World’s largest asteroid impacts found in central Australia

Studies of drill core rock samples up to 2 km below the ground in north-eastern South Australia, Queensland and the Northern Territory suggest the largest asteroid impact zone yet found (Glikson et al. 2015).

The impact zone was found in the ~400,000 km2 Warburton Basin (that has East and West sub-basins) in two parts, each 200 km wide, potentially forming a total 400 km diameter impact area. The asteroid that created them may have been as large as 10 km in diameter, before splitting into two and then striking the Earth.

The evidence of the impact was discovered during drilling as part of geothermal research, in the Warburton Basin. The impact zones seem to extend down through the Earth’s crust, to the mantle. The surrounding rocks are between 300 and 450 million years old, suggesting a similar age for the impact event.

Extent of the Warburton Basin based on the ‘C’ seismic horizon [Cretaceous marker]. (Credit: A. Glikson and colleagues)

Quartz grains have been found that have layers that are curved or bent which can be caused by (normal) tectonic deformation or by crustal rebound due to an impact. Such shocked quartz is considered a tell-tale sign of an asteroid impact. Both the Warburton East and Warburton West Basins also have large magnetic bodies at depths of 6-10 km, and gravity anomalies. Seismic studies suggest fracturing of the crust to 20 km depths or more.

One of the suggested scenarios that could explain the geology is a large asteroid impact that has caused central uplift in the area, causing the removal of Devonian (419-359 million years ago or 419-359 Ma) and Carboniferous (359-299 Ma) strata. Around this time period Australia, South America, Africa, India and Antarctica were joined into the Southern Hemisphere continent of Gondwana.

Part of the interest in the Warburton Basin findings is the relationship between large asteroid impacts and mass extinctions on Earth. When large (km-sized) asteroids hit Earth, usually at very high velocity, dust and debris rain down, disrupting the climate and causing extinction on a global, rather than local, scale.

Arguably the most famous impact event is the K-T (Cretaceous-Tertiary boundary) event. It is typically related to the disappearance of dinosaurs on Earth. At this time all large vertebrates on Earth suddenly became extinct about 65 Ma, the end of the Cretaceous Period. As well most plankton became extinct, and land plants were diminished. This event marks a defining moment in Earth’s history. The K-T extinctions were worldwide, affecting all the major continents and oceans. The K-T impact crater is a roughly oval geological structure called Chicxulub, deeply buried under the sediments of the Yucatán peninsula of Mexico. The structure is about 180 km across, and the igneous rock under Chicxulub contains high levels of iridium. Iridium is usually rare, yet in rocks near the boundary it is concentrated and abundant. Meteorites have a similar, high abundance in iridium.

There is evidence of mass extinctions around the inferred time of the Warburton Basin impact. One of the five major extinctions events that have affected Earth’s biota occurred ~360 Ma. Three quarters of all species died out in this Late Devonian mass extinction, though past research suggests a series of extinctions over several million years, with two main periods, rather than a single event.

Schematic of diversity declines during the Late Devonian mass extinction. (Credit: Dennis C. Murphy)

In particular the Kellwasser Event (about 375–360 Ma) marks when 50% of all genera went extinct. Major victims included ammonites, brachiopods, corals, jawless fishes, sponges and trilobites. The primary Devonian reef-builders (tabulate corals and stromatoporoids) never truly recovered from the extinctions and the changes in reef ecology were profound for at least 100 million years.

The recent results of the Warburton Basin suggest more study and further exploration is required. “When we know more about the age of the impact, then we will know whether it correlates with one of the large mass extinctions”, says research team leader Dr Andrew Glikson.

For more information, see

[Glen Mackie]

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Dark Energy Survey finds new dwarf galaxies

Up to nine new dwarf galaxies close to the Galaxy and the Magellanic Clouds have recently been found. Three of the new objects are definitely dwarf galaxies whilst the others may be fainter dwarfs or even globular clusters. Imaging data taken from the Dark Energy Survey (DES),  a five-year effort to photograph a large portion of the southern sky, has been analysed and these new objects were found. Two separate teams of astronomers (eight objects reported in Bechtol et al. 2015 and nine reported in Koposov et al. 2015) announced these findings after looking at the first year of DES data.

