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The VOID: Next Door: An enormous region of near-emptiness starts on our doorstep, giving astronomers their best chance to probe one of these gigantic cosmic structures.

Galaxies are gregarious, most living near others. For example, our galaxy and its great spiral neighbor 2.5 million light-years away, Andromeda, rule the Local Group, which boasts more than 100 galaxies.

But on the far side of our galaxy, starting less than 4 million light-years from Earth, lurks a dark domain that bears almost no galaxies at all. Moreover, this void is gargantuan, spanning a quarter billion light-years. Its center hides behind the galactic plane in Aquila or Sagittarius.

At first glance, voids might seem as uninviting to astronomers as treeless meadows are to arborists. After all, you're not likely to dazzle your friends by showing them a void through your telescope.

But voids are important, because most of the space of the universe is in one void or another. Voids help sculpt the large-scale structure of the universe, as galaxies flow out of them and into the glowing sheets and filaments that crisscross the cosmos. In addition, the few galaxies that still inhabit voids may evolve differently from galaxies elsewhere.

Because of its proximity to us, the Local Void provides our best chance to study all of these effects in action. Here, astronomers can search for galaxies so feeble they would be undetectable in more distant voids. And because the few galaxies in the Local Void are nearby, we can scrutinize them to see whether they differ from galaxies that grew up in more urban locations.

Discovering the Local Void

Two decades before R. Brent Tully (University of Hawai'i) and J. Richard Fisher (National Radio Astronomy Observatory) discovered the Local Void, the pair had met in graduate school. "I was the data guy," Fisher says, "and Brent was the interpreter." Following the completion of their PhD theses, Fisher set out to observe more than 1,000 galaxies and measure their redshifts, a sign of their distances from the Milky Way. Tully then plotted the three-dimensional positions of these and other galaxies.

"There was a whole sector of sky that was really, really empty of galaxies," Tully says. Unfortunately, the dusty galactic plane cuts right through this empty sector, obscuring any galaxies it might possess. "But it pokes out on each side, well above and below the plane of the galaxy," he adds. In their 1987 Nearby Galaxies Atlas, which contains information on 2,367 nearby galaxies, Tully and Fisher called this barren region the Local Void.

By then, astronomers had realized that most galaxies link up via dark matter to form a filamentary network across the universe. Between the bright nodes and filaments are dark voids, the sparsely populated countryside of the cosmos (S&T: Feb. 2015, p. 20). According to standard cosmology, voids are regions that started off with a bit less matter than average and then expanded.

The overall universe is also expanding, of course, but different parts do so at different speeds. Some regions, such as the Local Group, don't expand at all, thanks to the gravitational pull of the member galaxies. In contrast, voids contain so little matter that their gravity barely brakes the expansion, so they expand faster than the average. Over time, galaxy groups and clusters use their gravitational pull to lure additional galaxies into their lairs, tugging galaxies out of the centers of voids and toward their glowing edges.

"The rich get richer and the poor get poorer," Tully says. Just as gravity causes water to flow from mountaintops to valleys, so gravity makes galaxies stream from voids to groups and clusters. Voids therefore assist groups and clusters in shaping the universe's large-scale structure.

Recent searches behind the Milky Way's dusty veil confirm the Local Void's existence. "With a radio telescope, we can actually peer straight through the stars and the dust that obliterate the optical and even the infrared view," says Lister Staveley-Smith (University of Western Australia).

His team used the Parkes Observatory to seek unseen galaxies by the 21-centimeter radiation that their neutral hydrogen gas emits. "Astonishingly, we find very few external galaxies behind the central bulge of the Milky Way within quite a large radius," he says.

Voyage to the Void

Visiting this dark realm is easy. Just aim your starship into the Milky Way's disk, keeping Aquila on your left and Sagittarius on your right. After 27,000 light-years you speed past the Milky Way's center in Sagittarius, then reach the far side of the galaxy. As you zoom past the disk's edge, you look beneath the galactic plane to see the Sagittarius Dwarf Spheroidal, 85,000 light-years from Earth. This galaxy is a satellite of the Milky Way that our galaxy's tidal forces are tearing apart.

