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The Deuterium-to-Hydrogen Ratio in a Low-Metallicity Cloud Falling onto
the Milky Way
(U. Wisconsin),
(U. Massachusetts),
(Osservatorio Astrofisico di Arcetri),
(Johns Hopkins),
(U. Wisconsin),
(U. Colorado),
(U. Chicago),
(NASA/GSFC),
(Princeton)
Abstract: Using Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope
observations of the QSO PG, we detect D I Lyman-series absorption in
high velocity cloud Complex C, a low-metallicity gas cloud falling onto the
Milky Way. This is the first detection of atomic deuterium in the local
universe in a location other than the nearby regions of the Galactic disk. A
new Westerbork Synthesis Radio Telescope (WSRT) interferometer map of the H I
21 cm emission toward PG indicates that the sight line passes through a
compact concentration of neutral gas in Complex C. We find D/H =
(2.2+/-0.7)x10^-5, O/H = (8.0+/-2.5)x10^-5, and D/O = 0.28+/-0.12. The
metallicity of Complex C gas toward PG is approximately 1/6 solar, as
inferred from the oxygen abundance [O/H] = -0.79 (+0.12, -0.16). While we
cannot rule out a value of D/H similar to that found for the local ISM, we can
confidently exclude values as low as those determined recently for extended
sight lines in the Galactic disk. Combined with the sub-solar metallicity
estimate and the low nitrogen abundance, this conclusion lends support to the
hypothesis that Complex C is located outside the Milky Way, rather than inside
in material recirculated between the Galactic disk and halo. The value of D/H
for Complex C is consistent with the primordial abundance of deuterium inferred
from recent Wilkinson Microwave Anisotropy Probe observations of the cosmic
microwave background and simple chemical evolution models that predict the
amount of deuterium astration as a function of metallicity. [Abbreviated
Astrophysics (astro-ph)
for this version)
Submission history
From: Kenneth R. Sembach []
[v1] Fri, 7 Nov :21 GMTAstronomers Spy Mysterious Gas Cloud at Milky Way’s Galactic Center | Astronomy |
In 2011, VLT discovered a gas cloud with several times the mass of the Earth accelerating towards the black hole. This cloud is now making its closest approach and new observations show that it is being stretched by the black hole’s extreme gravitational field.
Dr Stefan Gillessen from Max Planck Institute for Extraterrestrial Physics, Germany, who is a lead author of a paper reporting the results in the
( / ), said that the gas at the head of the cloud is now stretched over more than 160 billion km around the closest point of the orbit to the black hole. And the closest approach is only a bit more than 25 billion km from the black hole itself – barely escaping falling right in.
“The cloud is so stretched that the close approach is not a single event but rather a process that extends over a period of at least one year.”
As the gas cloud is stretched its light gets harder to see. But by staring at the region close to the black hole for more than 20 hours of total exposure time, the astronomers were able to measure the velocities of different parts of the cloud as it streaks past the black hole.
“The most exciting thing we now see in the new observations is the head of the cloud coming back towards us at more than 10 million km/h along the orbit — about 1% of the speed of light. This means that the front end of the cloud has already made its closest approach to the black hole,” said co-author Dr Reinhard Genzel of the University of California, Berkeley.
This simulation of a gas cloud passing close to the supermassive black hole at the centre of the galaxy shows the situation in mid-2013 (ESO / S. Gillessen / MPE / Marc Schartmann)
The origin of the gas cloud remains mysterious, although there is no shortage of ideas. The new observations narrow down the possibilities.
“Like an unfortunate astronaut in a science fiction film, we see that the cloud is now being stretched so much that it resembles spaghetti. This means that it probably doesn’t have a star in it. At the moment we think that the gas probably came from the stars we see orbiting the black hole,” Dr Gillessen concluded.
Bibliographic information: Stefan Gillessen et al. 2013. Pericenter passage of the gas cloud G2 in the Galactic Center. ApJ, arXiv:
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相关词典网站：Mysterious G2 Cloud Near Black Hole Identified | W. M. Keck Observatory
<meta name="description" content="MAUNA KEA, Hawaii –&The mystery about a thin, bizarre
object in the center of the Milky Way headed toward our galaxy’s enormous black
hole has been solved by UCLA" />
Mysterious G2 Cloud Near Black Hole Identified
Credit: Andrea Ghez, Gunther Witzel/UCLA Galactic Center Group/W. M. Keck ObservatoryAn image from W. M. Keck Observatory near infrared data shows that G2 survived its closest approach to the black hole and continues happily on its orbit. The green circle just to its right depicts the location of the invisible supermassive black hole.