The Magellanic Clouds and the Auxiliary Telescopes at the Paranal Observatory in Chile. Six of the nine newly discovered satellites are shown (insets and enlarged). The other three possible dwarfs are outside the field of view. The insets show images of the three most visible objects (Eridanus 2, Horologium 1 and Pictoris 1) and each span 13×13 arcminutes on the sky. (Credit: V. Belokurov and S. Koposov, IoA, Cambridge; Y. Beletsky, Carnegie Observatories)

The closest of these new objects is 97,000 light-years away, about halfway to the Magellanic Clouds, in the constellation Reticulum. The most distant and most luminous of these objects is 1.2 million light-years away in the constellation Eridanus. The newly discovered objects are a billion times dimmer than the Milky Way, and a million times less massive. Their dimness and small size makes them incredibly difficult to find. Automatic object finding software is used to detect slight stellar overdensities. Colour information can then be used to filter out background stars to produce a candidate dwarf galaxy.

The projected positions of the objects are close to the Magellanic Clouds, suggesting some may be physically associated with the Clouds. They may even be linked with the extensive Magellanic Stream of cold, atomic hydrogen gas that spans a large part of the southern sky near the two Clouds. Spectroscopic confirmation of stars in each new object is still required to accurately determine their distances, chemical composition and structure

This animation shows how difficult it is to spot dwarf galaxy candidates in the Dark Energy Camera’s images. The first image is a snapshot of DES J0335.6-5403. This object sits roughly 100,000 light-years from Earth, and contains very few stars — only about 300 could be detected with DES data. The second image shows the detectable stars that likely belong to this object. (Credit: Fermilab/Dark Energy Survey)

Why are these discoveries significant? These ultra-faint galaxies are the smallest, least luminous and most dark matter dominated galaxies known. Most simulations of structure formation, using cold dark matter, predict hundreds of dwarf galaxies should be in orbit around the Galaxy. Until recently, only about 30 dwarfs were known. The current DES results increase this number by about 30%. The complete 5 year DES survey area is predicted to contain between 19-37 dwarf galaxies. It is unclear what the total number of dwarfs linked to the Galaxy is, due to incompleteness in depth (faintness) and sky coverage of various surveys. Thus it is still unclear the discrepancy between the number of dwarfs detected and that predicted by simulations.

Strangely, the majority of dwarf galaxies in orbit around the Galaxy appear to lie on a “great circle” on the sky. That is they do not seem to be isotropically distributed around the Galaxy. Recently, similar (planar) distributions of dwarf galaxies have been detected around our Local Group neighbour M31 (Andromeda), and many dwarfs appear to have the same sense of rotation about their host.  It is not yet clear whether these peculiar dwarf galaxy distributions are caused by ‘normal’ structure formation or other processes. Koposov et al. and Bechtol et al. both suggest that some of the new dwarfs found in DES may have been associated with the Magellanic Clouds in the past (harking back to previous suggestions of this nature by Donald Lynden-Bell in the mid-1970s). Perhaps they are Magellanic dwarfs not Galaxy dwarfs.

All-sky image showing the locations of known dwarf galaxies associated with the Galaxy. The new DES dwarfs are close to the positions of the LMC and SMC (bottom, left). (Credit: A. Frebel, MIT)

Finally, the dominance of dark matter in dwarf galaxies make these objects excellent targets for indirect dark matter searches. Dark matter annihilation produces gamma rays. Geringer-Sameth et al. (2015) have looked at Fermi-LAT data for the newly discovered DES dwarf, Reticulum 2. They detect a signal between 2 GeV and 10 GeV that exceeds the background. The detection is between 2.3 and 3.7 sigma, depending on the background model used. Despite this uncertainty, Reticulum 2 has the most significant gamma-ray signal of any known dwarf galaxy. Other sources of gamma rays (e.g. a background extragalactic source, or internal millisecond pulsars or young massive stars) still need to be ruled out.