The Milky Way is mighty, far larger and brighter than most of its peers, but you soon leave our galaxy and its many satellites behind. You hurtle past two more Local Group galaxies in Sagittarius. The first, 1.6 million light-years from Earth, is Barnard's Galaxy, also known as NGC 6822. Farther out, at 3.4 million light-years, you spy the dim Sagittarius Dwarf Irregular Galaxy patrolling the Local Group's lonely frontier.

Then you plunge into darkness. The vast emptiness of the Local Void stretches out ahead of you for another 250 million light-years. That's right: 100 times the distance between the Milky Way and the Andromeda Galaxy, with almost nothing to see, left or right, up or down. It's the intergalactic equivalent of driving through western Kansas.

Nothing Really Matters

The Local Void is essential for understanding our motion through the universe. Astronomers measure our movement relative to the cosmic microwave background, the Big Bang's afterglow, which cosmologists define to be at rest relative to the universe. This radiation is blueshifted in the direction we're approaching and redshifted in the direction we're racing away from.

These measurements reveal that the Milky Way and its neighbors are zipping through the universe at 630 kilometers per second, or 1.4 million mph. That's fast: Earth circles the Sun at only 30 km/s, the Milky Way moves toward the Andromeda Galaxy at only 110 km/s, and the Sun revolves around the galactic center at approximately 250 km/s.

Astronomers have long known of two strong gravitational tugs that explain most of our motion through the universe. One comes from Virgo, the nearest galaxy cluster at 54 million light-years away, whose gravitational force tries to pull us toward it. The other tug comes from a grander gathering of more distant galaxies in a direction nearly perpendicular to Virgo--the Great Attractor and, beyond it, the Shapley Concentration. Plus, a newfound void dubbed the Dipole Repeller, much farther than the Local Void and on the opposite side of us from these clusters, helps them in this effort, making us flow down a steeper gravitational slope toward the clusters (S&T: May 2017, p. 8).

But these mighty structures explain only two of the three dimensions of our motion through space. The Local Void supplies the third.

In 2008, Tully's team found that we stream away from the gravitational hill the Local Void creates at about 260 km/s. That's greater than the Virgo-induced velocity, which the astronomers put at 185 km/s, but less than the speed toward the Great Attractor and the Shapley Concentration, which amounts to 455 km/s.

Furthermore, the Local Void's "push" is nearly perpendicular to the pulls from both Virgo and the Great Attractor. To picture the situation, you must know an amazing fact about the eight great galaxies nearest our own: Every galactic goliath within 20 million light-years of us resides in nearly the same plane (see box on page 16).

These great galaxies constitute part of what Tully calls the Local Sheet, which is a wall of the Local Void. The Local Void is above the Local Sheet and causes it to move down. As evidence for this movement, he points to the Leo Spur, a filament some 35 million light-years below the Local Sheet. Because of the Local Sheet's downward movement, the Leo Spur's galaxies have smaller redshifts than they would otherwise.

Void Where Prohibited

Despite its name, the Local Void is not completely free of galaxies. "The void is partitioned by these wispy filaments that trace through it," Tully says. The filaments contain chains of galaxies that glow like dimly lit towns on a rural road. Nor is the Local Void perfectly round. Instead, it's lumpy.

The two nearest-known inhabitants of the Local Void are KK 246 and ALFAZOA J1952+1428; the two galaxies lie about 25 million light-years away, in Sagittarius and Aquila, respectively. Both galaxies are dwarf irregulars, miniature versions of the two brightest satellites of the Milky Way, the Large and Small Magellanic Clouds. The void galaxies emit only a few percent as much light as the Large Magellanic Cloud, but like the Magellanic Clouds they both possess gas that is spawning new stars.

In fact, it was the gas, not the stars, that first signaled the presence of the void galaxy in Aquila. Travis McIntyre, then a graduate student at the University of New Mexico, was searching for 21-centimeter radiation from neutral hydrogen gas in galaxies hiding behind the galactic plane. He found lots of new galaxies, but the one in Aquila stood out by virtue of its small redshift. "It was special because it was so close to our own galaxy," McIntyre says. Only later did he and his colleagues spot the galaxy's stars.

Because these two void galaxies are nearby, the Hubble Space Telescope can detect their red giants, aging stars whose apparent magnitudes reveal precise distances to their galactic homes. That's important, because by comparing the galaxies' distances with their velocities derived via redshifts, astronomers can see whether the void is actually expanding.

