Topic: Late July Astronomy Bulletin

[Clive]

ASTEROID WITH SHORTEST ORBIT
Caltech

Astronomers have discovered a very rare asteroid, one with an orbit entirely inside the Earth's orbit around the Sun, and consequently the asteroid with the shortest 'year' found so far. It takes only 151 days to orbit the Sun.  Rocks like that are called Atira asteroids, and as a class they are very difficult to observe. Because they are always inside our orbit, they tend to be near the Sun in the sky. That means that the only times we can observe them are very soon after sunset or just before sunrise. The newly discovered asteroid, called 2019 LF6, is even more difficult. Its orbit is highly elliptical and stretches from just inside Mercury's orbit to just
outside that of Venus. That means that it never strays far at all from the Sun, and can be seen only within a half hour or so of sunset or sunrise.  Worse, its orbit is tilted by about 30 degrees to that of the Earth, so it is not even generally due east or west of the Sun, where most  asteroids would lurk. That is certainly why it has evaded astronomers for so long; at about 1 kilometre wide it would have been discovered a long time ago were it not hiding near the Sun. It was about 19th magnitude when it was found, so detecting this asteroid at all was quite a feat, given that it was against a bright-ish sky background. The short year and tilted orbit suggests that the object has had an encounter or two with Mercury or even more likely Venus; it gets quite close to Venus, and that planet is big enough to affect the orbit more than Mercury could at the same distance. We know very little about LF6 so far, but it may have been a main-belt asteroid dropped into the inner solar system by Jupiter, then had its orbit altered by Venus. Perhaps further observations will help to determine how it got where it is. The short orbital period of 151 days just beats the previous record-holder for the shortest year: 2019 AQ3, discovered earlier just this year. That asteroid has a period of 165 days, and is bigger than LF6 at about 2 kilometres across.

 
HUBBLE HELPS SOLVE INTERSTELLAR MYSTERY
NASA/Goddard Space Flight Center

Scientists using the Hubble Space Telescope have confirmed the presence of electrically-charged molecules in space shaped like soccer balls, shedding light on the mysterious contents of the interstellar medium (ISM) -- the gas and dust that fills interstellar space. Since stars and planets form from collapsing clouds of gas and dust in space, the diffuse ISM can be considered as the starting point for the chemical processes that ultimately gave rise to planets and life. So fully identifying its contents provides information on the ingredients available to create stars and planets. The molecules identified by the team are a form of carbon called "Buckminster-fullerene", also known as Buckyballs", which consists of 60 carbon atoms (C60) arranged in a hollow sphere. C60 has been found in some rare cases  on Earth in rocks and minerals, and can also turn up in high-temperature combustion soot. C60 has been seen in space before. However, this is the first time an electrically charged (ionized) version has been confirmed to be present in the diffuse ISM. The C60 gets ionized when ultraviolet light from stars tears off an electron from the molecule, giving the C60 a positive charge (C60+). The diffuse ISM was historically considered too harsh and tenuous an environment for appreciable abundances of large molecules to occur. Prior to the detection of C60, the largest known molecules in space were only 12 atoms in size. Our confirmation of C60+ shows just how complex astrochemistry can get, even in the lowest-density, most strongly ultraviolet-irradiated environments in the Galaxy.

Life as we know it is based on carbon-bearing molecules, and this discovery shows that complex carbon molecules can form and survive in the harsh environment of interstellar space. In some ways, life can be thought of as the ultimate in chemical complexity. The presence of C60 unequivocally demonstrates a high level of chemical complexity intrinsic to space environments, and points towards a strong likelihood for other extremely complex, carbon-bearing molecules arising spontaneously in space. Most of the ISM is hydrogen and helium, but it is spiked with many compounds that have not been identified. Since interstellar space is so remote, scientists study how it affects the light from distant stars to identify its contents.  As starlight passes through space, elements and compounds in the ISM absorb and block certain wavelengths of the light. When scientists analyze star light from its spectrum, the colours that have been absorbed appear dim or are absent. Each element or compound has a unique absorption pattern that acts as a fingerprint allowing it to be identified. However, some absorption patterns from the ISM cover a broader range of colours, which appear different from any known atom or molecule on Earth. Those absorption patterns are called Diffuse Interstellar Bands (DIBs). Their identity has remained a mystery ever since they were discovered by Mary Lea Heger, who published observations of the first two DIBs in 1922. A DIB can be assigned by finding a precise match with the absorption fingerprint of a substance in the laboratory. However, there are millions of different molecular structures to try, so it would take many lifetimes to test them all. Today, more than 400 DIBs are known, but (apart from the few newly attributed to C60+), none has been conclusively identified. Together, the appearance of the DIBs indicate the presence of a large amount of carbon-rich molecules in space, some of which may eventually participate in the chemistry that gives rise to life.  However, the composition and characteristics of this material will remain unknown until the remaining DIBs are assigned.

