From Discover Magazine: “Hubble Reveals New Evidence for Controversial Galaxies Without Dark Matter”


From Discover Magazine

October 21, 2019
Jake Parks

This new and incredibly deep image from Hubble shows the dim and diffuse galaxy NGC 1052-DF4. New research presents the strongest evidence yet that this strange galaxy is basically devoid of dark matter. (Credit: NASA/ESA/STScI/S. Danieli et al.)

Astronomers have all but confirmed the universe has at least one galaxy that’s woefully deficient in dark matter. The new finding not only indicates that galaxies really can exist without dark matter, but also raises fundamental questions about how such oddball galaxies form in the first place.

The research, posted October 16 on the preprint site arXiv, used Hubble’s keen eye to take new, deep images of the ghostly galaxy NGC 1052-DF4 (or DF4 for short). Equipped with fresh observations, the researchers identified the bizarre galaxy’s brightest red giant stars (called the Tip of the Red Giant Branch, or TRGB). Because TRGB stars all shine with the same true brightness when viewed in infrared, the only thing that should affect how bright they appear is their distance.

So, by identifying the galaxy’s TRGB and using that to determine DF4’s distance, the new data essentially confirms the galaxy is located some 61 million light-years away. And according to the researchers, this essentially debunks other studies that claim DF4 is much closer and therefore contains a normal amount of dark matter.

“I think this is definitive,” co-author Pieter van Dokkum of Yale University told Astronomy via email.

The Debate Over Galaxies Without Dark Matter

Over the last few years, there’s been a controversy brewing in the astronomical community. In 2018, van Dokkum and his team stumbled upon a ghostly galaxy, nicknamed DF2, that seemed to lack any significant amount of dark matter [Nature]. And because dark matter is thought to account for about 85 percent of all matter in the universe, the apparent discovery of the first galaxy without the elusive substance raised a lot of eyebrows.

One such skeptic was Ignacio Trujillo of the Instituto de Astrofisica de Canarias. Intrigued by the extraordinary claim of a galaxy without dark matter, Trujillo and his team quickly carried out their own analysis of DF2. Based on a variety of methods, Trujillo’s team determined that DF2 was actually much closer than van Dokkum’s team claimed — some 42 million light-years away rather than 61 million light-years. This, Trujillo argued in a 2019 study [MNRAS], meant that DF2 wasn’t as strange as initially thought, and instead hides about as much dark matter as you would expect from your average, run-of-the-mill galaxy.

But then, just six days later, van Dokkum’s team published yet another study [The Astrophysical Journal Letters]identifying a second galaxy, named DF4, that was located about the same distance away as DF2 and likewise lacked dark matter. Yet again, Trujillo and his colleagues went about calculating their own distance to DF4. Based on the Hubble data available at the time, the non-dark-matter camp identified what they thought was DF4’s TRGB. But according to the newly presented Hubble data — which picked up many more, much fainters stars — Trujillo’s team may have misidentified the TRGB.

“In the new data, there really is no ambiguity,” says study author Shany Danieli of Yale University. “We think the new data really rule out the [the closer distance derived by Trujillo’s group]. The TRGB is generally seen as definitive, as its physics is well understood.”

What Does a Galaxy Without Dark Matter Mean?

If these latest results hold up to the scrutiny that’s likely to come, then discovering the first (and possibly second) galaxy without dark matter would fundamentally change our understanding of how we think galaxies form and evolve.

“[DF4 and DF2] point to an alternative channel for building galaxies — and they even raise the question whether we understand what a galaxy is,” van Dokkum says. Right now, he says, we think that galaxies begin with dark matter, which is how they’re able to gravitationally attract the massive amounts of gas and dust needed to kick-start star formation.

“The thing is, we have no idea how star formation would proceed in the absence of dark matter,” van Dokkum says. “All we can say is that there must have been very dense gas early on in their history,” otherwise, the galaxies couldn’t create new stars.

But is this latest distance determination to DF4 really robust enough to start exploring the implications of finding a galaxy without dark matter?

“Yes, that’s our hope. We’d love to move to discuss what these galaxies mean, rather than whether our measurements were correct,” Danieli says.

“That said,” she added, “we fully agree with everyone that ‘extraordinary claims require extraordinary evidence”‘

See the full article here .


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From Discover Magazine: “Some Volcanoes Create Undersea Bubbles Up to a Quarter Mile Wide”


From Discover Magazine

October 18, 2019
Meeri Kim

A plume of steam flows upward from Bogoslof volcano, a partially submerged volcano that created giant underwater bubbles when it erupted in 2017. (Credit: Dave Withrow, Alaska Volcano Observatory)

As a geophysicist at the Alaska Volcano Observatory, John Lyons spends much of his days trying to decipher the music of volcanic eruptions. Sensitive microphones scattered across the Aleutian Arc — a chain of over 80 volcanoes that sweeps westward from the Alaskan peninsula — eavesdrop on every explosion, tremor and burp.

In 2017, the partially submerged volcano Bogoslof erupted, sending clouds of ash and water vapor as high as 7 miles above sea level and significantly disrupting air traffic in the area. Throughout the nine months that the volcano remained active, the observatory’s microphones picked up a strange, low-and-slow melody that repeated over 250 times.