It's not easy, though. "Observing empty space is very hard," says Luca Rizzi (W. M. Keck Observatory).

Fortunately, the two void galaxies nearby are like glowing flotsam tracing the currents of a dark sea. "They are in the void, but they are closer to us than to the center" of the void, Rizzi says. In a study published last year, his team reported that both void galaxies are racing away from the Local Void's center and toward its wall at hundreds of km/s, a sign that the void is indeed expanding and growing ever emptier. "Voids like to get rid of galaxies as fast as they can," he says, as galaxies outside the voids tug the void galaxies away.

Both void galaxies are extremely isolated. Whereas lots of galaxies dwell close to our own, KK 246 doesn't have a single known neighbor within 10 million light-years.

And yet, despite their extreme isolation, the void galaxies seem normal. They have gas, just like most of the Local Group's outlying members, and probably for the same reason: because there's no giant galaxy nearby to steal it. In fact, KK 246's gas disk extends five times farther out than its stars do. Even that isn't unique, however, as a few dwarf galaxies elsewhere have equally impressive gaseous envelopes.

Thus, as long as a dwarf galaxy steers clear of gas-grabbing giants, it can lead a normal life, whether or not it's in a void. Internal processes govern the galaxy's evolution.

Nothing Ventured, Nothing Gained

Still, the Local Void poses a possible problem for standard cosmology. "The Local Void is fascinating," says P. James Peebles (Princeton University). "From the point of view of a cosmologist, the enigma is how few stars there are in this void." Numerical simulations of cosmic evolution predict that voids should have a mass density that is about 10% of the mean density of the universe.

"That density--10% of mean--is far greater than the number density of galaxies in the Local Void," says Peebles. He estimates the actual number density of Local Void galaxies is only about 1% of the cosmic mean.

So where are all the galaxies? Giants like our own don't form in such a sparse environment, but Peebles says dwarfs should still arise, albeit in smaller numbers than elsewhere. Because they are dim, dwarfs are harder to find than giants. However, the Local Void is so close that astronomers should have found more dwarf inhabitants than they have.

Not everyone thinks this "void phenomenon" spells trouble, though. Jeremy Tinker (New York University) says the apparent lack of dwarf galaxies agrees with numerical simulations of cosmic evolution. The matter that exists in voids consists primarily of dark matter plus a smattering of gas. In voids, this material may have gathered into dark galaxies that produced few if any stars.

"If there was literally no matter inside of the voids, that would completely blow the doors off" standard cosmology, Tinker says. "You would have to come up with some extra force to evacuate the mass out of the void, because gravity itself can't do it."

Peebles wonders whether amateur astronomers making long exposures with CCDs at dark sites might discern new dwarf galaxies in the Local Void.

"Wouldn't it be lovely?" he asks. "Amateurs pointing at the Local Void and just painfully stitching across it could pick up much fainter galaxies." He cites the work of amateur R. Jay GaBany, whose superb images have revealed dim structures around other galaxies (S&T: Jan. 2009, p. 92).

The ideal hunting ground would probably lie away from the Milky Way's dusty plane. Because the Local Void is so large, it juts out well above the galactic plane in Ophiuchus, northernmost Scorpius, and southernmost Libra. It can also be seen well below the galactic plane in Capricornus and southeastern Sagittarius. As a result, patient observers of these constellations could help cosmologists reconcile reality with theory by turning up new void galaxies in the vast expanse of darkness next door.

Meanwhile, voids themselves will keep expanding, evicting their remaining residents and thereby helping to build the glittering galactic architecture that spans the cosmos. Sometimes you really do get something from nothing.

THE EIGHT GREAT GALAXIES NEXT DOOR

The Milky Way is such a colossus that only eight other galaxies within 20 million light-years rival Its size, mass, and luminosity. Remarkably, all of the great galaxies nearby occupy the same plane, one that is proportionately even thinner than an American dime: the Milky Way and Andromeda Galaxies, the two superpowers of the Local Group; the giant spiral galaxy M81 in Ursa Major; the giant elliptical galaxy Maffei 1 and its giant spiral neighbors Maffei 2 and IC 342, which all hide behind interstellar dust in Cassiopeia and Camelopardalis; the giant edge-on spiral galaxy NGC 253 in Sculptor; and the giant elliptical galaxy Centaurus A and the giant barred spiral galaxy M83 in Hydra.

