Astronomers probe 'outstanding mystery' of how galaxies stop building stars

September 12, 2018, University of Kansas
Color Hubble Space Telescope image of 'self-quenching' galaxies 6 billion light years away. The bright dot in the center is the compact area, and some fuzz around the edges tells researchers it's a merger remnant. Credit: KU

Galaxies like our own Milky Way are factories that use gravity to fashion new stars out of molecular hydrogen gas.

"The Milky Way is turning gas into at about an average of the mass of the sun every year," said Gregory Rudnick, professor of physics & astronomy at the University of Kansas. "The galaxy is filled with gas, and we're constantly getting new gas from outside the galaxy. This gas falls into the galaxy under the force of gravity, gets formed into stars—and some of the gas gets blown back out of the galaxy."

But some have stopped this process of star formation, and astronomers have been hard-pressed to explain why.

"We see plenty of galaxies that don't form stars," Rudnick said. "For whatever reason, they don't have much gas in them so they can't make new stars. The big question is why. Why do some galaxies shut off? When they shut off, they're left with the stars they already have, but they don't make new ones."

A key to solving this puzzle may be a strange new class of galaxies about 6 billion light years from Earth that are in the process of violently expelling their own gas. Now, Rudnick has teamed up with scientists from around the U.S. under a new grant from the National Science Foundation to study these galaxies as a means to find out why some galaxies no longer have the gas necessary to form new stars.

Part of the process is to question recent conventional ideas about the cessation of star formation.

"One of the ways people have come up with to shut off star formation in a galaxy is to explosively remove the gas through an active galactic nucleus," Rudnick said. "Every galaxy, including our own, has a supermassive black hole at its center. As gas falls into the black hole, right before it hits the black hole, it gets super-hot, and the energy it gives off can actually blow out the rest of the gas from the galaxy. People like this idea because it's a mechanism that's energetic enough to do the job of expelling all the gas from the galaxy."

However, in 2007 a team of astronomers discovered a set of "self-quenching" galaxies that have shut down star formation by some other mechanism. Rudnick subsequently joined this team, which has led a multiyear investigation of these objects.

"Our research has found this kind of galaxy that has gas being blown out of it at thousands of kilometers per second—that's over 3,500 times faster than a jet plane—but there's absolutely no evidence of any kind of gas falling into a black hole," the KU researcher said. "So, there's a question of whether or not that black-hole process is required or if there are other ways of doing it. You realize, 'Wait a minute, the universe isn't that simple.' This could tell us really new and cool things about how galaxies evolve."

Astronomers probe 'outstanding mystery' of how galaxies stop building stars
CO J(2 → 1) maps of the galaxy SDSS J1341?0321 averaged over 333 km s-1channels spanning ΔV = ± 1500 km s-1 (labels give channel centers) relative to the systemic redshift. Contours start at 2σ and are logarithmically spaced at 0.2 dex multiples of σ. Dashed contours are negative equivalents. The crosshair indicates the peak of the stellar emission in the HST/F814W imaging. Black ellipses show the fwhm of the synthesized beam. Credit: KU

With better images of the galaxies from the Hubble Space Telescope, Rudnick and his collaborators realized gas could be expelled from the galaxies purely by the concentrated light of the stars in the galaxies, with no need for any extra energy from gas falling onto the black hole.

"Using HST, these fuzzy dots we saw before from our telescopes on the ground now showed these features that looked like a collision of galaxies—they had lots of streams of stars around. Most surprisingly, they were incredibly compact," Rudnick said. "The Milky Way has all its stars and gas spread out over 100,000 light years, meaning it takes light 100,000 years to get from one side to another. These galaxies, which are as massive as the Milky Way, look like they had most of their mass crammed into something like 1,000 light years. So, they're massive, but they're also super-concentrated. The idea we started developing is that maybe these galaxies are so compact that all the starlight from all the stars in these galaxies jammed into this small space is intense enough by itself to drive the gas out of the galaxies."

Rudnick said the idea is that stars emit light particles that would bump into particles of gas and "give them a little push. The sum of many of these little pushes is enough to push all the gas out of the galaxy at incredible speeds."

"When you condense an entire Milky Way into a small spot because of a merger of galaxies, it can cause hundreds of billions of stars to be in a very compact place," he said. "When that happens, you can put enough light into a small enough space, and that can be enough to push all of the gas out of a galaxy, with no extra energy needed from gas falling into a supermassive black hole."

The NSF grant work will enable further observation and study of this class of self-quenching galaxies. Rudnick said he and his colleagues will be studying the galaxies in as many ways as possible, using telescopes such as NASA's CHANDRA X-ray Observatory, the Keck Observatories in Hawaii and the Atacama Large Millimeter Array (ALMA) in Chile.

"How common is it that these galaxies are shutting themselves off this way?" he asked. "Is the galaxy really able to blow out all its gas completely—or are we seeing really fast, small stream? It's like you have a teakettle on the stove and it's hot and a stream of gas is shooting out of the teakettle. You know this gas is shooting out, and we'd like to know if there's enough gas shooting out that it will empty the teakettle completely."

Rudnick has just co-authored a paper appearing in Astrophysical Journal that used the ALMA telescope to look within a self-quenching galaxy at "the densest, coldest gas—the stuff that's actually forming the stars." In that paper, they ask, "What evidence do we have for that gas blowing out?"

The KU researcher and his co-authors found the galaxy to be the compact remains of a violent merger between two galaxies. This "merger remnant" also hosts huge winds of dense molecular gas but without any trace of an active galactic nucleus. They wanted to determine how the fast the dense gas was gas being driven out of the galaxy.