For more information, see

[Glen Mackie]

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Mature galaxy in the young Universe

A collaboration of astronomers, lead by Darach Watson from the University of Copenhagen, have discovered one of the most distant galaxies to date named A1689-zD1. The galaxy is at a redshift of 7.5, which means it is about 13 billion light years away and the light we detect is from when the Universe was only about 700 million years old. However, A1689-zD1’s distance is not the only thing that makes it interesting.

The discovery of A1689-zD1 was made possible by the existence of a gravitational lens that magnified the light of the galaxy by a factor of 9.3. Gravitational lenses are created when massive foreground objects – like galaxy clusters – distort space-time around them. When this happens light no longer travels in a straight line, but is bent around the intervening massive object, as if the light had passed through a convex lens. Depending on the object’s position behind the lens it can appear multiply imaged (Einstein cross) or is smudged into an arc of light around the lens (Einstein arc).

Without gravitational lensing it would be very difficult to observe galaxies at the distance of A1689-zD1. This is because the light emitted from A1689-zD1 is redshifted quite a bit by the time it reaches an observer on Earth. Redshifting of light occurs because the Universe is expanding, and as light travels through space it is literally stretched along with space.

When light is stretched its wavelength becomes longer. Light that has longer wavelengths is redder. Light from very distant galaxies is fainter than that from similar galaxies closer to us at first order due to the inverse-square law (but at such extreme distances other cosmological dimming effects dominate). Therefore, the light sent out from A1689-zD1 is very red and faint because it has had to travel such a large distance to reach us.

Hubble Space Telescope image of the rich galaxy cluster Abell 1689. The enormous concentration of mass from the cluster acts as a lens to bend light from more distant objects, increasing their apparent brightness. The insert box shows the lens background galaxy A1689-zD1. (Credit: NASA; ESA; L. Bradley (JHU); R. Bouwens (U. California, Santa Cruz); H. Ford (JHU); and G. Illingworth (U. California, Santa Cruz))

The other reason A1689-zD1 is important is because of its properties and how they were determined. Watson’s team observed A1689-zD1 using two instruments. The first instrument was the X-shooter spectrograph, which is part of the Very Large Telescope in Chile. A spectrograph spreads a galaxy’s light into a spectrum, similar to how a prism spreads white light into a rainbow. The amount of light in each part of the spectrum reveals several characteristics about the galaxy. Sometimes astronomer note a chunk of the spectrum is missing, which occurs when material is absorbing light emitted at those wavelengths.

Schematic of an absorption spectrum, showing how intervening gas can “remove” specific transition lines from the resulting spectrum.

Galaxies that are vigorously forming stars have such a chunk missing from their spectrum because young stars emit light at a wavelength that is just right to be absorbed by the neutral hydrogen in the galaxy. When hydrogen absorbs light, this boosts its energy and the hydrogen atoms undergoes what is called an energy transition. The strongest of these transitions is called the Lyman-α transition. The wavelength where this transition occurs is where light will be missing from the galaxy’s spectrum and is called the Lyman-α break. Because the galaxy’s light is redshifted based on how far it has travelled, astronomers can measure the wavelength the break occurs at in the galaxy and therefore determine how far away it is.

The spectrum of a galaxy is also very important because it allows astronomers to determine the age of the galaxy, the mass of the galaxy, and estimate its star-formation rate, which is how many stars it forms each year. Watson’s team found thatA1689-zD1 is quite a typical galaxy; it is not forming stars at an alarming rate and its mass is on the light side (much less than our own Milky Way galaxy).

The other tool used to study A1689-zD1 was the Atacama Large Submillimeter Array (ALMA), which can be used to measure the dust content of the distant galaxy. When stars evolve and die, they eject their material into space and enrich the galaxy with new elements. After many cycles of star formation, the galaxy will become enriched with dust and hence dust is a characteristic of a mature star-forming galaxy.  When Watson’s team observed A1689-zD1 with ALMA, the galaxy appeared very bright, indicating the it contains a significant amount of dust.

