Andromeda galaxy. Credit: Robert Gendler
An international team of astronomers has
identified for the first time a thick stellar disc in the Andromeda galaxy, the
nearest large spiral galaxy to our own Milky Way.
The discovery of the thick disc, a major
result from a five-year investigation, will help astronomers better understand
the processes involved in the formation and evolution of large spiral galaxies
like ours, according to the team, which includes UCLA research astronomer
Michael Rich and colleagues from Europe and Australia.
Using the Keck Telescope in Hawaii, the
astronomers analyzed the velocities of individual bright stars within the
Andromeda galaxy and were able to observe a group of stars tracing a thick disc—distinct
from those comprising the galaxy’s already-known thin disc—and assessed how
these stars differ from thin-disc stars in height, width, and chemistry.
Approximately 70% of Andromeda’s stars are
contained in the galaxy’s thin stellar disc. This disc structure contains the
spiral arms traced by regions of active star formation, and it surrounds a
central bulge of old stars at the core of the galaxy.
“From observations of our own Milky Way
and other nearby spirals, we know that these galaxies typically possess two
stellar discs, both a ‘thin’ and a ‘thick’ disc,” said Michelle Collins, a
doctoral student at the University of Cambridge’s Institute of Astronomy, who
led the study.
The thick disc consists of older stars whose
orbits take them along a “thicker” path—one that extends both above
and below the galaxy’s thin disc.
“The classical thin stellar discs that
we typically see in Hubble imaging result from the accretion of gas towards the
end of a galaxy’s formation, whereas thick discs are produced in a much earlier
phase of the galaxy’s life, making them ideal tracers of the processes involved
in galactic evolution,” Collins said.
The formation process of thick discs is not
yet well understood. Previously, the best hope for understanding this structure
was by studying the thick disc present in our own Milky Way. However, much of
our galaxy’s thick disc is obscured from view. The discovery of a similar thick
disc in Andromeda presents a much clearer view of spiral structure.
Astronomers will be able to determine the
properties of the disc across the galaxy and will search for signatures of the
events related to its formation, the researchers said.
“Our initial study of this component
already suggests that it is likely older than the thin disc, with a different
chemical composition,” said UCLA’s Rich, who was the principal
investigator at the Keck Observatory for the observations. “Future, more
detailed observations should enable us to unravel the formation of the disc
system in Andromeda, with the potential to apply this understanding to the formation
of spiral galaxies throughout the universe.”
“This result is one of the most
exciting to emerge from the larger parent survey of the motions and chemistry
of stars in the outskirts of Andromeda,” said Scott Chapman of the Institute
of Astronomy at Cambridge. “Finding this thick disc has afforded us a
unique and spectacular view of the formation of the Andromeda system and will
undoubtedly assist in our understanding of this complex process.”
The study is currently available in the
online version of Monthly Notices of the Royal Astronomical Society and will be
published in a print edition of the journal later this year.