Dots
show locations of stars in the Keck Observatory spectroscopic survey
superimposed on an image of Andromeda. The stars are color coded
according to their velocity relative to Earth, as measured from the
spectra (negative velocity = moving towards Earth). The center of
Andromeda is moving towards Earth at a speed of −300 km/s, whereas stars
to the northeast (upper left) of the map have less negative velocities,
indicating that they are moving away from Earth, relative to
Andromeda's center. Credit: Claire Dorman and the European Space Agency. Hi-res image
This
Hubble image of a crowded star field in the disk of the Andromeda
galaxy shows that stars of different ages can be distinguished from one
another on the basis of temperature (as indicated by color) and
brightness. Credit: Ben Williams, Julianne Dalcanton, and the PHAT collaboration. Hi-res image
MAUNA
KEA, HI – A detailed
study of the motions of different stellar populations in Andromeda galaxy by UC
Santa Cruz scientists using W. M. Keck Observatory data has found striking
differences from our own Milky Way, suggesting a more violent history of
mergers with smaller galaxies in Andromeda's recent past. The findings are
being presented on Thursday, January 8, at the winter
meeting of the American Astronomical Society in Seattle.
The
structure and internal motions of the stellar disk of a spiral galaxy hold
important keys to understanding the galaxy's formation history. The Andromeda
galaxy, also called M31, is the closest spiral galaxy to the Milky Way and the
largest in the local group of galaxies.
“In the
Andromeda galaxy we have the unique combination of a global yet detailed view
of a galaxy similar to our own. We have lots of detail in our own Milky Way,
but not the global, external perspective,” said Puragra Guhathakurta, professor
of astronomy and astrophysics at the University of California, Santa Cruz.
The new
study, led by UC Santa Cruz graduate student Claire Dorman and Guhathakurta,
combined data from two large surveys of stars in Andromeda conducted at the
Keck Observatory in Hawaii as well as data from the Hubble Space
Telescope.
The
Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH)
survey used data from the 10-meter Keck II telescope, fitted with the DEIMOS
multi-object spectrograph to measure radial velocities of more than 10,000
individual bright stars in Andromeda.
“The
sheer light-gathering power of the Keck Observatory, the superb quality of
DEIMOS spectra, free of instrumental/atmospheric artifacts, and its ability to
obtain spectra of as many as 300 stars at once were crucial to the success of
this experiment,” said Guhathakurta. “The Andromeda galaxy is about 2.5 million
light years away so even its most luminous stars generally appear quite faint
from our vantage point. To measure precise stellar velocities, the white light
of each of these faint stars must be subdivided into thousands of wavelengths.
The Keck/DEIMOS combination is the only one in the world capable of making
these velocity measurements for large numbers of Andromeda stars.”
The
recently completed Panchromatic Hubble Andromeda Treasury (PHAT) survey
provided high-resolution imaging at six different wavelengths for more than
half of these stars, Dorman said. The study presents the velocity dispersion of
young, intermediate-age, and old stars in the disk of Andromeda, the first such
measurement in another galaxy.
Dorman's
analysis revealed a clear trend related to stellar age, with the youngest stars
showing relatively ordered rotational motion around the center of the Andromeda
galaxy, while older stars displayed much more disordered motion. Stars in a “well
ordered” population are all moving coherently, with nearly the same velocity,
whereas stars in a disordered population have a wider range of velocities,
implying a greater spatial dispersion.
“If you
could look at the disk edge-on, the stars in the well-ordered, coherent
population would lie in a very thin plane, whereas the stars in the disordered
population would form a much puffier layer,” Dorman explained.
The researchers
considered different scenarios of galactic disk formation and evolution that
could account for their observations. One scenario involves the gradual
disturbance of a well-ordered disk of stars as a result of mergers with small
satellite galaxies. Previous studies have found evidence of such mergers in
tidal streams of stars in the extended halo of Andromeda, which appear to be
remnants of cannibalized dwarf galaxies. Stars from those galaxies can also
accrete onto the disk, but accretion alone cannot account for the observed
increase in velocity dispersion with stellar age, Dorman said.
An
alternate scenario involves the formation of the stellar disk from an initially
thick, clumpy disk of gas that gradually settled. The oldest stars would then
have formed while the gas disk was still in a puffed up and disordered
configuration. Over time, the gas disk would have settled into a thinner
configuration with more ordered motion, and the youngest stars would then have
formed with the disk in that ordered configuration.
According
to Dorman, a combination of these mechanisms could account for the team's
observations. “Our findings should motivate theorists to carry out more
detailed computer simulations of these scenarios,” she said.
The
comparison to the Milky Way revealed substantial differences suggesting that
Andromeda has had a more violent accretion history in the recent past. “Even
the most well ordered Andromeda stars are not as well ordered as the stars in
the Milky Way's disk,” Dorman said.
In the currently
favored “Lambda Cold Dark Matter” paradigm of structure formation in the
universe, large galaxies such as Andromeda and the Milky Way are thought to
have grown by cannibalizing smaller satellite galaxies and accreting their
stars and gas. Cosmologists predict that 70 percent of disks the size of
Andromeda's and the Milky Way's should have interacted with at least one
sizable satellite in the last 8 billion years. The Milky Way's disk is much too
orderly for that to have happened, whereas Andromeda's disk fits the prediction
much better.
“In this
context, the motion of the stars in Andromeda's disk is more normal, and the
Milky Way may simply be an outlier with an unusually quiescent accretion
history,” Guhathakurta said.
Other
researchers who collaborated with Dorman and Guhathakurta on this study include
Anil Seth at the University of Utah; Daniel Weisz, Julianne Dalcanton, Alexia
Lewis, and Benjamin Williams at the University of Washington; Karoline Gilbert
at the Space Telescope Science Institute; Evan Skillman at the University of
Minnesota; Eric Bell at the University of Michigan; and Katherine Hamren and
Elisa Toloba at UC Santa Cruz. This research was funded by the National Science
Foundation and NASA.
The W.
M. Keck Observatory operates the largest, most scientifically productive
telescopes on Earth. The two, 10-meter optical/infrared telescopes near the
summit of Mauna Kea on the Island of Hawaii feature a suite of advanced
instruments including imagers, multi-object spectrographs, high-resolution
spectrographs, integral-field spectrographs and world-leading laser guide star
adaptive optics systems.
DEIMOS
(the DEep Imaging and Multi-Object Spectrograph) boasts the largest field of
view (16.7 arcmin by 5 arcmin) of any of the Keck instruments, and the largest
number of pixels (64 Mpix). It is used primarily in its multi-object mode,
obtaining simultaneous spectra of up to 130 galaxies or stars. Astronomers
study fields of distant galaxies with DEIMOS, efficiently probing the most
distant corners of the universe with high sensitivity.
Keck
Observatory is a private 501(c) 3 non-profit organization and a scientific
partnership of the California Institute of Technology, the University of
California and NASA.
Media Contact:
Steve Jefferson
Communications Officer
W. M. Keck Observatory
808.881.3827
sjefferson@keck.hawaii.edu
Science Contacts:
Claire Dorman
UC Santa Cruz
cdorman@ucolick.org
Puragra (Raja) Guhathakurta
(408) 455-3036
UC Santa Cruz
raja@ucolick.org
Source: W.M. Keck Observatory