“Instead of happening very fast and with high frequencies, which is typical for explosive eruptions, these signals were really low frequency, and some of them had periods up to 10 seconds,” said Lyons.

The source of the odd sounds remained a mystery for months, until one of Lyons’ colleagues stumbled upon a striking description of the ocean’s surface during a 1908 Bogoslof eruption, observed from a Navy ship. As reported in a 1911 issue of The Technical World Magazine, officers reported seeing a “gigantic dome-like swelling, as large as the dome of the capitol at Washington [D.C.].” The dome shrank and grew until finally culminating in “great clouds of smoke and steam … gradually growing in immensity until the spellbound spectators began to fear they would be engulfed in a terrific cataclysm.”

Lyons and his colleagues wondered if the low-frequency signals they heard could correspond to huge bubbles of gas forming just under the surface of the ocean. They modeled the sounds as overpressurized gas bubbles near the water-air interface, inspired by studies of magmatic bubbles that formed at the air-magma interface of Italy’s Stromboli volcano, which emitted similar signals but of shorter duration.

Their results, published in the journal Nature Geoscience on Monday, suggest that submerged volcanic explosions can indeed produce Capitol dome-sized bubbles — and according to their calculations, these would be considered on the smaller side. The bubble diameters from the 2017 Bogoslof eruption were estimated to range from 100 to 440 meters (328 to 1,444 feet), with the largest stretching more than a quarter-mile across.

“It’s hard to imagine a bubble so big, but the volumes of gas that we calculated to be inside the bubbles are similar to the volumes of gas that have been calculated for [open air] explosions,” said Lyons. “Take the big cloud of gas and ash that’s emitted from a volcano and imagine sticking that underwater. It has to come out somehow.”

The researchers propose that gargantuan bubbles would arise from the unique interaction between cold seawater and hot volcanic matter. As magma begins to ascend from the submarine vent, the seawater rapidly chills the outer layer, producing a gas-tight cap over the vent. This rind of semicooled lava eventually pops like a champagne cork as a result of the pressure in the vent, releasing the gases trapped underneath as a large bubble. The bubble in the water grows larger and eventually pokes out into the air. After a few rounds of expansion and contraction, the bubble breaks, releasing the gas and producing eruption clouds in the atmosphere.

The low-frequency sounds come from the bubble alternately growing and shrinking as it attempts to find an equilibrium between the expansion of the gas inside and the constriction of the shell, made up of mostly seawater and volcanic ash. The findings represent the first time such activity has been recorded with infrasound monitoring, which detects sound waves traveling in the air below the threshold of human hearing. Researchers are increasingly turning to the technique as a way to supplement traditional seismic data and gain more insight into eruption dynamics.

“I find the work groundbreaking and impactful,” said Jeffrey B. Johnson, a geophysicist at Boise State University in Idaho who was not involved in the study. “Giant bubbles which defy the imagination are able to oscillate and produce sound that you can record several kilometers away.”

Aside from the 1908 Bogoslof eruption, two other recorded observations match this phenomenon of giant bubbles emerging from the sea: the 1952-53 eruption of the Myojin volcano in Japan and the 1996 eruption of the Karymsky volcano in Russia. A report on the latter event describes “a rapidly rising, dark grey, smooth-surfaced bulbous mass of expanding gas and pyroclasts, probably maintained by surface tension within a shell of water.” The bubble grew to an estimated height of 450 meters above the sea surface.

To witness these bubbles in real life is a challenge, since submerged volcanoes are often remote and surrounded by lots of ocean — not to mention, one’s timing has to be perfect. But Lyons hopes to follow up on this work by studying the dynamics of similar systems that are more approachable and directly observable, such as geysers or mud pots. He envisions listening in on the sounds coming from these types of bubbles to check the validity of certain assumptions they had to make in their model, such as the viscosity of the water.

See the full article here .


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From Discover Magazine: “The Cosmos’ Most Powerful Magnets May Form When Stars Collide”


From Discover Magazine

October 14, 2019
Jake Parks

These snapshots of two merging stars in action show the overall strength of the magnetic field in color (yellow is more magnetic), as well as the magnetic field lines (hatching). The stars on the left, which don’t have very strong magnetic fields, are just about to merge into a more massive and magnetic star (right). According to new research, such mergers can dramatically bolster the strength of the final star’s magnetic field. (Credit: F. Schneider et al./Nature volume 574, pages 211–214 (2019))

More than 60 years ago, astronomers realized about 10 percent of massive stars have powerful magnetic fields bursting from their surfaces. But the exact origins of these magnetic fields —which can reach hundreds to thousands of times the strength of the Sun’s — has so far remained a mystery.

The answer, it turns out, may be due to a collision between two normal stars.

A team of scientists recently used cutting-edge simulations to uncover an evolutionary path they think explains the formation of extremely magnetic stars. And as a cherry on top, their findings may also shed light on the origins of a slew of other astronomical oddities. These mysteries include magnetars (a rare type of hyper-magnetic neutron star), blue stragglers (massive stars that appear too young for their age), and maybe even enigmatic cosmic events like fast radio bursts and super-luminous supernovae.

The research was published October 9 in the journal Nature.

Magnetic mergers

When two stars collide, it sends their surfaces spinning and simultaneously kicks off enormous amounts of turbulence. This dramatically boosts the final star’s magnetic field.

