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Artist impression of a progenitor of Milky Way-like galaxy in the early universe with a background quasar shinning through a 'super halo' of hydrogen gas surrounding the galaxy. New ALMA observations of two such galaxies reveal that these vast halos extend well beyond the galaxies' dusty, star-forming disks. The galaxies were initially found by the absorption of background quasar light passing through the galaxies. ALMA was able to image the ionized carbon in the galaxies' disks, revealing crucial details about their structures. Credit: A. Angelich (NRAO/AUI/NSF)
Composite ALMA and optical image of a young Milky Way-like galaxy 12 billion light-years away and a background quasar 12.5 billion light-years away. Light from the quasar passed through the galaxy's gas on its way to Earth, revealing the presence of the galaxy to astronomers. New ALMA observations of the galaxy's ionized carbon (green) and dust continuum (blue) emission show that the dusty, star-forming disk of the galaxy is vastly offset from the gas detected by quasar absorption at optical wavelengths (red). This indicates that a massive halo of gas surrounds the galaxy. The optical data are from the Keck I Telescope at the W.M. Keck Observatory. Credit: ALMA (ESO/NAOJ/NRAO), M. Neeleman & J. Xavier Prochaska; Keck Observatory
Credit: Produced by Alexandra Angelich (NRAO/AUI/NSF); Written and narrated by Charles Blue (NRAO/AUI/NSF); Animations and footage courtesy of Alexandra Angelich (NRAO/AUI/NSF); NASA/Goddard Space Flight Center/Cruz deWilde and the Advanced Visualization Laboratory at the National Center for Supercomputing and B. O'Shea, M. Norman; ESO/C.Malin; Science images courtesy of M. Neeleman & J. Xavier Prochaska; Keck Observatory; Music by Geodesium.Vimeo
By harnessing the extreme sensitivity of the Atacama Large
Millimeter/submillimeter Array (ALMA), astronomers have directly
observed a pair of Milky Way-like galaxies seen when the universe was
only eight percent of its current age. These progenitors of today’s
giant spiral galaxies are surrounded by "super halos" of hydrogen gas
that extend many tens of thousands of light-years beyond their dusty,
Astronomers initially detected these galaxies
by studying the intense light from even-more-distant quasars. As this
light travels through an intervening galaxy on its way to Earth, it can
pick up the unique spectral signature from the galaxy’s gas. This
technique, however, normally prevents astronomers from seeing the actual
light emitted by the galaxy, which is overwhelmed by the much brighter
emission from the background quasar.
"Imagine a tiny firefly
next to a high-power search light. That’s what astronomers are up
against when it comes to observing these youthful versions of our home
galaxy," said Marcel Neeleman a postdoctoral fellow at the University of
California, Santa Cruz, and lead author on a paper appearing in the
journal Science. "We can now see the galaxies themselves, which
gives us an amazing opportunity to learn about the earliest history of
our own galaxy and others like it."
With ALMA, the astronomers
were finally able to observe the natural millimeter-wavelength "glow"
emitted by ionized carbon in the dense and dusty star-forming regions of
the galaxies. This carbon signature, however, is considerably offset
from the gas first detected by quasar absorption. This extreme
separation indicates that the galaxies’ gas content extends well beyond
their star-filled disks, suggesting that each galaxy is embedded in a
monstrous halo of hydrogen gas.
"We had expected we would see
faint emission right on top of the quasar, and instead we saw bright
galaxies at large separations from the quasar," said J. Xavier
Prochaska, professor of astronomy and astrophysics at UC Santa Cruz and
coauthor of the paper. The separation from the quasar to the observed
galaxy is about 137,000 light-years for one galaxy and about 59,000
light-years for the other.
According to the researchers, the
neutral hydrogen gas revealed by its absorption of quasar light is most
likely part of a large halo or perhaps an extended disk of gas around
the galaxy. "It's not where the star formation is, and to see so much
gas that far from the star-forming region means there is a large amount
of neutral hydrogen around the galaxy," Neeleman said.
ALMA data show that these young galaxies are already rotating, which is
one of the hallmarks of the massive spiral galaxies we see in the
universe today. The ALMA observations further reveal that both galaxies
are forming stars at moderately high rates: more than 100 solar masses
per year in one galaxy and about 25 solar masses per year in the other.
galaxies appear to be massive, dusty, and rapidly star-forming systems,
with large, extended layers of gas," Prochaska said.
solved a decades-old question on galaxy formation," said Chris Carilli,
an astronomer with the National Radio Astronomy Observatory in Socorro,
N.M., and co-author on the paper. "We now know that at least some very
early galaxies have halos that are much more extended that previously
considered, which may represent the future material for galaxy growth."
galaxies, which are officially designated ALMA J081740.86+135138.2 and
ALMA J120110.26+211756.2, are each about 12 billion light-years from
Earth. The background quasars are each roughly 12.5 billion light-years
The National Radio Astronomy Observatory is a
facility of the National Science Foundation, operated under cooperative
agreement by Associated Universities, Inc.
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Atacama Large Millimeter/submillimeter Array (ALMA), an international
astronomy facility, is a partnership of ESO, the U.S. National Science
Foundation (NSF) and the National Institutes of Natural Sciences (NINS)
of Japan in cooperation with the Republic of Chile. ALMA is funded by
ESO on behalf of its Member States, by NSF in cooperation with the
National Research Council of Canada (NRC) and the National Science
Council of Taiwan (NSC) and by NINS in cooperation with the Academia
Sinica (AS) in Taiwan and the Korea Astronomy and Space Science
ALMA construction and operations are led by ESO
on behalf of its Member States; by the National Radio Astronomy
Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on
behalf of North America; and by the National Astronomical Observatory of
Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO)
provides the unified leadership and management of the construction,
commissioning and operation of ALMA.
This research is presented
in a paper titled "[C II] 158-μm emission from the host galaxies of
damped Lyman alpha systems," by M. Neeleman et al., scheduled for
publication in the journal Science on 24 March 2017.