hidden supercluster could solve milky way mystery magazine november 21, 2017 hidden supercluster...

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https://www.quantamagazine.org/hidden-supercluster-could-solve-milky-way-mystery-20171121/ November 21, 2017

Hidden Supercluster Could Solve Milky WayMysteryAstronomers generally stay away from the “Zone of Avoidance.” When one astronomer didn’t, shefound a giant cosmic structure that could help explain why our galaxy moves so fast.

By Liz Kruesi

Mike Zeng for Quanta Magazine

An illustration of the Vela Supercluster peeking out from behind the Milky Way’s Zone of Avoidance.

Glance at the night sky from a clear vantage point, and the thick band of the Milky Way will slashacross the sky. But the stars and dust that paint our galaxy’s disk are an unwelcome sight toastronomers who study all the galaxies that lie beyond our own. It’s like a thick stripe of fog across awindshield, a blur that renders our knowledge of the greater universe incomplete. Astronomers callit the Zone of Avoidance.

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Renée Kraan-Korteweg has spent her career trying to uncover what lies beyond the zone. She firstcaught a whiff of something spectacular in the background when, in the 1980s, she found hints of apotential cluster of objects on old photographic survey plates. Over the next few decades, the hintsof a large-scale structure kept coming.

Late last year, Kraan-Korteweg and colleagues announced that they had discovered an enormouscosmic structure: a “supercluster” of thousands upon thousands of galaxies. The collection spans300 million light years, stretching both above and below the galactic plane like an ogre hidingbehind a lamppost. The astronomers call it the Vela Supercluster, for its approximate positionaround the constellation Vela.

University of Cape Town

Renée Kraan-Korteweg, an astronomer at the University of Cape Town, has spent decades trying to peer throughthe Zone of Avoidance.

Milky Way MoversThe Milky Way, just like every galaxy in the cosmos, moves. While everything in the universe isconstantly moving because the universe itself is expanding, since the 1970s astronomers haveknown of an additional motion, called peculiar velocity. This is a different sort of flow that we seemto be caught in. The Local Group of galaxies — a collection that includes the Milky Way, Andromeda

http://www.ast.uct.ac.za/~kraan/

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and a few dozen smaller galactic companions — moves at about 600 kilometers per second withrespect to the leftover radiation from the Big Bang.

Over the past few decades, astronomers have tallied up all the things that could be pulling andpushing on the Local Group — nearby galaxy clusters, superclusters, walls of clusters and cosmicvoids that exert a non-negligible gravitational pull on our own neighborhood.

The biggest tugboat is the Shapley Supercluster, a behemoth of 50 million billion solar masses thatresides about 500 million light years away from Earth (and not too far away in the sky from the VelaSupercluster). It accounts for between a quarter and half of the Local Group’s peculiar velocity.

ESA/Gaia/DPAC/CU5/DPCI/CU8

The Milky Way as seen by the Gaia satellite shows the dark clouds of dust that obscure the view of galaxies in theuniverse beyond.

The remaining motion can’t be accounted for by structures astronomers have already found. Soastronomers keep looking farther out into the universe, tallying increasingly distant objects thatcontribute to the net gravitational pull on the Milky Way. Gravitational pull decreases withincreasing distance, but the effect is partly offset by the increasing size of these structures. “As themaps have gone outward,” said Mike Hudson, a cosmologist at the University of Waterloo in Canada,“people continue to identify bigger and bigger things at the edge of the survey. We’re looking outfarther, but there’s always a bigger mountain just out of sight.” So far astronomers have only beenable to account for about 450 to 500 kilometers per second of the Local Group’s motion.

Astronomers still haven’t fully scoured the Zone of Avoidance to those same depths, however. Andthe Vela Supercluster discovery shows that something big can be out there, just out of reach.

In February 2014, Kraan-Korteweg and Michelle Cluver, an astronomer at the University of Western

http://sci.esa.int/gaia/59406-gaias-density-map-companion-to-the-preliminary-colour-sky-map/

https://uwaterloo.ca/scholar/mjhudson/home

http://www.ast.uct.ac.za/michellecluver

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Cape in South Africa, set out to map the Vela Supercluster over a six-night observing run at theAnglo-Australian Telescope in Australia. Kraan-Korteweg, of the University of Cape Town, knewwhere the gas and dust in the Zone of Avoidance was thickest; she targeted individual spots wherethey had the best chance of seeing through the zone. The goal was to create a “skeleton,” as shecalls it, of the structure. Cluver, who had prior experience with the instrument, would read off thedistances to individual galaxies.

That project allowed them to conclude that the Vela Supercluster is real, and that it extends 20 by25 degrees across the sky. But they still don’t understand what’s going on in the core of thesupercluster. “We see walls crossing the Zone of Avoidance, but where they cross, we don’t havedata at the moment because of the dust,” Kraan-Korteweg said. How are those walls interacting?Have they started to merge? Is there a denser core, hidden by the Milky Way’s glow?

And most important, what is the Vela’s Supercluster’s mass? After all, it is mass that governs thepull of gravity, the buildup of structure.

