Astronomers Discover Dizzying Spin of the Milky Way Galaxy’s “Halo”

Astronomers at the University of Michigan’s College of Literature, Science, and the Arts (LSA) discovered for the first time that the hot gas in the halo of the Milky Way galaxy is spinning in the same direction and at comparable speed as the galaxy's disk, which contains our stars, planets, gas, and dust. This new knowledge sheds light on how individual atoms have assembled into stars, planets, and galaxies like our own, and what the future holds for these galaxies.

Our Milky Way galaxy and its small companions are surrounded by a giant halo of million-degree gas (seen in blue in this artists' rendition) that is only visible to X-ray telescopes in space. University of Michigan astronomers discovered that this massive hot halo spins in the same direction as the Milky Way disk and at a comparable speed. Credits: NASA/CXC/M.Weiss/Ohio State/A Gupta et al

“This flies in the face of expectations,” says Edmund Hodges-Kluck, assistant research scientist. 

“People just assumed that the disk of the Milky Way spins while this enormous reservoir of hot gas is stationary – but that is wrong. This hot gas reservoir is rotating as well, just not quite as fast as the disk.”

The new NASA-funded research using the archival data obtained by XMM-Newton, a European Space Agency telescope, was recently published in the Astrophysical Journal. The study focuses on our galaxy’s hot gaseous halo, which is several times larger than the Milky Way disk and composed of ionized plasma.

Because motion produces a shift in the wavelength of light, the U-M researchers measured such shifts around the sky using lines of very hot oxygen. What they found was groundbreaking: The line shifts measured by the researchers show that the galaxy’s  halo spins in the same direction as the disk of the Milky Way and at a similar speed—about 400,000 mph for the halo versus 540,000 mph for the disk.

“The rotation of the hot halo is an incredible clue to how the Milky Way formed,” said Hodges Kluck. “It tells us that this hot atmosphere is the original source of a lot of the matter in the disk.”

Scientists have long puzzled over why almost all galaxies, including the Milky Way, seem to lack most of the matter that they otherwise would expect to find. Astronomers believe that about 80% of the matter in the universe is the mysterious “dark matter” that, so far, can only be detected by its gravitational pull. But even most of the remaining 20% of “normal” matter is missing from galaxy disks. More recently, some of the “missing” matter has been discovered in the halo. The U-M researchers say that learning about the direction and speed of the spinning halo can help us learn both how the material got there in the first place, and the rate at which we expect the matter to settle into the galaxy.

“Now that we know about the rotation, theorists will begin to use this to learn how our Milky Way galaxy formed – and its eventual destiny,” says Joel Bregman, a U-M LSA professor of astronomy.

“We can use this discovery to learn so much more – the rotation of this hot halo will be a big topic of future X-ray spectrographs,” Bregman says.

For more information, please visit: http://heasarc.gsfc.nasa.gov/docs/xmm/xmmgof.html

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By Felicia Chou
NASA Headquarters, Washington, D.C.

Editor: Ashley Morrow

Source: NASA/Galaxies

Stellar Motions in Outer Halo Shed New Light on Milky Way Evolution

This illustration shows the disk of our Milky Way galaxy, surrounded by a faint, extended halo of old stars. Astronomers using the Hubble Space Telescope to observe the nearby Andromeda galaxy serendipitously identified a dozen foreground stars in the Milky Way halo. They measured the first sideways motions (represented by the arrows) for such distant halo stars. The motions indicate the possible presence of a shell in the halo, which may have formed from the accretion of a dwarf galaxy. This observation supports the view that the Milky Way has undergone continuing growth and evolution over its lifetime by consuming smaller galaxies.

Illustration Credit: NASA, ESA, and A. Feild (STScI)

Science Credit: NASA, ESA, A. Deason and P. Guhathakurta (University of California, Santa Cruz), and R. van der Marel, T. Sohn, and T. Brown (STScI)

Peering deep into the vast stellar halo that envelops our Milky Way galaxy, astronomers using NASA's Hubble Space Telescope have uncovered tantalizing evidence for the possible existence of a shell of stars that are a relic of cannibalism by our Milky Way.

Hubble was used to precisely measure, for the first time ever, the sideways motions of a small sample of stars located far from the galaxy's center. Their unusual lateral motion is circumstantial evidence that the stars may be the remnants of a shredded galaxy that was gravitationally ripped apart by the Milky Way billions of years ago. These stars support the idea that the Milky Way grew, in part, through the accretion of smaller galaxies.

"Hubble's unique capabilities are allowing astronomers to uncover clues to the galaxy's remote past. The more distant regions of the galaxy have evolved more slowly than the inner sections. Objects in the outer regions still bear the signatures of events that happened long ago," said Roeland van der Marel of the Space Telescope Science Institute (STScI) in Baltimore, Md.