Credit: Ethan Tweedie PhotographyTelescopes from Hawaii’s W.M. Keck Observatory use a powerful technology called adaptive optics, which enabled UCLA astronomers to discover that G2 is a pair of binary stars that merged together, cloaked in gas and dust.
Media ContactSteve JeffersonCommunications OfficerW. M. Keck Observatory808.881.3827
MAUNA KEA, Hawaii –&The mystery about a thin, bizarre
object in the center of the Milky Way headed toward our galaxy’s enormous black
hole has been solved by UCLA astronomers using the W. M. Keck Observatory, home
of the two largest telescopes on Earth. The scientists studied the object,
known as G2, during its closest approach to the black hole this summer, and
found the black hole did not dine on it. The research is published today in the journal Astrophysical
Journal Letters.While some scientists believed the
object was a cloud of hydrogen gas that would be torn apart in a fiery show,
Ghez and her team proved&it was much more interesting. “G2 survived and continues
a gas cloud would not do that,” said Andrea Ghez, UCLA
professor of physics and astronomy who holds the Lauren B. Leichtman and Arthur
E. Levine Chair in Astrophysics, and directs the . “G2
was completely unaffect no fireworks.”Instead, the team has
demonstrated it is a pair of binary stars that had been orbiting the black hole
in tandem and merged together into an extremely large star, cloaked in gas and
dust, and choreographed by the black hole’s powerful gravitational field. “G2 is not alone,” said Ghez,
who uses Keck Observatory to study thousands of stars in the neighborhood of
the supermassive black hole. “We’re seeing a new class of stars near the black
hole, and as a consequence of the black hole.” Ghez and her colleagues — who
include lead author Gunther
Witzel, a UCLA postdoctoral scholar in Ghez’s research group, and Mark Morris,
a UCLA professor of physics and astronomy
— studied the event with both of the 10-meter telescopes at Keck
Observatory.Keck Observatory employs a powerful technology&called adaptive optics,
which Ghez helped to pioneer, to correct the distorting
effects of the Earth's atmosphere in real time, and to reveal the region of
space around the black hole. With adaptive optics, Ghez
and her colleagues have revealed many surprises about the environments
surrounding supermassive black holes, discovering, for example, young stars
where none were expected and seeing a lack of old stars where many were
anticipated.“The Keck Observatory has been
the leader in adaptive optics for more than a decade and has enabled us to
achieve tremendous progress in correcting the distorting effects of the Earth’s
atmosphere using high–angular resolution imaging techniques,” Ghez said.The researchers wouldn’t have been able to arrive at their conclusions
without the Keck’s advanced technology. “It is a result that in its precision
was possible only with these incredible tools, the Keck Observatory’s 10-meter
telescopes,” Witzel said.“We are seeing phenomena about
black holes that you can’t watch anywhere else in the universe,” Ghez added.
“We are starting to understand the physics of black holes in a way that has
never been possible before, and is possible only at the center of the galaxy.”Massive stars in our galaxy, she
noted, primarily come in pairs. When the two stars merge into one, the star expands
for more than one million years “before it settles back down,” Ghez said. “This may
be happening
the stars at the center of the galaxy are
massive and mostly binaries. It’s possible that many of the stars we’ve been
watching and not understanding may be the end product of a merger that are calm
now.”G2, in that explosive stage now,
has been an object of fascination. “Its closest approach to the black hole was
one of the most watched events in astronomy in my career,” Ghez said.G2 makes an unusual, 300-year elliptical
orbit around the black hole and Ghez’s group calculated its closest approach occurred
this summer — later than other astronomers believed —and they were in place at
Keck Observatory to gather the data. Black holes, which form out of
the collapse of matter, have such high density that nothing can escape their
gravitational pull, not even light. They cannot be seen directly, but their
influence on nearby stars is visible and provides a signature, said Ghez, a
2008 MacArthur Fellow.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 spectographs and
world-leading laser guide star adaptive optics systems. NIRC2 (the
Near-Infrared Camera, second generation) works in combination with the Keck II
adaptive optics system to obtain very sharp images at near-infrared
wavelengths, achieving spatial resolutions comparable to or better than those
achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is
probably best known for helping to provide definitive proof of a central
massive black hole at the center of our galaxy. Astronomers also use NIRC2 to
map surface features of solar system bodies, detect planets orbiting other
stars, and study detailed morphology of distant galaxies.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.
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The sky is ready for LRIS-ADC and ESI. Our observers from UC-lbnl and UH are also ready.
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Keck 1 LRIS-ADC is scheduled with UC-lbnl observers. Keck 2 DEIMOS is scheduled for NASA observers.
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