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Hunting the galaxy killer.

Half the stars in the universe live in "dead" galaxies, where star formation has ground to a halt. What great force has stopped it in its tracks?

If we could travel back in time 10 billion years, we would experience a universe at the height of fertility. Galaxies underwent tremendous growth spurts as stars formed at prodigious rates: Hundreds or even thousands of them ignited each year in every galaxy. Deep within these galaxies, black holes turned into cosmic lighthouses, gorging on so much gas that the material they were trying to consume grew bright enough to be seen across the universe as quasars.

Since then, there's been a decline. Many black holes have shut off their surrounding lighthouses, while in general the creation of new stars has become less efficient. Our own Milky Way struggles to turn dense clouds containing hundreds of solar masses of molecular gas into a measly Sun's worth of stars each year. But at least our Milky Way is still forming stars. Some galaxies have ceased star formation altogether; they're now composed of stars that are cool, red, and old compared to more massive and short-lived stars. Such galaxies are evocatively described as "red and dead."

Part of the reason for this is that there's less gas available for star formation today than there was 10 billion years ago. Yet three-quarters of red-and-dead galaxies have no shortage of gas--it's just that it's too warm to make stars. These galaxies contain half the stars in the universe yet will make hardly any more--something seems to have happened to them. An unknown force, powerful enough to affect entire galaxies, is on a killing spree. Three suspects have been implicated, each with its own compelling means and opportunity.

Death by Blunt Force

Most red-and-dead galaxies are found in clusters, which house ellipticals near their centers and dusty, disk-shaped lenticular galaxies cruising through their suburbs. Could it be that the environment around these galaxies determined their fate?

Red galaxies have been synonymous with galaxy clusters for as long as there have been galaxy clusters. "Ellipticals seem to be in place in clusters quite early in the history of the universe," says James Geach (University of Hertfordshire, UK). "But lenticulars only appear more recently, over the last five billion years."

Lenticular galaxies might have evolved from spiral galaxies. Although they're disk-shaped, they have little, if any, spiral structure. While they've lost most of their gas, they're obscured by plenty of dust and are rich in old, red stars. They also seem to congregate mostly in and around galaxy clusters.

Jeffrey Kenney (Yale University) thinks there's a connection. He spends his time researching the relationship between galaxies and their environments. When the Hubble Space Telescope imaged the face-on galaxy NGC 4921, which features ghostly spiral arms, Kenney noticed something odd on one side of the galaxy's disk. A prominent ridge of gas and dust, thousands of light-years long, appears to be fraying like loose threads on a sweater.

NGC 4921 is currently plunging deep into the Coma Cluster, 310 million light-years distant. Its journey takes it through the hot intracluster medium, a thick soup of X-ray-emitting plasma where temperatures soar to more than 10 million kelvin. This plasma acts like a harsh wind, scouring the leading edge of the infalling galaxy and stripping gas into long tails. The frayed filaments are denser, magnetically bound clumps of gas that more strongly resist the stripping pressure as NGC 4921 rams into the intracluster medium.

Right now, this ram-pressure stripping is operating tens of thousands of light-years from the galaxy's center, but it won't stop there. "It will eventually eat its way in and probably remove all the gas from the galaxy," says Kenney. "We see lots of disk galaxies in the Coma Cluster that have almost no gas, no star formation. Most of these were likely spiral galaxies that were completely ram-pressure stripped. [NGC 4921] is just the one we've caught in the act; for most of them the action is already over."

Ram pressure's effect depends heavily on the cluster's mass. The Virgo Cluster, 54 million light-years away, is less massive than Coma. Since its galaxies orbit at a slower pace and its intracluster medium is less dense, its ram pressure is consequently lower. Virgo can completely strip its dwarf galaxies of their star-forming gas, but larger spiral galaxies only lose gas on the outer edge. "In Coma," Kenney says, by way of comparison, "almost all spiral galaxies will be completely stripped on their first passage towards the cluster center."

While ram-pressure stripping may produce lenticular galaxies in massive clusters, it cannot satisfactorily explain why every cluster, big or small, contains an ancient, red elliptical galaxy at its hub. "It's not entirely clear what quenches star formation in elliptical galaxies," admits Kenney.

Stellar Suicide

Suspicion has therefore fallen on other suspects, such as the stars within galaxies. In 2014 Geach and colleagues observed outflows from intense star formation in an extremely compact galaxy. Measuring less than 600 light-years across, this tiny starburst probably arose during a gas-rich galactic merger. Now it's spewing away a third of its gas reservoir.

