In a tidal disruption event, an unfortunate star passes too close to a dormant supermassive black hole and gets torn apart by tidal forces, feeding the black hole for a short time. Astronomers use distinctive observational signatures to detect these events, but they are not seeing nearly as many tidal disruption events as theory says they should.

When a black hole tears a star apart, the star's material is stretched out into what's known as a tidal stream. That stream continues on a trajectory around the black hole, with roughly half the material eventually falling back on the black hole, whipping around it in a series of orbits. Where those orbits intersect each other, the material smashes together and circularizes, forming a disk that then accretes onto the black hole.

Astronomers don't observe anything until after the tidal streams collide and the material begins to accrete onto the black hole. At that point, they observe a sudden peak in luminosity, which then gradually decreases as the tail end of what's left of the star accretes and the black hole's food source eventually runs out.

So why have astronomers only been observing about a tenth as many tidal disruption events (TDEs) as theory predicts they should see? By studying the structure of tidal streams in TDEs, Guillochon and Ramirez-Ruiz have found a potential reason, and the culprit is general relativity.

"It is an effect of general relativity that is modulating the digestion process of the black hole, so the digestion rate depends strongly on the mass of the black hole," Ramirez-Ruiz said.

The researchers ran a series of simulations of tidal disruption events around black holes of varying masses and spins to see what form the resulting tidal streams take over time. They found that precession of the tidal stream due to the black hole's gravitational effects changes how the stream interacts with itself, and therefore what astronomers observe. Some cases behave as expected for what's currently considered a "typical" event, but some do not.

For cases where the relativistic effects are small (such as black holes with masses less than a few million solar masses), the tidal stream collides with itself after only a few windings around the black hole, quickly forming a disk — but the disk forms far from the black hole, so it takes a long time to accrete. As a result, the observed flare can take 100 times longer to peak than typically expected, so these sources may not be identified as tidal disruption events.

Furthermore, for cases where the black hole is both massive and has a spin greater than a certain value (about 20 percent of its maximum allowed spin), the tidal stream doesn't collide with itself right away. Instead, it can take many windings around the black hole before the first intersection. In these cases, it may potentially be years after a star gets ripped apart before the material accretes and astronomers are able to observe the event.

The Daily Galaxy via UC Santa Cruz

Working in a cave complex deep beneath South Africa's Malmani dolomites, an international team of scientists has brought to light an unprecedented trove of hominin fossils -- more than 1,500 well-preserved bones and teeth -- representing the largest, most complete set of such remains found to date in Africa. The discovery, cached in a barely accessible chamber in a subterranean labyrinth not far from Johannesburg, adds a new branch to the human family tree, a creature dubbed Homo naledi.

Skeletal fossils of Homo naledi are pictured above in the Wits bone vault at the Evolutionary Studies Institute at the University of the Witwatersrand, Johannesburg, South Africa, on Sept. 13, 2014. The fossils are among nearly 1,700 bones and teeth retrieved from a nearly inaccessible cave near Johannesburg. The fossil trove was created, scientists believe, by Homo naledi repeatedly secreting the bodies of their dead companions in the cave. Analysis of the fossils -- part of a project known as the Rising Star Expedition -- was led in part by paleoanthropologist John Hawks, professor of anthropology at the University of Wisconsin-Madison.

So far, parts of at least 15 skeletons representing individuals of all ages have been found and the researchers believe many more fossils remain in the chamber. It is part of a complex of limestone caves near what is called "The Cradle of Humankind," a World Heritage Site in Gauteng province well known for critical paleoanthropological discoveries of early humans, including the 1947 discovery of 2.3 million-year-old Australopithecus africanus.

"We have a new species of Homo, with all of its interesting characteristics," says Hawks, one of the leaders of a team that painstakingly retrieved the fossils under excruciatingly cramped and difficult conditions. "We now have the biggest discovery in Africa for hominins."

The find was reported today (Sept. 10, 2015) with the publication of two papers in the open access journal eLife by a group led by paleoanthropologist Lee R. Berger of the University of Witwatersrand. The expedition to retrieve the fossils and their subsequent analysis was supported by the National Geographic Society.