Decades of laboratory studies have failed to find a precise match with any DIBs until the work on C60+. In the new work, the team was able to match the absorption pattern seen from C60+ in the laboratory to that from Hubble observations of the ISM, confirming the recently claimed assignment by a team from University of Basel, whose laboratory studies provided the required C60+ comparison data. The big problem for detecting C60+ using conventional, ground-based telescopes, is that atmospheric water vapour blocks the view of the C60+ absorption pattern. However, orbiting above most of the atmosphere in space, the Hubble telescope has a clear, unobstructed view. Nevertheless, the observers still had to push Hubble far beyond its usual sensitivity limits to stand a chance of detecting the faint fingerprints of C60+. The observed stars were all blue supergiants, located in the plane of our Galaxy, the Milky Way. The Milky Way's interstellar material is primarily located in a relatively flat disk, so lines of sight to stars in the Galactic plane traverse the greatest quantities of interstellar matter, and therefore show the strongest absorption features due to interstellar molecules. The detection of C60+ in the diffuse ISM supports the team's expectations that very large, carbon-bearing molecules are likely candidates to explain many of the remaining, unidentified DIBs.  That suggests that laboratory efforts should be made to measure the absorption patterns of compounds related to C60+, to help identify some of the remaining DIBs. The team is seeking to detect C60+ in more environments to see just how widespread buckyballs are in the Universe. It seems that C60+ is very widespread in the Galaxy.


SUPERMASSIVE BLACK HOLE ON COLLISION COURSE
Simons Foundation

Astronomers have observed a distant pair of titanic black holes headed for a collision. Each black hole's mass is more than 800 million times that of the Sun. As the two gradually draw closer together in a death spiral, they will begin sending gravitational waves rippling through space-time. Those cosmic ripples will join the as-yet-undetected background noise of gravitational waves from other supermassive black holes. Even before the destined collision, the gravitational waves emanating from the supermassive black hole pair will dwarf those previously detected from the mergers of much smaller black holes and neutron stars. Astronomers say that gravitational waves from supermassive black hole pairs are a million times louder than those detected by LIGO. The two supermassive black holes are especially interesting because they are around 2.5 billion light-years away from Earth. Since looking at distant objects in astronomy is like looking back  in time, the pair belongs to a Universe 2.5 billion years younger than our own. Coincidentally, that's roughly the same amount of time the astronomers estimate the black holes will take to begin producing powerful gravitational waves. In the present-day Universe, the black holes are already emitting such gravitational waves, but even at light speed the waves won't reach us for billions of years. The duo is still useful, though. Its discovery can help scientists estimate how many nearby supermassive black holes are emitting gravitational waves that we could detect right now.

Detecting the gravitational-wave background will help resolve some of the biggest unknowns in astronomy, such as how often galaxies merge and whether supermassive black hole pairs merge at all or become stuck in a near-endless waltz around each other. Supermassive black holes contain millions or even billions of suns' worth of mass. Nearly all galaxies, including the Milky Way, contain at least one of the behemoths at their core. When galaxies merge, their supermassive black holes begin orbiting one another. Over time, their orbit tightens as gas and stars pass between the black holes and steal energy. Once the supermassive black holes get close enough, though, that energy theft all but stops. Some theoretical studies suggest that
black holes then stall at around 1 parsec (roughly 3.2 light-years) apart. That slowdown lasts nearly indefinitely and is known as the final parsec problem. In this scenario, only very rare groups of three or more supermassive black holes result in mergers. Astronomers can't just look for stalled pairs because long before the black holes are 1 parsec apart, they're too close to distinguish as two separate objects. Moreover, they don't produce strong gravitational waves until they overcome the final-parsec hurdle and get closer together. Observed now as they were 2.5 billion years ago, the newfound supermassive black holes appear about 430
parsecs apart.

