Galaxies don't grow forever. At some point, even the most prolific star-forming galaxies start to slow down, then stall, then settle into a long quiet retirement. Astronomers have known about this transition for a long time, but we haven't had a clean physical explanation for why it happens, and why it happens at the particular mass scale that it does.

A new paper led by Preetish Mishra of the Korea Institute for Advanced Study, along with an international team of scientists, makes a clear and testable proposal: that the slowdown in galaxy growth is caused by the birth of a stable cloud of hot gas surrounding the galaxy, and that cloud forms at a very specific mass: roughly 10^12.5 solar masses. Above that threshold, galaxies stop being efficient stellar factories, no matter how much raw material they have on hand.

The question is: what flips the switch?

To get to that calculation, the team used the Horizon Run 5 simulation, one of the largest cosmological simulations ever created. It takes a chunk of virtual universe roughly a gigaparsec across, models the full physics of gas, gravity, star formation, supernovas, and supermassive black holes from shortly after the Big Bang to the present day, and lets researchers track individual galaxies through their entire histories. Mishra and colleagues picked out roughly 20,000 of the most massive central galaxies and watched what happened to them over cosmic time.

The key quantity they tracked is the stellar-to-total mass ratio. It's a measure of how much of a galaxy’s entire mass budget (stars, gas, dark matter, black holes, everything) actually ends up locked into stars. Think of it as a galaxy's star-formation efficiency report.

The team found that this ratio peaks sharply in galaxies with total masses between about 10^12.4 and 10^12.7 solar masses. Below that range, galaxies are turning gas into stars roughly as fast as the gas comes in. Above it, they slow down by more than a factor of three. That peak is the critical mass.

Mishra's theory as to why galaxies stop growing is the formation of a hot gas halo that has reached gravitational equilibrium. As a galaxy grows, the gas falling into it gets shock-heated. Up to a certain mass, that gas cools quickly enough to keep raining down and feeding new star formation.

Past the critical mass, the halo gets dense and hot enough to hold itself up against gravity for billions of years. The gas can no longer cool fast enough to fall in and the galaxy is suddenly cut off from its fuel supply. It keeps gobbling up dark matter and dragging in satellite galaxies, but the cool gas that actually makes stars stops arriving.

The paper also rules out a competing explanation. One natural guess is that galaxies above the critical mass simply lose more of their normal matter to outflows from supernovas and active galactic nuclei. The team checked this directly by computing how much of each galaxy's baryon budget actually stayed bound to the system. The variation turned out to be no more than 30 percent. That isn't nothing, but it can't account for the factor-of-three drop in star formation efficiency. The decisive change is on the inflow side, not the outflow side.

A few caveats are worth flagging. Horizon Run 5 is a simulation, not a telescope, and its results depend on the sub-grid physics used to model star formation, supernovas, and black hole feedback. The authors did sensitivity tests and the basic result holds up, but the precise numerical value of the critical mass scale could shift as those prescriptions improve.

The analysis also restricts itself to galaxies above 10^10.8 solar masses to make sure each one has enough simulation particles to be reliably resolved. Smaller galaxies are a story for another simulation.

What makes this work satisfying is that it pins a famous observational pattern to a single, specific physical mechanism. Not just that galaxies above a certain mass quench, but that they quench because their hot gas halos become self-supporting. That is the kind of statement that can be checked against future surveys of galaxy clusters and the warm-hot intergalactic medium, the gas and dust between galaxies.

We will know whether they got the right answer once those surveys roll in.

Astronomers have obtained their most detailed look yet at young galaxies in the early universe using the James Webb Space Telescope, Hubble Space Telescope and Atacama Large Millimeter/submillimeter Array. The conclusion? These cosmic adolescents grew up incredibly fast.

The team behind this research observed 18 galaxies located around 12.5 billion light-years away over a range of wavelengths of light. Existing just over 1 billion years after the Big Bang, these galaxies were in the midst of rapid star formation and were therefore undergoing explosive growth.

The team's most important discovery was the fact that these galaxies seem to have matured faster than previously expected in more than one way — but most strikingly, the galaxies are richer in elements heavier than hydrogen and helium, or "metals," as astronomers call them, particularly carbon and oxygen.