With a small head and brain, hunched shoulders, powerful hands and thin limbs, Homo naledi was built for long-distance walking, says Hawks, an expert on early humans. Fully grown, it stood about five feet tall, was broad chested, walked upright and had a face, including a smile that was probably more human than apelike. Powerful hands imply it was also a climber.

The fossils have yet to be dated. The unmineralized condition of the bones and the geology of the cave have prevented an accurate dating, says Hawks. "They could have been there 2 million years ago or 100,000 years ago, possibly coexisting with modern humans. We don't yet have a date, but we're attempting it in every way we can."

So far, the remains of newborns to the aged have been retrieved from the cave and the researchers expect that many more bones remain in the chamber, which is nearly 100 feet underground and accessible only after squeezing, clambering and crawling 600 feet to a large chamber where the brittle fossils cover the floor.

"We know about every part of the anatomy, and they are not at all like humans," notes Hawks, who co-directed the analysis of the fossils. "We couldn't match them to anything that exists. It is clearly a new species."

The astonishing find was made initially by amateur cavers and thought at the time to be a single hominin skeleton. The fossils were retrieved by a band of diminutive paleoanthropologists, all women, recruited for their size.

"Naledi" means star in the Sesotho language and is a reference to the Rising Star cave system that includes the chamber, known as the Dinaledi Chamber, where the fossils were found. The circuitous and difficult passage to the chamber narrows at one point to a bare seven inches.

In addition to identifying an entirely new species in the genus Homo, the collections of fossils, which bear no marks from predators or scavengers, are strong evidence that Homo naledi was deliberately depositing its dead in the cave, according to Hawks, a UW-Madison professor of anthropology.

"We think it is the first instance of deliberate and ritualized secreting of the dead," says Hawks. "The only plausible scenario is they deliberately put bodies in this place."

The cave, according to Hawks, was likely more accessible to Homo naledi than it is today for modern humans. Geochemical tests, however, show that the cave was never open to the surface, raising intriguing questions about the behavior and technologies available to the creatures.

"We know it was not a death trap," says Hawks, referring to natural features like hidden sinkholes that sometimes trap and doom creatures over long periods of time. "There are no bones from other animals aside from a few rodents. And there are no marks on the bones from predators or scavengers to suggest they were killed and dragged to the chamber. We can also rule out that it was a sudden mass death."

Instead, Hawks, Berger and their colleagues believe the chamber was something like a repository. "It seems probable that a group of hominins was returning to this place over a period of time and depositing bodies," Hawks explains, adding that the supposition is akin to discovering similar behavior in chimpanzees. "It would be that surprising."

The way the bodies are arranged and their completeness suggests they were carried to the cave intact. "The bodies were not intentionally covered and we're not talking about a religious ceremony, but something that was repeated and repeated in the same place. They clearly learned to do this and did it as a group over time. That's cultural. Only humans and close relatives like Neandertals do anything like this."

So far, no other organic materials or evidence of fire have been found in the cave complex.

Dating the fossils remains a key problem to solve, says Hawks. "We depend on the geology to help us date things, and here the geology isn't much like other caves in South Africa. And the fossils don't have anything within them that we can date. It's a problem for us."

One hope, he says, is finding the remains of an animal that may have been a contemporary of Homo naledi. The fossils are embedded in a matrix of soft sediment and there are layers that remain unexcavated.

According to Hawks, years of work remain at the site and to document and analyze all of the materials excavated from the Dinaledi Chamber. Plans, he says, include bringing many new technologies to bear on analyzing the fossils to help determine diet, rate of aging and where on the landscape the creatures may have been from.

The project to excavate the fossils and the May 2014 scientific workshop to analyze them were supported by the National Geographic Society, the South African National Research Foundation, the Gauteng Provincial Government, and Wits University. The Wisconsin Alumni Research Foundation also provided support, as did the Texas A&M College of Liberal Arts Seed Grant Program.

Berger led the Rising Star expedition as National Geographic Explorer-in-Residence. The expedition involved an international team of scientists, including six "underground astronauts" who descended into the Dinaledi chamber to excavate and retrieve the fossils of Homo naledi.