If the final-parsec problem doesn't exist, then astronomers expect that the Universe is filled with the clamour of gravitational waves from supermassive black-hole pairs. This noise is called the gravitational wave background, and it's a bit like a chaotic chorus of crickets chirping in the night. You can't discern one cricket from another, but the volume of the noise helps you estimate how many crickets are out there. When two supermassive black holes finally collide and combine, they send out a thundering chirp that dwarfs all others. Such an event is brief and extraordinarily rare, though, so scientists don't expect to detect one any time soon. The gravitational waves generated by supermassive black hole pairs are outside the frequencies currently observable by experiments such as LIGO and Virgo. Instead, gravitational wave hunters rely on arrays of special stars called pulsars that act like metronomes. The rapidly spinning stars send out radio waves in a steady rhythm. If a passing gravitational wave stretches or compresses the space between Earth and the pulsar, the rhythm is slightly thrown off.  Detecting the gravitational-wave background using one of these pulsar timing arrays takes patience and plenty of monitored stars. A single pulsar's rhythm might be disrupted by only a few hundred nanoseconds over a decade.  The louder the background noise, the bigger the timing disruption and the sooner the first detection will be made. The team detected the two titans with the Hubble Space Telescope. Although supermassive black holes are not directly visible through an optical telescope, they are surrounded by bright clumps of luminous stars and warm gas drawn in by the powerful gravitational tug. For its time in history, the galaxy harbouring the new-found super-massive black hole pair is basically the most luminous galaxy in the Universe. What's more, the galaxy's core is shooting out two unusually colossal plumes of gas. After the researchers pointed the Hubble Space Telescope at the galaxy to uncover the origins of its spectacular gas clouds, they discovered that the system contained not one but two massive black holes. On the basis of the findings, the team predicts that in an optimistic scenario there are about 112 nearby supermassive black holes emitting gravitational waves. The first detection of the gravitational- wave background from supermassive black holes should therefore come within the next five years or so. If such a detection is not made, that would be evidence that the final-parsec problem may be insurmountable. The team is currently looking at other galaxies similar to the one harbouring the newfound supermassive black hole pair. Finding additional pairs will help them further hone their predictions.


EARLIEST EXAMPLE OF MERGING GALAXIES
Waseda University

A team of astronomers used ALMA to observe B14-65666, an object located 13 billion light-years away in the constellation Sextans. Owing to the finite speed of light, the signals we receive from B14-65666 today had to travel for 13 billion years to reach us. In other words they show us the image of what the galaxy looked like 13 billion years ago, less than 1 billion years after the Big Bang. ALMA detected radio emissions from oxygen, carbon, and dust in B14-65666. This is the earliest galaxy where all three of those signals have been detected. The detection of multiple signals is important because they carry complementary information. Data analysis showed that the emissions are divided into two blobs. Previous observations with the Hubble Space Telescope (HST) had revealed two star clusters in B14-65666. Now with the three emission signals detected by ALMA, the team was able to show that the two blobs do in fact form a single system, but they have different speeds. That indicates that the blobs are two galaxies in the process of merging. They are the earliest known example of merging galaxies. The research team estimated that the total stellar mass of B14-65666 is less than 10% that of the Milky Way. That means that B14-65666 is in the earliest phases of its evolution. Despite its youth, B14-65666 is producing stars 100 times more actively than the Milky Way. Such active star-formation is another important signature of galactic mergers, because the gas compression in colliding galaxies naturally leads to bursty star-formation.

With rich data from ALMA and HST, combined with advanced data analysis, the team could put the pieces together to show that B14-65666 is a pair of merging galaxies in the earliest era of the Universe. Detection of radio waves from three components in such a distant object clearly demonstrates ALMA's high capability to investigate the distant Universe. Modern galaxies like our Milky Way have experienced countless, often violent, mergers.  Sometimes a larger galaxy swallowed a smaller one. In rare cases, galaxies with similar sizes merged to form a new, larger galaxy. Mergers are essential for galaxy evolution, so many astronomers are eager to trace back the history of mergers. The next step is to search for nitrogen, another major chemical element, and even the carbon monoxide molecule. Ultimately, astronomers hope to understand observationally the circulation and accumulation of elements and material in the context of galaxy formation and evolution.


