Category: Asteroids & Comets

Astronomers Discover 83 Supermassive Black Holes In The Early Universe

Astronomers from Japan, Taiwan and Princeton University have discovered 83 quasars powered by supermassive black holes in the distant universe, from a time when the universe was less than 10 percent of its present age.

“It is remarkable that such massive dense objects were able to form so soon after the Big Bang,” said Michael Strauss, a professor of astrophysical sciences at Princeton University who is one of the co-authors of the study. “Understanding how black holes can form in the early universe, and just how common they are, is a challenge for our cosmological models.”

This finding increases the number of black holes known at that epoch considerably, and reveals, for the first time, how common they are early in the universe’s history. In addition, it provides new insight into the effect of black holes on the physical state of gas in the early universe in its first billion years. The research appears in a series of five papers published in The Astrophysical Journal and the Publications of the Astronomical Observatory of Japan.

Supermassive black holes, found at the centers of galaxies, can be millions or even billions of times more massive than the sun. While they are prevalent today, it is unclear when they first formed, and how many existed in the distant early universe. A supermassive black hole becomes visible when gas accretes onto it, causing it to shine as a “quasar.” Previous studies have been sensitive only to the very rare, most luminous quasars, and thus the most massive black holes. The new discoveries probe the population of fainter quasars, powered by black holes with masses comparable to most black holes seen in the present-day universe.

The research team used data taken with a cutting-edge instrument, “Hyper Suprime-Cam” (HSC), mounted on the Subaru Telescope of the National Astronomical Observatory of Japan, which is located on the summit of Maunakea in Hawaii. HSC has a gigantic field-of-view — 1.77 degrees across, or seven times the area of the full moon — mounted on one of the largest telescopes in the world. The HSC team is surveying the sky over the course of 300 nights of telescope time, spread over five years.

The team selected distant quasar candidates from the sensitive HSC survey data. They then carried out an intensive observational campaign to obtain spectra of those candidates, using three telescopes: the Subaru Telescope; the Gran Telescopio Canarias on the island of La Palma in the Canaries, Spain; and the Gemini South Telescope in Chile. The survey has revealed 83 previously unknown very distant quasars. Together with 17 quasars already known in the survey region, the researchers found that there is roughly one supermassive black hole per cubic giga-light-year — in other words, if you chunked the universe into imaginary cubes that are a billion light-years on a side, each would hold one supermassive black hole.

The sample of quasars in this study are about 13 billion light-years away from the Earth; in other words, we are seeing them as they existed 13 billion years ago. As the Big Bang took place 13.8 billion years ago, we are effectively looking back in time, seeing these quasars and supermassive black holes as they appeared only about 800 million years after the creation of the (known) universe.

It is widely accepted that the hydrogen in the universe was once neutral, but was “reionized” — split into its component protons and electrons — around the time when the first generation of stars, galaxies and supermassive black holes were born, in the first few hundred million years after the Big Bang. This is a milestone of cosmic history, but astronomers still don’t know what provided the incredible amount of energy required to cause the reionization. A compelling hypothesis suggests that there were many more quasars in the early universe than detected previously, and it is their integrated radiation that reionized the universe.

“However, the number of quasars we observed shows that this is not the case,” explained Robert Lupton, a 1985 Princeton Ph.D. alumnus who is a senior research scientist in astrophysical sciences. “The number of quasars seen is significantly less than needed to explain the reionization.” Reionization was therefore caused by another energy source, most likely numerous galaxies that started to form in the young universe.

The present study was made possible by the world-leading survey ability of Subaru and HSC. “The quasars we discovered will be an interesting subject for further follow-up observations with current and future facilities,” said Yoshiki Matsuoka, a former Princeton postdoctoral researcher now at Ehime University in Japan, who led the study. “We will also learn about the formation and early evolution of supermassive black holes, by comparing the measured number density and luminosity distribution with predictions from theoretical models.”

