Thursday, October 04, 2007

Dwarf galaxies

Smallest Galaxies Ever Seen Solve a Big Problem
Mauna Kea scientists may have solved a discrepancy between the number of extremely small, faint galaxies predicted to exist near the Milky Way and the number actually observed. In an attempt to resolve the “Missing Dwarf Galaxy” problem, two astronomers used the W. M. Keck Observatory to study a population of the darkest, most lightweight galaxies known, each containing 99% dark matter. The findings suggest the “Missing Dwarf Galaxy” problem is not as severe as previously thought, and may have been solved completely.

“It seems that very small, ultra-faint galaxies are far more plentiful than we thought,” said Dr. Marla Geha, co-author of the study and a Plaskett Research Fellow at the Herzberg Institute of Astrophysics in Canada. “If you asked me last year whether galaxies this small and this dark existed, I would have said no. I’m astonished that so many tiny, dark matter-dominated galaxies have now been discovered.”

I don't really have a lot to add to this, except to remark that this is probably a more significant result than may be immediately obvious. After all, so a lot of tiny, lightweight galaxies have been found in orbit around the Milky Way – so what?

The significance is that this observation goes a long way towards solving a problem with the hypothesis that the universe contains large amounts of "cold dark matter" – perhaps 4 times as much mass in the form of "dark matter" than there is in the form of more ordinary "baryonic" matter. There is already a huge amount of evidence for the existence of dark matter, as discussed here and here.

The problem is that simulations which have been done on the evolution of galaxies under the assumption of a ratio of 4:1 dark matter to baryonic matter predict the existence of many more small "dwarf" satellite galaxies around the Milky Way than are actually observed. Intuitively, one would expect many small galaxies, since if visible galaxies consist of stars made of baryonic matter inside blobs of dark matter, there ought to be a large range of sizes, from the smallest to the largest. Instead, what has been observed until now is far too few of the smallest sizes.

The solution suggested by the results here is that galaxies that formed inside the smallest blobs of dark matter have far fewer stars than would be expected, and hence they are intrinsically dim and hard to detect, so that most very small galaxies simply haven't been noticed.

But why would such galaxies have so few stars? This question remains to be answered, but a plausible hypothesis is that most of the gas of these galaxies, from which stars could form, may have been literally blown away by the intense light radiated by the first very large stars that formed in the Milky Way itself:
Based on the masses measured for the new dwarf galaxies, Drs. Simon and Geha concluded the fierce ultraviolet radiation given off by the first stars, which formed just a few hundred million years after the Big Bang, may have blown all of the hydrogen gas out of the dwarf galaxies forming at that time. The loss of gas prevented the galaxies from creating new stars, leaving them very faint, or in many cases completely dark. When this effect is included in theoretical models, the numbers of expected and observed dwarf galaxies agree.

“One of the implications of our results is that up to a few hundred completely dark galaxies really should exist in the Milky Way’s cosmic neighborhood,” said Dr. Geha. “If the Cold Dark Matter model is correct they have to be out there, and the next challenge for astronomers will be finding a way to detect their presence.”

Other reports on this research: here, here

Preprint of the research paper: The Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem

Preprint of a subsequent research paper describing how dark matter content of very faint dwarf galaxies might be confirmed: The Most Dark Matter Dominated Galaxies: Predicted Gamma-ray Signals from the Faintest Milky Way Dwarfs

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