They also offer a new opportunity for measuring the "hidden" mass of our galaxy, which is in the form of dark matter (an invisible form of matter that does not emit or reflect radiation). In a universe full of 100 billion galaxies, our Milky Way "home" offers the closest and therefore best site for detailed study of the history and architecture of a galaxy.

A team of astronomers led by Alis Deason of the University of California, Santa Cruz, and van der Marel identified 13 stars located roughly 80,000 light-years from the galaxy's center. They lie in the Milky Way's outer halo of ancient stars that date back to the formation of our galaxy.

The team was surprised to find that the stars showed more of a sideways, or tangential, amount of motion than they expected. This movement is different from what astronomers know about the halo stars near the Sun, which move predominantly in radial orbits. Stars in these orbits plunge toward the galactic center and travel back out again. The stars' tangential motion can be explained if there is an over-density of stars at 80,000 light-years, like cars backing up on an expressway. This traffic jam would form a shell-like feature, as seen around other galaxies.

Deason and her team plucked the outer halo stars out of seven years' worth of archival Hubble telescope observations of our neighboring Andromeda galaxy. In those observations, Hubble peered through the Milky Way's halo to study the Andromeda stars, which are more than 20 times farther away. The Milky Way's halo stars were in the foreground and considered as clutter for the study of Andromeda. But to Deason's study they were pure gold. The observations offered a unique opportunity to look at the motion of Milky Way halo stars.

Finding the stars was meticulous work. Each Hubble image contained more than 100,000 stars. "We had to somehow find those few stars that actually belonged to the Milky Way halo," van der Marel said. "It was like finding needles in a haystack."

The astronomers identified the stars based on their colors, brightnesses, and sideways motions. The halo stars appear to move faster than the Andromeda stars because they are so much closer. Team member Sangmo Tony Sohn of STScI identified the halo stars and measured both the amount and direction of their slight sideways motion. The stars move on the sky only about one milliarcsecond a year, which would be like watching a golf ball on the Moon moving one foot per month. Nonetheless, this was measured with 5 percent precision, made possible in visible-light observations because of Hubble's razor-sharp view and instrument consistency.

"Measurements of this accuracy are enabled by a combination of Hubble's sharp view, the many years' worth of observations, and the telescope's stability. Hubble is located in the space environment, and it's free of gravity, wind, atmosphere, and seismic perturbations," van der Marel said.

Stars in the inner halo have highly radial orbits. When the team compared the tangential motion of the outer halo stars with their radial motion, they were very surprised to find that the two were equal. Computer simulations of galaxy formation normally show an increasing tendency towards radial motion if one moves further out in the halo. These observations imply the opposite trend. The existence of a shell structure in the Milky Way halo is one plausible explanation of the researchers' findings. Such a shell can form by accretion of a satellite galaxy. This is consistent with a picture in which the Milky Way has undergone continuing evolution over its lifetime due to the accretion of satellite galaxies.

The team compared their results with data of halo stars recorded in the Sloan Digital Sky Survey. Those observations uncovered a higher density of stars at about the same distance as the 13 outer halo stars in their Hubble study. A similar excess of halo stars exists across the Triangulum and Andromeda constellations. Beyond that radius, the number of stars plummets.

Deason immediately thought the two results were more than just coincidence. "What may be happening is that the stars are moving quite slowly because they are at the apocenter, the farthest point in their orbit about the hub of our Milky Way," Deason explained. "The slowdown creates a pileup of stars as they loop around in their path and travel back towards the galaxy. So their in and out or radial motion decreases compared with their sideways or tangential motion."

Shells of stars have been seen in the halos of some galaxies, and astronomers predicted that the Milky Way may contain them, too. But until now there was limited evidence for their existence. The halo stars in our galaxy are hard to see because they are dim and spread across the sky.

Encouraged by this study, the team hopes to search for more distant halo stars in the Hubble archive. "These unexpected results fuel our interest in looking for more stars to confirm that this is really happening," Deason said. "At the moment we have quite a small sample. So we really can make it a lot more robust with getting more fields with Hubble." The Andromeda observations only cover a very small "keyhole view" of the sky.

The team's goal is to put together a clearer picture of the Milky Way's formation history. By knowing the orbits and motions of many halo stars it will also be possible to calculate an accurate mass for the galaxy. "Until now, what we have been missing is the stars' tangential motion, which is a key component. The tangential motion will allow us to better measure the total mass distribution of the galaxy, which is dominated by dark matter. By studying the mass distribution, we can see whether it follows the same distribution as predicted in theories of structure formation," Deason said.

