The SOCIETY for POPULAR ASTRONOMY
Electronic News Bulletin No. 406 2015 August 30
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SCIENTISTS STUDY PLUTO'S ATMOSPHERE
Southwest Research Institute
The latest data from the New Horizons spacecraft reveal diverse features on Pluto's surface and an atmosphere dominated by nitrogen. However, Pluto's small mass allows hundreds of tons of atmospheric nitrogen to escape into space every hour. More nitrogen has to come from somewhere to re-supply both the nitrogen ice that is moving around Pluto's surface in seasonal cycles, and the nitrogen that is escaping off the top of the atmosphere as the result of heating by ultraviolet light from the Sun. Astronomers wondered if comets could deliver enough nitrogen to Pluto's surface to re-supply what is escaping from its atmosphere. They also looked at whether craters made by the comets hitting the surface could excavate enough nitrogen -- but that would require a very deep layer of nitrogen ice at the surface, which is not proven. The team also studied whether craters could expose more surface area, by punching through surface deposits that might be built up over time.
They found that none of those effects, which are the major ones from cratering, seems able to supply enough nitrogen to maintain the escaping atmosphere. While it is possible that the escape rate was not as high in the past as it is now, it is thought that tectonic activity may be helping by bringing nitrogen up from Pluto's interior. The newest images of Pluto show landforms that suggest that heat is rising beneath the surface, with troughs of dark matter either collecting, or bubbling up, between flat segments of crust. The pre-flyby prediction was that it is most likely that Pluto is actively re-supplying nitrogen from its interior to its surface, possibly through ongoing geysers or cryo-vulcanism. As data from New Horizons come in, scientists will be very interested to see if that proves true.
ASTRONOMERS DISCOVER NEW PLANET ORBITING TWO STARS
San Francisco State University
A team of astronomers has discovered another planet orbiting a pair of stars, the 10th 'circumbinary' planet discovered by the Kepler mission. The planet, known as Kepler-453b, is located within its host stars' 'habitable zone', the region around the stars where life could potentially exist. The fortuitous nature of its discovery may suggest that there could be more planets like it than previously believed. If the planet had been looked for earlier or later than it was, astronomers would have seen nothing and assumed that there was no planet there. So there may be many more such planets than have been supposed, and we have just been looking at the wrong times. Researchers typically detect 'exo-planets' -- planets outside the Solar System -- by observing the decrease in starlight as the planets transit between their host stars and the Earth. But because Kepler-453b is affected by the gravitational pulls of two stars, not just one, its orbit is erratic -- "like a spinning top". As a result, its transits are only visible to astronomers 9% of the time. In fact, had researchers not detected the planet now, their next chance to do so would not have come until 2066. Kepler-453b blocked 0.5% percent of its host stars' light during the transit, which enabled researchers to calculate that the planet's radius is 6.2 times that of the Earth, or about 60% larger than Neptune. Its size indicates that it is a gas giant, rather than a rocky planet, and thus unable to have life despite being in the habitable zone. But it could have moons that are rocky. Any sentient inhabitants of the system would see two suns in their sky orbiting one another every 27 days. The larger star is almost the size of the Sun, the smaller one only a fifth the size of the Sun and much cooler, emitting less than 1% of the larger star's energy. Kepler-453b takes 240 days to orbit its ho st stars.
DARK-ENERGY SURVEY FINDS MORE NEARBY DWARF GALAXIES
DOE/Fermi National Accelerator Laboratory
Scientists on the Dark-Energy Survey, using one of the world's most powerful digital cameras, have discovered eight more faint celestial objects hovering near our Milky Way galaxy. Signs indicate that they, like the objects found by the same team earlier this year, are likely to be dwarf satellite galaxies, the smallest and closest known form of galaxies. Satellite galaxies are small celestial objects that orbit larger galaxies, such as the Milky Way. Dwarf galaxies can be found with fewer than 1,000 stars, in contrast to the Milky Way, an average-size galaxy containing billions of stars. Scientists have believed that larger galaxies are built from smaller ones, which are thought to be especially rich in dark matter, the substance that is said by some to make up about 25% of the total matter and energy in the Universe. Dwarf satellite galaxies, therefore, are considered to be the key to understanding dark matter and the process by which larger galaxies form. The main goal of the Dark-Energy Survey (DES), as its name suggests, is to understand the nature of dark energy, the mysterious stuff that makes up about 70% of the matter and energy in the Universe. Scientists believe that dark energy is the key to understanding why the expansion of the Universe is speeding up. To carry out its dark-energy mission, DES takes snapshots of hundreds of millions of distant galaxies. However, some of the DES images also contain stars in dwarf galaxies much closer to the Milky Way. The same data can therefore be used to probe both dark energy, which scientists think is driving galaxies apart, and dark matter, which is thought to hold galaxies together. Scientists can see the faintest dwarf galaxies only when they are 'nearby', and had previously found only a few of them. If the new discoveries are representative of the entire sky, there could be many more galaxies hiding in our cosmic neighbourhood. Just this year, more than 20 dwarf satellite galaxy candidates have been found, 17 of them in DES data. That has nearly doubled the number of such objects we know about in just one year.
