases, we can find examples of two massive objects moving towards each other with high transverse motion. Namely, astronomers have found two massive galaxy clusters that we think have collided. Think two American football teams comprised of apricots with helmets in a free-for-all charge.

In this case we do measure a separation of the center of the dark matter (the fruit), from the center of the stars (the pit). As a result, we are starting to learn about how dark matter acts differently from ordinary matter, although we still cannot point to the dark matter and tell which substance it actually is.

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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How Galaxies Are Like Apricots — 1 Comment

  1. A few years ago I read in a text book that the Milky Way may contain 100 to 400 billion stars. Has that changed and is it now thought that all galaxies fall into a range of 10 to 100 billion? If so, what would be your current estimate as to where the Milky Way falls in that range?

    Thank you and I enjoy your blogs.

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