The Hubble study will appear in an upcoming issue of the Astrophysical Journal.

The science team consists of A. Deason and P. Guhathakurta of UCO/Lick Observatory, University of California, Santa Cruz, Calif., and R.P. van der Marel, S.T. Sohn, and T.M. Brown of the Space Telescope Science Institute, Baltimore, Md.

CONTACT

Donna Weaver
Space Telescope Science Institute, Baltimore, Md.
410-338-4493
dweaver@stsci.edu

Alis Deason
University of California, Santa Cruz, Calif.
831-459-3841
alis@ucolick.org

Roeland van der Marel
Space Telescope Science Institute, Baltimore, Md.
410-338-4931
marel@stsci.edu

NASA's Hubble Confirms that Galaxies Are the Ultimate Recyclers

Distant quasars serve as distant lighthouse beacons that shine through the gas-rich "fog" of hot plasma encircling galaxies. At ultraviolet wavelengths, Hubble's Cosmic Origins Spectrograph (COS) is sensitive to absorption from many ionized heavy elements, such as nitrogen, oxygen, and neon. COS's high sensitivity allows many galaxies that happen to lie in front of the much more distant quasars to be studied. The ionized heavy elements serve as proxies for estimating how much mass is in a galaxy's halo. Illustration Credit: NASA, ESA, and A. Feild (STScI). Science Credit: NASA, ESA, N. Lehner (University of Notre Dame), T. Tripp (University of Massachusetts, Amherst), and J. Tumlinson (STScI)

The color and shape of a galaxy is largely controlled by gas flowing through an extended halo around it. All modern simulations of galaxy formation find that they cannot explain the observed properties of galaxies without modeling the complex accretion and "feedback" processes by which galaxies acquire gas and then later expel it after chemical processing by stars. Hubble spectroscopic observations show that galaxies like our Milky Way recycle gas while galaxies undergoing a rapid starburst of activity will lose gas into intergalactic space and become "red and dead." Illustration Credit: NASA, ESA, and A. Feild (STScI). Science Credit: NASA, ESA, N. Lehner (University of Notre Dame), T. Tripp (University of Massachusetts, Amherst), and J. Tumlinson (STScI)

Galaxies learned to "go green" early in the history of the universe, continuously recycling immense volumes of hydrogen gas and heavy elements to build successive generations of stars stretching over billions of years.

This ongoing recycling keeps galaxies from emptying their "fuel tanks" and therefore stretches out their star-forming epoch to over 10 billion years. However, galaxies that ignite a rapid firestorm of star birth can blow away their remaining fuel, essentially turning off further star-birth activity.

This conclusion is based on a series of Hubble Space Telescope observations that flexed the special capabilities of its comparatively new Cosmic Origins Spectrograph (COS) to detect otherwise invisible mass in the halo of our Milky Way and a sample of more than 40 other galaxies. Data from large ground-based telescopes in Hawaii, Arizona, and Chile also contributed to the studies by measuring the properties of the galaxies.

This invisible mass is made up of normal matter — hydrogen, helium, and heavier elements such as carbon, oxygen, nitrogen, and neon — as opposed to dark matter that is an unknown exotic particle pervading space.

The results are being published in three papers in the November 18 issue of Science magazine. The leaders of the three studies are Nicolas Lehner of the University of Notre Dame in South Bend, Ind.; Jason Tumlinson of the Space Telescope Science Institute in Baltimore, Md.; and Todd Tripp of the University of Massachusetts at Amherst.

The Key Findings

The color and shape of a galaxy is largely controlled by gas flowing through an extended halo around it. All modern simulations of galaxy formation find that they cannot explain the observed properties of galaxies without modeling the complex accretion and "feedback" processes by which galaxies acquire gas and then later expel it after processing by stars. The three studies investigated different aspects of the gas-recycling phenomenon.

"Our results confirm a theoretical suspicion that galaxies expel and can recycle their gas, but they also present a fresh challenge to theoretical models to understand these gas flows and integrate them with the overall picture of galaxy formation," Tumlinson says.

The team used COS observations of distant stars to demonstrate that a large mass of clouds is falling through the giant corona halo of our Milky Way, fueling its ongoing star formation. These clouds of ionized hydrogen reside within 20,000 light-years of the Milky Way disk and contain enough material to make 100 million suns. Some of this gas is recycled material that is continually being replenished by star formation and the explosive energy of novae and supernovae, which kicks chemically enriched gas back into the halo; the remainder is gas being accreted for the first time. The infalling gas from this vast reservoir fuels the Milky Way with the equivalent of about a solar mass per year, which is comparable to the rate at which our galaxy makes stars. At this rate the Milky Way will continue making stars for another billion years by recycling gas into the halo and back onto the galaxy. "We now know where is the missing fuel for galactic star formation," Lehner concludes. "We now have to find out its birthplace."

