Gemini Observatory with NSF’s National Optical-Infrared Astronomy Research Laboratory
Astronomers have uncovered two historic events in which the Andromeda Galaxy underwent major changes to its structure. The findings shed light not only on the evolution and formation of the Andromeda Galaxy, but to our own Milky Way Galaxy as well. Two of the facilities in NSF's National Optical-Infrared Astronomy Research Laboratory, Kitt Peak National Observatory and the International Gemini Observatory, played critical roles in the research, now published in the latest issue of the journal Nature.
Large galaxies like the one we live in, the Milky Way, are believed
to grow through repeated merging with smaller, dwarf galaxies. Gas and
dwarf galaxies in the vast cosmic web follow the gravitational paths
laid out by dark matter — traversing filaments, they migrate slowly
toward collections of dark matter and assemble into large galaxies. As
dwarf galaxies are pulled in by gravity, they are also pulled apart,
leaving behind long trailing streams of stars and compact star clusters.
Astronomers have uncovered evidence for two major migration events in
the history of our large galactic neighbor, the Andromeda Galaxy (also
known as M31). The more recent migration event occurred a few billion
years ago and the older event many billions of years before that. The
evidence for the two events comes from “galactic archaeology,” the use
of the motions and properties of stars and stellar clusters to
reconstruct the formation and evolutionary history of galaxies.
In the case of the Andromeda Galaxy, the team of galactic
archaeologists, led by Dr. Dougal Mackey (Australian National
University) and Professor Geraint Lewis (University of Sydney), measured
the velocities of 77 of the Andromeda Galaxy’s compact star clusters,
using the 4-meter Mayall telescope at Kitt Peak National Observatory, the 8-meter Gemini North telescope
on Maunakea, Hawai‘i, and other facilities. The star clusters are all
located in the outer halo of the galaxy. The outer regions of the galaxy
are of particular interest because the dynamical signature of migration
events persists longer there.
“By tracing the faint remains of dwarf galaxies with star clusters,
we’ve been able to recreate the way the Andromeda Galaxy drew them in at
different times, from what’s known as the ‘cosmic web’ of matter that
threads the Universe,” Lewis said.
The team found that the star clusters divide into two populations, a
young group associated with stellar streams, and an older group that has
no such association. The two populations both orbit the Andromeda
Galaxy, but their orbital axes are nearly perpendicular to each other.
The different orbits are evidence for two distinct accumulation
events. The stellar streams associated with the more recent event are
still present, but streams from the older event are long gone.
According to Mackey, reconstructing the formation history of the
Andromeda Galaxy provides insights into the history of our own galaxy,
the Milky Way.
“One of our main motivations in studying astronomy is to understand
our place in the Universe. A way of learning about the Milky Way is to
study galaxies that are similar to it, and try to understand how these
systems formed and evolved.” Studying the Andromeda Galaxy, “can
actually be easier than looking at the Milky Way. Because we live inside
it, that can make certain types of observations quite difficult,”
Mackey said.
Astronomer Knut Olsen of NSF's National Optical-Infrared Astronomy
Research Laboratory, who studies the formation of galaxies but was not
part of the study said, “This work shows that galaxies as massive as the
Large Magellanic Cloud have merged with the Andromeda Galaxy at least
twice in its history.” The Large Magellanic Cloud is a companion galaxy
to the Milky Way that is easily visible to the naked eye in the Southern
hemisphere. Olsen added, “If we could have observed these events taking
place billions of years ago, we would have been treated to a real
display of cosmic fireworks as new stars formed!”
“This is a great example of NSF-sponsored facilities being used in
unison to unravel the mysteries of our neighbor galaxy M31, something
that NSF’s National Optical-infrared Astronomy Research Laboratory
should make much easier,” noted Ralph Gaume, Division Director for NSF’s
Division of Astronomical Sciences.
Source: Gemini Observatory
More Information
The study, published in Nature, analyzed data from the Pan-Andromeda Archaeological Survey (PAndAS):“Two major accretion epochs in M31 from two distinct populations of globular clusters,” Mackey et al. 2019, Nature.
NSF’s National Optical-Infrared Astronomy Research Laboratory, the US center for ground-based optical-infrared astronomy, operates the Gemini Observatory, Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and the Large Synoptic Survey Telescope (LSST). It is managed by the Association of Universities for Research in Astronomy (AURA)
under a cooperative agreement with NSF and is headquartered in Tucson,
Arizona. The astronomical community is honored to have the opportunity
to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona,
on Maunakea in Hawai’i, and on Cerro Tololo and Cerro Pachón in Chile.
We recognize and acknowledge the very significant cultural role and
reverence that these sites have to the Tohono O’odham Nation, to the
Native Hawaiian community, and to the local Communities in Chile,
respectively.
Science Contacts:
Dr. Dougal Mackey
Research School of Astronomy and Astrophysics
Australian National University, College of Science
Email: dougal.mackey@anu.edu.au
Desk: +61 2 6125 0214
Cell: +61 457 871 313
Professor Geraint Lewis
Sydney Institute for Astronomy
School of Physics, University of Sydney
Email: geraint.lewis@sydney.edu.au
Cell: +61 424 254 551
Dr. Joan Najita
NSF’s National Optical-Infrared Astronomy Research Laboratory
950 N. Cherry Ave. Tucson, AZ 85719 USA
Email: najita@noao.edu
Desk: +1 520-318-8416
