Topic: Late November Astronomy Bulletin

[Clive]

NEPTUNE MOONS LOCKED IN STRANGE ORBITS
NASA

Even by the wild standards of the outer solar system, the strange orbits that carry Neptune's two innermost moons are unprecedented. Orbital dynamics experts are calling it a "dance of avoidance" performed by the tiny moons Naiad and Thalassa. The two are true partners, orbiting only about 1,850 kilometres apart. But they never get that close to each other; Naiad's orbit is tilted and perfectly timed. Every time it passes the slower-moving Thalassa, the two are about 3,540 kilometres apart. In this perpetual choreography, Naiad swirls around the ice giant every seven hours, while Thalassa, on the outside track, takes seven and a half hours. An observer sitting on Thalassa would see Naiad in an orbit that varies wildly in a zigzag pattern, passing by twice from above and then twice from below. This up, up, down, down pattern repeats every time Naiad gains four laps on Thalassa. Although the dance may appear odd, it keeps the orbits stable.  Astronomers refer to this repeating pattern as a resonance. There are many different types of 'dances' that planets, moons and asteroids can follow, but this one has never been seen before.

Far from the pull of the Sun, the giant planets of the outer solar system are the dominant sources of gravity, and collectively, they boast dozens upon dozens of moons. Some of those moons formed alongside their planets and never went anywhere; others were captured later, then locked into orbits dictated by their planets. Some orbit in the opposite direction their planets rotate; others swap orbits with each other as if to avoid collision.  Neptune has 14 confirmed moons. Neso, the farthest-flung of them, orbits in a wildly elliptical loop that carries it nearly 74 million kilometres away from the planet and takes 27 years to complete. Naiad and Thalassa are small and shaped like Tic Tacs, spanning only about 100 kilometres in length. They are two of Neptune's seven inner moons, part of a closely packed system that is interwoven with faint rings. It's thought that the original satellite system was disrupted when Neptune captured its giant moon, Triton, and that these inner moons and rings formed from the leftover debris. The unusual orbital pattern was discovered using analysis of observations by NASA's Hubble Space Telescope. The work also provides the first hint about the internal composition of Neptune's inner moons.  Researchers used the observations to compute their mass and, thus, their densities - which were close to that of water ice.


HYPER-FAST STAR EJECTED BY SUPERMASSIVE BLACK HOLE
RAS

Astronomers have observed an ultrafast star, travelling at a blistering 6 million km/h, ejected by the supermassive black hole at the heart at the Milky Way five million years ago. The discovery of the star, known as S5-HVS1, was made using the Southern Stellar Stream Spectroscopic Survey (S5). Located in the constellation of Grus, S5-HVS1 was found to be moving ten times faster than most stars in the Milky Way. The velocity of the discovered star is so high that it will inevitably leave the Galaxy and never return. Astronomers have wondered about high-velocity stars since their discovery only two decades ago. S5-HVS1 is unprecedented because of its high speed and relatively close passage to the Earth, 'only' 29,000 light-years away. With that information, astronomers could track its journey back into the centre of the Milky Way, where a 4-million-solar-mass black hole, known as Sagittarius A*, lurks. Astronomers have long suspected that black holes can eject stars with very high velocities. However, they never before had a clear association of such a fast star with the Galactic Centre.  This ejection happened at the time when humanity's ancestors were just learning to walk on two feet. Superfast stars can be ejected by black holes via the Hills Mechanism, proposed by astronomer Jack Hills thirty years ago.  Originally, S5-HSV1 lived with a companion in a binary system, but they strayed too close to Sagittarius A*. In the gravitational tussle, the companion star was captured by the black hole, while S5-HVS1 was thrown out at extremely high speed. This is the first clear demonstration of the Hills Mechanism in action. The discovery of S5-HVS1 was made with the 3.9-metre Anglo-Australian Telescope, coupled with observations from the European Space Agency's Gaia satellite that allowed the astronomers to reveal the full speed of the star and its journey from the centre of the Milky Way.


