Scientists elucidate the origin of the darkest galaxies in the universe (Forwarded)

Stanford University
Stanford, California

Contact:

Neil Calder
Stanford Linear Accelerator Center
(650) 926-8707

Comment:

Stelios Kazantzidis
Kavli Institute for Particle Astrophysics and Cosmology
(650) 926-4786

February 12, 2007

Scientists elucidate the origin of the darkest galaxies in the universe
BY Kelen Tuttle

Ghostly galaxies composed almost entirely of dark matter speckle the
universe. Unlike normal galaxies, these extreme systems contain very few
stars and are almost devoid of gas. Most of the luminous matter, so common
in most galaxies, has been stripped away, leaving behind a dark matter
shadow. These intriguing galaxies -- known as dwarf spheroidals -- are so
faint that, although researchers believe they exist throughout the universe,
only those relatively close to Earth have ever been observed. And until
recently, no scientific model proposed to unravel their origin could
simultaneously explain their exceptional dark matter content and their
penchant for existing only in close proximity to much larger galaxies.

Now, Stelios Kazantzidis, a researcher at Stanford University's Kavli
Institute for Particle Astrophysics and Cosmology (KIPAC), in collaboration
with Lucio Mayer of the Swiss Federal Institute of Technology in Zurich and
the University of Zurich, Chiara Mastropietro of the University of Munich in
Germany and James Wadsley of McMaster University in Canada, has developed an
elegant explanation for how galaxies come to be dominated by dark matter.
Kazantzidis, who completed part of the study as a fellow at the University
of Chicago's Kavli Institute for Cosmological Physics, reports his findings
in the Feb. 15 issue of Nature.

"These results are so exciting because they are based on a combination of
physical effects that has never before been postulated," said Kazantzidis.
"This is one step toward a more complete understanding of the formation of
structure in the universe, which is one of the fundamental goals of
astrophysics."

Using supercomputers to create novel simulations of galaxy formation,
Kazantzidis and his collaborators found that a dark matter-dominated galaxy
begins life as a normal system. But when it approaches a much more massive
galaxy, it simultaneously encounters three environmental effects -- "ram
pressure," "tidal shocking" and the cosmic ultraviolet background -- that
transform it into a mere dark-matter shadow of its former self.

About 10 billion years ago, when the gas-rich progenitors of dark
matter-dominated galaxies originally fell into the Milky Way, the universe
was hot with a radiation called the cosmic ultraviolet background. As a
small satellite galaxy traveled along its elliptical path around a more
massive galaxy, called the host, this radiation made the gas within the
smaller galaxy hotter. This state allowed ram pressure -- a sort of "wind
resistance" a galaxy feels as it speeds along its path -- to strip away the
gas within the satellite galaxy.

Simultaneously, as the satellite galaxy moved closer to the massive system,
it encountered the overwhelming gravitational force of the much larger mass.
This force wrenched luminous stars from the small galaxy. Over billions of
years of evolution, the satellite passed by the massive galaxy several times
as it traversed its orbital path. Each time its stars shook and the
satellite lost some of them as a result of a mechanism called tidal
shocking. These effects conspired to eventually strip away nearly all the
luminous matter -- gas and stars -- and left behind only a dark-matter
shadow of the original galaxy.

The dark matter, on the other hand, was nongaseous and therefore unaffected
by the ram pressure force or the cosmic ultraviolet background, the
scientists posit. It did experience tidal shocking, but this force alone was
not strong enough to pull away a substantial amount of dark matter.

The numerical simulations conducted by Kazantzidis and his collaborators
constitute the most extensive calculations ever performed on this topic,
consuming up to two months of supercomputing time each at the University of
Zurich, the Pittsburgh Supercomputing Center and elsewhere.

"Computer models of galaxy formation in the last decade or so have focused
on modeling the properties of dark matter rather than those of the more
familiar baryonic [luminous] matter," said co-author Mayer. "Instead, our
work suggests that we cannot understand the origin of galaxies without
modeling the detailed physics of baryonic matter, even in a dark
matter-dominated universe."

The scientists say this new understanding of the origin of the darkest
galaxies in the universe may soon lead to fundamental insights into the
nature of dark matter.

"Elucidating the nature of dark matter is one of the grandest challenges of
modern cosmology," said Kazantzidis. "In the next several years, numerous
experiments will attempt to detect dark matter using dwarf spheroidal
galaxies as targets." Kazantzidis' work will benefit these studies by
offering a better explanation of the origin of ghostly galaxies.

Mystery of the missing satellites

Additionally, the work may help to explain a long-standing discrepancy
between theory and observation. The leading modern cosmological model,
Lambda Cold Dark Matter (lambda-CDM), predicts that many more small galaxies
surround massive galaxies like the Milky Way and Andromeda than are
currently observed. This mismatch, which is often referred to as the
"missing satellites problem," has been traditionally regarded as one of the
toughest challenges to the lambda-CDM paradigm. Kazantzidis' work suggests
that the process by which small galaxies are stripped of their luminous
matter is common, and implies that the "missing" galaxies could exist in the
form of dark matter-dominated satellites.

"These galaxies could just be too dark to detect," he said. "But their
possible existence will substantially alleviate the missing satellites
problem with profound implications for the predictive power of the
lambda-CDM theory." Coincidentally, in the last few months, one of the most
advanced observational programs ever undertaken, the Sloan Digital Sky
Survey, has revealed in the vicinity of the Milky Way a number of what
appear to be ultra-faint satellite galaxies. If this finding is confirmed by
follow-up observations and analysis, these newly discovered systems would be
explained by Kazantzidis' calculations and would contribute to solving the
long-standing missing satellites problem, he says.

This research has been supported by the Zwicky Prize Fellowship program at
the Swiss Federal Institute of Technology in Zurich and by the U.S.
Department of Energy through a KIPAC Fellowship at Stanford University and
the Stanford Linear Accelerator Center. This project also was supported in
its initial stages by the Swiss National Science Foundation and the Kavli
Institute for Cosmological Physics at the University of Chicago.

Editor Note:

Please note that the Lambda Cold Dark Matter theory acronym, lambda-CDM,
should begin with the Greek letter lambda, which may not come through in
this document.

Photos of Kazantzidis and the cosmos are available on the web at
http://newsphotos.stanford.edu/Kaz/

Relevant Web URLs:

* Stanford Linear Accelerator Center
http://www.slac.stanford.edu/
* Kavli Institute for Particle Astrophysics and Cosmology
http://www-group.slac.stanford.edu/kipac/