Based on the results achieved so far, the team is looking forward to finding yet more distant black holes and discovering when the first supermassive black hole appeared in the universe.

LAMP Instrument Sheds Light On Lunar Water Movement

Using the Southwest Research Institute-led Lyman Alpha Mapping Project (LAMP) aboard NASA’s Lunar Reconnaissance Orbiter (LRO), scientists have observed water molecules moving around the dayside of the Moon. A paper published in Geophysical Research Letters describes how LAMP measurements of the sparse layer of molecules temporarily stuck to the surface helped characterize lunar hydration changes over the course of a day.

Up until the last decade or so, scientists thought the Moon was arid, with any water existing mainly as pockets of ice in permanently shaded craters near the poles. More recently, scientists have identified surface water in sparse populations of molecules bound to the lunar soil, or regolith. The amount and locations vary based on the time of day. This water is more common at higher latitudes and tends to hop around as the surface heats up.

“This is an important new result about lunar water, a hot topic as our nation’s space program returns to a focus on lunar exploration,” said SwRI’s Dr. Kurt Retherford, the principal investigator of the LRO LAMP instrument. “We recently converted the LAMP’s light collection mode to measure reflected signals on the lunar dayside with more precision, allowing us to track more accurately where the water is and how much is present.”

Water molecules remain tightly bound to the regolith until surface temperatures peak near lunar noon. Then, molecules thermally desorb and can bounce to a nearby location that is cold enough for the molecule to stick or populate the Moon’s extremely tenuous atmosphere, or “exosphere,” until temperatures drop and the molecules return to the surface. SwRI’s Dr. Michael Poston, now a research scientist on the LAMP team, had previously conducted extensive experiments with water and lunar samples collected by the Apollo missions. This research revealed the amount of energy needed to remove water molecules from lunar materials, helping scientists understand how water is bound to surface materials.

“Lunar hydration is tricky to measure from orbit, due to the complex way that light reflects off of the lunar surface,” Poston said. “Previous research reported quantities of hopping water molecules that were too large to explain with known physical processes. I’m excited about these latest results because the amount of water interpreted here is consistent with what lab measurements indicate is possible. More work is needed to fully account for the complexities of the lunar surface, but the present results show that work is definitely worth doing!”

Scientists have hypothesized that hydrogen ions in the solar wind may be the source of most of the Moon’s surface water. With that in mind, when the Moon passes behind the Earth and is shielded from the solar wind, the “water spigot” should essentially turn off. However, the water observed by LAMP does not decrease when the Moon is shielded by the Earth and the region influenced by its magnetic field, suggesting water builds up over time, rather than “raining” down directly from the solar wind.

“These results aid in understanding the lunar water cycle and will ultimately help us learn about accessibility of water that can be used by humans in future missions to the Moon,” said Amanda Hendrix, a senior scientist at the Planetary Science Institute and lead author of the paper. “A source of water on the Moon could help make future crewed missions more sustainable and affordable. Lunar water can potentially be used by humans to make fuel or to use for radiation shielding or thermal management; if these materials do not need to be launched from Earth, that makes these future missions more affordable.”

The funding for this research came from NASA Goddard Space Flight Center’s LRO program office, including an LRO LAMP subcontract between SwRI and PSI, and the team received additional support from a NASA Solar System Exploration Research Virtual Institute (SSERVI) cooperative agreement.

New Surprises From Jupiter And Saturn

The latest data sent back by the Juno and Cassini spacecraft from giant gas planets Jupiter and Saturn have challenged a lot of current theories about how planets in our solar system form and behave.

The detailed magnetic and gravity data have been “invaluable but also confounding,” said David Stevenson from Caltech, who will present an update of both missions this week at the 2019 American Physical Society March Meeting in Boston.

“Although there are puzzles yet to be explained, this is already clarifying some of our ideas about how planets form, how they make magnetic fields and how the winds blow,” Stevenson said.

Cassini orbited Saturn for 13 years before its dramatic final dive into the planet’s interior in 2017, while Juno has been orbiting Jupiter for two and a half years.