As the star spins, its inner layers rotate faster than its outer layers — a process called differential rotation. Running through and connecting each of these layers are magnetic field lines, Fabian Schneider of Heidelberg University and author of the new study told Astronomy. Because each layer rotates at a different speed, the magnetic field lines connecting the layers get twisted and tangled up, Schneider says. This serves to amplify the overall strength of the magnetic field.

“Now comes the turbulence,” Schneider explained. During a merger event, stellar material gets violently sloshed around. This turbulence further stirs the magnetic field lines, exponentially increasing the star’s magnetism.

But the new research doesn’t just describe how colliding stars can form insanely magnetized stars, it also may explain the origins of a bizarre class of strange objects called blue stragglers.

Blue straggler stars within the globular cluster M55 are circled in this color-magnitude diagram, which plots the color and overall brightness of stars of the same age. Middle-aged, main-sequence stars fall along the thick band spanning from the lower-right to the center of this image. When they evolve to red giants, they start climbing up the thin band to the upper-right. Rejuvenated blue stragglers are stars so massive (and therefore short-lived) that they seem like they should already be on the red giant branch, but they instead form their own extended population along the main sequence. (Credit: B.J. Mochejska, J. Kaluzny (CAMK), 1-m Swope Telescope)

Carnegie Institution 1-meter Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena. near the north end of a 7 km (4.3 mi) long mountain ridge. Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile


Making a blue straggler

Blue stragglers are a unique class of stars that masquerade as stars younger than they truly are. These “rejuvenated” stars are much hotter — making them bluer — and brighter than your average main-sequence (or middle-aged) star of a similar apparent age.

But what is the fountain of youth that keeps blue stragglers looking so fresh? A leading theory is that merging with another star will do the trick. And this new research supports that notion.

First off, typical main-sequence stars power themselves by fusing hydrogen into helium in their cores. But when the hydrogen in their cores runs out, they move on to fusing concentric shells of hydrogen around their now-inert cores. This causes the star to balloon up into a red giant, moving it off the main-sequence and into the so-called red giant branch.

But if two main-sequence stars collide, their material gets mixed together. The resulting merged product now has a restocked reservoir of hydrogen in its core, which allows it to chug along as a more massive — yet still main-sequence — blue straggler instead of evolving into a red giant.

“This just means that post-merger stars have more nuclear fuel to then live longer,” Schneider says. “In other words, their internal clock has been set back.”

But that only makes the newly formed star appear younger. “The point is simply that the blue straggler could have lived for a long time as lower-mass stars and then merged to become this more massive blue straggler,” Schneider says. “It’s high mass fooling us into thinking it must be younger.”

Scientists first suggested a collision between two stars could generate strong magnetic fields more than a decade ago. “But until now, we weren’t able to test this hypothesis because we didn’t have the necessary computational tools,” said co-author Sebastian Ohlmann of the computing center at the Max Planck Society near Munich in a press release.

But thanks to the powerful AREPO simulation code, the researchers were finally able to show that two merging stars, which originally lacked much magnetism, can join forces and create a new, highly magnetize star with a face lift.

According to the study’s abstract, “This can explain the properties of the magnetic ‘blue straggler’ star τ Sco,” which is located less than 500 light-years away in the constellation Scorpius. Because τ Sco has an apparent age of less than five million years, while its birth cluster appears to be closer to 10 million years old, the researchers think the oddball star may be a prime example of what stellar mergers can produce.

But that’s not all.

The study’s abstract goes on to state: “Such massive blue straggler stars seem likely to be the progenitors of magnetars, perhaps giving rise to some of the enigmatic fast radio bursts observed, and their supernovae may be affected by their strong magnetic fields.”

Magnetars are a rare breed of neutron stars with absurdly powerful magnetic fields that reach some 5 quadrillion (one quadrillion is 1,000 trillions) times stronger than Earth’s. “Magnetars are thought to have the strongest magnetic fields in the universe,” said co-author Friedrich Röpke of the Heidelberg Institute for Theoretical Studies in a press release.

“We are suggesting that magnetars could be the natural end product of [main-sequence] and probably also pre-[main-sequence] mergers,” Schneider says. “The biggest and yet-unsolved question is whether the magnetic field produced in the merger can survive up to the supernova stage, and then whether the magnetic field is indeed maintained in the forming neutron star when the core of the star collapses.”

“This still needs to be seen,” Schneider says, “but I think our suggestion is a very promising channel to finally understand the origin of magnetars and their strong magnetic fields.”

See the full article here .


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From Discover Magazine: “The Milky Way’s Supermassive Black Hole Erupted With a Violent Flare a Few Million Years Ago”


From Discover Magazine

October 9, 2019
Erika K. Carlson

A flare erupted from the Milky Way’s center some 3.5 million years ago. While Earth wouldn’t be in any danger if it happened today, the light would be clearly visible. (Credit: James Josephides/ASTRO 3D)

Astronomers believe supermassive black holes probably lurk in the centers of most large galaxies. These gargantuan black holes can gather swirling disks of material around them as their gravity attracts stars and gases. In some cases, these disks can emit vast amounts of light and even shoot huge jets of matter into space. The center of such an eventful galaxy is called an active galactic nucleus, or AGN.