Following up with the Institute of Millimeter Radio Astronomy's Plateau de Bure Interferometer in the French Alps, the astronomers observed a Doppler shift in emission lines from ionized magnesium and doubly ionized oxygen, indicating that powerful outflows are blowing gas out of this galaxy at up to 1,000 km/s (2 million mph). Astronomers ordinarily attribute such velocities to fierce winds of radiation from supermassive black holes, but there's no evidence of an active black hole in this galaxy. It must be the stars that are driving the gas out.

In the same way that sunlight can impart momentum to push a solar sail, photons emitted from stars can also drive away gas molecules. The more massive a star, the hotter it is and the greater its radiation pressure.

"There's enough energy in stellar radiation pressure to drive quite a lot of gas out of a galaxy," says Geach. He paints a picture where a frenzied burst of star formation converts gas into stars, which in turn produce photon winds that are powerful enough to sweep out the remaining gas and bring star formation to a halt.

So how many stars can a galaxy form before stellar feedback kicks in? It depends how crowded the stars are. Compact galaxies concentrate their star formation rather than spreading it across a large disk, so even a few stars can kick out large amounts of gas. That's not the case for the Milky Way, where the bubbles that individual star-forming regions blow are tiny compared to the scale of our galaxy.

Newborn stars aren't the only ones to create bubbles; dying stars do, too. Intense bursts of star formation produce massive stars that inevitably go supernova. A star cluster might see rapid-fire stellar destruction over the course of just a few million years. While supernova shocks can trigger starbirth by compressing the surrounding gas, ultimately they heat and clear out gas in superbubbles spanning hundreds of light-years. In large galaxies such as our Milky Way, this can create pockets of infertile space. And in smaller galaxies, superbubbles may completely remove all potentially starforming material, sterilizing the entire system.

If ram-pressure stripping only operates under certain circumstances, and stellar feedback has only limited range in larger galaxies, then only one alternative remains to explain giant, red-and-dead ellipticals: black holes.

Death by Black Hole

At the hub of most large galaxies is a supermassive black hole millions or billions of times more massive than the Sun. Some are dormant, while others are raging beasts consuming huge amounts of gas. What they don't swallow, they shoot out along magnetic field lines coiled tightly as gun barrels. The material bursts out of the galaxy as powerful jets of charged particles and radiation. While superheated matter spirals inward to await being swallowed or spat out, it settles into a disk, which may launch its own wind. The jets and disk together provide sufficient power to blow a huge bubble into a galaxy's stores of molecular gas--heating or sweeping up gas that might otherwise have formed stars. Molecular hydrogen must be at most a few tens of degrees above absolute zero to condense into stars, so it doesn't take much for the black hole to disturb the gas's delicate thermal condition.

At least, that's the theory. Now, there's some evidence to back it up, from the SDSS project Mapping Nearby Galaxies at Apache Point Observatory (MANGA) that's mapping 10,000 nearby galaxies. Among this immense collection, an international team of scientists led by Edmond Cheung (University of Tokyo) identified a new, rare type of galaxy: red geysers. They contain supermassive black holes that are only nibbling on surrounding gas; nevertheless, their low feeding rates are enough to launch a wind that spews into the galaxy.

The prototype red geyser observed by Cheung's group, nicknamed Akira, is ripping cool gas from a small companion dubbed Tetsuo. Normally, cool gas would turn into stars, but the red geyser's wind, revealed in velocity measurements of ionized gas billowing out from the central black hole, heats the gas and blows it away, leading the galaxy to turn red.

Many large elliptical galaxies are ensconced within a gaseous halo that, like the intracluster medium, is hot and emits X-rays. Similar halos have been discovered encapsulating spiral galaxies, too--in 2012, for example, Chandra detected an X-ray halo around our Milky Way with a temperature between 1 million and 2.5 million kelvin and a mass between 10 and 60 billion Suns. The halos hold material left over from galaxy formation, as well as gas accumulated from the intergalactic medium. Feedback processes also feed and heat the halos and can maintain a red-and-dead state even after the black hole has become largely inactive.

"Once it becomes too strong, the black hole begins to quench its own gas supply," explains Megan Donahue (Michigan State University), who has a particular interest in how black holes influence star formation.

Cheung estimates that up to 10% of galaxies with quiescent black holes are red geysers--indeed, even our own Milky Way Galaxy fits the bill. Two gigantic cavities above and below its disk, called the Fermi bubbles, are ancient geysers that might be relics of our black hole's rollicking past (S&T: April 2014, p. 26).