But now there was a conundrum; the amount of dust detected within the galaxy is much larger than expected for its age (since many cycles of star formation are needed to build up the dust content of a galaxy). This indicates that the galaxy must have been forming stars for a long time, which is tricky because the Universe itself is not that old at this point – recall that the light we detect from A1689-zD was when the Universe was only about 700 million years old). Alternatively, it must have under gone a very intense period of star-formation, where its star formation rate was very high for a short period of time and during this short timeframe the stars in the galaxy could have produced a lot of dust. This is not the first time astronomers have discovered objects that appear to be much older than expected at high redshift. Last year, Caroline Straatman of Leiden University found a population of very mature compact galaxies at a redshift of 4, when the Universe was 1.6 billion years old, which means they too must have been vigorously forming stars while the Universe was very young.

This shows how little we still know about the early universe and the first galaxies. Understanding the formation and evolution of these distant galaxies is one of the main goals of astronomers, but is only possible if these objects can be detected. Hopefully, in the next decade more advanced telescopes will allow astronomers to look back to these epochs and answer fundamental questions about the galaxies which populated the Universe 13 billion years ago.

For more information, see

[Rebecca Allan]

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Dawn mission reaches Ceres

The NASA Dawn satellite has arrived at Ceres, the largest body in the asteroid belt, and is the first mission to orbit a dwarf planet.  On Friday 6 March, the Dawn satellite was captured by the gravity of Ceres and successfully went into orbit.

Ceres: as seen by the Dawn spacecraft on 1 March 2015 as a distance of 48,000 km from the dwarf planet. (Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

The Dawn spacecraft was launched in September 2007, and after receiving a gravity assist from March in February 2009, it arrived at the asteroid Vesta on 16 July 2011.  Dawn orbited and mapped the surface of Vesta for just over a year, sending back over 30,000 images of the giant asteroid. It continued on its journey for another 3.5 years,  finally arriving at the dwarf planet Ceres in February 2015. Now in orbit, Dawn will spend its first 15 days completing an orbit of Ceres. It will then slowly spiral closer towards the surface and begin mapping the dwarf planet. The mission is currently scheduled to end in mid-2016.

The trajectory of the Dawn spacecraft over the lifetime of its 8 year mission. (Credit: NASA/JPL/Marc Rayman)

Dawn has arrived at ‘dark side’ of Ceres, approached the night side of the dwarf planet that faces away from the Sun. Our next daytime view of Ceres will be in mid-April.  Images taken on the approach to Ceres in mid-February have already present some very interesting surprises: two extremely bright spots were imaged in a 92 km-wide crater, possible due to liquid or highly reflective material on the surface. The central spot is twice as bright as its smaller companion and is unresolved in the images, which means it is less than 4 km in size. We’ll have to wait for higher resolution images before we can determine the nature of these intriguing bright spots!

Dawn’s image of Ceres on 19 February 2015 at a distance of nearly46,000 km, showing two bright spots in a 92-km crater. (Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

Both the asteroid Vesta (with a diameter of just over 520 km) and the dwarf planet Ceres (almost twice as large with a diameter of 950 km) are both thought to be surviving protoplanets left over from the formation of the Solar System.  The three main scientific drivers of the Dawn mission are:  (1) to capture the earliest moments in the origin of the solar system enabling us to understand the conditions under which these objects formed; (2) to determine the nature of the building blocks from which the terrestrial planets formed; and (3) to contrast the formation and evolution of two small planets that followed very different evolutionary paths so that we understand what controls that evolution.

 

For more information, see

[Sarah Maddison]

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S1-2015 has begun!

The wheels fell off last semester, but we’re back! So get ready for another fabulous semester of SAO astro news :-)

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New definition of our local supercluster: Laniakea

After many years of cosmic map making, Brent Tully and his colleagues have (re)defined our local supercluster of galaxies and given it a new name: Laniakea (from the Hawaiian: “lani” is heaven, and “akea” is spacious or immeasurable).

