In a landmark
study, scientists using NASA’s Hubble Space Telescope have mapped the immense
envelope of gas, called a halo, surrounding the Andromeda galaxy, our nearest
large galactic neighbor. Scientists were surprised to find that this tenuous,
nearly invisible halo of diffuse plasma extends 1.3 million light-years from
the galaxy — about halfway to our Milky Way — and as far as 2 million
light-years in some directions. This means that Andromeda’s halo is already bumping
into the halo of our own galaxy.
They also found that the halo has a layered structure,
with two main nested and distinct shells of gas. This is the most comprehensive
study of a halo surrounding a galaxy.
“Understanding the huge halos of gas surrounding
galaxies is immensely important,” explained co-investigator Samantha Berek of
Yale University in New Haven, Connecticut. “This reservoir of gas contains fuel
for future star formation within the galaxy, as well as outflows from events
such as supernovae. It’s full of clues regarding the past and future evolution
of the galaxy, and we’re finally able to study it in great detail in our
closest galactic neighbor.”
“We find the inner shell that extends to about a half
million light-years is far more complex and dynamic,” explained study leader
Nicolas Lehner of the University of Notre Dame in Indiana. “The outer shell is
smoother and hotter. This difference is a likely result from the impact of
supernova activity in the galaxy’s disk more directly affecting the inner
halo.”
A signature of this activity is the team’s discovery
of a large amount of heavy elements in the gaseous halo of Andromeda. Heavier
elements are cooked up in the interiors of stars and then ejected into space —
sometimes violently as a star dies. The halo is then contaminated with this
material from stellar explosions.
The Andromeda galaxy, also known as M31, is a majestic
spiral of perhaps as many as 1 trillion stars and comparable in size to our
Milky Way. At a distance of 2.5 million light-years, it is so close to us that
the galaxy appears as a cigar-shaped smudge of light high in the autumn sky. If
its gaseous halo could be viewed with the naked eye, it would be about three
times the width of the Big Dipper. This would easily be the biggest feature on
the nighttime sky.
Through a program called Project AMIGA (Absorption Map
of Ionized Gas in Andromeda), the study examined the light from 43 quasars —
the very distant, brilliant cores of active galaxies powered by black holes —
located far beyond Andromeda. The quasars are scattered behind the halo,
allowing scientists to probe multiple regions. Looking through the halo at the
quasars’ light, the team observed how this light is absorbed by the Andromeda
halo and how that absorption changes in different regions. The immense
Andromeda halo is made of very rarified and ionized gas that doesn’t emit
radiation that is easily detectable. Therefore, tracing the absorption of light
coming from a background source is a better way to probe this material.
The researchers used the unique capability of Hubble’s
Cosmic Origins Spectrograph (COS) to study the ultraviolet light from the
quasars. Ultraviolet light is absorbed by Earth’s atmosphere, which makes it
impossible to observe with ground-based telescopes. The team used COS to detect
ionized gas from carbon, silicon, and oxygen. An atom becomes ionized when
radiation strips one or more electrons from it.
Andromeda’s halo has been probed before by Lehner’s
team. In 2015, they discovered that the Andromeda halo is large and massive.
But there was little hint of its complexity; now, it’s mapped out in more
detail, leading to its size and mass being far more accurately determined.
“Previously, there was very little information — only
six quasars — within 1 million light-years of the galaxy. This new program
provides much more information on this inner region of Andromeda’s halo,”
explained co-investigator J. Christopher Howk, also of Notre Dame. “Probing gas
within this radius is important, as it represents something of a gravitational
sphere of influence for Andromeda.”
Because we live inside the Milky Way, scientists
cannot easily interpret the signature of our own galaxy’s halo. However, they
believe the halos of Andromeda and the Milky Way must be very similar since
these two galaxies are quite similar. The two galaxies are on a collision
course, and will merge to form a giant elliptical galaxy beginning about 4
billion years from now.
Scientists have studied gaseous halos of more distant
galaxies, but those galaxies are much smaller on the sky, meaning the number of
bright enough background quasars to probe their halo is usually only one per
galaxy. Spatial information is therefore essentially lost. With its close
proximity to Earth, the gaseous halo of Andromeda looms large on the sky,
allowing for a far more extensive sampling.
“This is truly a unique experiment because only with
Andromeda do we have information on its halo along not only one or two
sightlines, but over 40,” explained Lehner. “This is groundbreaking for
capturing the complexity of a galaxy halo beyond our own Milky Way.”
In fact, Andromeda is the only galaxy in the universe
for which this experiment can be done now, and only with Hubble. Only with an
ultraviolet-sensitive future space telescope will scientists be able to
routinely undertake this type of experiment beyond the approximately 30
galaxies comprising the Local Group.
“So Project AMIGA has also given us a glimpse of the
future,” said Lehner.
Journal article: https://iopscience.iop.org/article/10.3847/1538-4357/aba49c
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