Our own Milky Way seems to have a relatively calm center, but astronomers suspect this wasn’t always the case.

Some clues suggest that a flare of energetic radiation burst from our galaxy’s center within the last few million years. In a new study, a team of researchers now describe another piece of evidence that the Milky Way burped out such a flare, with the research also pointing to the supermassive black hole in our galaxy’s center, called Sagittarius A*, or Sgr A*, as being responsible.

The team estimated this event occurred about 3.5 million years ago, give or take a million years. That would mean the Milky Way’s center transitioned from an active to a quiet phase pretty recently in Earth’s history, possibly when early human ancestors were roaming the planet.

The flare would have been visible to the naked eye, shining about 10 times fainter than the full moon across a broad spectrum of light wavelengths.

“It would look like the cone of light from a movie projector as it passes through a smoky theater,” University of Sydney astrophysicist and lead study author Jonathan Bland-Hawthorn said in an email.

The researchers describe their findings in an upcoming paper in The Astrophysical Journal.

Following the Trail

Clues to the Milky Way’s active history include giant bubbles of gas ballooning out from the disk of the galaxy. The bubbles, which emit high-energy X-ray and gamma-ray radiation, could have formed when jets of matter shot out from the galaxy’s center.

The new piece of evidence comes from examining a stream of gas that arcs around the Milky Way. This stream is like a trail that two dwarf galaxies, called the Large and Small Magellanic Clouds, leave as they orbit the Milky Way. The research team studied ultraviolet light coming from this gas trail, called the Magellanic Stream.

The characteristics of the UV light indicate that gases in some sections of the stream are in an excited state. Only a very energetic event, like a beam of radiation from an active galactic nucleus, could have done this, according to Bland-Hawthorn. This means that our own home galaxy had an active galactic nucleus phase in the past.

“I think AGN flickering is what goes on for all of cosmic time,” Bland-Hawthorn said via email. “All galaxies are doing this” — like volcanoes that can lie quietly for long stretches of time but suddenly erupt.

Learning more about the central black hole of our galaxy is an exciting area of research, he added.

“I think Sgr A* is the future of astrophysics, like searching for life signatures around planets,” Bland-Hawthorn said. “I am excited by what we will learn over the next 50 years.”

See the full article here .


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From Discover Magazine: “Giant Bubbles Spotted Rushing Out from Milky Way’s Center”


From Discover Magazine

September 11, 2019
Mara Johnson-Groh

The MeerKAT telescope is superimposed on a radio image of the Milky Way’s center. Radio bubbles extend from between the two nearest antennas to the upper right corner, with filaments running parallel to the bubbles. (Credit: South African Radio Astronomy Observatory/MeerKAT)

The Milky Way is blowing bubbles. Two giant radio bubbles, extending out from the galaxy for over 1,400 light years, were just discovered in X-ray data. Astronomers think the bubbles started forming a few million years ago due to some type of cataclysmic event near the galaxy’s central supermassive black hole.

The bubbles’ location also closely matches the range of over 100 narrow, magnetized filaments of radio emissions that stretch for tens of light years in length. First discovered 35 years ago, these filaments’ origins have remained a mystery, but the bubbles’ discovery may now provide an answer.

“The filaments have been a mystery for a long time,” said Ian Heywood, astronomer at the University of Oxford and lead author on the new discovery. He says their results hint that the event that created the bubbles could have also produced high-energy charged particles that created the filaments.

The symmetry of the bubbles billowing above and below the galaxy suggests they were formed by an extremely energetic explosion near the supermassive black hole at the center of the Milky Way. The most likely explanation is a flare up in the black hole’s activity as it gobbled up extra nearby material and burped out other particles and radiation. The bubbles could also have been created by an extreme burst in star formation that sent a shock wave across the galactic center. Or possibly, it was a combination of both events.

The discovery, published on September 11 in the journal Nature, used the MeerKAT telescope, a radio telescope with 64 antennas, at the South African Radio Astronomy Observatory (SARAO) in South Africa.

SKA SARAO Meerkat telescope(s), 90 km outside the small Northern Cape town of Carnarvon, SA

Astronomers there were taking some of the first science images with the new telescope, looking at the radio emissions of the central galactic region, when they made the surprising discovery.

“These enormous bubbles have until now been hidden by the glare of extremely bright radio emission from the center of the galaxy,” said Fernando Camilo of SARAO in Cape Town, and a co-author on the paper, in a press release.

The astronomers were specifically looking at a type of radio emission called synchrotron radiation. This type or radiation is created when relativistic electrons — those traveling at nearly the speed of light — encounter strong magnetic fields, which imparts a particular signature on the light. Astronomers often use this type of radiation to pinpoint highly energetic regions in space.

The new discovery isn’t the first giant bubble seen escaping from the Milky Way. In 2010, astronomers discovered two similar giant bubbles of gamma ray radiation blossoming above and below the galaxy, extending a combined length of 50,000 light-years. Now known as the Fermi bubbles, the origin of these balloons of radiation is still unexplained, but likely linked to the galaxy’s central supermassive black hole. The astronomers on this latest research think that the new radio bubbles they’ve discovered may have been caused by a smaller but similar event.