The Laniakea supercluster is shown, where green areas are densely populated rich with galaxies (white dots) and white lines indicate motion towards the supercluster center. An outline of Laniakea is given in orange, while the blue dot shows the location of the Milky Way. Outside the orange line, galaxies flow towards other overdensities of galaxies (other superclusters). The Laniakea supercluster has a diameter of about 500 million light years and contains about 100,000 times the mass of our Milky Way Galaxy. (Credit: R. Brent Tully et al., SDvision, DP, CEA/Saclay)

Rather than simply plotting positions of galaxies near to us, Tully and his team took into account the radial velocities of the galaxies, which in turn allowed them to derive the peculiar velocities (which is the “true” velocity of the galaxies in local rest frame, which in this case is the difference between the radial velocity and the velocity expected purely from the general expansion of the universe).

These peculiar velocities are driven by gravitational effects, and are better tracers of total mass (for all types of matter, both luminous and dark) than simple positions of individual galaxies, which primarily show the location of baryonic matter. By mapping the peculiar velocities of many galaxies in our region of the Universe, information on galaxy motions and large-scale flows can be derived and a more accurate definition of the shape and extent of our local supercluster can be derived.

Laniakea is larger than the historical local supercluster defined up until now. Our Milky Way galaxy is at the origin.  Regions with well known names like the Great Attractor, Pavo-Indus, the Antlia wall, Fornax-Eridanus cloud, as well as the clusters Norma, Hydra, Centaurus, Virgo, Ophiuchus, Abell 2870, Abell 3581 and Abell 3656 are all embedded within the new Laniakea supercluster. Shapley, Hercules, Coma and Perseus–Pisces are complexes of galaxies outside Laniakea.

The supercluster region includes 13 Abell clusters, including the well known Virgo cluster. Local velocity flows within the region converge towards the Norma and Centaurus clusters, in good approximation to the location of what has been called the ‘Great Attractor’, after survey work done in the 1980s and 1990s. This volume includes the historical Local and Southern superclusters, the important Pavo-Indus filament, an extension to the Ophiuchus cluster, the Local Void, and the Sculptor and other bounding voids. This region of inflow towards a local basin of attraction can be reasonably called a supercluster.

If we approximate the supercluster as round, it has a diameter of 12,000 km/s (recession velocity) or a distance of 160 megaparsecs, and encompasses ~1017 solar masses.

The present work uses a new catalogue of galaxy distances and peculiar velocities, that extends to recession velocities of 30,000 km/s (redshift z = 0.1) and with 8,161 galaxy entries. The new catalogue is called Cosmicflows-2 and the six main methodologies for the distance estimates rely on the characteristics of Cepheid star pulsations, the luminosity cut-off of stars at the tip of the red giant branch, surface brightness fluctuations of stars in elliptical galaxies, the standard candle nature of supernovae type Ia, the adherence of elliptical galaxies to a fundamental plane in luminosity, radius, and velocity dispersion, and the correlation between the luminosities of spirals and their rates of rotation.

The above animation takes you through the Laniakea supercluster, pointing out various feaures. (The 2013 work on Cosmicflows-2 also produced a 17 minute video that explores the data set in a spectacular way. It is well worth 17 mins of your time!)

Suspicions of higher order galaxy clustering (i.e. cluster-cluster rather than just galaxy-galaxy clustering) had been aired from as early as the 18th century.  Nearby overdensities had been noted in the work of Messier and the Herschels and obvious overdensities were clearly seen in associated figures of the Shapley-Ames catalog (Shapley and Ames 1932). Shapley and Ames refer to a notable overdensity as the “elongated Virgo supersystem”.  The Local Supercluster revealed itself as nearby clusters tended to favor the northern Galactic hemisphere with the Virgo cluster dominant (de Vaucouleurs 1953) although de Vaucouleurs at the time used the term ‘supergalaxy’.