"With this sample, we are uniquely poised to study galaxy evolution during a key epoch in the universe that has been hard to image until now," team member Andreas Faisst of the California Institute of Technology (Caltech) said in a statement. "Thanks to these exceptional telescopes, we have spatially resolved these galaxies and can observe the stages of star formation as they were happening and their chemical properties when our universe was less than a billion years old."

Galaxies grow up too fast

When the first galaxies in the universe formed, the cosmos was filled with hydrogen and helium and just a smattering of heavier elements. The first stars and their home galaxies were correspondingly metal-poor. These stars forged metals during their lives and then dispersed them throughout their galactic homes in supernova explosions that marked their deaths. These heavy elements became the building blocks of the next generation of stars, which were more metal-rich than their predecessors.

However, this process of enrichment should take longer than 1 billion years, meaning the prematurely mature state of these early galaxies is curious, to say the least.

"It was a surprise to see such chemically mature galaxies," Faisst added. "It's like seeing 2-year-old children act like teenagers. How do metals form in less than 1 billion years?"

Anyone living with human teenagers will tell you they have quite the appetites, and that is also true of these premature cosmic teens. The team found that the supermassive black holes in these galaxies are rapidly feeding, or accreting, surrounding matter. That means these black holes are also growing rapidly.

In addition to their anachronistically metal-rich nature, Faisst and colleagues discovered that many of the galaxies they studied had rotating stellar disks, similar to the spiral arms of our much more mature galaxy, the Milky Way. These features had also developed much earlier than previous models had predicted.

"Now, with this new survey, we can show that some of these galaxies were both structurally and chemically evolved," Faisst said.

It wasn't just the galaxies studied by these scientists that were unexpectedly metal-rich. The surrounding gas, the circumgalactic medium, was also similarly enriched.

"The galaxies show very flat gradients in their metal abundances, reaching out to more than 30,000 light-years," team member Wuji Wang of Caltech's Infrared Processing & Analysis Center said in the statement.

The team now intends to match their observations of these galaxies using simulations of galactic growth and metal enrichment.

"The combination of observations and simulations provides a powerful synergy to understand the details of star formation, and dust and metal production mechanisms," Faisst said. "The knowledge of these will ultimately help us understand the formation of the first stars and planets and how our own Milky Way came into being."

The team's research was presented at the 247th meeting of the American Astronomical Society in Phoenix on Tuesday (Jan.6), and was published in The Astrophysical Journal Supplement.

Astronomers have discovered that large flows of cold gas created by collisions between galaxies in the early universe may have forged some of the most monstrous star systems.

The formation of ancient gigantic galaxies that bulge like footballs compared to our relatively flat spiral galaxy, the Milky Way, has confused astronomers for decades.

Now, a team led by scientists from the University of Southampton may have scored a touchdown in this quest. They believe their research may finally solve this long-standing galactic puzzle.

"Two disk galaxies smashing together caused gas – the fuel from which stars are formed – to sink towards their center, generating trillions of new stars," team member and the University of Southampton researcher Anna Puglisi said in a statement. "These cosmic collisions happened some eight to 12 billion years ago when the universe was in a much more active phase of its evolution.

"Our findings take us closer to solving a long-standing mystery in astronomy that will redefine our understanding of how galaxies were created in the early universe."

The team's research was published on Wednesday (Dec. 4) in the journal Nature.

The team made their breakthrough using the Atacama Large Millimeter Array (ALMA). ALMA is the largest astronomical project in existence comprised of 66 radio antennas situated in the Atacama Desert of northern Chile.

The team gathered high-quality observations of many distant galaxies using ALMA and data from the A3COSMOS and A3GOODSS archival projects. In particular, they analyzed over 100 galaxies that are currently in the process of intensely forming stars.

Team leader Qing-Hua Tan of the Purple Mountain Observatory explained that the project used a new technique to examine the distribution of light from distant and extremely bright galaxies.

"This is the first real evidence that spheroids form directly through intense episodes of star formation located in the cores of distant galaxies," Tan said. "Astrophysicists have sought to understand this process for decades.

"These galaxies form quickly – gas is sucked inwards to feed black holes and triggers bursts of stars, which are created at rates ten to 100 times faster than our Milky Way."

