A Milky Way “Seed” Discovered?

It is well known that our Milky Way galaxy has a spiral shape. Perhaps it is less well-known that the Milky Way is surrounded by about 150 dense star clusters. To picture this better, if we imagine that a beehive is the Milky Way galaxy, then the bees orbiting the beehive in all directions would be the dense star clusters.

It was in the year 1919 that Harlow Shapley counted these star clusters, also known as globular clusters. Interestingly, he found that there were about twice as many stars in one direction compared to the anti-direction, from which he inferred that the Sun + Earth system lies about two-thirds of the way out from the center.

This resulted in a shift in our philosophical outlook, for with this one exercise humans were humbled out of their assumed ‘central’ placement in our own galaxy.

A new use for globular clusters is discussed in this week’s issue of Nature magazine. In this study, the speeds of rapidly-rotating dead stars or “pulsars” are measured. The question is then asked, “How much mass needs to be at the center of the star cluster in order to explain their rapid speeds?” It turns out that the central masses need to be very large, around millions of times the mass of the Sun.

In the study they find that this large mass must be, in turn, confined to a very small space. As no such large and bright object is seen in the cluster center, the most likely option is that there is an intermediate mass black hole at the center of this one globular cluster.

It is not a new idea to test for potential intermediate mass black holes, but it is an idea that is famously hard to test given that black holes are well......black! If it is true that intermediate mass black holes exist, then it is our best bet for finding what was the “seed” of our own Milky Way galaxy when it was born more than 13 billion years ago.

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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A Milky Way “Seed” Discovered? — 1 Comment

  1. Nature Article:
    An intermediate-mass black hole in the centre of the globular cluster 47 Tucana
    by: Bülent Kiziltan, Holger Baumgardt & Abraham Loeb

    globular cluster 47 Tucana centre
    contains 2200 solar mass 4.4E+36 gram
    within 4*pi/3*(1 parsec)^3 or 1.2E+56 cubic cm
    for a density of 3.6E-20 gram/cubic cm

    This low density does not seem indicative of a black hole.
    Since this central region is reported to be electromagnetically invisible,
    could it be in the form of dark matter?

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