Quasar's Light Yields Clues to Outflow
At a time when our earliest human ancestors had recently mastered
walking upright, the heart of our Milky Way galaxy underwent a titanic
eruption, driving gases and other material outward at 2 million miles
per hour.
Now, at least 2 million years later, astronomers are witnessing the
aftermath of the explosion: billowing clouds of gas towering about
30,000 light-years above and below the plane of our galaxy.
The enormous structure was discovered five years ago as a gamma-ray
glow on the sky in the direction of the galactic center. The
balloon-like features have since been observed in X-rays and radio
waves. But astronomers needed NASA's Hubble Space Telescope to measure
for the first time the velocity and composition of the mystery lobes.
They now seek to calculate the mass of the material being blown out of
our galaxy, which could lead them to determine the outburst's cause
from several competing scenarios.
Astronomers have proposed two possible origins for the bipolar lobes:
a firestorm of star birth at the Milky Way's center or the eruption of
its supermassive black hole. Although astronomers have seen gaseous
winds, composed of streams of charged particles, emanating from the
cores of other galaxies, they are getting a unique, close-up view of
our galaxy's own fireworks.
"When you look at the centers of other galaxies, the outflows appear
much smaller because the galaxies are farther away," said Andrew Fox of
the Space Telescope Science Institute in Baltimore, Maryland, lead
researcher of the study. "But the outflowing clouds we're seeing are
only 25,000 light-years away in our galaxy. We have a front-row seat.
We can study the details of these structures. We can look at how big
the bubbles are and can measure how much of the sky they are covering."
Fox's results will be published in The Astrophysical Journal Letters
and will be presented at the American Astronomical Society meeting in
Seattle, Washington.
The giant lobes, dubbed Fermi Bubbles, initially were spotted using
NASA's Fermi Gamma-ray Space Telescope. The detection of high-energy
gamma rays suggested that a violent event in the galaxy's core
aggressively launched energized gas into space. To provide more
information about the outflows, Fox used Hubble's Cosmic Origins
Spectrograph (COS) to probe the ultraviolet light from a distant quasar
that lies behind the base of the northern bubble. Imprinted on that
light as it travels through the lobe is information about the velocity,
composition, and temperature of the expanding gas inside the bubble,
which only COS can provide.
Fox's team was able to measure that the gas on the near side of the
bubble is moving toward Earth and the gas on the far side is travelling
away. COS spectra show that the gas is rushing from the galactic
center at roughly 2 million miles an hour (3 million kilometers an
hour).
"This is exactly the signature we knew we would get if this was a
bipolar outflow," explained Rongmon Bordoloi of the Space Telescope
Science Institute, a co-author on the science paper. "This is the
closest sightline we have to the galaxy's center where we can see the
bubble being blown outward and energized."
The COS observations also measure, for the first time, the
composition of the material being swept up in the gaseous cloud. COS
detected silicon, carbon, and aluminum, indicating that the gas is
enriched in the heavy elements produced inside stars and represents the
fossil remnants of star formation.
COS measured the temperature of the gas at approximately 17,500
degrees Fahrenheit, which is much cooler than most of the super-hot gas
in the outflow, thought to be at about 18 million degrees Fahrenheit.
"We are seeing cooler gas, perhaps interstellar gas in our galaxy's
disk, being swept up into that hot outflow," Fox explained.
This is the first result in a survey of 20 faraway quasars whose
light passes through gas inside or just outside the Fermi Bubbles —
like a needle piercing a balloon. An analysis of the full sample will
yield the amount of mass being ejected. The astronomers can then
compare the outflow mass with the velocities at various locations in the
bubbles to determine the amount of energy needed to drive the outburst
and possibly the origin of the explosive event.
One possible cause for the outflows is a star-making frenzy near the
galactic center that produces supernovas, which blow out gas. Another
scenario is a star or a group of stars falling onto the Milky Way's
supermassive black hole. When that happens, gas superheated by the
black hole blasts deep into space. Because the bubbles are short-lived
compared to the age of our galaxy, it suggests this may be a repeating
phenomenon in the Milky Way's history. Whatever the trigger is, it
likely occurs episodically, perhaps only when the black hole gobbles up
a concentration of material.
"It looks like the outflows are a hiccup," Fox said. "There may have
been repeated ejections of material that have blown up, and we're
catching the latest one. By studying the light from the other quasars
in our program, we may be able to detect the fossils of previous
outflows."
Galactic winds are common in star-forming galaxies, such as M82,
which is furiously making stars in its core. "It looks like there's a
link between the amount of star formation and whether or not these
outflows happen," Fox said. "Although the Milky Way overall currently
produces a moderate one to two stars a year, there is a high
concentration of star formation close to the core of the galaxy."
CONTACT
Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Andrew Fox
Space Telescope Science Institute, Baltimore, Md.
410-338-5083
afox@stsci.edu
Source: HubbleSite
