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Topic Name: Dark, But Light: Smallest Galaxies Ever Seen Solve a Big Problem
Category: Geo sciences & technology
Research persons: Dr. Taft Armandroff, Marla Geha, Joshua D. Simon
Location: W. M. Keck Observatory , California Association for Research in Astronomy , 65-1120 Mamalahoa Hwy, Kamuela, HI 96743, United States
Details
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 in Hawaii 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."
The Missing Dwarf Galaxy puzzle comes from a prediction of the
"Cold Dark Matter" model, which explains the growth and evolution of the
universe. It predicts large galaxies like the Milky Way should be surrounded by
a swarm of up to several hundred smaller galaxies known as "dwarf galaxies."
However, until recently, only 11 such companions were known to be orbiting the
Milky Way. To explain this large discrepancy, theorists suggested that while
hundreds of dwarf galaxies near the Milky Way may indeed exist, the majority
might have few, if any, stars. If so, the galaxies would be comprised almost
entirely of dark matter-a mysterious type of matter that has gravitational
effects on ordinary atoms, but which does not produce any light. But proving the
existence of a large number of nearly invisible galaxies seemed problematic,
until now.
Dr. Josh Simon, a Millikan Postdoctoral Scholar at the
California Institute of Technology, and Dr. Geha used the 10-meter Keck II
telescope with the DEIMOS spectrograph
to conduct follow-up studies of eight new dwarf galaxies first discovered with
the Sloan Digital Sky Survey. The results enabled the duo to calculate precisely
the total mass of each galaxy. To their surprise, each system was among the
smallest ever measured, more than 10,000 times smaller than the Milky Way.
"The formation of such small galaxies is not very well
understood from a theoretical perspective," said Dr. Simon. "Explaining how
stars form inside these remarkably tiny galaxies is difficult, and so it is hard
to predict exactly how many dwarfs we should find near the Milky Way. Our work
narrows the gap between the Cold Dark Matter theory and observations by
significantly increasing the number of Milky Way dwarf galaxies and telling us
more about the properties of these galaxies. We also now know that dwarf
galaxies can be even smaller than we thought possible."
Numerous, repeated measurements of 814 stars in the eight dwarf
galaxies were obtained at W. M. Keck Observatory. The stars were found to be
moving much slower than stars in any other known galaxy (about 4 to 7 km/sec.)
For comparison, the Sun orbits the center of the Milky Way at a speed of about
220 km/sec. In all, the astronomers measured precise speeds for 18 to 214 stars
in each galaxy, about three times more stars per galaxy than any previous study.
"This is a significant paper," said Dr. Taft Armandroff,
director of the W. M. Keck Observatory, whose own research includes the study of
dwarf galaxies. "It is a compelling example of how large, ground-based
telescopes can precisely measure the orbits of distant stars on the sky to just
a few kilometers per second. I expect DEIMOS will soon tell us about the
chemical composition of these stars to help us better understand how star
formation takes place in such small galaxies."
Some parameters of the Cold Dark Matter theory can now be
updated to match observed conditions in the local universe. 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."
Because the Sloan Digital Sky Survey only covered about 25
percent of the sky, future surveys of the remainder of the sky are expected to
discover as many as 50 more dark matter‑dominated dwarf galaxies orbiting the
Milky Way. Telescopes for one such survey, the Pan-STARRS project on Maui, are
now under construction.
The paper, "Kinematics of the Ultra-Faint Milky Way Satellites:
Solving the Missing Satellite Problem," will be published in the November 10
issue of the Astrophysical Journal. Funding for the project was provided by the
California Institute of Technology under the Millikan Fellowship program and the
Herzberg Institute of Astrophysics of the National Research Council of Canada.
Data reduction software was made possible by the support of the National Science
Foundation (AST 0071048).
About The Researcher:
Dr. Taft Armandroff
Director
W. M. Keck Observatory
Phone: (808) 881-3855
newsletter@keck.hawaii.edu
Marla Geha
National Research Council of Canada, Hertzberg Institute of Astrophysics
Victoria, BC Canada
Phone: 1-250-363-8103
marla.geha@nrc-cnrc.gc.ca
Joshua D. Simon
California Institute of Technology
Pasadena, California USA
Phone: 1-626-395-3693
jsimon@astro.caltech.edu
About W. M. Keck Observatory
Observations were conducted at the W. M. Keck Observatory (www.keckobservatory.org)
in Hawaii, a non-profit 501 (c) (3) organization. The governing board of
Keck Observatory consists of directors from the California Institute of
Technology and the University of California. In addition, the National
Aeronautics and Space Administration and the W. M. Keck Foundation each have
liaisons to the board. Construction of the twin Keck telescopes and domes
was made possible with generous grants totaling more than $140 million from
the W. M. Keck Foundation in Los Angeles.
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