JOURNEY TO AMAZING HOMININ DISCOVERY STARTED ON FACEBOOK

"We need perhaps three or four individuals with excellent archaeological/paleontological excavation skills for a short term project that may kick off as early as November 1st 2013 and last the month if all logistics go as planned. The catch is this - the person must be skinny and preferably small. They must not be claustrophobic, they must be fit, they should have some caving experience, climbing experience would be a bonus. They must be willing to work in cramped quarters, have a good attitude and be a team player."

Although University of Wisconsin-Madison anthropology graduate student Alia Gurtov didn't quite know why she was raising her hand in response, she fit the bill. Slight in stature with a background in paleoanthropology, including work at famed Olduvai Gorge in Tanzania, Gurtov took a flyer.

"I just applied," Gurtov recalls. "In a very bizarre email, I gave my dimensions and my CV" and soon after, the Wisconsin researcher was on a flight to South Africa -- where she would play a lead role in discovering and retrieving the largest, most complete store of hominin fossils on a continent famous for such discoveries.

A bantam frame was needed because Gurtov and five other small women scientists were about to enter a difficult and dangerous subterranean labyrinth. Going down into the South African earth, Gurtov and her diminutive colleagues would clamber, crawl, climb and, finally, drop into a space where they could enter a chamber last visited hundreds of thousands or perhaps millions of years ago by creatures that, scientists believe, were repeatedly secreting their dead companions, protecting them from scavengers and, at the same time, creating an astonishing paleontological record of a lost member of the tribe of humanity, a hominin species dubbed Homo naledi.

"It was very, very narrow," recalls Gurtov. "There is a 7-inch chokepoint. The only way I could fit in is if I had my head turned to the side."

The grueling 20- to 25-minute commute from daylight to the chamber of fossils included technical climbing, where the researchers, aided by an experienced caver, donned harnesses to mount a feature called the "Dragon's Back." Gurtov and her colleagues were forced to navigate several "squeezes," including one 15-foot section called the "Superman Crawl," where forward progress required wriggling on one's belly with arms extended like the soaring Man of Steel. The final leg of the underground foray, "the worst chokepoint," involved slithering down a 12-foot, crag-studded chute in the dark.

The expedition, overseen by noted paleoanthropologist Lee Berger of the University of Witwatersrand, was organized to investigate what was believed to be a single hominin skeleton, first spotted by cavers.

"You're in this initial chamber and then you have to squeeze through a crevice that opens into the chamber where the skull was observed," Gurtov explains.

Entering for the first time, she recalls, was a solemn moment. "It had the feeling of a tiny cathedral. It was just so still and dynamic at the same time. There was a sense of ages. It was absolutely silent. The floor was covered in skeletal material."

"We knew there was a skull in there. We had no idea we were going to find more than that," says the Wisconsin researcher.

Typically at the site of a hominin discovery, the first thing examined are the teeth, because they are telltale and -- as the hardest materials in the body -- tend to preserve well, says Gurtov, who spends much of her time at Tanzania's Olduvai Gorge looking at the teeth of animals preyed upon by ancient hominins. "We saw pretty quick that we had more teeth than would fit in a single mouth," Gurtov happily notes. When not in the cave, she lent her expertise on dentition to the group assembled to analyze the fossils.

Over that month in the field, Gurtov and her companions returned many times to the chamber, now known as the Dinaledi Chamber. Working 6- to 8-hour shifts below ground, the team recovered more than 1,500 pieces of bone by clearing the floor and excavating one small section of soft cave floor sediment, a puzzle box of fossils. "It was like pick-up sticks," says Gurtov. "You couldn't get one thing out without excavating something else. The sheer volume of material makes it unique."

The fossils were excavated according to the forensic techniques prescribed by paleontology, often with a toothpick, from the cave's clumpy, wet sediment. Wrapped in paper and nested in plastic containers, the bones of Homo naledi were prepped for the journey out to another kind of vault where they would be measured, documented, studied and reconstructed to reveal an entirely new species of hominin.