Juno’s success as a mission to Jupiter is a tribute to innovative design. Its instruments are powered by solar energy alone and protected so as to withstand the fierce radiation environment.

Stevenson says the inclusion of a microwave sensor on Juno was a good decision.

“Using microwaves to figure out the deep atmosphere was the right, but unconventional, choice,” he said. The microwave data have surprised the scientists, in particular by showing that the atmosphere is evenly mixed, something conventional theories did not predict.

“Any explanation for this has to be unorthodox,” Stevenson said.

Researchers are exploring weather events concentrating significant amounts of ice, liquids and gas in different parts of the atmosphere as possible explanations, but the matter is far from sealed.

Other instruments on board Juno, gravity and magnetic sensors, have also sent back perplexing data. The magnetic field has spots (regions of anomalously high or low magnetic field) and also a striking difference between the northern and southern hemispheres.

“It’s unlike anything we have seen before,” Stevenson said.

The gravity data have confirmed that in the midst of Jupiter, which is at least 90 percent hydrogen and helium by mass, there are heavier elements amounting to more than 10 times the mass of Earth. However, they are not concentrated in a core but are mixed in with the hydrogen above, most of which is in the form of a metallic liquid.

The data has provided rich information about the outer parts of both Jupiter and Saturn. The abundance of heavier elements in these regions is still uncertain, but the outer layers play a larger-than-expected role in the generation of the two planets’ magnetic fields. Experiments mimicking the gas planets’ pressures and temperatures are now needed to help the scientists understand the processes that are going on.

For Stevenson, who has studied gas giants for 40 years, the puzzles are the hallmark of a good mission.

“A successful mission is one that surprises us. Science would be boring if it merely confirmed what we previously thought,” he said.

Crater Counts On Pluto, Charon Show Small Kuiper Belt Objects Surprisingly Rare

Using New Horizons data from the Pluto-Charon flyby in 2015, a Southwest Research Institute-led team of scientists have indirectly discovered a distinct and surprising lack of very small objects in the Kuiper Belt. The evidence for the paucity of small Kuiper Belt objects (KBOs) comes from New Horizons imaging that revealed a dearth of small craters on Pluto’s largest satellite, Charon, indicating that impactors from 300 feet to 1 mile (91 meters to 1.6 km) in diameter must also be rare.

The Kuiper Belt is a donut-shaped region of icy bodies beyond the orbit of Neptune. Because small Kuiper Belt objects were some of the “feedstock” from which planets formed, this research provides new insights into how the solar system originated. This research was published in the March 1 issue of the journal Science.

“These smaller Kuiper Belt objects are much too small to really see with any telescopes at such a great distance,” said SwRI’s Dr. Kelsi Singer, the paper’s lead author and a co-investigator of NASA’s New Horizons mission. “New Horizons flying directly through the Kuiper Belt and collecting data there was key to learning about both large and small bodies of the Belt.”

“This breakthrough discovery by New Horizons has deep implications,” added the mission’s principal investigator, Dr. Alan Stern, also of SwRI. “Just as New Horizons revealed Pluto, its moons, and more recently, the KBO nicknamed Ultima Thule in exquisite detail, Dr. Singer’s team revealed key details about the population of KBOs at scales we cannot come close to directly seeing from Earth.”

Craters on solar system objects record the impacts of smaller bodies, providing hints about the history of the object and its place in the solar system. Because Pluto is so far from Earth, little was known about the dwarf planet’s surface until the epic 2015 flyby. Observations of the surfaces of Pluto and Charon revealed a variety of features, including mountains that reach as high as 13,000 feet (4 km) and vast glaciers of nitrogen ice. Geologic processes on Pluto have erased or altered some of the evidence of its impact history, but Charon’s relative geologic stasis has provided a more stable record of impacts.

