This diagram shows how scientists determined the size of the halo of the
Andromeda galaxy. Because the gas in the halo is dark, the team
measured it by using the light from quasars, the very distant bright
cores of active galaxies powered by black holes. They observed the
quasars' light as it traveled through the intervening gas. The halo's
gas absorbed some of that light and made the quasar appear darker in a
very small wavelength range. By measuring the tiny dip in brightness at
that specific range, scientists could tell how much gas is between us
and each quasar. Some quasars showed no dip in brightness, and this
helped define the size of the halo. Illustration Credit: NASA, ESA, and A. Feild (STScI). Science Credit: NASA, ESA, N. Lehner and J.C. Howk (University of Notre Dame), and B. Wakker (University of Wisconsin, Madison). Image Releases
Scientists using NASA's Hubble Space Telescope have discovered that the immense halo of gas enveloping the Andromeda galaxy, our nearest massive galactic neighbor, is about six times larger and 1,000 times more massive than previously measured. The dark, nearly invisible halo stretches about a million light-years from its host galaxy, halfway to our own Milky Way galaxy. This finding promises to tell astronomers more about the evolution and structure of majestic giant spirals, one of the most common types of galaxies in the universe.
"Halos are the gaseous atmospheres of galaxies. The properties of
these gaseous halos control the rate at which stars form in galaxies
according to models of galaxy formation," explained the lead
investigator, Nicolas Lehner of the University of Notre Dame, Indiana.
The gargantuan halo is estimated to contain half the mass of the stars
in the Andromeda galaxy itself, in the form of a hot, diffuse gas. If
it could be viewed with the naked eye, the halo would be 100 times the
diameter of the full Moon in the sky. This is equivalent to the patch
of sky covered by two basketballs held at arm's length.
The Andromeda galaxy lies 2.5 million light-years away and looks like
a faint spindle, about 6 times the diameter of the full Moon. It is
considered a near-twin to the Milky Way galaxy.
Because the gas in Andromeda's halo is dark, the team looked at
bright background objects through the gas and observed how the light
changed. This is a bit like looking at a glowing light at the bottom of
a pool at night. The ideal background "lights" for such a study are
quasars, which are very distant bright cores of active galaxies powered
by black holes. The team used 18 quasars residing far behind Andromeda
to probe how material is distributed well beyond the visible disk of
the galaxy. Their findings were published in the May 10, 2015, edition
of The Astrophysical Journal.
Earlier research from Hubble's Cosmic Origins Spectrograph
(COS)-Halos program studied 44 distant galaxies and found halos like
Andromeda's, but never before has such a massive halo been seen in a
neighboring galaxy. Because the previously studied galaxies were much
farther away, they appeared much smaller on the sky. Only one quasar
could be detected behind each faraway galaxy, providing only one light
anchor point to map their halo size and structure. With its close
proximity to Earth and its correspondingly large footprint on the sky,
Andromeda provides a far more extensive sampling of a lot of background
quasars.
"As the light from the quasars travels toward Hubble, the halo's gas
will absorb some of that light and make the quasar appear a little
darker in just a very small wavelength range," explains co-investigator
J. Christopher Howk, also of Notre Dame. "By measuring the dip in
brightness in that range, we can tell how much halo gas from M31 there
is between us and that quasar."
The scientists used Hubble's unique capability to study the
ultraviolet light from the quasars. Ultraviolet light is absorbed by
Earth's atmosphere, which makes it difficult to observe with a
ground-based telescope. The team drew from about 5 years' worth of
observations stored in the Hubble data archive to conduct this research.
Many previous Hubble campaigns have used quasars to study gas much
farther away than — but in the general direction of — Andromeda, so a
treasure trove of data already existed.
But where did the giant halo come from? Large-scale simulations of
galaxies suggest that the halo formed at the same time as the rest of
Andromeda. The team also determined that it is enriched in elements
much heavier than hydrogen and helium, and the only way to get these
heavy elements is from exploding stars called supernovae. The
supernovae erupt in Andromeda's star-filled disk and violently blow
these heavier elements far out into space. Over Andromeda's lifetime,
nearly half of all the heavy elements made by its stars have been
expelled far beyond the galaxy's 200,000-light-year-diameter stellar
disk.
What does this mean for our own galaxy? Because we live inside the
Milky Way, scientists cannot determine whether or not such an equally
massive and extended halo exists around our galaxy. It's a case of not
being able to see the forest for the trees. If the Milky Way does
possess a similarly huge halo, the two galaxies' halos may be nearly
touching already and quiescently merging long before the two massive
galaxies collide. Hubble observations indicate that the Andromeda and
Milky Way galaxies will merge to form a giant elliptical galaxy
beginning about 4 billion years from now.
Contact
Space Telescope Science Institute, Baltimore, Md.
410-338-4488 / 410-338-4514
jenkins@stsci.edu / villard@stsci.edu
Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov
Nicolas Lehner
University of Notre Dame, Notre Dame, Indiana
574-631-5755
nlehner@nd.edu
Source: HubbleSite