“These fascinating radio bubbles provide a new window into understanding recent activity at the galactic center,” Andrew Fox, astronomer at the Space Telescope Science Institute, in Baltimore, Maryland, who was not involved with the new research, said via email. “Other observations taken across the electromagnetic spectrum have revealed evidence for a burst of activity several million years ago, and these new observations provide another clue. Taken together, the results show that the Milky Way blows bubbles on different scales.”

By connecting the origin location of the bubbles to the central black hole region of the galaxy, astronomers are starting to learn more about the processes in this dynamic region. It may also help them learn about events unfolding in other galaxies. Evidence for giant gamma ray bubbles, like the Fermi bubbles, have also been seen outside the Milky Way in its nearest neighbor, the Andromeda galaxy.

See the full article here .


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From Discover Magazine: “Massive Clouds Colliding in Space Could be Birthing Huge Stars”


From Discover Magazine

September 4, 2019
Mara Johnson-Groh

This star-forming region is one of many in M33 that’s birthing new stars from massive clouds of dust and gas. (Credit: ESA/Hubble and NASA)

Astronomers have witnessed a rare event: the birth of massive stars 2.73 million light-years away in the Triangulum Galaxy (Messier 33). At the center of two giant colliding gas clouds are some 10 young stars with masses tens of times that of the Sun. Their discovery indicates that such cloud-cloud collisions are a main pathway to creating giant stars in the nearby universe, which could help answer the long-standing question of how big stars form.

Cosmic Collision

High-mass stars — those at least eight times the mass of the Sun — are the celebrities of galaxies. Although they’re relatively rare, they produce most of a galaxy’s visible light. They also strongly influence the environment around them through the radiation they release during their lifetimes and the heavy elements they scatter upon their explosive deaths. Their formation, however, remains debated.

New research submitted to the Publications of the Astronomical Society of Japan uses the Atacama Large Millimeter/submillimeter Array to study two giant clouds in Messier 33.

The Triangulum Galaxy, Messier 33, via The VLT Survey Telescope (VST) at ESO’s Paranal Observatory in Chile. This beautifully detailed image of the galaxy Messier 33. This nearby spiral, the second closest large galaxy to our own galaxy, the Milky Way, is packed with bright star clusters, and clouds of gas and dust. This picture is amongst the most detailed wide-field views of this object ever taken and shows the many glowing red gas clouds in the spiral arms with particular clarity.

ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

The clouds are 190,000 and 240,000 times more massive than the Sun, respectively, and contain molecules such as molecular hydrogen and carbon dioxide. The two clouds collided at supersonic speeds around 500,000 years ago. (Here, “supersonic” means faster than the speed of sound in the clouds’ environment. In dense regions of space, the speed of sound can be a few miles per second or more; on Earth at sea level, the speed of sound is just over 1,100 feet [340 meters] per second).

The researchers looked specifically for signatures of carbon monoxide, which can be easily seen in radio observations, to chart the denser filamentary structures in the clouds. They also looked for a specific signature of hydrogen that indicates the presence of massive stars. At the center of the collision, they found 10 objects that appear to be young, massive stars. That makes it highly likely that the collision caused changes in the clouds’ gas that made it collapse to form the stars.

Go Big

Massive stars, which are harder to form than smaller stars, aren’t seen everywhere low-mass star formation occurs. So, the question is: Why not?

Astronomers think massive star formation must require some sort of additional triggering mechanism, such as cloud-cloud collisions, strong winds blown off active stars, expanding gas heated by other massive stars or shockwaves sent out by exploding supernovae. But until recently, there was scant observational evidence supporting cloud-cloud collisions. This study, however, now bolsters that option as a way to form massive stars.

“We have a number of different ideas of how massive star formation is initiated,” says Harold Yorke, an astronomer at the NASA Ames Research Center in Mountain View, California. “We know that molecular clouds are turbulent, so you would suspect massive stars could form in those conditions.”

“Recently, there has been a lot of observational, theoretical evidence of the cloud-to-cloud collision as the formation mechanism of massive stars,” says Toshikazu Onishi at the Osaka Prefecture University in Osaka, Japan, and co-author on the new study. “This paper provides the first observational evidence of [cloud-to-cloud collision] for massive star formation in the [Triangulum Galaxy].”

See the full article here .


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From Discover Magazine: “How Galaxies Live, Breathe and Die”


From Discover Magazine

August 28, 2019
Ann Finkbeiner

Gas glows white, lit by a stellar nursery, in this view of a region within the Large Magellanic Cloud, the Milky Way’s largest satellite galaxy. Most cosmic gas is not so visible and lies outside of galaxies — in halos surrounding galaxies and in the vast spaces in between. Yet the gas determines galactic life cycles. (Credit: ESA/Hubble & NASA)

NASA/ESA Hubble Telescope

Most of what astronomers know about the universe comes from what they can see. So their ideas have been prejudiced toward stars and galaxies, which are bright. But most of the regular matter in the universe is in the form of gas, which is dim. Gas called the intergalactic medium fills the space between galaxies; the gas of the circumgalactic medium surrounds galaxies more closely. The gas in both places regulates the birth, life and death of the galaxies, and holds a detailed history of the universe. Only lately have astronomers been able to detect it.