This plane nearly coincides with the so-called supergalactic plane, which includes the Virgo Cluster. In supergalactic coordinates, the SGX and SGY coordinates describe the supergalactic plane, and SGZ is perpendicular to it. The Great Attractor and Shapley Concentration lie at negative SGX and the newfound distant void lies at positive SGX; the Virgo Cluster lies at positive SGY; and the Local Void lies at positive SGZ, above the supergalactic plane. These clusters and voids move us toward negative SGX, positive SGY, and negative SGZ, explaining the Local Group's high speed through space.

The Leo Spur is a filament of galaxies beneath the supergalactic plane and thus on the other side of us from the Local Void. Members include the irregular galaxy NGC 2337, which is 37 million light-years away in Lynx, and the edge-on spiral NGC 2683, which is 31 million light-years from Earth, also in Lynx.

KEN CROSWELL earned his PhD at Harvard University for studying the Milky Way Galaxy and is the author of The Alchemy of the Heavens: Searching for Meaning in the Milky Way, which was a Los Angeles Times Book Prize finalist.

Caption: AN EMPTY EXPANSE Just beyond the Milky Way begins a vast near-emptiness of space some 250 million light-years wide. Graduate student Johan Hidding visualized this void using computational geometry, starting with galaxies cataloged in the 2MASS Redshift Survey and then recreating the filametary cosmic web of dark matter that shapes how these galaxies are distributed.

Caption: HOME This view shows the Milky Way, as well as several of the dwarf galaxies around it, relative to the supergalactic plane (circular white lines). This plane contains the Local Sheet of galaxies as well as the Virgo Cluster.

BARNARD'S GALAXY

This dwarf irregular galaxy, also known as NGC 6822, lies 1.6 million light-years from Earth.

SAGITTARIUS DWARF IRREGULAR GALAXY Not to be confused with the Sagittarius Dwarf Spheroidal near the Milky Way, this galaxy floats 3.4.million light-years away at the edge of the Local Void.

Caption: ALL AROUND US Several cosmic structures determine the motion of our galaxy, and indeed of the whole Local Group of galaxies, through space. We can understand this motion in three dimensions relative to the supergalactic plane: Our galaxy is moving back along the X axis, sliding down the gravitational hill created by the dense collections of galaxies in the Great Attractor and the Shapley Concentration on one end, and the Dipole Repeller void on the other end. The Virgo galaxy cluster pulls us along they axis; meanwhile, the Local Void steepens the gravitational hill that our galaxy slides down on the z axis.

Caption: VOIDS IN THE NEIGHBORHOOD Using computational geometry, Johan Hidding and others visualized the flow of matter near the Milky Way, which serves to outline the Local Void and several other nearby cosmic voids. The walls and filaments shown here represent dark matter structures, based on the distribution of galaxies cataloged in the 2MASS Redshift Survey.

Caption: KK 246 The gaseous disk of this void galaxy in Sagittarius, marked here as contours, extends five times farther out than its stars do, as seen in the background black-and-white image.

Caption: HIDING IN PLAIN SIGHT In this all-sky view of nearby galaxies, the plane of the Milky Way (gray) is almost directly perpendicular to the supergalactic plane, which extends from left to right. The Local Void (black oval) sits directly behind the galactic plane, which makes finding galaxies in this vast expanse a challenge.

Caption: ALFAZOA J1952+1428 The second nearest galaxy known in the Local Void also abounds with gas. It's 20 million light-years from KK 246 and 27 million light-years from Earth. Searchers found the galaxy not by its stars but by the radio glow from its neutral hydrogen gas.

Caption: LOCAL SHEET Nine galaxies, including the Milky Way, dominate the Local Sheet, which sits below the Local Void from the perspective of the supergalactic plane. Circles on this 1 plane mark 6.5 million, 13 million, and 20 million light-years. Two galaxies, KK 246 and ALFAZOA J1952 + 1428. are flying out of the void and toward the plane.
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Title Annotation:Our Local Void
Author:Croswell, Ken
Publication:Sky & Telescope
Date:Oct 1, 2018
Words:3121
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