Astronomers have witnessed similar behavior in elliptical galaxies using the European Space Agency's retired Herschel Space Observatory, which observed the universe at far-infrared wavelengths. Herschel detected cold gas reservoirs in a half-dozen giant elliptical galaxies. Yet the gas wasn't able to cool enough to form stars. Observations by NASA's Chandra X-ray Observatory show that the central black hole was agitating the gas.

The Galactic Phoenix

But Donahue had suspicions that there was more to the story. She began to apply for time on Hubble to search elliptical galaxies for ultraviolet light from hot, young stars.

Her requests for Hubble time were politely turned down. "It was hard," she says of her attempts. "The received wisdom was that [elliptical galaxies] are red and dead, so why would you waste time pointing an ultraviolet telescope at them?"

As it happens, the Cluster Lensing and Supernova Survey with Hubble (CLASH) was studying how 25 massive clusters act as gravitational lenses, magnifying the light of much more distant galaxies. The CLASH project was interested in these magnified galaxies and so observed each cluster--and the ellipticals within them--through 16 different filters, including ultraviolet.

"I was extremely happy because I could have proposed for years and never gotten that kind of coverage," says Donahue. It was worth the wait. Hubble's observations of many of the brightest cluster galaxies (BCGs), the most luminous, largest, and exclusively elliptical galaxies at the center of each cluster, revealed a delightful variety of ultraviolet-emitting knots and filaments. Stars are forming at a rate of up to 80 solar masses per year--in galaxies that were presumed to be red and dead --and simulations suggest the central black holes are contributing to the rebirth. Like a phoenix arising from the ashes, the galaxies were being brought back to life. So were they ever really dead, or were they just faking it?

"Quenching [of star formation] might be too simplistic a concept," Donahue acknowledges.

Instead she describes a scenario in which feedback engages in a subtle interplay with a galaxy's gas reservoir. Whether feedback comes from an active black hole or from newborn and dying stars, it may act as a galactic thermostat. First it heats surrounding gas, preventing it from falling onto the black hole or condensing into more stars. But as activity shuts down, the gas has a chance to cool and fall inward, which in turn re-ignites the black hole's activity or forms stars (or both). And so on.

That's what Donahue observed in the BCGs: With feedback stalled, the gas was able to cool and fall back onto the galaxy, some of it forming new stars, the rest heading towards the black hole where it will kick off the next round of activity. Furthermore, although this rebirth was only observed in the BCGs, Donahue suspects that it's at work in other galaxies, too, though harder to observe.

"The physics of cooling, precipitation, infall, and the creation of cold molecular gas is not unique to clusters. It's just that we can see it all in a cluster," she says. "We have all the pieces of the puzzle in front of us."

Perhaps rather than being truly dead, many galaxies instead enter a state of hibernation. Or it may be even more complicated than that: Some forms of feedback may actually resuscitate galaxies rather than kill them. Recent evidence from the European Southern Observatory's Very Large Telescope in Chile has shown new stars born within the very winds that ought to halt their formation. Observing a galaxy collision 600 million light-years away, Roberto Maiolino (University of Cambridge, UK) and colleagues found evidence of infant stars in cold gas that had been swept up by outflows moving through one of the colliding galaxies.

Different feedback mechanisms are also important for different galaxies, so looking for a single cosmic killer may be the wrong way to go about things. In smaller galaxies, stellar outflows and supernovae play a dominant role, and black holes only have to step in once in a while, says Donahue. In larger galaxies, stellar feedback isn't enough. "The black holes have to step up almost all the time."

So are external processes, such as ram-pressure stripping or interactions with other galaxies, ever important? To answer that question, astronomers surveyed 70,000 galaxies in the Cosmological Evolution Survey (COSMOS), where some of the most powerful telescopes on Earth and space have imaged a 2-square-degree patch of sky in the constellation Sextans. The COSMOS team found that internal processes, such as black hole and stellar feedback, quenched most star formation until about 8 billion years ago. Since then, external forces have come to the fore. This result isn't surprising, since star formation and quasar activity were reaching their peaks in the early universe, whereas most clusters took longer to fully form and influence galaxies.