How to See Through the HazeWhile the Zone’s dust and stars block out light in optical and infrared wavelengths, radio waves canpierce through the region. With that in mind, Kraan-Korteweg has a plan to use a type of cosmicradio beacon to map out everything behind the thickest parts of the Zone of Avoidance.

The plan hinges on hydrogen, the simplest and most abundant gas in the universe. Atomic hydrogenis made of a single proton and an electron. Both the proton and the electron have a quantumproperty called spin, which can be thought of as a little arrow attached to each particle. In hydrogen,these spins can line up parallel to each other, with both pointing in the same direction, orantiparallel, pointing in opposite directions. Occasionally a spin will flip — a parallel atom will switchto antiparallel. When this happens, the atom will release a photon of light with a particularwavelength.

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SKA South Africa

One of the 64 antenna dishes that will make up the MeerKAT telescope in South Africa.

The likelihood of one hydrogen atom’s emitting this radio wave is low, but gather a lot of neutralhydrogen gas together, and the chance of detecting it increases. Luckily for Kraan-Korteweg and hercolleagues, many of Vela’s member galaxies have a lot of this gas.

During that 2014 observing session, she and Cluver saw indications that many of their identifiedgalaxies host young stars. “And if you have young stars, it means they recently formed, it meansthere’s gas,” Kraan-Korteweg said, because gas is the raw material that makes stars.

The Milky Way has some of this hydrogen, too — another foreground haze to interfere withobservations. But the expansion of the universe can be used to identify hydrogen coming from theVela structure. As the universe expands, it pulls away galaxies that lie outside our Local Group andshifts the radio light toward the red end of the spectrum. “Those emission lines separate, so you canpick them out,” said Thomas Jarrett, an astronomer at the University of Cape Town and part of theVela Supercluster discovery team.

While Kraan-Korteweg’s work over her career has dug up some 5,000 galaxies in the VelaSupercluster, she is confident that a sensitive enough radio survey of this neutral hydrogen gas willtriple that number and reveal structures that lie behind the densest part of the Milky Way’s disk.

That’s where the MeerKAT radio telescope enters the picture. Located near the small desert town ofCarnarvon, South Africa, the instrument will be more sensitive than any radio telescope on Earth. Its64th and final antenna dish was installed in October, although some dishes still need to be linkedtogether and tested. A half array of 32 dishes should be operating by the end of this year, with thefull array following early next year.

Kraan-Korteweg has been pushing over the past year for observing time in this half-array stage, butif she isn’t awarded her requested 200 hours, she’s hoping for 50 hours on the full array. Bothoptions provide the same sensitivity, which she and her colleagues need to detect the radio signalsof neutral hydrogen in thousands of individual galaxies hundreds of light years away. Armed withthat data, they’ll be able to map what the full structure actually looks like.

Cosmic BasinsHélène Courtois, an astronomer at the University of Lyon, is taking a different approach to mappingVela. She makes maps of the universe that she compares to watersheds, or basins. In certain areasof the sky, galaxies migrate toward a common point, just as all the rain in a watershed flows into asingle lake or stream. She and her colleagues look for the boundaries, the tipping points of wherematter flows toward one basin or another.

https://www.ipnl.in2p3.fr/perso/courtois/

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Eric Leroux, University Lyon Claude Bernard Lyon 1.

Hélène Courtois, an astronomer at the University of Lyon, maps cosmic structure by examining the flow of galaxies.

A few years ago, Courtois and colleagues used this method to attempt to define our local large-scalestructure, which they call Laniakea. The emphasis on defining is important, Courtois explains,because while we have definitions of galaxies and galaxy clusters, there’s no commonly agreed-upondefinition for larger-scale structures in the universe such as superclusters and walls.

Part of the problem is that there just aren’t enough superclusters to arrive at a statistically rigorousdefinition. We can list the ones we know about, but as aggregate structures filled with thousands ofgalaxies, superclusters show an unknown amount of variation.

Now Courtois and colleagues are turning their attention farther out. “Vela is the most intriguing,”Courtois said. “I want to try to measure the basin of attraction, the boundary, the frontier of Vela.”She is using her own data to find the flows that move toward Vela, and from that she can infer howmuch mass is pulling on those flows. By comparing those flow lines to Kraan-Korteweg’s mapshowing where the galaxies physically cluster together, they can try to address how dense of asupercluster Vela is and how far it extends. “The two methods are totally complementary,” Courtoisadded.

The two astronomers are now collaborating on a map of Vela. When it’s complete, the astronomershope that they can use it to nail down Vela’s mass, and thus the puzzle of the remaining piece of theLocal Group’s motion — “that discrepancy that has been haunting us for 25 years,” Kraan-Kortewegsaid. And even if the supercluster isn’t responsible for that remaining motion, collecting signals

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through the Zone of Avoidance from whatever is back there will help resolve our place in theuniverse.

hidden supercluster could solve milky way mystery magazine november 21, 2017 hidden supercluster...

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