"ALMA uses light with much a longer wavelength than visible light—with a wavelength of a tenth of a millimeter," Rudnick said. "It turns out carbon monoxide molecules in this gas give off light you can see with a telescope from Earth. Most of the gas is actually hydrogen, but hydrogen is hard to see from Earth, so we pick a trace gas. It's a lot like natural gas, you can't smell it, so they put something in it to make it smell like rotten eggs. Carbon monoxide works like that, in that it tells us where the hard-to-see molecular hydrogen is. We use ALMA to detect carbon monoxide and use that to detect how much total molecular gas. We found this gas, which makes up much of what the galaxy has, is moving from the center of the galaxy at 1,000 kilometers per second. It's really the stuff that forms stars that's being blown out."

Explore further: Research reveals the real cause of death for some starburst galaxies

More information: J. E. Geach et al, Violent Quenching: Molecular Gas Blown to 1000 km s−1 during a Major Merger, The Astrophysical Journal (2018). DOI: 10.3847/2041-8213/aad8b6

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Tuxford
1.6 / 5 (13) Sep 12, 2018
"Our research has found this kind of galaxy that has gas being blown out of it at thousands of kilometers per second—that's over 3,500 times faster than a jet plane—but there's absolutely no evidence of any kind of gas falling into a black hole,"

Strike 56. Indeed.

The core star has grown so large and therefore so active in ejecting new matter therefrom, that stars simply migrate away from center, and the ejecting winds are so strong that stars can no longer form close enough to the core to provide sufficient luminal density within the galaxy to be observed from this great distance. Whew.

So the stars are scattered outward at lower spacial density and disappear into the background apart from our insensitive view at such distances.

So again, we left with the uncomfortable conclusion that the winds are generated from new matter being formed and ejected from within the core star itself. A merger maniacs nightmare!
jonesdave
3.1 / 5 (15) Sep 12, 2018
^^^^Always a crank on hand whenever we need the mysteries of the Universe explained! Where would we be without them? :)
Surveillance_Egg_Unit
3.2 / 5 (9) Sep 12, 2018
^^^^Always a crank on hand whenever we need the mysteries of the Universe explained! Where would we be without them? :)


So instead of adding something scientifically substantial to the mix, you had nothing else to say but that? Waste of space. Short on relevancy. Something dug out of your own storehouse of crank pottery, eh?
Surveillance_Egg_Unit
2.6 / 5 (5) Sep 12, 2018
"...realized gas could be expelled from the galaxies purely by the concentrated light of the stars in the galaxies".

More likely it was the HEAT from the Stars that expelled the gas from the galaxies. A million flashlights on the ON position inside a room full of gas would not cause the gas to explode or rush out. But light a match and B-O-O-M!!
theredpill
2 / 5 (4) Sep 13, 2018
"So instead of adding something scientifically substantial to the mix, you had nothing else to say but that? Waste of space. Short on relevancy. Something dug out of your own storehouse of crank pottery, eh?"

IOW...his status quo...
TorbjornLarsson
5 / 5 (4) Sep 13, 2018
^^^^Always a crank on hand whenever we need the mysteries of the Universe explained! Where would we be without them? :)


So instead of adding something scientifically substantial to the mix, you had nothing else to say but that? Waste of space. Short on relevancy. Something dug out of your own storehouse of crank pottery, eh?


What would he add but the implicit, that the science was already explained in the article with references, and Tuxford was just spouting nonsense without any shred of evidence?

But thanks goes to you and the later commentators that had nothing to add but strawman criticism and unearned invective, making his and everyone actually interested in the science point. ("Crank" = crackpot, and it is obvious that Tuxford would score high on the often used Crackpot Index.)
Tuxford
1.7 / 5 (6) Sep 14, 2018
The all too common intellectual merger maniac has a hermetically sealed mind, quite content already in his intellectual superiority. Fearing shown otherwise, he is quick to condemn alternate ideas that threaten his own inflated view of himself, as demonstrated by posters here. The maniac simply cannot consider that he is wrong. It would be too disturbing to his fragile ego. And so there is no use in arguing. Our president suffers the same affliction. Egomania is rampant in society.

There is plenty of observational evidence in support of LaViolette's continuous creation model. Is anyone really interested? No. Egomania prevails, along with some DoD counter-intelligence support. Who do you think owns this website?? Why does it exist?
jonesdave
1 / 5 (3) Sep 14, 2018
Who do you think owns this website?? Why does it exist?


Salvation Army? In praise of baby Jesus? Who knows?
Meanwhile, back at the ranch, who is the fruitloop who owns the following website, and why does it exist?

(Warning; wear sunglasses and ear defenders before clicking on link, or disallow Javascript!)

http://etheric.com/
granville583762
5 / 5 (2) Sep 14, 2018
Colliding Galaxies
The Milky Way has been producing stars for 13billion years and is still producing stars and is in the process of merging with the Andromeda galaxy
The point being - galaxies merge every 4billion years or so, that for one particular galaxy to cease star production, every 4billion years it is involved in a merger with colliding galaxies - therefore it is not possible every galaxy this dead galaxy merges with is also a dead galaxy!
That is stretching incredulity a might, don't you think old chap.
691Boat
5 / 5 (2) Sep 14, 2018
"...realized gas could be expelled from the galaxies purely by the concentrated light of the stars in the galaxies".

More likely it was the HEAT from the Stars that expelled the gas from the galaxies. A million flashlights on the ON position inside a room full of gas would not cause the gas to explode or rush out. But light a match and B-O-O-M!!


@SEU:
What's the medium that all this "heat" is being transferred through to expel gas? If only there was something akin to radiation pressure that could be a viable explanation...like when concentrated light from millions of stars in a single galaxy.......oh yeah...that is radiation pressure.

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