“A major part of the mission of New Horizons is to better understand the Kuiper Belt,” said Singer, whose research background studying the geology of the icy moons of Saturn and Jupiter positions her to understand the surface processes seen on KBOs. “With the successful flyby of Ultima Thule early this year, we now have three distinct planetary surfaces to study. This paper uses the data from the Pluto-Charon flyby, which indicate fewer small impact craters than expected. And preliminary results from Ultima Thule support this finding.”

Typical planetary models show that 4.6 billion years ago, the solar system formed from the gravitational collapse of a giant molecular cloud. The Sun, the planets and other objects formed as materials within the collapsing cloud clumped together in a process known as accretion. Different models result in different populations and locations of objects in the solar system.

“This surprising lack of small KBOs changes our view of the Kuiper Belt and shows that either its formation or evolution, or both, were somewhat different than those of the asteroid belt between Mars and Jupiter,” said Singer. “Perhaps the asteroid belt has more small bodies than the Kuiper Belt because its population experiences more collisions that break up larger objects into smaller ones.”

Using Stardust Grains, Scientists Build New Model For Nova Eruptions

What do tiny specks of silicon carbide stardust, found in meteorites and older than the solar system, have in common with pairs of aging stars prone to eruptions?

A collaboration between two Arizona State University scientists — cosmochemist Maitrayee Bose and astrophysicist Sumner Starrfield, both of ASU’s School of Earth and Space Exploration — has uncovered the connection and pinpointed the kind of stellar outburst that produced the stardust grains.

Their study has just been published in The Astrophysical Journal.

The microscopic grains of silicon carbide — a thousand times smaller than the average width of a human hair — were part of the construction materials that built the Sun and planetary system. Born in nova outbursts, which are repeated cataclysmic eruptions by certain types of white dwarf stars, the silicon carbide grains are found today embedded in primitive meteorites.

“Silicon carbide is one of the most resistant bits found in meteorites,” Bose said. “Unlike other elements, these stardust grains have survived unchanged from before the solar system was born.”

Violent birth

A star becomes a nova — a “new star” — when it suddenly brightens by many magnitudes. Novae occur in pairs of stars where one star is a hot, compact remnant called a white dwarf. The other is a cool giant star so large its extended outer atmosphere feeds gas onto the white dwarf. When enough gas collects on the white dwarf, a thermonuclear eruption ensues, and the star becomes a nova.

Although powerful, the eruption doesn’t destroy the white dwarf or its companion, so novae can erupt over and over, repeatedly throwing into space gas and dust grains made in the explosion. From there the dust grains merge with clouds of interstellar gas to become the ingredients of new star systems.

The Sun and solar system were born about 4.6 billion years ago from just such an interstellar cloud, seeded with dust grains from earlier stellar eruptions by many different kinds of stars. Almost all the original grains were consumed in making the Sun and planets, yet a tiny fraction remained. Today these bits of stardust, or presolar grains, can be identified in primitive solar system materials such as chondritic meteorites.

“The key that unlocked this for us was the isotopic composition of the stardust grains,” Bose said. Isotopes are varieties of chemical elements that have extra neutrons in their nuclei. “Isotopic analysis lets us trace the raw materials that came together to form the solar system.”

She added, “Each silicon carbide grain carries a signature of the isotopic composition of its parent star. This provides a probe of that star’s nucleosynthesis — how it made elements.”

Bose collected published data on thousands of grains, and found that nearly all the grains grouped naturally into three main categories, each attributable to one kind of star or another.

But there were about 30 grains that couldn’t be traced back to a particular stellar origin. In the original analyses, these grains were flagged as possibly originating in nova explosions.

But did they?

Making stardust

As a theoretical astrophysicist, Starrfield uses computer calculations and simulations to study various kinds of stellar explosions. These include novae, recurrent novae, X-ray bursts, and supernovae.

Working with other astrophysicists, he was developing a computer model to explain the ejected materials seen in the spectrum of a nova discovered in 2015. Then he attended a colloquium talk given by Bose before she had joined the faculty.

