A new study seeking to answer the
question of why some galaxies are extremely dark compared with others may
eventually help to explain the formation of all galaxies, according to
researchers at the Harvard-Smithsonian Center for Astrophysics (CfA).

While the inner structure of most
galaxies is dominated by their visible components, the small, spheroid-shaped
galaxies scientists refer to as dwarf spheroidals are dominated by an invisible
component that scientists refer to as dark matter.

“The mass in these dwarf spheroidal
systems is mostly dark matter,” says Elena D’Onghia, a Research Associate at
the CfA and author of the study that was published in the journal Nature. “We
want to know why these systems are so dark-matter dominated compared to normal,
bigger galaxies like our own Milky Way or Andromeda, which have less dark
matter relative to stars and gas.”

Stars, gas, and dust make up about 10
percent of the mass of the average galaxy. “But these spheroidal dwarf systems
have an even smaller percentage of this visible matter,” D’Onghia says.
“There must be some physical process that has removed the gas and the
stars very efficiently, leaving these systems as mostly dark matter” and
consequently extremely faint to the astronomer’s eye, she adds.

Mainstream astrophysics holds that
structure in the Universe in small systems is probably similar to that in
larger systems. “We think that galaxies build up their mass hierarchically —
smaller systems combining to make bigger things,” explains co-author of the
study Gurtina Besla, a graduate student in Harvard’s Department of Astronomy.

“In large galaxies like our Milky Way,
stars, gas, and dust form a disk structure and rotate at a certain speed around
its center — the galaxy’s core — depending on their distance from it,” Besla
says. Perhaps smaller, spheroidal galaxies, satellites to larger galaxies (much
like the moon to the Earth), originally shared the same characteristic.

D’Onghia and collaborators simulated
“encounters” between such dwarf galaxies. “We found that when a small dwarf
(the victim) felt the attraction of the gravitational potential of a larger
dwarf, the frequency of rotation of the material at some position in the
victim’s disk matches the frequency at which the victim orbited around the
bigger object,” says D’Onghia. “This is referred to as a ‘resonance,’ and
provides an additional kick that strips the stars and the gas off of the victim
dwarf.”

The team believes that resonance
stripping explains how these dwarf spheroidals lost most of their stars, dust,
and gas material, while keeping all of their dark matter.

“The dark matter remained because it is
not rotating. It doesn’t feel the additional kick the gas and stars do,” says
D’Onghia. Stripping the disk of most of its stars and gas leaves behind a very
faint nucleus, or core. “Those are the dwarf spheroidal galaxies we observe.”

So far, researchers have seen these old
and faint spheroidals in both the neighborhood of our galaxy and that of nearby
Andromeda. “There are around 20 or 30 of these systems,” D’Onghia says.
Understanding how these systems formed may give researchers a better idea of
how the universe looked at an earlier age.

According to the cold dark matter theory
in astrophysics, the universe today should contain a greater number of dwarfs —
but it doesn’t appear to.

“In the Milky Way alone, theory predicts
about 200 of these galaxies near the center of ours,” says D’Onghia. “So there
are still a lot of ‘missing’ dwarfs. The ones we see haven’t been stripped of all their gas yet, and the
‘missing’ ones may actually be there but we can’t see them because all they
have is dark matter,” she says, adding, “Resonance stripping might explain
this, but we haven’t pushed the theory to this extreme yet because we are
trying to determine the efficiency of our model.”

Although other models have been proposed to
explain the origin of dwarf spheroidals, they require Milky Way-sized galaxies
in the vicinity of dwarfs-to-be. As such, they fail to explain the existence of
dwarf spheroidals in the outskirts of our galactic neighborhood — the so-called
Local Group — that have never been close to a large galaxy. “So the next step
is to see if this process can explain many different scenarios,” D’Onghia says.

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