Shortly after its birth, the universe was filled with gas, mostly hydrogen. Over time, here and there, gravity pulled the gas into clouds which turned into galaxies and in which stars ignited. Stars shine by thermonuclear burning of the gas; of those that die in explosions, some blow the gas back out of the galaxies. Out in intergalactic space, the gas cools and gets denser, until gravity pulls it back into the galaxy where new stars form. The process repeats: Gravity condenses gas into galaxies and stars, stars blow up and kick the gas out, gravity cycles the gas back in and makes new stars.

In time, any given galaxy begins to run out of recyclable gas. Without gas, it can’t form new stars; the old stars live out their lives and die, and eventually the galaxy dies too. Galaxies sit in a bath of gas, the medium from which they were born and which fuels them. The galaxies breathe gas in and out, and their stars burn until their gas is gone.

Within a galaxy, relatively dense gases fuel star birth. Just outside, the gases thin in the circumgalactic medium, and become even less dense farther out into the intergalactic medium. Astronomers were unable to study the gases between galaxies until the 1960s, when they began to gather light from distant quasars, filtered through the gas, for spectral analysis. In the last decade they have turned their focus to the circumgalactic medium. (Credit: J Tumlinson et al./AR Astronomy and Astrophysics 2017;ESO/M. Kornmesser)

This is theory. The problem with verifying it has been that astronomers’ instruments could barely detect signs of gas, let alone map its comings and goings. With more sensitive instruments and dogged surveys, astronomers now know more. Convincing evidence suggests that the intergalactic medium is rich in gas, which fills the universe and seeds galaxies. Less-convincing and sometimes puzzling evidence in the circumgalactic medium shows that galaxies live by recycling gas into and out of stars. And astronomers have only preliminary evidence supporting arguments for how galaxies might run out of gas, stop forming stars and die.

Connecting Gas and Galaxies

Part of the problem has been that, though galaxies and gas are intrinsically related, the astronomers studying one didn’t talk to those looking at the other. Historically, astronomers studying galaxies, which were easier to see, were in a separate community from those studying gas, which was harder. The arrangers of scientific meetings, says Charles Steidel of the California Institute of Technology, who studied gas, would “put us on the last day when the galaxy people went home, before we could tell them about the rest of the universe.”

In 1989, Steidel used a technique (pioneered by his mentor at Caltech, Wallace Sargent) that allowed gas to be observed at distances at which galaxies couldn’t be seen. He collected enough evidence to argue that he’d found, as had others, gas between galaxies, out in the intergalactic medium. He also found evidence that clouds of gas in the vicinity of those otherwise invisible galaxies showed signs of having once been inside the galaxies, further linking the gas between galaxies with the galaxies themselves. When he wrote his doctoral thesis, he carefully put both “intergalactic medium” and “galaxies” in the title. “Once I finished my degree,” he says, “my goal has been to connect galaxies with gas.”

By 2013 when Steidel’s student Gwen Rudie, now at the Carnegie Observatories in Pasadena, California, wrote her own doctoral thesis, observational techniques had improved enough that at the same distances as Steidel’s gas clouds, she could find the previously invisible galaxies.

The galaxies were young, forming stars furiously and using gas fast. She found that the gas immediately around these galaxies, in the circumgalactic medium, was a thousand times denser than the average of gas in the intergalactic medium; like others, she also found signs of gas flowing out of galaxies.

By now, gas and galaxies were inextricably connected, and the study of galaxies now commonly includes the study of the gas around and between them, out of which they’re created and by which they live.

The Intergalactic Medium: Making Galaxies Out of Gas

Galaxies shine, gas barely glows. Gas becomes visible when it sits in front of something bright — most notably quasars, the cores of extremely distant and extraordinarily brilliant galaxies — and absorbs its light. To astronomers analyzing the light that reaches Earth, the gas shows up as dark lines in the spectra of the quasars’ light. The pattern of the dark absorption lines held a surprising amount of information, including the distance (and so the age) of the gas: It was visible at distances vastly greater, and therefore at times vastly earlier, than normal galaxies then were. Because spectra also reveal the gas’s chemical components, density, temperature, and motion toward or away from Earth, for the last 50 years quasar absorption line studies have remained one of the best ways to study cosmic gas.

Most noticeable in the quasars’ spectra were crowds of dark absorption lines at distances reaching back to the early universe and packed so closely together that they looked, says Charles Danforth, then at the University of Colorado, Boulder, “like tree trunks, boom, boom, boom.”

The trees were called the Lyman alpha forest — the gas absorbing the light was hydrogen in a specific transition between states called Lyman alpha — and showed a young universe full of airy hydrogen clouds.

By the mid-1990s, writes Matthew McQuinn of the University of Washington in the 2016 Annual Review of Astronomy and Astrophysics, astronomers had come to understand the Lyman alpha forest as gas between the earliest galaxies — the intergalactic medium.

The intergalactic medium has been around from early on: The Lyman alpha forest begins when the universe is around a billion years old. “Run the Lyman alpha forest forward” in simulations, says McQuinn, “and it looks like today’s intergalactic medium.”

Gas clouds show up as dark absorption lines in the spectrum of a quasar’s light, which can be analyzed to better understand the distance and nature of the gas. At high resolutions, the absorption lines show up at a range of wavelengths as distinct “trees” in what was termed the Lyman alpha forest. (Credit: M. Rauch/AR Astronomy and Astrophysics 1998)

The intergalactic medium of the young universe accounted for 98 percent of its regular matter: “People usually think of the universe as the stuff that lights up,” says Molly Peeples of the Space Telescope Science Institute (STScI) in Baltimore, but the quasar absorption line studies show that in the gas outside the stars and galaxies are “most of the atoms of the universe.”