In March, researchers with the DES and an independent team from the University of Cambridge jointly announced the discovery of nine such objects in snapshots taken by the Dark-Energy Camera. Two of those have been confirmed as dwarf satellite galaxies so far. Before 2015, scientists had located only about two dozen such galaxies around the Milky Way. DES is finding galaxies so faint that they would have been very difficult to recognize in previous surveys. The discovery of so many new galaxy candidates in one-eighth of the sky implies that there are more to be found around the Milky Way. The closest of the newly discovered objects is about 80,000 light-years away, and the furthest roughly 700,000 light-years away. The objects are, on average, around a billion times less bright than the Milky Way and a million times less massive. The faintest of the new dwarf-galaxy candidates has about 500 stars. Most of the newly discovered objects are in the southern half of the DES survey area, in close proximity to the Magellanic Clouds. Those are the two largest satellite galaxies associated with the Milky Way, about 158,000 light-years and 208,000 light-years away. It is possible that many of the new objects could be satellite galaxies of those larger satellite galaxies, which would be a discovery in itself. Satellites of satellites are predicted by models of dark matter. Since dwarf galaxies are thought to be made mostly of dark matter, with very few stars, they are excellent prospects to explore the properties of dark matter. Further analysis will confirm whether the new objects are indeed dwarf satellite galaxies and whether signs of dark matter can be detected in them.
NEAREST GALAXY COLLISION
A spectacular galaxy collision, that has created a complete ring of bright new stars, has been discovered lurking behind the Milky Way. The galaxy is 30 million light-years away. Such systems are rare and arise from 'bull's-eye' collisions between two galaxies of similarmass. Shock waves from the collision compress reservoirs of gas in each galaxy and trigger the formation of new stars, creating a spectacular ring of intense emission, and lighting up the system like a Catherine-Wheel firework on Bonfire Night. Galaxies grow through collisions, but it is rare to catch one in the process, and extremely rare to see a bull's-eye collision in progress. Fewer than 20 systems with complete rings are known. The object, dubbed Kathryn's Wheel, was discovered during a special wide-field survey of the southern Milky Way undertaken with the UK Schmidt telescope in Australia. It used a narrow-wavelength optical region centred on the red H-alpha emission line of hydrogen. The colliding pair was discovered during a search of the survey images for remnants of dying stars in the Milky Way. Astronomers were very surprised to find also the spectacular cosmic ring, sitting remotely behind the dust and gas of the Milky Way in the southern constellation Ara.
The newly discovered ring galaxy is seven times closer than anything similar found before, and forty times closer than the famous 'Cartwheel' galaxy. The ring is located behind a dense star field and close to a bright foreground star, which is why it had not been noticed before. There are very few other galaxies in its neighbourhood; the odds against a collision in such an empty region of space are very high. Not only is the system visually stunning, but it is close enough to be accessible to detailed study. The ring is also quite low in mass -- a few thousand million Suns, or less than 1% of the Milky Way -- so the discovery shows that collision rings can form around much smaller galaxies than was thought. Smaller galaxies are more common than large ones, implying that collisional rings could be ten times as common as previously supposed. The team intends to carry out more detailed studies with larger telescopes, since the newly discovered galaxy is currently the only one of its kind close enough to be studied in detail.
SUPERNOVAE DISCOVERED IN 'WRONG PLACE'
Space Telescope Science Institute (STScI)
A new analysis of 13 supernovae -- including archived data from Hubble -- is helping astronomers explain how some young stars exploded sooner than expected, hurling them far from their host galaxies. It is a complicated story of double-star systems, merging galaxies, and twin black holes that began in 2000 when the first such supernova was discovered. Astronomers knew that such stars had to be far from their origins where they exploded as supernovae and wanted to find out how they arrived at their current locations. It was thought that the doomed stars had somehow migrated to their final resting spots. To prove that idea, scientists studied data from the Lick and Keck observatories and the Subaru telescope, to determine how fast the stars were travelling. To their surprise, they discovered that the stars were moving at about the same speed as stars that have been tossed out of the Milky Way galaxy by its central supermassive black hole, at more than 2000 km/s. Then attention turned to the ageing galaxies in the area of the speeding supernovae. Hubble archival images confirmed that many are massive elliptical galaxies that were merging or had recently merged with other galaxies. The lanes are the shredded remnants of a cannibalized galaxy. Other observations provided circumstantial evidence for such encounters, showing that the cores of many of the galaxies had active supermassive black holes fuelled by the collision. Many of the galaxies also exist in dense environments at the hearts of clusters, a prime area for mergers. The telltale clue was strong dust lanes piercing the centres of several of them. The location of the supernovae in relation to ancient galaxies indicates that the original stars must have been old, too, and if the stars were old, then they must have had companions with them that provided enough material to trigger a supernova blast.