SUPERMASSIVE BLACK HOLES IN EARLY UNIVERSE
University of Western Ontario

Astrophysicists have found evidence for the direct formation of black holes that do not need to emerge from a star remnant. The production of black holes in the early Universe, formed in that manner, may provide scientists with an explanation for the presence of extremely massive black holes at a very early stage in the history of our Universe. The team has developed an explanation for the observed distribution of supermassive black hole masses and luminosities, for which there was previously no scientific explanation.  The model is based on a very simple assumption: supermassive black holes form very, very quickly over very, very short periods of time and then suddenly, they stop. That explanation contrasts with the current understanding of how stellar-mass black holes are formed, which is they emerge when the centre of a very massive star collapses in upon itself. This is indirect observational evidence that black holes originate from direct collapses and not from stellar remnants. The team developed the new mathematical model by calculating the mass function of supermassive black holes that form over a limited time period and undergo a rapid exponential growth of mass. The mass growth can be regulated by the Eddington limit that is set by a balance of radiation and gravitation forces or can even exceed it by a modest factor. Supermassive black holes only had a short time period where they were able to grow fast and then at some point, because of all the radiation in the Universe created by other black holes and stars, their production came to a halt. That is the direct-collapse scenario. During the last decade, many supermassive black holes that are a billion times more massive than the Sun have been discovered at high redshifts, meaning that they were in place in our Universe within 800 million years after the Big Bang. The presence of those young and very massive black holes question our understanding of black-hole formation and growth. The direct-collapse scenario allows for initial masses that are much greater than implied by the standard stellar remnant scenario, and  can go a long way to explaining the observations. This new result provides evidence that such direct-collapse black holes were indeed produced in the early Universe. The team believes that the new results can be used with future observations to infer the formation history of the extremely massive black holes that existed at very early times in our Universe.


PLAN FOR KEEPING EARLIEST NASA EXPLORERS GOING
NASA

With careful planning and dashes of creativity, engineers have been able to keep NASA's Voyager 1 and 2 spacecraft flying for nearly 42 years -- longer than any other spacecraft in history. To ensure that those vintage robots continue to return the best science data possible from the frontiers of space, mission engineers are implementing a new plan to manage them. That involves making difficult choices, particularly about instruments and thrusters. One key issue is that both Voyagers, launched in 1977, have less and less power available over time to run their instruments and the heaters that keep them warm in the coldness of deep space. Engineers have had to decide what parts get power and what parts have to be turned off on both spacecraft. But those decisions must be made sooner for Voyager 2 than Voyager 1 because Voyager 2 has one more instrument collecting data -- and drawing power -- than its sibling. Not only are Voyager mission findings providing humanity with observations of truly uncharted territory, but they help us understand the very nature of energy and radiation in space -- key information for protecting NASA's missions and astronauts even when closer
to home. Mission-team members can now preliminarily confirm that Voyager 2's cosmic-ray instrument is still returning data, despite dropping to a chilly minus 59 degrees Celsius. That is lower than the temperatures at  which CRS was tested more than 42 years ago (down to minus 45 degrees Celsius). Another Voyager instrument also continued to function for years after it dropped below temperatures at which it was tested.

Voyager 2 continues to return data from five instruments as it travels through interstellar space. In addition to the cosmic-ray instrument, which detects fast-moving particles that can originate from the Sun or from sources outside our Solar System, the spacecraft is operating two instruments dedicated to studying plasma (a gas in which atoms have been ionized and electrons float freely) and a magnetometer (which measures magnetic fields) for Understanding the sparse clouds of material in interstellar space. Taking data from a range of directions, the low-energy charged-particle instrument is particularly useful for studying the probe's transition away from our heliosphere. Because CRS can look only in certain fixed directions, the Voyager science team decided to turn off CRS's heater first. Launched separately in 1977, the two Voyagers are now over 18 billion kilometres from the Sun and far from its warmth. Engineers have to control temperature on both spacecraft to keep them operating. For instance, if fuel lines powering the thrusters that keep the spacecraft oriented were to freeze, the Voyagers' antennae could stop pointing at Earth. That would prevent engineers from sending commands to the spacecraft or receiving scientific data. So the spacecraft were designed to heat themselves. But running heaters -- and instruments -- requires power, which is constantly diminishing on both Voyagers.