The Daily Galaxy via University of Wisconsin-Madison

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Although the Universe may seem spacious most galaxies are clumped together in groups or clusters and a neighbor is never far away. But a rare galaxy, known as NGC 6503, has found itself in a lonely position, shown here at the edge of a strangely empty patch of space called the Local Void, a patch of space thought to be about 150 million light-years across that seems to be curiously devoid of galaxies. Astronomers using Hubble discovered that the emptiness of this region has quite an effect on the space around us -- the Milky Way is being strongly pulled away from it by the gentle but relentless tug of other nearby galaxies.

This galaxy does not just offer poetic inspiration; it is also the subject of ongoing research. The Hubble Legacy ExtraGalactic UV Survey (LEGUS) is exploring a sample of nearby galaxies, including NGC 6503, to study their shape, internal structure, and the properties and behaviour of their stars. This survey uses 154 orbits of time on Hubble; by contrast, a typical Hubble observing programme lasts from a few to a few tens of orbits.

NGC 6503 lies right on the edge of this void. It has an almost non-existent central bulge surrounded by a massive halo of gas. The galaxy's central region is a good example of something known as a "low ionisation nuclear emission region", or LINER. These are less luminous than some of the brightest galaxies. Emission from NGC 6503's heart is believed to be the result of a starved black hole that is only just being kept active, receiving a very small amount of infalling gas to keep its large appetite at bay.

A previous image of NGC 6503 was released as a Hubble Picture of the Week back in 2010, taken by Hubble's Advanced Camera for Surveys. However, this new image, taken using Hubble's Wide Field Camera 3 (WFC3), shows NGC 6503 in striking detail and with a richer set of colours. Bright red patches of gas can be seen scattered through its swirling spiral arms, mixed with bright blue regions that contain newly-forming stars. Dark brown dust lanes snake across the galaxy's bright arms and centre, giving it a mottled appearance.

Installed in 2009 during the final Hubble servicing mission, SM4, WFC3 covers a wide range of the spectrum, from the ultraviolet all the way through to the near-infrared. Compared with its predecessor, the Wide Field and Planetary Camera 2 (WFPC2), it offers improved resolution and a wider field of view, and has led to a large number of stunning Hubble images since its installation.

The Daily Galaxy via NASA/ESA Hubble Space Telescope

A nearby dwarf galaxy poses an intriguing mystery: How is it able to form brilliant star clusters without the dusty, gas-rich environments found in larger galaxies? The answer, astronomers believe, lies in densely packed and previously unrecognized nuggets of star-forming material sprinkled throughout the galaxy. An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) has discovered an unexpected population of compact interstellar clouds hidden within the nearby dwarf irregular galaxy Wolf—Lundmark—Melotte, more commonly known as WLM.

WLM is a relatively isolated dwarf galaxy located approximately 3 million light-years away on the outer edges of the Local Group: the collection of galaxies that includes the Milky Way, the Magellanic Clouds, Andromeda, M33, and dozens of smaller galaxies.

Irregular galaxies lack the distinctive shapes of spiral and elliptical galaxies. Dwarf irregulars, like WLM, are hundreds of times smaller than the larger variety and contain only a few hundred million stars instead of tens of billions. Though small, some are now known to harbor massive black holes at their centers.

These clouds, which are nestled within a heavy blanket of interstellar material, help explain how dense star clusters are able to form in the tenuous environs of a galaxy thousands of times smaller and far more diffuse than our own Milky Way.

Star clusters, like the Pleiades found in our own Milky Way galaxy, are made up of hundreds of stars. Others, like globular clusters, can contain hundreds of thousands to a few million stars. Though many stars in the Milky Way originally form in clusters, some – like the Sun – drift away from their stellar nurseries and move freely throughout their home galaxy. Stars in the largest and densest clusters, like those observed in WLM, remain relatively close together.

"For many reasons, dwarf irregular galaxies like WLM are poorly equipped to form star clusters," noted Monica Rubio, an astronomer with the University of Chile and lead author on a paper to appear in the scientific journal Nature. "These galaxies are fluffy with very low densities. They also lack the heavy elements that contribute to star formation. Such galaxies should only form dispersed stars rather than concentrated clusters, but that is clearly not the case."

By studying this galaxy with ALMA, the astronomers were able to locate, for the first time, compact regions that appear able to emulate the nurturing environments found in larger galaxies.

