Trajectory of Smith Cloud
This diagram shows the 100-million-year-long trajectory of the Smith
Cloud as it arcs out of the plane of our Milky Way galaxy and then
returns like a boomerang. Hubble Space Telescope measurements show that
the cloud, because of its chemical composition, came out of a region
near the edge of the galaxy's disk of stars 70 million years ago. The
cloud is now stretched into the shape of a comet by gravity and gas
pressure. Following a ballistic path, the cloud will fall back into the
disk and trigger new star formation 30 million years from now. Illustration Credit: NASA, ESA, and A. Feild (STScI) - Science Credit: NASA, ESA, and A. Fox (STScI)
Hubble Space Telescope astronomers are finding that the old adage "what goes up must come down" even applies to an immense cloud of hydrogen gas outside our Milky Way galaxy. The invisible cloud is plummeting toward our galaxy at nearly 700,000 miles per hour.
Size of Smith Cloud on the Sky
This composite image shows the size and location of the Smith Cloud on
the sky. The cloud appears in false-color, radio wavelengths as observed
by the Robert C. Byrd Green Bank Telescope in West Virginia. The
visible-light image of the background star field shows the cloud's
location in the direction of the summer constellation Aquila. The cloud
is 15 degrees across in angular size — the width of an outstretched
hand at arm's length. The apparent size of the full moon is added for
comparison.
Hubble Characterizes the High-Velocity Smith Cloud
The infalling Smith Cloud does not emit light at wavelengths that the
Hubble Space Telescope is sensitive to. However, Hubble's Cosmic Origins
Spectrograph can measure how the light from distant background objects
is affected as it passes through the cloud. These measurements yield
clues to the chemical composition of the cloud. By using these
intergalactic forensics, Hubble astronomers trace the cloud's origin to
the disk of our Milky Way. Combined ultraviolet and radio observations
correlate to the cloud's infall velocities, providing solid evidence
that the spectral features link to the cloud's dynamics. Illustration Credit: NASA, ESA, and A. Feild (STScI) - Science Credit: NASA, ESA, and A. Fox (STScI) Hubble Space Telescope astronomers are finding that the old adage "what goes up must come down" even applies to an immense cloud of hydrogen gas outside our Milky Way galaxy. The invisible cloud is plummeting toward our galaxy at nearly 700,000 miles per hour.
Though hundreds of enormous, high-velocity gas clouds whiz around the
outskirts of our galaxy, this so-called "Smith Cloud" is unique
because its trajectory is well known. New Hubble observations suggest
it was launched from the outer regions of the galactic disk, around 70
million years ago. The cloud was discovered in the early 1960s by
doctoral astronomy student Gail Smith, who detected the radio waves
emitted by its hydrogen.
The cloud is on a return collision course and is expected to plow
into the Milky Way's disk in about 30 million years. When it does,
astronomers believe it will ignite a spectacular burst of star
formation, perhaps providing enough gas to make 2 million suns.
"The cloud is an example of how the galaxy is changing with time,"
explained team leader Andrew Fox of the Space Telescope Science
Institute in Baltimore, Maryland. "It's telling us that the Milky Way
is a bubbling, very active place where gas can be thrown out of one
part of the disk and then return back down into another."
"Our galaxy is recycling its gas through clouds, the Smith Cloud
being one example, and will form stars in different places than before.
Hubble's measurements of the Smith Cloud are helping us to visualize
how active the disks of galaxies are," Fox said.
Astronomers have measured this comet-shaped region of gas to be
11,000 light-years long and 2,500 light-years across. If the cloud
could be seen in visible light, it would span the sky with an apparent
diameter 30 times greater than the size of the full moon.
Astronomers long thought that the Smith Cloud might be a failed,
starless galaxy, or gas falling into the Milky Way from intergalactic
space. If either of these scenarios proved true, the cloud would
contain mainly hydrogen and helium, not the heavier elements made by
stars. But if it came from within the galaxy, it would contain more of
the elements found within our sun.
The team used Hubble to measure the Smith Cloud's chemical
composition for the first time, to determine where it came from. They
observed the ultraviolet light from the bright cores of three active
galaxies that reside billions of light-years beyond the cloud. Using
Hubble's Cosmic Origins Spectrograph, they measured how this light
filters through the cloud.
In particular, they looked for sulfur in the cloud which can absorb
ultraviolet light. "By measuring sulfur, you can learn how enriched in
sulfur atoms the cloud is compared to the sun," Fox explained. Sulfur
is a good gauge of how many heavier elements reside in the cloud.
The astronomers found that the Smith Cloud is as rich in sulfur as
the Milky Way's outer disk, a region about 40,000 light-years from the
galaxy's center (about 15,000 light-years farther out than our sun and
solar system). This means that the Smith Cloud was enriched by material
from stars. This would not happen if it were pristine hydrogen from
outside the galaxy, or if it were the remnant of a failed galaxy devoid
of stars. Instead, the cloud appears to have been ejected from within
the Milky Way and is now boomeranging back.
Though this settles the mystery of the Smith Cloud's origin, it
raises new questions: How did the cloud get to where it is now? What
calamitous event could have catapulted it from the Milky Way's disk,
and how did it remain intact? Could it be a region of dark matter — an
invisible form of matter — that passed through the disk and captured
Milky Way gas? The answers may be found in future research.
The team's research appears in the January 1, 2016, issue of The Astrophysical Journal Letters.
Contact:
Space Telescope Science Institute, Baltimore, Maryland
410-338-4488 / 410-338-4514
jenkins@stsci.edu / villard@stsci.edu
Andrew Fox
Space Telescope Science Institute, Baltimore, Maryland
410-338-5083
afox@stsci.edu
Source: HubbleSite