Even in the young universe, however, the gas is not uniform. Mostly it’s cold, between 100 and 1,000 kelvins. But scattered patches of the intergalactic medium are hot, reaching 20,000 kelvins or more — evidence of stars turning on and galaxies forming.

The intergalactic medium is also not pure hydrogen: It is salted sparingly with elements heavier than hydrogen, created when stars blow up and die. The intergalactic medium is “clumpy,” says Michael Shull of the University of Colorado, Boulder, in places where gravity has pulled slightly denser gas into even denser clumps.

Despite the pockets of hot gas, the intergalactic medium is generally cooling, says Anson D’Aloisio at the University of California, Riverside, “because the universe is expanding.” With time, on average, the gas has also thinned out: “As you go toward today,” says Jason Prochaska at the University of California, Santa Cruz, “you can see by eyeball in the spectra, you can see the forest thins.”

This ancient, clumpy, cooling, rarefied intergalactic medium, says Prochaska, “is a pretty well understood entity” that holds a convincing picture of when and from what galaxies emerged.

The Circumgalactic Medium: Regulating Galaxies’ Lives

In the quasar spectra data, the Lyman alpha forest’s hydrogen clouds were just the most rarefied and chemically the purest. Scientists found other clouds, too, that were denser and sprinkled with heavier elements that astronomers call metals — such as carbon, oxygen, silicon, iron and magnesium. Astronomers reasoned that because these metals are made only by stars, and because all stars are in galaxies, then these metal-rich, denser clouds must be somehow associated with galaxies. They classified the types of clouds into a little zoo: Denser, more metallic clouds were called Lyman limit systems, and the densest clouds with higher metallicities were called damped Lyman alpha systems. The systems looked like a progression — the Lyman alpha forest through the Lyman limit to the damped Lyman alpha systems — of gas closer to and more intimately associated with galaxies.

Confirmation of these ideas had to wait for more sensitive instruments and for the beginning, at least 10 years ago, of painstaking and systematic surveys still using quasar absorption lines. Researchers showed (to no one’s surprise) that, in the maturing universe, if the Lyman alpha forest gas was the intergalactic medium, the Lyman limit and damped Lyman alpha systems were the circumgalactic.

One survey, the Keck Baryonic Structure Survey (KBSS), grew out of Steidel and company’s mission of connecting gas with galaxies. The KBSS team chose the 15 brightest quasars and found in their absorption lines evidence of 5,000 galaxies. Within those, the team looked for gas around galaxies from 10 billion to 11 billion years ago. A few billion years after the universe began, this was a time when stars were forming furiously — “cosmic noon,” astronomers call it.

Another large survey, using the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope, was called COS-Halos. COS-Halos was essentially KBSS for nearby galaxies; it began with 44 local galaxies — both active ones still forming stars and quiescent ones — whose surrounding gas was pierced by the sight lines to quasars.

Together the surveys characterized wholesale the density, temperature, and metallicity of galaxies’ circumgalactic media. The circumgalactic gas was up to 1,000 times denser than gas in the intergalactic medium, and ranged in temperature from cooler than the intergalactic medium to much, much hotter, from 10,000 to 1 million kelvins. And the closer to the host galaxy, the more metallic the gas.

There’s no agreement on where the intergalactic medium ends and the circumgalactic medium begins. “It’s nomenclature,” Peeples says.

Her colleague Jason Tumlinson, also of STScI, concurs: “The arguments about the boundary are all human. Nature has stuff crossing any boundary you can set. What was once in the intergalactic medium will be in the circumgalactic medium, and what’s in the circumgalactic medium will make it back out into the intergalactic medium.”

That is, though the gas in the intergalactic and circumgalactic media changes with time and proximity to a galaxy, it’s still all the same gas. And in flowing between the two, somehow or other, it keeps galaxies alive. “What’s not understood,” Prochaska says, “is the astrophysics of how the intergalactic medium fuels the circumgalactic medium and the galaxies.”

One possible scenario of this fueling flow, called galactic recycling, is simple: Gas falls into galaxies and fuels stars, then is blown back out, then falls back in to fuel more stars. Gathering evidence to back the scenario is painstaking and so far inconclusive. Infalling streams of gas are hard to see — they come into galaxies as narrow rivers — though some observers think they’ve seen them.

But, says Crystal Martin of the University of California, Santa Barbara, “the inflow signal often overlaps with the signal from the galaxy itself,” that is, the infall can be hard to see against the galaxy.

On the other hand, observers commonly detect outflows, gas heavy in metals flowing in wide swaths out of the galaxies. “Essentially every spectrum we take of a star-forming galaxy has evidence of winds being driven out of the galaxy,” says Rudie.

No one knows for sure what could be driving the outflows — maybe supernova explosions, or the massive jets shot out of the chaos around black holes, or the winds from hot stars. Nor does anyone know whether the gas is recycling locally between the galaxy and the circumgalactic medium, or whether it circulates more widely with the intergalactic medium; evidence exists for both scenarios.

