Artist impression of ring of cool, interstellar gas
surrounding the supermassive black hole at the center of the Milky Way.
New ALMA observations reveal this structure for the first time. Credit: NRAO/AUI/NSF; S. Dagnello. Hi-res image
ALMA image of the disk of cool hydrogen gas flowing around
the supermassive black hole at the center of our galaxy. The colors
represent the motion of the gas relative to Earth: the red portion is
moving away, so the radio waves detected by ALMA are slightly stretched,
or shifted, to the "redder" portion of the spectrum; the blue color
represents gas moving toward Earth, so the radio waves are slightly
scrunched, or shifted, to the "bluer" portion of the spectrum.
Crosshairs indicate location of black hole. Credit: ALMA (ESO/NAOJ/NRAO), E.M. Murchikova; NRAO/AUI/NSF, S. Dagnello. Hi-res image
Through decades of study, astronomers have developed a clearer picture of the chaotic and crowded neighborhood surrounding the supermassive black hole at the center of the Milky Way. Our galactic center is approximately 26,000 light-years from Earth and the supermassive black hole there, known as Sagittarius A* (A “star”), is 4 million times the mass of our Sun.
We now know that this region is brimming with roving stars, interstellar dust clouds, and a large reservoir of both phenomenally hot and comparatively colder gases. These gases are expected to orbit the black hole in a vast accretion disk that extends a few tenths of a light-year from the black hole’s event horizon.
Until now, however, astronomers have been able to image only the
tenuous, hot portion of this flow of accreting gas, which forms a
roughly spherical flow and showed no obvious rotation. Its temperature
is estimated to be a blistering 10 million degrees Celsius (18 million
degrees Fahrenheit), or about two-thirds the temperature found at the
core of our Sun. At this temperature, the gas glows fiercely in X-ray
light, allowing it to be studied by space-based X-ray telescopes, down
to scale of about a tenth of a light-year from the black hole.
In addition to this hot, glowing gas, previous observations with
millimeter-wavelength telescopes have detected a vast store of
comparatively cooler hydrogen gas (about 10 thousand degrees Celsius, or
18,000 degrees Fahrenheit) within a few light-years of the black hole.
The contribution of this cooler gas to the accretion flow onto the black
hole was previously unknown.
Although our galactic center black hole is relatively quiet, the
radiation around it is strong enough to cause hydrogen atoms to
continually lose and recombine with their electrons. This recombination
produces a distinctive millimeter-wavelength signal, which is capable of
reaching Earth with very little losses along the way.
With its remarkable sensitivity and powerful ability to see fine details, the Atacama Large Millimeter/submillimeter Array (ALMA)
was able to detect this faint radio signal and produce the first-ever
image of the cooler gas disk at only about a hundredth of a light-year
away (or about 1000 times the distance from the Earth to the Sun) from
the supermassive black hole. These observations enabled the astronomers
both to map the location and trace the motion of this gas. The
researchers estimate that the amount of hydrogen in this cool disk is
about one tenth the mass of Jupiter, or one ten-thousandth of the mass
of the Sun.
By mapping the shifts in wavelengths of this radio light due to the
Doppler effect (light from objects moving toward the Earth is slightly
shifted to the “bluer” portion of the spectrum while light from objects
moving away is slightly shifted to the “redder” portion), the
astronomers could clearly see that the gas is rotating around the black
hole. This information will provide new insights into the ways that
black holes devour matter and the complex interplay between a black hole
and its galactic neighborhood.
“We were the first to image this elusive disk and study its
rotation,” said Elena Murchikova, a member in astrophysics at the
Institute for Advanced Study in Princeton, New Jersey, and lead author
on the paper. “We are also probing accretion onto the black hole. This
is important because this is our closest supermassive black hole. Even
so, we still have no good understanding of how its accretion works. We
hope these new ALMA observations will help the black hole give up some
of its secrets.”
The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc.
Contact:
Charles E. Blue
Public Information Officer
cblue@nrao.edu; 434-296-0314
Reference:
E.M. Murchikova, et al., “A cool accretion disk around the Galactic Center black hole,” Nature, 06 June 2019
The Atacama Large Millimeter/submillimeter Array (ALMA), an
international astronomy facility, is a partnership of the European
Organisation for Astronomical Research in the Southern Hemisphere (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
Ministry of Science and Technology (MOST) and by NINS in cooperation
with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and
Space Science Institute (KASI).
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.