The team will now combine their findings with data collected by the James Webb Space Telescope (JWST) and the Euclid satellite.

This should help them to chart the populations of stars within target galaxies to shine further light on the mystery of giant galaxy formation.

"This will give us a more complete picture of early galaxy formation and deepen our understanding of how the universe has evolved since the beginning of time," Puglisi concluded.

What happens when a cosmic bully collides with a galactic dwarf? The big guy comes out on top, according to a new study.

Sometimes size does matter: In a survey of more than 20,000 merged galaxies, researchers found that star formation sped up in the larger of two galaxies, but nearly came to a halt in the smaller one. When the two galaxies are of roughly equal mass, they both see an increase in star formation. You can watch an adorable video explaining the new results here on Space.com.

The cause of this unbalanced outcome may be due to the larger galaxy slurping up the smaller galaxy's supply of gas, from which new stars form.

When two galaxies collide, astronomers suspect that clouds of gas inside the galaxy get churned up and seed new star birth faster than if the two galaxies remained separate, and tranquil. But as the new study shows, this isn't the case for smaller galaxies.

"This might be because the bigger galaxy strips away its smaller companion's gas, leaving it without star-forming fuel or because it stops the smaller galaxy obtaining the new gas required to form more stars," said Luke Davies, of the International Centre for Radio Astronomy Research (ICRAR) at the University of Western Australia, and author of the new research.

The findings may one day have big implications for star formation in the Milky Way galaxy, which is on a collision course with the nearby Andromeda galaxy. The two neighbors are moving toward each other at 248,548 mph (400,000 kilometers per hour).

"Don't panic yet, the two won't smash into each other for another four billion years or so," Davies said in a statement.

The two relatively large galaxies ("cosmic tanks," he calls them) are of a similar mass, so it's more likely that the collision will spur star formation in both of them.

"Investigating such cosmic collisions lets us better understand how galaxies grow and evolve," Davies said.  

The new research employed the Galaxy and Mass Assembly (GAMA) survey using the Anglo-Australian Telescope in New South Wales, and was published online on July 12 in the journal Monthly Notices of the Royal Astronomical Society.

Follow Calla Cofield @callacofield.Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

Astronomershave found nine of the faintest, tiniest and most compact galaxies ever seen.

The littleobjects are hundreds to thousands of times smaller and vastly younger than ourMilky Way, lending support to a ?building block? theory in which hundreds ofthe tiny galaxies merge together and form larger bodies of stars.

"Theseare among the lowest mass galaxies ever directly observed in the earlyuniverse," said Nor Pirzkal of the Space Telescope Science Institute in Baltimore, Md.

Pirzkalsaid their petite mass, observed by the Hubble Space Telescope and confirmed bythe Spitzer Space Telescope, shows these galaxies are some of the smallestbuilding blocks of the universe, aside from stars themselves.

The twotelescopes saw light emitted from the galaxies only 1 billion years after thetheoretical Big Bang, giving arare glimpse into the past. Sangeeta Malhotra, an astronomer at Arizona State University who helped make the discovery, said the absence of infrared lightin the sensitive Spitzer images showed the stars are first-generation and onlya few million years old.

"Theseare truly young galaxies without an earlier generation of stars," Malhotrasaid.

Hubbledetected hot blue stars within the nine galaxies, indicating that the youthfulstars are in the process of turning hydrogen and helium into heavier elementslike carbon, oxygen and silicon necessary forplanet-building--and life. The astronomers speculated, however, that suchstars probably haven't begun to "pollute" space with the crucialelements forging within their cores.

Thedevelopment of three of the galaxies appears to be slightly disrupted; ratherthan taking on a rounded-blob shape typical of the youngest galaxies, they're stretchedinto tadpole-like forms. Astronomers think it may signal their first fusionwith neighboring galaxies to form larger, cohesive structures.

Thegalaxies were observed in the Hubble Ultra Deep Field (HUDF) image with Hubble'sAdvanced Camera for Surveys and the Near Infrared Camera and Multi-ObjectSpectrometer. Observations were also done with Spitzer's Infrared Array Cameraand the European Southern Observatory's Infrared Spectrometer and Array Camera.

• GALLERY: Hubble's Greatest Hits
• VIDEO: Fixing Hubble
• VOTE: The Best of Hubble Images