Phantoms of the Milky Way --"Galactic Quakes Reveal Invisible Starless Galaxies Orbiting Our Galaxy"
The outer edges of the Milky Way's appear to be orbited by innumerable invisible galaxies. Three bright pulsating stars on the outskirts of the Milky Way galaxy could be beacons from an invisible dwarf galaxy that astronomers predicted was there based on its effects of galactic quakes in our galaxy. These galactic quakes, ripples in gas at the outer disk of our galaxy, have puzzled astronomers since they were first revealed by radio observations a decade ago. Now, astronomers believe these stars mark the location of a dark matter-dominated dwarf galaxy far beyond the edge of the Milky Way disk, which terminates at 60,000 light-years.
“Of course we aren’t talking about a pond, but our galaxy, which is tens of thousands of light years across, and made of stars and gas, but the result is the same – ripples!” Chakrabarti adds that this work is part of a new discipline called galactoseismology, “This is really the first non-theoretical application of this field, where we can infer things about the unseen composition of galaxies from analyzing galactic-quakes.”
The prediction was the first to come out of the new field of galactoseismology, which uses ripples in the distribution of hydrogen gas in the plane of the Milky Way to infer the presence of invisible satellite galaxies, thousands of which may be buzzing around or through the Milky Way. The technique was pioneered by former UC Berkeley postdoctoral fellow Sukanya Chakrabarti, now an assistant professor of astronomy at the Rochester Institute of Technology, and her UC Berkeley mentor, Leo Blitz, a professor of astronomy.
While some of the Milky Way’s unseen satellite galaxies are hidden from view by dust, many are invisible because they’re composed mostly of dark matter, a so-far mysterious substance that dominates the matter in the universe: 85 percent of all matter in the universe is dark matter. Where it concentrates, normal matter – mostly gas – congregates and condenses into stars and galaxies that can be seen. While the normal matter in the Milky Way is large enough to produce hundreds of billions of bright stars, however, the normal matter in dark matter-dominated galaxies is apparently too small to produce enough stars to be visible over large distances.
Chakrabarti thought of looking for the effects these galaxies have on the gas distribution in the galaxy, and using this to pinpoint their location. Just as seismologists analyze waves traveling through the earth to infer properties of our planet’s interior, she uses waves in the galactic disk to map the interior structure and mass of galaxies.
“We have made significant progress into this new field of galactoseismology, whereby you can infer the dark matter content of dwarf galaxies, where they are, as well as properties of the interior of galaxies by looking at observable disturbances in the gas disk,” she said.
In 2009, Chakrabarti and Blitz used these techniques to predict the existence of a dwarf satellite galaxy in the direction of the constellation Norma, and last year she and her team used the Gemini South Telescope in Chile and Magellan telescopes to search for stars in that region that might be part of the galaxy. They found three pulsating stars called Cepheid variables, typically used as yardsticks to measure distance, that are at approximately the same distance from the sun: 300,000 light-years.
Using spectroscopic analysis, they were able to show that the stars also have about the same velocity and that they are moving too fast to be part of our galaxy. They are racing away from the center of the galaxy at 450,000 miles per hour (200 kilometers per second), whereas the average Milky Way star has a radial velocity of only about 25,000 miles per hour (12 kilometers per second).
“The radial velocity of the Cepheid variables is the last piece of evidence that we’ve been looking for,” she said. “You can immediately conclude that they are not part of our galaxy.”
“These observations basically confirm that the galaxy Sukanya predicted but couldn’t see is there,” Blitz said.
Astronomers discovered the first evidence of mysterious dark galaxies with no starlight in 2005 - VirgoHI 21 - a cloud of hydrogen in the Virgo Cluster 50 million light-years from the Earth was found to be colliding with our galaxy. Virgohi 21 revealed its existence from radio waves from neutral hydrogen coming from a rotating cloud containing enough hydrogen gas to spawn 100 million stars like the sun and fill a small galaxy.
The rotation of VirgoHI21 is far too fast to be consistent with the gravity of the detected hydrogen. Rather, it implies the presence of a dark matter halo with tens of billions of solar masses. Given the very small number of stars detected, this implies a mass-to-light ratio of about 500, far greater than that of a normal galaxy (which would be around 50). The large gravity of the dark matter halo in this interpretation explains the perturbed nature of the nearby spiral galaxy NGC 4254 and the bridge of neutral hydrogen extending between the two entities.
VirgoHI21 proved to be the first discovery of the dark galaxies anticipated by simulations of dark-matter theories. Although other dark-galaxy candidates have previously been observed, follow-up observations indicated that these were either very faint ordinary galaxies or tidal tails.
According to her Blitz, searching for satellite galaxies with this method is like inferring the size and speed of a ship by looking at its wake. “You see the waves from a lot of boats, but you have to be able to separate out the wake of a medium or small ship from that of an ocean liner,” he says.
Simulations of galaxy formation suggest a galaxy the size of the Milky Way should feature about 1000 dwarf galaxies, but only a few dozen have been found so far. Some of the missing dwarfs may be dark galaxies that are all but invisible, he says.
The models Blitz developed in 2014 predict that the universe should contain far more dwarf galaxies than the tiny fraction that astronomers can identify.
If so, Blitz thinks he knows how to find the dark galaxies. "Imagine them plopping through the gas of the outer Milky Way," he says. "They might create some sort of splash or ripple."
These distant reaches are relatively calm, making such disturbances possible to detect. Blitz explains, "It's like throwing darts at a board. As these dark galaxies come at the Milky Way, they're likely going to hit the outer parts because there's more surface area there."
To pinpoint any dark galaxy hot spots, Blitz and his research group are mapping the structure of the Milky Way. In the process, they have been able to characterize the warping of our generally flat galaxy: "It's like hitting cymbals; it's held in the middle and the outer parts are free to vibrate," he says.
"That's exactly the kind of signature we look for if the Milky Way were being hit by these dark matter galaxies," he says.As promising as the mapping looks, Blitz is hedging his bets with a second approach: seeking gassy cores that could be embedded even in dark galaxies.
"We're trying to survey regions of the sky to see if there are concentrations of atomic hydrogen that are not associated with known galaxies," he says. "I'm hoping that by making a large enough survey of the sky, we'll be able to find galaxies that contain only hydrogen and no stars. By looking at the motions of the hydrogen, we'll be able to determine the properties of the dark matter that's within it as well."
The resulting map of interstellar hydrogen could help answer another paradox in astronomy: why today's galaxies haven't yet run out of gas. According to observations, most galaxies have just enough fuel left to make stars for another billion years or so. Yet galaxies have endured for most of the age of the universe, making it unlikely that so many should blink out at once.
Blitz thinks they could be topping up their tanks with interstellar gases. As galaxies interact gravitationally, gases from their edges will get torn loose. These gases may eventually fall onto other galaxies, just as water vapor gets recycled back into rain. "There should be enough material between galaxies to be able to make up for the stars that are currently being formed," he says. "That's measurable with the Allen Telescope."
The image at the top of the page shows contours of HI column density obtained from the ALFALFA observations of the field around VirgoHI21 and NGC 4254 superposed on an optical image. The HI tail extends from NGC 4254 (visible in the lower left) more than 250 kpc to the north (assuming it lies at the Virgo distance of 16.7 Mpc).
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The Daily Galaxy via UC Berkeley and Lawrence Berkeley National Laboratory