Distant galaxy seems filled with dark matter | Science News for Students

Distant galaxy seems filled with dark matter

Earlier studies suggested such mega-clusters of stars would lack the dark stuff
Sep 21, 2018 — 6:45 am EST
Atacama telescope array

With these telescopes of the Atacama Large Millimeter/submillimeter Array in Chile, astronomers discovered the most distant galaxy that seems to be filled with dark matter.

C. Malin/ESO

Cosmic Seagull is a galaxy — a massive community of stars — some 11.3 billion light-years away. This galaxy also hosts a mystery. Its outermost stars race way too fast to be propelled just by the gravity of the galaxy’s gas and stars. Instead, they move as if urged on by an invisible force. And that appears to point to the presence of dark matter, scientists now report.

Most of the universe appears to be filled with dark matter, a mysterious, unseen material. Astronomers know it exists by the way its mass interacts — via gravity — with visible matter.

If the Cosmic Seagull is one such repository, it will be the most distant galaxy to be filled with dark matter.

Verónica Motta works at the University of Valparaíso in Chile. This astrophysicist and her colleagues shared their new findings August 8 at arXiv.org.

“In our nearby universe, you see these halos of dark matter around galaxies like ours,” she says. “So we should expect that in the past, that halo was there, too.” And because the light from distant galaxies is just reaching us from long, long ago, these new data point to the presence of dark matter back in those very ancient times.

Motta and her team used radio telescopes in Chile. They’re known as ALMA, which is short for the Atacama Large Millimeter/submillimeter Array. These telescopes have measured the speed of gas across the Cosmic Seagull’s visible disk, which spans out from the center across a distance of about 9,800 light-years. ALMA’s findings show that the galaxy’s stars speed up the farther they get from the galaxy’s center.

That’s a strange setup for most orbiting objects. When planets orbit a star, for instance, the most distant planets move the most slowly. But the Cosmic Seagull’s gas speeds can be explained if the galaxy’s far reaches are dominated by dark matter, which speeds things along.

New data challenges some earlier ones

cosmic seagull
Known as the Cosmic Seagull, this galaxy lies behind the Bullet Cluster of galaxies, whose regular matter (red, depicting X-rays emitted by hot gas) seems separate from its dark matter (blue, inferred by distortion of galaxies in the background). The Bullet Cluster’s mass magnified the Seagull’s image. This let astronomers observe the smaller galaxy in detail.

Similar measurements of the Milky Way and its neighboring galaxies provided one of the first signs that dark matter may truly exist. But those data can’t reveal what dark matter is actually made of. Physicists still are still trying to detect directly the proposed particles that make up dark matter.

The new finding by Motta’s team does challenge one recent claim. Last year, researchers reported such distant galaxies are oddly lacking in dark matter. Astronomer Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany had led a team looking into the issue. More than 100 distant galaxies keep their slower stars at the edges and faster stars closer in, they found. That means these galaxies would have required little to no dark matter.

The report by Genzel’s group “has been viewed with both excitement and skepticism,” says Richard Ellis. He’s a cosmologist at University College London who was not involved in either study. “It makes a lot of sense for others to examine galaxies at these [distances] in different ways,” Ellis now concludes.

Motta and her colleagues were able to focus their probe of dark matter thanks to a massive galactic train wreck. Called the Bullet Cluster, this cosmic collision acted like a huge telescope. Cosmic Seagull lies behind the Bullet Cluster (at least from Earth’s perspective). The cluster’s mass alters the Seagull’s light in a phenomenon known as gravitational lensing.

Indeed, that distortion earned the disk-shaped galaxy its name: The first images of it had reminded Motta’s team of the seagull logo for a popular music festival in Viña del Mar, Chile. More importantly, that distortion magnified the galaxy’s light 50-fold — a new record.

The Motta team “have exquisite data,” Ellis wrote in an e-mail. Their observations, however, are limited. For instance, the team looked at only one galaxy. And that galaxy is much smaller and less massive than those that seem to host little dark matter. What’s more, the telescopes’ observations don’t cover the entire galactic disk. That means that stars beyond what the team could view may be slower than those they were able to see.

Motta agrees that a distant slowdown is possible. Still, she notes, her observations cover the same portion of the Cosmic Seagull’s disk as had the study of galaxies that seemed short on dark matter.

“We are roughly at the place in which we should see the turning point,” if it exists, from fast to slow stars, she says. To be sure, “we need to extend the study.” In fact, her team has been granted more time with ALMA next year to keep looking.

Power Words

(more about Power Words)

array     A broad and organized group of objects. Sometimes they are instruments placed in a systematic fashion to collect information in a coordinated way. Other times, an array can refer to things that are laid out or displayed in a way that can make a broad range of related things, such as colors, visible at once. The term can even apply to a range of options or choices.

arXiv     A website that posts research papers — often before they are formally published — in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics. Anyone can read a posted paper at no charge.

astronomy     The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.

astrophysics     An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.

colleague     Someone who works with another; a co-worker or team member.

cosmic     An adjective that refers to the cosmos — the universe and everything within it.

cosmology     The science of the origin and development of the cosmos, or universe. People who work in this field are known as cosmologists.

dark matter     Physical objects or particles that emit no detectable radiation of their own. They are believed to exist because of unexplained gravitational forces that they appear to exert on other, visible astronomical objects.

distort     (n. distortion) To change the shape or image of something in a way that makes it hard to recognize, or to change the perception or characterization of something (as to mislead).

extraterrestrial     (ET) Anything of or from regions beyond Earth.

factor     Something that plays a role in a particular condition or event; a contributor.

force     Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

galaxy     A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

gravitational lensing     The distortion of light by an intense gravitational force, such as what can be exerted by clusters of galaxies — the most massive things in the universe. The gravity can bend or focus light, making it appear brighter and in one or more different places in the sky.

gravity     The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

light-year     The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

matter     Something that occupies space and has mass. Anything on Earth with matter will have a property described as "weight."

Milky Way     The galaxy in which Earth’s solar system resides.

orbit     The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

phenomenon     Something that is surprising or unusual.

physics     The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.

radio     To send and receive radio waves, or the device that receives these transmissions.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

telescope     Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

universe     The entire cosmos: All things that exist throughout space and time. It has been expanding since its formation during an event known as the Big Bang, some 13.8 billion years ago (give or take a few hundred million years).

X-ray     A type of radiation analogous to gamma rays, but having somewhat lower energy.


Journal:​ V. Motta et al. The ‘Cosmic Seagull’: A highly magnified disk-like galaxy at z~2.8 behind the Bullet Cluster. arXiv:1808.02828v1. Posted August 8, 2018.