Further evidence for the importance of internal feedback processes in the early universe comes from research led by Sandro Tacchella (Swiss Federal Institute of Technology, Zurich) in 2015. Using the Hubble Space Telescope and the Very Large Telescope, Tacchella and colleagues mapped the distribution of old and new stars in 22 young elliptical galaxies that existed about 10 billion years ago, discovering that their star formation was ending from the inside out, rather than being brought to an end by external processes.

While the identity of the galactic killer may vary in time and space, it's the galaxies themselves that get the last laugh: They're capable of coming back to life, albeit in subdued fashion, hundreds of millions of years after the cosmic crime has been committed. The interplay between quenching mechanisms is going to be crucial to a greater understanding of how galaxies evolve.

KEITH COOPER, a British freelance science journalist, was editor of Astronomy Now magazine from 2006 to 2015. You can follow him via @21stCenturySETI on Twitter.

Green Valley Galaxies

Amidst fertile blue spirals and elderly red ellipticals exist a small group of in-between galaxies that aren't quite one or the other. These galaxies lie within the "green valley" on a color-magnitude diagram, which, a bit like a Hertzsprung-Russell diagram, plots galaxies' colors against their luminosities. The green valley is nestled between the swarm of blue galaxies and the curve rich with red-and-dead galaxies, implying that the green galaxies represent a brief intermediate stage.

"The green valley galaxies are where feedback is happening now," Geach says. Something is quenching these galaxies, whether it be black hole feedback, stellar outflows, or ram-pressure stripping. This stage of a galaxy's life might be relatively brief, which explains why green galaxies are so rare. "If you can map out where the green valley galaxies are and what their properties are, it will help pin down that evolutionary transition," says Geach.

Caption: FACING DISSOLUTION As NGC 4921, a typical "green" galaxy, plunges deep into the Coma Cluster, it rams into thin, hot gas. Gradually, this ram-pressure stripping will tear the gas from its star-forming spiral arms, transforming it into a red-and-dead galaxy.

Caption: A BLACK DEATH The supermassive black hole ensconced in a galaxy's center may ultimately be responsible for its demise.

Caption: RED & DEAD Both elliptical (left) and lenticular (right) galaxies largely contain red, aging stars. Although lenticulars are dusty and disk-shaped, like many spiral galaxies, they're more like ellipticals in terms of their star formation. Ellipticals are often found in galaxy clusters' centers, while lenticulars more often turn up in clusters' outer regions.

Caption: COMA CLUSTER The hot gas between galaxies in a cluster such as Coma (pictured here) will strip away the gas inside the galaxies that might have formed stars, leaving them aging and sterile.

Caption: A GALACTIC FOUNTAIN A bubble of hot gas rises from the core of a spiral galaxy--perhaps driven by star formation.

Caption: STARBURST WIND A millimeter-wavelength image from the IRAM Plateau de Bure Interferometer shows cool gas flowing out of a small galaxy at speeds up to 2 million mph. Winds from regions of intense star formation appear to be the culprit.

Caption: BLOWING BUBBLES A star 45 times the mass of our Sun produces powerful winds to create this ionized bubble. Stellar particles and radiation sweep into the interstellar medium, heating and pushing aside the cooler gas. (The star, the bright purple source at 10 o'clock, appears offset from the center because its winds encountered denser gas on one side than on the other.)

Caption: BLACK HOLE ANNIHILATION This artist's impression portrays a feeding black hole and the powerful wind it generates, which may heat and blow out a galaxy's potentially star-forming gas.

Caption: FERMI BUBBLES Two giant bubbles on either side of the Milky Way's disk, seen in microwaves, X-rays, and gamma-rays (pictured here), point to a violent event in our galaxy's past: either the ancient antics of the now-quiescent supermassive black hole or a previous burst of star formation and stellar feedback.

Caption: SPEEDING STARBIRTH Astronomers found stars forming within the galactic wind pouring out of the galaxy IRAS F23128-5919, depicted here in an artist's conception. The stars, forming at a rate of 15 Suns' worth per year, are flying out of the galaxy along with the outflow, though at a somewhat slower pace.

Caption: HOT HALO A huge X-ray-emitting halo of gas surrounds the Milky Way Galaxy and many other spiral and elliptical galaxies.

Caption: REBIRTH An artist depicts cold gas clumps that have condensed out of hot intergalactic surroundings to rain back onto a galaxy's supermassive black hole, such as was observed in the brightest galaxy of the cluster Abell 2597. The clumps are fueling both star formation and black hole activity.
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Title Annotation:CSI: GALAXY
Author:Cooper, Keith
Publication:Sky & Telescope
Date:Jul 1, 2017
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