The inset diagram at bottom left shows the measurement of gas moving toward and away from Earth, indicating the material is traveling at a high velocity.
Hubble also observed light from quasars that passed outside the northern bubble. The box at upper right reveals that the gas in one such quasar's light path is not moving toward or away from Earth. This gas is in the disk of the Milky Way and does not share the same characteristics as the material probed inside the bubble. Illustration Credit: NASA, ESA, and Z. Levy (STScI); Science Credit: NASA, ESA, and R. Bordoloi (MIT). Image release
For the supermassive black hole at the center of our Milky Way galaxy, it's been a long time between dinners. NASA's Hubble Space Telescope has found that the black hole ate its last big meal about 6 million years ago, when it consumed a large clump of infalling gas. After the meal, the engorged black hole burped out a colossal bubble of gas weighing the equivalent of millions of suns, which now billows above and below our galaxy's center.
The immense structures, dubbed the Fermi Bubbles, were first discovered in 2010 by NASA's Fermi Gamma-ray Space Telescope.
But recent Hubble observations of the northern bubble have helped
astronomers determine a more accurate age for the bubbles and how they
came to be.
"For the first time, we have traced the motion of cool gas throughout
one of the bubbles, which allowed us to map the velocity of the gas and
calculate when the bubbles formed," said lead researcher Rongmon
Bordoloi of the Massachusetts Institute of Technology in Cambridge.
"What we find is that a very strong, energetic event happened 6 million
to 9 million years ago. It may have been a cloud of gas flowing into the
black hole, which fired off jets of matter, forming the twin lobes of
hot gas seen in X-ray and gamma-ray observations. Ever since then, the
black hole has just been eating snacks."
The new study is a follow-on to previous Hubble observations that placed the age of the bubbles at 2 million years old.
A black hole is a dense, compact region of space with a gravitational
field so intense that neither matter nor light can escape. The
supermassive black hole at the center of our galaxy has compressed the
mass of 4.5 million sun-like stars into a very small region of space.
Material that gets too close to a black hole is caught in its
powerful gravity and swirls around the compact powerhouse until it
eventually falls in. Some of the matter, however, gets so hot it escapes
along the black hole's spin axis, creating an outflow that extends far
above and below the plane of a galaxy.
The team's conclusions are based on observations by Hubble's Cosmic
Origins Spectrograph (COS), which analyzed ultraviolet light from 47
distant quasars. Quasars are bright cores of distant active galaxies.
Imprinted on the quasars' light as it passes through the Milky Way
bubble is information about the speed, composition, and temperature of
the gas inside the expanding bubble.
The COS observations measured the temperature of the gas in the
bubble at approximately 17,700 degrees Fahrenheit. Even at those
sizzling temperatures, this gas is much cooler than most of the
super-hot gas in the outflow, which is 18 million degrees Fahrenheit,
seen in gamma rays. The cooler gas seen by COS could be interstellar gas
from our galaxy's disk that is being swept up and entrained into the
super-hot outflow. COS also identified silicon and carbon as two of the
elements being swept up in the gaseous cloud. These common elements are
found in most galaxies and represent the fossil remnants of stellar
evolution.
The cool gas is racing through the bubble at 2 million miles per
hour. By mapping the motion of the gas throughout the structure, the
astronomers estimated that the minimum mass of the entrained cool gas in
both bubbles is equivalent to 2 million suns. The edge of the northern
bubble extends 23,000 light-years above the galaxy.
"We have traced the outflows of other galaxies, but we have never
been able to actually map the motion of the gas," Bordoloi said. "The
only reason we could do it here is because we are inside the Milky Way.
This vantage point gives us a front-row seat to map out the kinematic
structure of the Milky Way outflow."
The new COS observations build and expand on the findings of a 2015
Hubble study by the same team, in which astronomers analyzed the light
from one quasar that pierced the base of the bubble.
"The Hubble data open a whole new window on the Fermi Bubbles," said study co-author Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland. "Before, we knew how big they were and how much radiation they emitted; now we know how fast they are moving and which chemical elements they contain. That's an important step forward."
The Hubble study also provides an independent verification of the
bubbles and their origin, as detected by X-ray and gamma-ray
observations.
"This observation would be almost impossible to do from the ground
because you need ultraviolet spectroscopy to detect the fingerprints of
these elements, which can only be done from space," Bordoloi said. "Only
with COS do you have the wavelength coverage, the sensitivity, and the
spectral resolution coverage to make this observation."
The Hubble results appeared in the January 10, 2017, edition of The Astrophysical Journal.
The Hubble Space Telescope is a project of international cooperation
between NASA and the 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.
Related links
- The science paper by R. Bordoloi et al.
- NASA's Hubble Portal
- Hubble Discovers that Milky Way Core Drives Wind at 2 Million Miles Per Hour
Contacts
Felicia Chou
NASA Headquarters, Washington, D.C.
felicia.chou@nasa.gov
202-358-0257
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Rongmon Bordoloi
Massachusetts Institute of Technology, Cambridge, Massachusetts
617-252-1736
bordoloi@mit.edu
Source: Hubble Site