These regions were discovered by pinpointing the almost imperceptible and highly localized millimeter wavelength light emitted by carbon monoxide (CO) molecules, which are typically associated with star-forming interstellar clouds.

Earlier, an affiliated team of astronomers led by Deidre Hunter at the Lowell Observatory in Flagstaff, Ariz., first detected CO in the WLM galaxy with the single-dish Atacama Pathfinder Experiment (APEX) telescope. These initial, low-resolution observations could not resolve where the molecules reside, but they did confirm that WLM contains the lowest abundance of CO ever detected in any galaxy. This lack of CO and other heavy elements should put a serious damper on star formation, the astronomers note.

"Molecules, and carbon monoxide in particular, play an important role in star formation," said Rubio. "As gas clouds begin to collapse, temperatures and densities rise, pushing back against gravity. That's where these molecules and dust particles come to the rescue by absorbing some of the heat through collisions and radiating it into space at infrared and submillimeter wavelengths." This cooling effect enables gravity to continue the collapse until a star forms.

The problem previously was that in WLM and similar galaxies with very low abundances of heavy elements, astronomers simply didn't see enough of this material to account for the new star clusters they observed.

The reason the CO was initially so difficult to see, the researchers discovered, is that unlike in normal galaxies, the WLM clouds are very tiny compared to their overlying envelopes of molecular and atomic gas.

To become viable star factories, the concentrated CO clouds need these enormous envelopes of transitional gas to bear down on them, giving the cores of CO a high enough density to allow them to form a normal cluster of stars.

"Like a diver being squeezed at the bottom of a deep abyss, these bundles of star-forming gas are under tremendous pressure, even though the surrounding ocean of interstellar gas is much more shallow," said Bruce Elmegreen, a co-author on the paper and researcher at the IBM T.J. Watson Research Center in Yorktown Heights, N.Y. "By discovering that the carbon monoxide is confined to highly concentrated regions within a vast expanse of transitional gas, we could finally understand the mechanisms that led to the impressive stellar neighborhoods we see in the galaxy today."

Further studies with ALMA will also help determine the conditions that formed the globular clusters found in the halo of the Milky Way. Astronomers believe these much larger clusters may have originally formed in dwarf galaxies and later migrated to the halo after their host dwarf galaxies dispersed.

The Daily Galaxy via The National Radio Astronomy Observatory

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"The Earth's atmosphere contains oxygen because plants continuously produce it through photosynthesis. This abundant supply of oxygen allows life forms like animals to flourish. Oxygen had been thought to be an essential biomarker for life on extrasolar planets.

Until now, it had been thought that if a planet has oxygen, that must mean that some form of plants are producing it through photosynthesis. Therefore, it had been assumed that when searching for signs of life on habitable extrasolar planets, the presence of oxygen in the atmosphere could be considered a definitive biomarker. However, non-biological chemical reactions can also affect atmospheric compositions of extrasolar planets. Now, the research team led by Dr. Narita has shown that, abiotic oxygen produced by the photocatalytic reaction of titanium oxide, which is known to be abundant on the surfaces of terrestrial planets, meteorolites, and the Moon in the Solar System, cannot be discounted.

For a planet with an environment similar to the Sun-Earth system, continuous photocatalytic reaction of titanium oxide on about 0.05 % of the planetary surface could produce the amount of oxygen found in the current Earth's atmosphere. In addition, the team estimated the amount of possible oxygen production for habitable planets around other types of host stars with various masses and temperatures. They found that even in the least efficient production case of a low-temperature star, the photocatalytic reaction of the titanium oxide on about 3% of the planetary surface could maintain this level of atmospheric oxygen through abiotic processes. In other words, it is possible that a habitable extrasolar planet could maintain an atmosphere with Earth-like oxygen, even without organisms to perform photosynthesis.

"To search for life on extrasolar planets through astronomical observation, we need to combine the knowledge from various scientific fields and to promote astrobiology researches to establish the decisive signs of life," said Narita. "Although oxygen is still one of possible biomarkers, it becomes necessary to look for new biomarkers besides oxygen from the present result."

The image above shows Earth clouds captured by cosmonaut Oleg Artemyev from the International Space Station.

The Daily Galaxy via Astrobiology Center, National Institutes of Natural Sciences

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