GALACTIC FOUNTAINS AND CAROUSELS
RAS

Scientists from Germany and the United States have unveiled the results of a newly-completed, state-of-the-art simulation of the evolution of galaxies.  TNG50 is the most detailed large-scale cosmological simulation yet. It allows researchers to study in detail how galaxies form, and how they have evolved since shortly after the Big Bang. For the first time, it reveals that the geometry of the cosmic gas flows around galaxies determines galaxies' structures, and vice versa. Astronomers running cosmological simulations face a fundamental trade-off: with finite computing power, typical simulations so far have been either very detailed or have spanned a large volume of virtual space, but have so far not been able to do both.  Detailed simulations with limited volumes can model no more than a few galaxies, making statistical deductions difficult. Large-volume simulations, in turn, typically lack the details necessary to reproduce many of the small-scale properties we observe in our own Universe, reducing their predictive power. The TNG50 simulation, which has just been published, manages to avoid this trade-off. For the first time, it combines the idea of a large-scale cosmological simulation -- a Universe in a box -- with the computational resolution of "zoom" simulations, at a level of detail
that had previously only been possible for studies of individual galaxies.  In a simulated cube of space that is more than 230 million light-years across, TNG50 can discern physical phenomena that occur on scales a million times smaller, tracing the simultaneous evolution of thousands of galaxies over 13.8 billion years of cosmic history. It does so with more than 20 billion particles representing dark (invisible) matter, stars, cosmic gas, magnetic fields, and supermassive black holes. The calculation itself required 16,000 cores on the Hazel Hen supercomputer in Stuttgart, working together, 24/7, for more than a year -- the equivalent of fifteen thousand years on a single processor, making it one of the most demanding astrophysical computations to date.

The first scientific results from TNG50 reveal unforeseen physical phenomena. Numerical experiments of this kind are particularly successful when you get out more than you put in. In the simulation, phenomena that had not been programmed explicitly into the simulation code could be seen.  Those phenomena emerge in a natural fashion, from the complex interplay of the basic physical ingredients of our model Universe. TNG50 features two prominent examples for this kind of emergent behaviour. The first concerns the formation of 'disc' galaxies like our own Milky Way. Using the simulation as a time machine to rewind the evolution of cosmic structure, researchers have seen how the well-ordered, rapidly rotating disc galaxies (which are common in our nearby Universe) emerge from chaotic, disorganised, and highly turbulent clouds of gas at earlier epochs.

As the gas settles down, newborn stars are typically found on more and more circular orbits, eventually forming large spiral galaxies -- galactic carousels. In practice, TNG50 shows that our own Milky Way galaxy with its thin disc is at the height of galaxy fashion: over the past 10 billion years, at least those galaxies that are still forming new stars have become more and more disc-like, and their chaotic internal motions have decreased considerably. The Universe was much messier when it was just a few billion years old! As these galaxies flatten out, researchers found another emergent phenomenon, involving the high-speed outflows and winds of gas flowing out of galaxies. That happened as a result of the explosions of massive stars (supernovae) and activity from supermassive black holes found at the heart of galaxies. Galactic gaseous outflows are initially also chaotic and flow away in all directions, but over time they begin to become more focused along a path of least resistance. In the late Universe, flows out of galaxies take the form of two cones, emerging in opposite directions -- like two ice-cream cones placed tip to tip, with the galaxy swirling at the centre. These flows of material slow down as they attempt to leave the gravitational well of the galaxy's halo of invisible -- or dark -- matter, and can eventually stall and fall back, forming a galactic fountain of recycled gas. That process redistributes gas from the centre of a galaxy to its outskirts, further accelerating the transformation of the galaxy itself into a thin disc: galactic structure shapes galactic fountains, and vice versa.