What is not a scenario but shown by hard evidence is that at some point, a galaxy runs out of fuel and dies — a process called “quenching.” Astronomers have known for nearly two decades, when the Sloan Digital Sky Survey classified galaxies into two general categories, that galaxies with lots of gas and actively forming stars are blue, and those with little gas and dying stars are red. Most galaxies are either blue or red, with almost nothing in between.

SDSS Telescope at Apache Point Observatory, near Sunspot NM, USA, Altitude 2,788 meters (9,147 ft)

Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

If galaxies are either living or dying, and if they die by running out of gas to make new stars, it means that however galaxies die, they run out of gas fast. How that happens remains unclear. Finding evidence for recycling has proved difficult, and finding evidence for a quenching mechanism is so far impossible. The circumgalactic medium should hold that evidence, but in fact observations have only made the problem more intractable.

COS-Halos found gas around red, dead galaxies that is bound gravitationally to the galaxies and at 10,000 to 100,000 kelvins — which should be cool enough to fall into them. But it doesn’t. Scientists have proposed that something somehow shuts off the infalling gas, or something else heats it up so it’s too feisty to fall in.

Whatever the answer is, it’s going to be found in the circumgalactic medium. Jessica Werk, at the University of Washington and on the COS-Halos team, is putting together a survey that will increase the number of red galaxies observed by a factor of 10. “A lot of the questions,” she says, “come down to what happens to galaxies that stop star formation and how that plays out in the circumgalactic medium.”

Re-creating Galactic Birth

So far, what observers have found doesn’t add up to a coherent story of how galaxies are born, live and die. Stories are theory’s job, and in astronomy, theory often comes in the form of computer simulations. Theorists put together gravity, hydrodynamics, regular matter that shines and dark matter that doesn’t, and let the simulation re-create the evolution of galaxies. Then they compare the simulated galaxies with real ones: shape, rates of star formation, assumed method of quenching, rates of outflows, evidence of infalls, temperatures, density and metallicity. At present, the simulations run on two scales, the large intergalactic and smaller circumgalactic; no one simulation can cover both.

Galaxies are not distributed evenly throughout the universe, but their distribution can’t be understood without also understanding the role of gas. Here, a still from a simulation shows one of the largest structures identified in the universe, a supercluster of galaxies, voids and galactic filaments called the BOSS Great Wall. (Credit: Max Planck Institute for Astrophysics/Wikimedia Commons)

Max Planck Institute for Astrophysics

The simulations help astronomers interpret their current observations, or suggest new ones. For instance, in theorist Molly Peeples’ simulation, metals show up unexpectedly, far outside the circumgalactic medium, so observer Charles Danforth can be a little more confident of his observations of metals out in the intergalactic medium.

In simulations, “infalling cold gas is unambiguous,” says Crystal Martin, but it’s not obvious to observers like her. So her group looks specifically for cold, low-pressure gas in the circumgalactic medium that moves slowly enough and with enough drag that it should spiral in to the galaxy. Most simulations show the intergalactic medium containing pockets of gas at warm-to-hot temperatures, called WHIM, that no observer has yet convincingly seen. “I love simulators, they’re the best,” says Werk, “but I’m not sure their universe is the real one.”

Real or not, the simulations (several of them, done by separate groups) are the clearest visualizations of how gas might have made galaxies, and they’re gorgeous. Here’s what they look like: Begin in a 200 million-year-old universe, before galaxies and stars. The gas has been cooling but is still very hot, around 100,000 kelvins; it looks like an uneven fog, clearing in places, thickening in others. Eventually, in the thickest places, stars form.

When the universe reaches an age of 500 million years, the cooling, condensing gas gravitationally falls in on itself into sheets; then the sheets narrow into splotchy filaments. The clearings in the spaces between grow larger and blanker. At around a billion years, the filaments intersect with other filaments and a network grows. At 1.5 billion years, gas runs down the filaments and at some nodes puddles up and forms into galaxies, huge and white-hot, heated to between 10 million and 100 million kelvins by shock waves and explosions from dying stars.

By 2 billion years, supermassive black holes at the galaxies’ centers and more exploding stars send shocks flooding into the intergalactic medium. At 3.5 billion years, within the spreading shock fronts are little knots of galaxies. The galaxies collect the intergalactic gas into their own circumgalactic media, and enrich it with metals splashed into it by exploding supernovae.

By 7 billion years, the intergalactic medium has noticeably thinned: Its fraction of all matter has fallen from 95 percent to 80 percent. At 10 billion years, the galaxies and circumgalactic media are more metallic, the filaments are ropier and still hot, the clearings are larger, blacker and cold.

And now, at present, 13.8 billion years after the universe began, only 60 percent of the gas remains in the intergalactic medium; the rest is in the circumgalactic media and in galaxies. The galaxies are strung around voids, looking like the lit-up interstates and cities of a dark fly-over country.

Stars have shot metals all over the place, both out into the circumgalactic medium and within the galaxy, ready to be reprocessed into other stars. New stars coalesce out of the metallic gas along with dust. Around them form protoplanetary disks, which here and there condense into planets, on one of which is us. “Every atom in your body,” says Werk, “cycled through the intergalactic medium and the circumgalactic medium.” So this history is a story, she says, not only about galaxies but also “about our cosmic origins.”

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