NASA’s Fermi telescope has detected a gamma-ray
excess at the center of the Andromeda galaxy that's similar to a
signature Fermi previously detected at the center of our own Milky Way.
Watch to learn more. Credits: NASA’s Goddard Space Flight Center/Scott Wiessinger, producer
The gamma-ray excess (shown in yellow-white) at the
heart of M31 hints at unexpected goings-on in the galaxy's central
region. Scientists think the signal could be produced by a variety of
processes, including a population of pulsars or even dark matter. Credits: NASA/DOE/Fermi LAT Collaboration and Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF
NASA’s Fermi Gamma-ray Space Telescope has found a signal at the center of the neighboring Andromeda galaxy that could indicate the presence of the mysterious stuff known as dark matter. The gamma-ray signal is similar to one seen by Fermi at the center of our own Milky Way galaxy.
Gamma rays are the highest-energy form of light, produced by the
universe’s most energetic phenomena. They’re common in galaxies like the
Milky Way because cosmic rays, particles moving near the speed of
light, produce gamma rays when they interact with interstellar gas
clouds and starlight.
Surprisingly, the latest Fermi data shows the gamma rays in Andromeda
— also known as M31 — are confined to the galaxy’s center instead of
spread throughout. To explain this unusual distribution, scientists are
proposing that the emission may come from several undetermined sources.
One of them could be dark matter, an unknown substance that makes up
most of the universe.
“We expect dark matter to accumulate in the innermost regions of the
Milky Way and other galaxies, which is why finding such a compact signal
is very exciting,” said lead scientist Pierrick Martin, an
astrophysicist at the National Center for Scientific Research and the
Research Institute in Astrophysics and Planetology in Toulouse, France.
“M31 will be a key to understanding what this means for both Andromeda
and the Milky Way.”
A paper describing the results will appear in an upcoming issue of The Astrophysical Journal.
Another possible source for this emission could be a rich
concentration of pulsars in M31’s center. These spinning neutron stars
weigh as much as twice the mass of the sun and are among the densest
objects in the universe. One teaspoon of neutron star matter would weigh
a billion tons on Earth. Some pulsars emit most of their energy in
gamma rays. Because M31 is 2.5 million light-years away, it’s difficult to find individual pulsars.
To test whether the gamma rays are coming from these objects,
scientists can apply what they know about pulsars from observations in
the Milky Way to new X-ray and radio observations of Andromeda.
Now that Fermi has detected a similar gamma-ray signature in both M31
and the Milky Way, scientists can use this information to solve
mysteries within both galaxies. For example, M31 emits few gamma rays
from its large disk, where most stars form, indicating fewer cosmic rays
roaming there. Because cosmic rays are usually thought to be related to
star formation, the absence of gamma rays in the outer parts of M31
suggests either that the galaxy produces cosmic rays differently, or
that they can escape the galaxy more rapidly. Studying Andromeda may
help scientists understand the life cycle of cosmic rays and how it is
connected to star formation.
“We don’t fully understand the roles cosmic rays play in galaxies, or
how they travel through them,” said Xian Hou, an astrophysicist at
Yunnan Observatories, Chinese Academy of Sciences in Kunming, China,
also a lead scientist in this work. “M31 lets us see how cosmic rays
behave under conditions different from those in our own galaxy.”
The similar discovery in both the Milky Way and M31 means scientists
can use the galaxies as models for each other when making difficult
observations. While Fermi can make more sensitive and detailed
observations of the Milky Way’s center, its view is partially obscured
by emission from the galaxy’s disk. But telescopes view Andromeda from
an outside vantage point impossible to attain in the Milky Way.
“Our galaxy is so similar to Andromeda, it really helps us to be able
to study it, because we can learn more about our galaxy and its
formation,” said co-author Regina Caputo, a research scientist at NASA’s
Goddard Space Flight Center in Greenbelt, Maryland. “It’s like living
in a world where there’s no mirrors but you have a twin, and you can see
everything physical about the twin.”
While more observations are necessary to determine the source of the
gamma-ray excess, the discovery provides an exciting starting point to
learn more about both galaxies, and perhaps about the still elusive
nature of dark matter.
“We still have a lot to learn about the gamma-ray sky,” Caputo said.
“The more information we have, the more information we can put into
models of our own galaxy.”
NASA's Fermi Gamma-ray Space Telescope is an astrophysics and
particle physics partnership, developed in collaboration with the U.S.
Department of Energy and with important contributions from academic
institutions and partners in France, Germany, Italy, Japan, Sweden and
the United States.
For more information on Fermi, visit: http://www.nasa.gov/fermi
Editor: Rob Garner
Source: NASA/Fermi Space Telescope