GALAXY DUPLICATED AT LEAST 12 TIMES
ESA/Hubble Information Centre

Astronomers using the NASA/ESA Hubble Space Telescope have observed a galaxy in the distant regions of the Universe which appears duplicated at least 12 times on the night sky. This unique sight, created by strong gravitational lensing, helps astronomers get a better understanding of the cosmic era known as the epoch of reionization. The galaxy, nicknamed the Sunburst Arc, is almost 11 billion light-years away from Earth and has been lensed into multiple images by a massive cluster of galaxies 4.6 billion light-years away. The mass of the galaxy cluster is large enough to bend and magnify the light from the more distant galaxy behind it. That process leads not only to a deformation of the light from the object, but also to a multiplication of the image of the lensed galaxy. In the case of the Sunburst Arc the lensing effect leads to at least 12 images of the galaxy, distributed over four major arcs. Hubble uses these cosmic magnifying glasses to study objects otherwise too faint and too small for even its extraordinarily sensitive instruments. The Sunburst Arc is no exception, despite being one of the brightest gravitationally lensed galaxies known.  The lens makes various images of the Sunburst Arc between 10 and 30 times brighter. It allows Hubble to view structures as small as 520 light-years across -- a rare detailed observation for an object that distant. This compares reasonably well with star-forming regions in galaxies in the local Universe, allowing astronomers to study the galaxy and its environment in great detail.

Hubble's observations showed that the Sunburst Arc is an analogue of galaxies which existed at a much earlier time in the history of the Universe -- a period known as the epoch of reionization -- an era which began 'only' 150 million years after the Big Bang. The epoch of reionization was a key era in the early Universe, one which ended the "dark ages", the epoch before the first stars were created when the Universe was dark and filled with neutral hydrogen. Once the first stars formed, they started to radiate light, producing the high-energy photons required to ionise the neutral hydrogen. This converted the intergalactic matter into the mostly ionised form in which it exists today. However, to ionise intergalactic hydrogen, high-energy radiation from those early stars would have had to escape their host galaxies without first being absorbed by interstellar matter. So far only a small number of galaxies have been found to "leak" high-energy photons into deep space. How such light escaped from the early galaxies remains a mystery. The analysis of the Sunburst Arc helps astronomers to
add another piece to the puzzle -- it seems that at least some photons can leave the galaxy through narrow channels in a gas-rich neutral medium.   This is the first observation of a long-theorised process. While this process is unlikely to be the main mechanism that led the Universe to become reionized, it may very well have provided a decisive push.
 

SCIENTISTS REFINE UNIVERSE EXPANSION
Clemson University

The concept of an expanding universe was advanced by the American astronomer Edwin Hubble (1889-1953). In the early 20th century, Hubble became one of the first astronomers to deduce that the Universe was composed of multiple galaxies. His subsequent research led to his most renowned discovery: that galaxies were moving away from each other at a speed in proportion to their distance. Hubble originally estimated the expansion rate to be 500 kilometres per second per megaparsec, with a megaparsec being equivalent to about 3.26 million light years. Hubble concluded that a galaxy two megaparsecs away from our galaxy was receding twice as fast as a galaxy only one megaparsec away. This estimate became known as the Hubble Constant, which proved for the first time that the Universe was expanding. Astronomers have been re-calibrating it -- with mixed results -- ever since. With the help of skyrocketing technologies, astronomers came up with measurements that differed significantly from Hubble's original calculations -- slowing the expansion rate down to between 50 and 100 kilometres per second per megaparsec. And in the past decade, ultra-sophisticated instruments, such as the Planck satellite, have increased the precision of Hubble's original measurements in relatively dramatic fashion. Now astronomers have compared the latest gamma-ray attenuation data from the Fermi Gamma-ray Space
Telescope and Imaging Atmospheric Cherenkov Telescopes to devise their estimates from extragalactic background light models. This novel strategy led to a measurement of approximately 67.5 kilometres per second per megaparsec.

Gamma rays are the most energetic form of light. Extragalactic background light (EBL) is a cosmic fog composed of all the ultraviolet, visible and infrared light emitted by stars or from dust in their vicinity. When gamma rays and EBL interact, they leave an observable imprint -- a gradual loss of flow -- that the scientists were able to analyze in formulating their hypothesis. A common analogy of the expansion of the universe is a balloon dotted with spots, with each spot representing a galaxy. When the balloon is blown up, the spots spread farther and farther apart. Some theorize that the balloon will expand to a particular point in time and then re-collapse. But the most common belief is that the Universe will continue to expand until everything is so far apart there will be no more observable light. At this point, the Universe will suffer a cold death. But this is nothing for us to worry about. If this happens, it will be trillions of years from now. But if the balloon analogy is accurate, what is it, exactly, that is blowing up the balloon? Matter -- the stars, the planets, even us -- is just a small fraction of the Universe's overall composition. The large majority of the Universe is made up of dark energy and dark matter. And astronomers believe it is dark energy that is 'blowing up the balloon. Dark energy is pushing things away from each other. Gravity, which attracts objects toward each other, is the stronger force at the local level, which is why some galaxies continue to collide. But at cosmic distances, dark energy is the dominant force.