Scientists think that a pair of supermassive black holes in the merging galaxies can provide the gravitational slingshot to rocket the binary stars into intergalactic space. Hubble observations reveal that nearly every galaxy has a massive black hole at its centre. According to that scenario, after two galaxies merge, their black holes migrate to the centre of the new galaxy, each trailing a cluster of stars. As the black holes orbit around one another, slowly getting closer, one of the binary stars in the entourage of one of the holes may come too close to the other hole. Many such stars will be flung far away, and those ejected stars in surviving binary systems will orbit one another even more closely after the encounter, speeding up their merger. With a single black hole, occasionally a star will get too close to it and have an extreme interaction. With two black holes, there are two reservoirs of stars being dragged close to another black hole, dramatically increasing the likelihood that a star is ejected. While the black hole at the centre of the Milky Way may eject about one star a century, a binary supermassive black hole may kick out 100 stars a year. After getting shot out of the galaxy, the binary stars move closer together as their orbits continue to accelerate, which speeds up the binary stars' ageing process. The binary stars are probably both white dwarfs, which are the burned-out relics of stars. Eventually, the white dwarfs get close enough for one to be ripped apart by tidal forces. As material from the dead star is quickly dumped onto the surviving star, an explosion occurs, creating the supernova. The time it takes for one of the ejected stars to explode is relatively short, about 50 million years. Normally, such binary stars would take a long time to merge, probably much longer than the present age of the Universe.
GAMMA RAYS FROM DWARF GALAXY
A newly discovered dwarf galaxy orbiting the Milky Way appears to be radiating gamma rays. The exact source of that high-energy light is uncertain, but it might be a signal of dark matter lurking at the galaxy's centre. The galaxy, named Reticulum 2, was discovered earlier this year in the data of the Dark-Energy Survey. At a distance of about 100,000 light-years, Reticulum 2 is one of the 'nearest' dwarf galaxies yet detected. Using publicly available data from the Fermi gamma-ray space telescope, astronomers have found more gamma rays coming from the direction of that galaxy than would be expected from the normal background. In the search for dark matter, gamma rays from a dwarf galaxy have long been considered a very strong signature, and it seems that we may now be detecting such a thing for the first time. The gravitational detection of dark matter tells us very little about the particle behaviour of the matter, but now we may have a non-gravitational detection that shows dark matter behaving like a particle, which is a holy grail of sorts.
A leading theory suggests that dark-matter particles are WIMPs -- Weakly Interacting Massive Particles. When pairs of WIMPs meet, they annihilate one another, giving off high-energy gamma rays. If that is true, then there should be a lot of gamma rays emanating from places where WIMPs are thought to be plentiful, like the dense centres of galaxies. The trouble is, the high-energy rays also originate frommany other sources, including black holes and pulsars, which makes it difficult to disentangle a dark-matter signal from the background noise. That is why dwarf galaxies are important in the hunt for the dark-matter particle. Dwarfs are thought to lack other gamma-ray-producing sources, so a gamma-ray flux from a dwarf galaxy would make a strong case for dark matter. They are basically very clean and quiet systems. Scientists have been looking at them with Fermi for signs of gamma rays for the last several years. There has never been a convincing signal until now. Over the last few years astronomers have been developing an analysis technique that searches the gamma-ray data for weak signals that could be due to dark-matter annihilation. With the discovery of Reticulum 2, they turned their attention to that part of the sky. They looked at all of the gamma rays coming from the direction of the dwarf galaxy, as well as those coming from adjacent areas of the sky to provide a background level.
SLOW DEATH OF THE UNIVERSE
An international team of astronomers studying more than 200,000 galaxies has measured the energy generated within a large portion of space more precisely than has been done before. The teams finds that the energy produced in a section of the Universe today is only about half what it was two billion years ago, and that the fading is occurring across all wavelengths. The Universe is slowly dying. The study involves many of the world's most powerful telescopes, including ESO's VISTA and VST survey telescopes at the Paranal Observatory in Chile. Supporting observations were made by two orbiting space telescopes operated by NASA (GALEX and WISE) and another belonging to ESA (Herschel). The research is part of the Galaxy And Mass Assembly (GAMA) project, the largest multi-wavelength survey ever put together. The survey data include measurements of the energy output of each galaxy at 21 wavelengths, from the ultraviolet to the far infrared. The data set will help scientists to understand how different types of galaxies form and evolve.
All the energy in the Universe was created in the Big Bang, with some portion locked up as mass. Stars shine by converting mass back into energy, as described by Einstein's famous equation E = mc2. The GAMA study sets out to map and model all of the energy generated within a large volume of space today and at different times in the past. While most of the energy in the Universe arose in the aftermath of the Big Bang, additional energy is constantly being generated by stars as they fuse elements like hydrogen and helium together. The new energy is either absorbed by dust as it travels through the host galaxy, or escapes into intergalactic space and travels until it hits something, such as another star, a planet, or, very occasionally, a telescope mirror. The Universe will decline from now on, sliding gently into old age. The team of researchers hopes to expand the work to map energy production over the entire history of the Universe, using a swathe of new facilities, including the world's largest radio telescope, the Square Kilometre Array, which is due to be completed in Australia and South Africa over the next decade.
Bulletin compiled by Clive Down
(c) 2015 the Society for Popular Astronomy
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