Surprising new data from NASA's Hubble Space Telescope suggests the
smooth, settled "brim" of the Sombrero galaxy's disk may be concealing a
turbulent past. Hubble's sharpness and sensitivity resolves tens of
thousands of individual stars in the Sombrero's vast, extended halo, the
region beyond a galaxy's central portion, typically made of older
stars. These latest observations of the Sombrero are turning
conventional theory on its head, showing only a tiny fraction of older,
metal-poor stars in the halo, plus an unexpected abundance of metal-rich
stars typically found only in a galaxy's disk, and the central bulge.
Past major galaxy mergers are a possible explanation, though the stately
Sombrero shows none of the messy evidence of a recent merger of massive
galaxies.
"The Sombrero has always been a bit of a weird galaxy,
which is what makes it so interesting," said Paul Goudfrooij of the
Space Telescope Science Institute (STScI), Baltimore, Maryland.
"Hubble's metallicity measurements (i.e.: the abundance of heavy
elements in the stars) are another indication that the Sombrero has a
lot to teach us about galaxy assembly and evolution."
"Hubble's
observations of the Sombrero's halo are turning our generally accepted
understanding of galaxy makeup and metallicity on its head," added
co-investigator Roger Cohen of STScI.
Long a favorite of
astronomers and amateur sky watchers alike for its bright beauty and
curious structure, the Sombrero Galaxy (M104) now has a new chapter in
its strange story — an extended halo of metal-rich stars with barely a
sign of the expected metal-poor stars that have been observed in the
halos of other galaxies. Researchers, puzzling over the data from
Hubble, turned to sophisticated computer models to suggest explanations
for the perplexing inversion of conventional galactic theory.
Those
results suggest the equally surprising possibility of major mergers in
the galaxy's past, though the Sombrero's majestic structure bears no
evidence of recent disruption. The unusual findings and possible
explanations are published in the Astrophysical Journal
"The absence of metal-poor stars was a big surprise," said Goudfrooij, "and the abundance of metal-rich stars only added to the mystery."
In a
galaxy's halo astronomers expect to find earlier generations of stars
with less heavy elements, called metals, as compared to the crowded
stellar cities in the main disk of a galaxy. Elements are created
through the stellar "lifecycle" process, and the longer a galaxy has had
stars going through this cycle, the more element-rich the gas and the
higher-metallicity the stars that form from that gas. These younger,
high-metallicity stars are typically found in the main disk of the
galaxy where the stellar population is denser — or so goes the
conventional wisdom.
Complicating the facts is the presence of
many old, metal-poor globular clusters of stars. These older, metal-poor
stars are expected to eventually move out of their clusters and become
part of the general stellar halo, but that process seems to have been
inefficient in the Sombrero galaxy. The team compared their results with
recent computer simulations to see what could be the origin of such
unexpected metallicity measurements in the galaxy's halo.
The
results also defied expectations, indicating that the unperturbed
Sombrero had undergone major accretion, or merger, events billions of
years ago. Unlike our Milky Way galaxy, which is thought to have
swallowed up many small satellite galaxies in so-called "minor"
accretions over billions of years, a major accretion is the merger of
two or more similarly massive galaxies that are rich in
later-generation, higher-metallicity stars.
The satellite galaxies
only contained low metallicity stars that were largely hydrogen and
helium from the big bang. Heavier elements had to be cooked up in
stellar interiors through nucleosynthesis and incorporated into later
generation stars. This process was rather ineffective in dwarf galaxies
such as those around our Milky Way, and more effective in larger, more
evolved galaxies.
The results for the Sombrero are surprising
because its smooth disk shows no sign.s of disruption. By comparison,
numerous interacting galaxies, like the iconic Antennae galaxies, get
their name from the distorted appearance of their spiral arms due to the
tidal forces of their interaction. Mergers of similarly massive
galaxies typically coalesce into large, smooth elliptical galaxies with
extended halos — a process that takes billions of years. But the
Sombrero has never quite fit the traditional definition of either a
spiral or an elliptical galaxy. It is somewhere in between — a hybrid.
For
this particular project, the team chose the Sombrero mainly for its
unique morphology. They wanted to find out how such "hybrid" galaxies
might have formed and assembled over time. Follow-up studies for halo
metallicity distributions will be done with several galaxies at
distances similar to that of the Sombrero.
The research team looks
forward to future observatories continuing the investigation into the
Sombrero's unexpected properties. The Wide Field Infrared Survey
Telescope (WFIRST), with a field of view 100 times that of Hubble, will
be capable of capturing a continuous image of the galaxy's halo while
picking up more stars in infrared light. The James Webb Space Telescope
will also be valuable for its Hubble-like resolution and deeper infrared
sensitivity.
The Hubble Space Telescope is a project of
international cooperation between NASA and ESA (European Space Agency).
NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the
telescope. The Space Telescope Science Institute (STScI) in Baltimore,
Maryland, conducts Hubble science operations. STScI is operated for NASA
by the Association of Universities for Research in Astronomy in
Washington, D.C.
Source: HubbleSite
Contact:
Leah Ramsay/ Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
667-218-6439 / 410-338-4514
lramsay@stsci.edu / villard@stsci.edu
Roger Cohen / Paul Goudfrooij
Space Telescope Science Institute, Baltimore, Maryland
rcohen@stsci.edu / goudfroo@stsci.edu
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