HAYABUSA-2 PROBE HEADS FOR EARTH
JAXA

Japan's Hayabusa-2 probe has left its orbit around a distant asteroid and is heading for Earth, carrying samples that could shed light on the origins of the Solar System. In late 2020 the probe will bring back to Earth "carbon and organic matter" that will provide data as to "how the matter is scattered around the Solar System, why it exists on the asteroid and how it is related to Earth. The mission took the fridge-sized probe some 300 million kilometres from Earth, where it explored the asteroid Ryugu, whose name means "Dragon Palace" in Japanese???a reference to a castle at the bottom of the ocean in an ancient fable. In April, Hayabusa-2 fired an "impactor" into the asteroid to stir up materials that had not previously been exposed to the atmosphere. It then made a "perfect" touchdown on the surface of the asteroid to collect the samples that scientists hope will provide clues into what the Solar System was like at its birth some 4.6 billion years ago. The six-year mission, which cost around £250 million, had exceeded expectations but had to overcome a host of technical problems.  It took the probe three-and-a-half years to get to the asteroid but the return journey should be significantly shorter because Earth and Ryugu will be much closer due to their current positions. The probe is the successor to JAXA's first asteroid explorer "Hayabusa", which means falcon in Japanese. The earlier probe returned with dust samples from a smaller, potato-shaped asteroid in 2010 despite various setbacks during its epic seven-year odyssey, and was hailed as a scientific triumph. Under the current plan, Hayabusa-2 will continue its journey in space after dropping off its capsule to Earth, and might carry out another asteroid exploration.


NORTH KOREAN NUCLEAR WEAPON EQUIVALENT TO 17 HIROSHIMAS.
RAS

North Korea withdrew from the Treaty on the Non-Proliferation of Nuclear Weapons in 2003. It subsequently developed nuclear weapons, with five underground nuclear tests culminating in a suspected thermonuclear explosion (a hydrogen bomb) on 3 September 2017. Now a team of scientists have used satellite data to augment measurements of tests on the ground. The researchers find that the most recent test shifted the ground by a few metres, and estimate it to be equivalent to 17 times the size of the bomb dropped on Hiroshima in 1945. Conventional detection of nuclear tests relies on seismic measurements using the networks deployed to monitor earthquakes. But there are no openly available seismic data from stations near this particular test site, meaning that there are big uncertainties in pinpointing the location and size of nuclear explosions taking place there.  The team turned to space for a solution. Using data from the ALOS-2 satellite and a technique called Synthetic Aperture Radar Interferometry (InSAR), the scientists measured the changes on the surface above the test chamber resulting from the September 2017 explosion, sited at Mount Mantap in the northeast of North Korea. InSAR uses multiple radar images to create maps of deformation over time, and allows direct study of the sub- surface processes from space. The new data suggest that the explosion was powerful enough to shift the surface of the mountain above the detonation point by a few metres, and the flank of the peak moved by up to half a metre. Analysing the InSAR readings in detail reveals that the explosion took place about 540 metres below the summit, about 2.5 kilometres north of the entrance of the tunnel used to access the test chamber. Based on the deformation of the ground, the team predict that the explosion created a cavity with a radius of 66 metres. It had a yield of between 245 and 271 kilotonnes, compared with the 15 kilotonnes of the "Little Boy" bomb used in the attack on Hiroshima in 1945. The present study demonstrates the value of space-borne InSAR data for measurement of the characteristics of underground nuclear tests, with greater precision than conventional seismic methods. At the moment nuclear explosions are rarely monitored from space due to a lack of data. The team argue that currently operating satellites such as Sentinel-1 and ALOS-2 along with the NASA-ISRO Synthetic Aperture Radar (NISAR) mission, due to launch in 2022, could be used for this purpose.