“I would not have pursued this if I hadn’t heard Maitrayee’s talk and then had our follow-up discussion,” he said. That drew him deeper into the details of nova eruptions in general and what presolar grains could say about these explosions that threw them into space.

A problem soon arose. “After talking with her,” Starrfield said, “I discovered our initial way of solving the problem was not agreeing with either the astronomical observations or her results.

“So I had to figure out a way to get around this.”

He turned to multidimensional studies of classical nova explosions, and put together a wholly new way of doing the model calculations.

There are two major composition classes of nova, Starrfield said. “One is the oxygen-neon class which I’ve been working on for 20 years. The other is the carbon-oxygen class which I had not devoted as much attention to.” The class designations for novae come from the elements seen in their spectra.

“The carbon-oxygen kind produce a lot of dust as part of the explosion itself,” Starrfield said. “The idea is that the nova explosion reaches down into the white dwarf’s carbon-oxygen core, bringing up all these enhanced and enriched elements into a region with high temperatures.”

That, he said, can drive a much bigger explosion, adding, “It’s really messy. It shoots out dust in tendrils, sheets, jets, blobs, and clumps.”

Starrfield’s calculations made predictions of 35 isotopes, including those of carbon, nitrogen, silicon, sulfur, and aluminum, that would be created by the carbon-oxygen nova outbursts.

It turned out that getting the right proportion of white dwarf core material and accreted material from the companion star was absolutely necessary for the simulations to work. Bose and Starrfield then compared the predictions with the published compositions of the silicon carbide grains.

This led them to a somewhat surprising conclusion. Said Bose, “We found that only five of the roughly 30 grains could have come from novae.”

While this may seem a disappointing result, the scientists were actually pleased. Bose said, “Now we have to explain the compositions of the grains that didn’t come from nova outbursts. This means there’s a completely new stellar source or sources to be discovered.”

And looking at the larger picture, she added, “We have also found that astronomical observations, computer simulations, and high-precision laboratory measurements of stardust grains are all needed if we want to understand how stars evolve. And this is exactly the kind of interdisciplinary science that the school excels at.”

Waves In Saturn’s Rings Give Precise Measurement Of Planet’s Rotation Rate

Saturn’s distinctive rings were observed in unprecedented detail by NASA’s Cassini spacecraft, and scientists have now used those observations to probe the interior of the giant planet and obtain the first precise determination of its rotation rate. The length of a day on Saturn, according to their calculations, is 10 hours 33 minutes and 38 seconds.

The researchers studied wave patterns created within Saturn’s rings by the planet’s internal vibrations. In effect, the rings act as an extremely sensitive seismograph by responding to vibrations within the planet itself.

Similar to Earth’s vibrations from an earthquake, Saturn responds to perturbations by vibrating at frequencies determined by its internal structure. Heat-driven convection in the interior is the most likely source of the vibrations. These internal oscillations cause the density at any particular place within the planet to fluctuate, which makes the gravitational field outside the planet oscillate at the same frequencies.

“Particles in the rings feel this oscillation in the gravitational field. At places where this oscillation resonates with ring orbits, energy builds up and gets carried away as a wave,” explained Christopher Mankovich, a graduate student in astronomy and astrophysics at UC Santa Cruz.

Mankovich is lead author of a paper, published January 17 in the Astrophysical Journal, comparing the wave patterns in the rings with models of Saturn’s interior structure.

Most of the waves observed in Saturn’s rings are due to the gravitational effects of the moons orbiting outside the rings, said coauthor Jonathan Fortney, professor of astronomy and astrophysics at UC Santa Cruz. “But some of the features in the rings are due to the oscillations of the planet itself, and we can use those to understand the planet’s internal oscillations and internal structure,” he said.

Mankovich developed a set of models of the internal structure of Saturn, used them to predict the frequency spectrum of Saturn’s internal vibrations, and compared those predictions with the waves observed by Cassini in Saturn’s C ring. One of the main results of his analysis is the new calculation of Saturn’s rotation rate, which has been surprisingly difficult to measure.