One goal of the studies was to study how other galaxies like our Milky Way accrete mass for star making. But instead of widespread accretion, the team found nearly ubiquitous halos of hot gas surrounding vigorous star-forming galaxies. These galaxy halos, rich in heavy elements, extend as much as 450,000 light-years beyond the visible portions of their galactic disks. The surprise was discovering how much mass in heavy elements is far outside a galaxy. COS measured 10 million solar masses of oxygen in a galaxy's halo, which corresponds to about 1 billion solar masses of gas — as much as in the entire interstellar medium between stars in a galaxy's disk. They also found that this gas is nearly absent from galaxies that have stopped forming stars. This is evidence that widespread outflows, rather than accretion, determine a galaxy's fate. "We didn't know how much mass was there in these gas halos, because we couldn't do these observations until we had COS," Tumlinson says. "This stuff is a huge component of galaxies but can't be seen in any images."

He points out that because so much of the heavy elements has been ejected into the halos instead of sticking around in the galaxies, the formation of planets, life, and other things requiring heavy elements could have been delayed in these galaxies.

The COS data also demonstrate that those galaxies forming stars at a very rapid rate, perhaps a hundred solar masses per year, can drive 2-million-degree gas very far out into intergalactic space at speeds of up to 2 million miles per hour. That's fast enough for the gas to escape forever and never refuel the parent galaxy. While hot plasma "winds" from galaxies have been known for some time, the new COS observations reveal that hot outflows extend to much greater distances than previously thought and can carry a tremendous amount of mass out of a galaxy. Some of the hot gas is moving more slowly and could eventually be recycled. The Hubble observations show how gas-rich star-forming spiral galaxies can evolve to quiescent elliptical galaxies that no longer have star formation. "So not only have we found that star-forming galaxies are pervasively surrounded by large halos of hot gas," says Tripp, "we have also observed that hot gas in transit — we have caught the stuff in the process of moving out of a galaxy and into intergalactic space."

The light emitted by this hot plasma is invisible, so the researchers used COS to detect the presence of the gas by the way it absorbs certain colors of light from background quasars. The brightest objects in the universe, quasars are the brilliant cores of active galaxies that contain rapidly accreting supermassive black holes. The quasars serve as distant lighthouse beacons that shine through the gas-rich "fog" of hot plasma encircling galaxies. At ultraviolet wavelengths, COS is sensitive to absorption from many ionized heavy elements, such as nitrogen, oxygen, and neon. COS's high sensitivity allows many galaxies that happen to lie in front of the much more distant quasars to be studied. The ionized heavy elements serve as proxies for estimating how much mass is in a galaxy's halo.

"Only with COS can we now address some of the most crucial questions that are at the forefront of extragalactic astrophysics," Tumlinson says.

CONTACT

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu

Jason Tumlinson
Space Telescope Science Institute, Baltimore, Md.
410-338-4553
tumlinson@stsci.edu

Fast Falling Clouds Fuel Milky Way Star Formation

Large-Scale Flows in the Milky Way Halo

Illustration Credit: NASA, ESA, and A. Feild (STScI)

Science Credit: NASA, ESA, and N. Lehner and C. Howk (University of Notre Dame)

This diagram shows the large-scale flows of gas in the Milky Way halo of faint stars and hot gas. Using the Hubble Space Telescope's Cosmic Origins Spectrograph and Space Telescope Imaging Spectrograph, Nicolas Lehner and Chris Howk at the University of Notre Dame were able to directly constrain the distance of the fast-moving ionized clouds responsible for flows of gas in the Milky Way halo. The invisible clouds were detected by finding their absorption signature in the ultraviolet spectra of distant background halo stars. The researchers found that the clouds are reservoirs of gas that enable stars to continue to form in the Milky Way. Without the replenishment from recycled gas and infalling extragalactic material, stars would have stopped forming in the Milky Way a long time ago. This study also suggests the clouds slow down as they approach the Milky Way.

The long-term forecast for the Milky Way is cloudy with gaseous rain. A study by Nicolas Lehner and Christopher Howk of the University of Notre Dame concludes that massive clouds of ionized gas are raining down from our galaxy's halo and intergalactic space and will continue to provide fuel for the Milky Way to keep forming stars. Using the Hubble Space Telescope’s Cosmic Origins Spectrograph they measured for the first time the distances to huge, fast-moving clouds of ionized gas previously seen covering a large fraction of the sky.

Source: Hubble Site