The authors of the recent Laniakea work stress that in the fullness of distance measurements on a much larger scale it will almost certainly be demonstrated that Laniakea is not at rest with respect to the cosmic expansion and is only a part of something very large that is at rest in the cosmic reference frame.

For more information:

[Glen Mackie]

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Rosetta prepares comet lander

After a long 10-year journey, the Rosetta spacecraft arrived at comet 67P/Churyumov-Gerasimenko in early August this year and has been busy mapping the comet’s surface to choose a landing site.  Rosetta is the first satellite to orbit a comet, and will soon be the first mission to place a lander on the surface of a comet.  The Rosetta satellite and the lander Philae, will follow the comet as it approaches the Sun over the next year. Five potential landing sites are currently being assessed and ranked, and next week at the ESA Headquarters in Paris the primary landing site for Philae will be announced, with the landing currently scheduled for mid November. 

Five potential landing sites on 67P/Churyumov-Gerasimenko. Three views of the comet, taken with the OSIRIS narrow-angle camera on 16 August from a distance of 100 km. (Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

The Rosetta mission is a joint ESA and NASA/JPL collaboration. The mission aims to keep Rosetta stay in close proximity to the icy nucleus of comet 67P/Churyumov-Gerasimenko as its orbit carries it into the warm, inner Solar System, where the comet will become more active.  The lander, which contains ten instruments, will be able to study in detail the physical properties of the comet’s surface, measuring its chemical, mineralogical and isotopic composition.  The spacecraft, which contains 11 of its own instruments, will orbit and map the comet as it gets closer to the Sun, investigating how the active comet interacts with the solar wind.

Rosetta’s journey began in March 2004 in French Guiana, when an Ariane 5 rocket launched the mission. During the 10-year journey to the comet, Rosetta has had its orbital energy boosted by four separate gravity assist flybys, three with the Earth and one with Mars. The satellite has crossed the asteroid belt twice, including a flyby of asteroid Steins in September 2008 and asteroid Lutetia in July 2010. It had almost 3 years of hibernation before being “woken” in January 2014 to prepare for the rendezvous sequence which began in May 2014, slowing the relative velocities between the satellite and the comet, allowing Rosetta to finally come alongside the comet in August 2014.

Comet 67P/Churyumov-Gerasimenko is a short period comet which orbits the Sun every 6.5 years. Its average distance from the Sun is about 3.5 AU, though its eccentric orbit (e=0.64) carries it between 1.2 AU, a little further from the Sun than the Earth, and 5.7 AU which is just beyond the orbit of Jupiter. Leading up to the August arrival, Rosetta revealed comet 67P/Churyumov-Gerasimenko’s strange shape: the irregular nucleus contained two distinct lobes, suggesting that it might be a contact comet, which form when two comets collide. The strange shape might also result from large amounts of ice evaporating from the surface during its close approach to the Sun, leaving behind the asymmetric shape we see today. (Follow this link for an animated sequence of comet views from Rosetta’s arrival.)

Rosetta’s view of comet t 67P/Churyumov–Gerasimenko 3 August 2014. (Credit: ESA/NASA JPL)

For the past month, Rosetta has been mapping the surface of the comet to learn about its structure and to select a landing site for Philea, while slowly manoeuvring closer to the surface. In November, Philea will be released from a height of about 1 km above the comet’s surface and unfold its 3 legs ready for a gentle touchdown at human walking speed.  Philea will then immediately fire a harpoon to anchor itself to the surface to ensure it doesn’t escape from the comet’s extremely weak gravity. Then both Rosetta and Philea will follow comet 67P/Churyumov-Gerasimenko as it approaches the Sun over the next 12 months or so. The current mission has a nominal end date of December 2015, while the lifetime of the Lander will depend on the cometary environment.

Rosetta’s planned journey as it accompanies 67P/Churyumov-Gerasimenko as it travels towards perihelion – its closest approach to the Sun – between January 2014 and December 2015. (Credit: JPL)

For more information about the mission, see

[Sarah Maddison]

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