As a gas giant planet, Saturn has no solid surface with landmarks that could be tracked as it rotates. Saturn is also unusual in having its magnetic axis nearly perfectly aligned with its rotational axis. Jupiter’s magnetic axis, like Earth’s, is not aligned with its rotational axis, which means the magnetic pole swings around as the planet rotates, enabling astronomers to measure a periodic signal in radio waves and calculate the rotation rate.

The rotation rate of 10:33:38 determined by Mankovich’s analysis is several minutes faster than previous estimates based on radiometry from the Voyager and Cassini spacecraft.

“We now have the length of Saturn’s day, when we thought we wouldn’t be able to find it,” said Cassini Project Scientist Linda Spilker. “They used the rings to peer into Saturn’s interior, and out popped this long-sought, fundamental quality of the planet. And it’s a really solid result. The rings held the answer.”

The idea that Saturn’s rings could be used to study the seismology of the planet was first suggested in 1982, long before the necessary observations were possible. Coauthor Mark Marley, now at NASA’s Ames Research Center in Silicon Valley, subsequently fleshed out the idea for his Ph.D. thesis in 1990, showed how the calculations could be done, and predicted where features in Saturn’s rings would be. He also noted that the Cassini mission, then in the planning stages, would be able to make the observations needed to test the idea.

“Two decades later, people looked at the Cassini data and found ring features at the locations of Mark’s predictions,” Fortney said.

First Evidence Of Gigantic Remains From Star Explosions

Astrophysicists have found the first ever evidence of gigantic remains being formed from repeated explosions on the surface of a dead star in the Andromeda Galaxy, 2.5 million light years from Earth. The remains or “super-remnant” measures almost 400 light years across. For comparison, it takes just 8 minutes for light from the Sun to reach us.

A white dwarf is the dead core of a star. When it is paired with a companion star in a binary system, it can potentially produce a nova explosion. If the conditions are right, the white dwarf can pull gas from its companion star and when enough material builds up on the surface of the white dwarf, it triggers a thermonuclear explosion or “nova,” shining a million times brighter than our Sun and initially moving at up to 10,000 km per second.

Astrophysicists including Dr Steven Williams from Lancaster University in the UK examined the nova M31N 2008-12a in the Andromeda Galaxy, one of our nearest neighbours.

They used Hubble Space Telescope imaging, accompanied by spectroscopy from telescopes on Earth, to help uncover the nature of a gigantic super-remnant surrounding the nova. This is the first time such a huge remnant has been associated with a nova, and their research appears in Nature.

Dr Williams worked on Liverpool Telescope observations of the nova as well as helping to interpret the results.

He said: “This result is significant, as it is the first such remnant that has been found around a nova. This nova also has the most frequent explosions of any we know — once a year. The most frequent in our own Galaxy in only once every 10 years.

“It also has potential links to Type Ia supernovae, as this is how we would expect a nova system to behave when it is nearly massive enough to explode as a supernova.”

A Type Ia supernova is caused when the entire white dwarf is blown apart when it reaches a critical upper mass, rather than an explosion on its surface as in the case of the nova in this work. Type Ia supernovae are relatively rare. We have not observed one in our own Galaxy since Kepler’s supernova of 1604, named after the famous astronomer Johannes Kepler, who observed it shortly after it exploded and for the following year.

The team simulated how such a nova can create a vast, evacuated cavity around the star, by continually sweeping up the surrounding medium within a shell at the edge of a growing super-remnant.

The models show that the super-remnant — larger than almost all known remnants of supernova explosions — is consistent with being built up by frequent nova eruptions over millions of years.

Dr Matt Darnley from Liverpool John Moores University in the UK, who led the work, said: “Studying M31N 2008-12a and its super-remnant could help us to understand how some white dwarfs grow to their critical upper mass and how they actually explode as a Type Ia Supernova once they get there. Type Ia supernovae are critical tools used to work out how the universe expands and grows.”