Each of the probes is powered by three radioisotope thermoelectric generators, or RTGs, which produce heat via the natural decay of plutonium-238 radio-isotopes and convert that heat into electrical power. Because the heat energy of the plutonium in the RTGs declines and their internal efficiency decreases over time, each spacecraft is producing about 4 fewer watts of electrical power each year. That means that the generators produce about 40% less than what they did at launch nearly 42 years ago, limiting the number of systems that can run on the spacecraft. The mission's new power-management plan explores multiple options for dealing with the diminishing power supply on both spacecraft, including shutting off additional instrument heaters over the next few years. Another challenge that engineers have faced is managing the degradation of some of the spacecraft thrusters, which fire in tiny pulses, or puffs, to rotate the spacecraft. That became an issue in 2017, when mission controllers noticed that a set of thrusters on Voyager 1 needed to give off more puffs to keep the spacecraft's antenna pointed at Earth. To make sure that the spacecraft could continue to maintain proper orientation, the team fired up another set of thrusters on Voyager 1 that had not been used in 37 years. Voyager 2's current thrusters have started to degrade, too. Mission managers have decided to make the same thruster switch on that probe this month. Voyager  2 last used those thrusters (known as trajectory-correction manoeuvre thrusters) during its encounter with Neptune in 1989. The engineers' plan to manage power and aging parts should ensure that Voyager 1 and 2 can continue to collect data from interstellar space for several years to come. Data from the Voyagers continue to provide scientists with never- before-seen observations of our boundary with interstellar space, complementing NASA's Interstellar Boundary Explorer (IBEX), a mission that is remotely sensing that boundary. NASA is also preparing the Interstellar Mapping and Acceleration Probe (IMAP), due to launch in 2024,to capitalize on the Voyagers' observations.


NEW THEORY COULD CHANGE HOW WE THINK ABOUT GRAVITY
Durham University

Supercomputer simulations of galaxies have shown that Einstein's theory of General Relativity might not be the only way to explain how gravity works or how galaxies form. Physicists simulated the cosmos using an alternative model for gravity -- f(R)-gravity, a so called Chameleon Theory. The resulting images produced by the simulation show that galaxies like our Milky Way could still form in the Universe even with different laws of gravity. The findings show the viability of Chameleon Theory -- so called because it changes behaviour according to the environment -- as an alternative to General Relativity in explaining the formation of structures in the universe. The research could also help further understanding of dark energy -- the mysterious substance that is accelerating the expansion rate of the Universe. General Relativity was developed by Albert Einstein in the early 1900s to explain the gravitational effect of large objects in space,  for example to explain the orbit of Mercury in the Solar System. It is the foundation of modern cosmology but also plays a role in everyday life, for example in calculating GPS positions in smartphones. Scientists already know from theoretical calculations that Chameleon Theory can reproduce the success of General Relativity in the Solar System. The team has now shown that the theory allows realistic galaxies like our Milky Way to form and can be distinguished from General Relativity on very large cosmological scales. Chameleon Theory allows for the laws of gravity to be modified so we can test the effect of changes in gravity on galaxy formation.

Simulations show that even if you change gravity, it would not prevent disc galaxies with spiral arms from forming. The research definitely does not mean that General Relativity is wrong, but it does show that it does not have to be the only way to explain gravity's role in the evolution of the Universe. The researchers looked at the interaction between gravity in Chameleon Theory and supermassive black holes that sit at the centres of galaxies. Black holes play a key role in galaxy formation because the heat and material they eject when swallowing surrounding matter can burn away the gas needed to form stars, effectively stopping star formation. The amount of heat spewed out by black holes is altered by changing gravity, affecting how galaxies form. However, the new simulations showed that even accounting for the change in gravity caused by applying Chameleon Theory, galaxies were still be able to form. General Relativity also has consequences for understanding the accelerating expansion of the Universe. Scientists believe that that expansion is being driven by dark energy and the researchers say that their findings could be a small step towards explaining the properties  of this substance. In General Relativity, scientists account for the accelerated expansion of the Universe by introducing a mysterious form of matter called dark energy -- the simplest form of which may be a cosmological constant, whose density is a constant in space and time. However, alternatives to a cosmological constant which explain the accelerated expansion by modifying the law of gravity, like f(R) gravity, are also widely considered, given how little is known about dark energy. The findings can be tested through observations with the Square Kilometre Array (SKA) telescope, based in Australia and South Africa, which is due to begin observations in 2020.