Half the Milky Way may be stolen material
Some galaxies may grow by swiping gas from their neighbors. Those thieving ways may be at work in even our galaxy, the Milky Way. New computer analyses suggest that nearly half the mass in the Milky Way might have started as gas released by other galaxies. That gas is a raw material from which galaxies build their bulk.
The finding is scheduled to appear in the Monthly Notices of the Royal Astronomical Society. The new analysis reveals an, unexpected way that galaxies may acquire mass. It also offers clue to how they evolve.
“These simulations show a huge amount of interaction among galaxies — a huge dance that’s going on,” says Romeel Davé. He is an astronomer in Scotland at the University of Edinburgh. That dance, and the subsequent exchange of atoms, could be what establishes a galaxy’s character. It could determine whether a galaxy is small or big, elliptical or spiral, quiet or bursting with star formation. Results from the new computer model need to be confirmed with observations. But this could be a major step toward understanding galaxy formation, says Davé.
It makes sense that much of the material in one galaxy actually came from others, says Claude-André Faucher-Giguère. “Still, the result was really unexpected,” says this coauthor. A theoretical astrophysicist, he works at Northwestern University in Evanston, Ill.
Astronomers had thought galaxies got their matter in two main ways.
In the first, atoms would clump together to form stars. These would then assemble, through gravity, to form galaxies. Much of this likely would have taken place not long after the Big Bang, some 13.8 billion years ago.
In the second way, some of those atoms would have eventually been ejected by exploding stars, known as supernovas. Those atoms likely would have rained back down into this same galaxy. That would have recycled the gas over and over.
The new simulations show a third way galaxies could obtain gas.
Powerful supernovas would eject atoms in the form of gas. These could travel far off into intergalactic space. Pushed by galactic winds, they would eventually arrive at new, distant galaxies. Those cosmic winds travel at several hundred kilometers (miles) per second. When the particles they carried neared some galaxy, its gravitational pull would suck them in. These particles would then serve as the basis of stars, planets, dust and other material in this new host galaxy.
Such an exchange of atoms is extremely difficult to spot in space. After all, these gas atoms would not give off light, as stars do.
Faucher-Giguère and his colleagues spotted the gas exchange in a computer model. That model predicted how galaxies would have formed just after the Big Bang. It also suggested how they likely have evolved over time. The team tracked gas atoms as they moved through this sim universe, as they formed stars, and as exploding stars later ejected those atoms again from their home galaxies.
In the model, up to half of the atoms in large galaxies came from other galaxies. More massive galaxies have more gravity. So they tended to pull atoms from the ejected material of small galaxies. This accumulation of foreign matter appears to take billions of years. It takes that long for atoms to travel the vast space between galaxies, the team notes.
“It’s that not surprising to see a galaxy kick out matter, which is then pulled in by other galaxies,” Davé says. What is surprising, he says, is how much matter can be transferred this way. Before seeing the simulations, he would have guessed that maybe a max of 5 percent of gas was transferred between galaxies, this way. “To see that it is up to 50 percent is pretty remarkable,” he says.
Already, astronomers are searching the cosmos for signs of such a swapping behavior. Faucher-Giguère and colleagues are working with researchers using the Hubble Space Telescope. They hope to observe the transfer of gas between galaxies soon.
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
atom The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
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.
Big Bang The rapid expansion of dense matter that, according to current theory, marked the origin of the universe. It is supported by physics’ current understanding of the composition and structure of the universe.
coauthor One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.
colleague Someone who works with another; a co-worker or team member.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
cosmos (adj. cosmic) A term that refers to the universe and everything within it.
elliptical Having the shape of an ellipse, which is an oval shape.
evolve (adj. evolving) To change gradually over generations, or a long period of time. In living organisms, the evolution usually involves random changes to genes that will then be passed along to an individual’s offspring. Nonliving things may also be described as evolving if they change over time. For instance, the miniaturization of computers is sometimes described as these devices evolving to smaller, more complex devices.
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.
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.
intergalactic Between galaxies.
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.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
particle A minute amount of something.
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.
sim An abbreviation for simulation or simulated.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.
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.
supernova (plural: supernovae or supernovas) A massive star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.
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.
theoretical An adjective for an analysis or assessment of something that based on pre-existing knowledge of how things behave. It is not based on experimental trials. Theoretical research tends to use math — usually performed by computers — to predict how or what will occur for some specified series of conditions. Experimental testing or observations of natural systems will then be needed to confirm what had been predicted.
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).
Journal: D. Anglés-Alcázar et al. The cosmic baryon cycle and galaxy mass assembly in the FIRE simulations. Monthly Notices of the Royal Astronomical Society. Published online June 20, 2017. doi: 10.1093/mnras/stx1517.