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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S2-2014 has begun!

Get ready for another fabulous semester of SAO astro news!

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SAO wins NOVA award

The SAO team is delighted to announce that our unit Galaxies and their Place in the Universe won a 2014 NOVA Awards for Online Learning Excellence sponsored by Open Universities Australia.

Interacting

Snapshot of SAO’s 3D PDF of two interacting disk galaxies. (Credit: SAO and Daisuke Kawata)

The unit design principles of Open Universities Australia for which the NOVA is awarded are centred on authentic, adaptive, personalised, collaborative and supported learning. Our SAO unit Galaxies and their Place in the Universe provides an interactive, authentic and supportive learning environment via (i) the innovative use of online interactions & collaboration, (ii) assessment in an authentic context, (iii) appropriate & comprehensive learner support, and (iv) the use of technology for sound education outcomes.

As well as providing real-world hands-on activities which allow students to explore and discover the universe for themselves, Galaxies and their Place in the Universe is one of three introductory SAO units that contains interactive 3D PDFs.  The 3D PDFs, created using the Swinburne-designed S2PLOT package by David Barnes, Christopher Fluke and collaborators, allow students to interact with and explore complex 3D geometries and astronomical datasets.  An example of one of our 3D PDFs of interacting galaxies can be found below.

The NOVA awards were announced at the 2014 Education Frontiers Summit in Melbourne last week.  The SAO team submission was authored by Glen Mackie, Sarah Maddison, Chris Fluke and Artem Bourov. 

For more information, see

[Glen Mackie & Sarah Maddison]

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Double trouble: the birth of the magnetar

Magnetars are bizarre and exotic stellar remnants formed by the gravitational collapse of massive stars that are tens of times more massive than our Sun. The strongest magnets in the Universe, these objects are a rare variant of the neutron star, which are the relics of supernova explosions.

Artist’s impression of a magnetar in a rich, young star cluster. The lines represent the very strong magnetic field of the magnetar. (Credit: ESO/L. Calçada)

Depending on its initial mass, a star will end its live as one of three types of stellar remnants. White dwarfs are the remnants of low to intermediate mass stars (less than about 8 times the mass of our Sun) that eject their outer envelopes towards the end of their lives, leaving behind the exposed core of the initial star.  More massive stars have more violet deaths, ending their lives in a supernova explosion. Stars between about 8 and 40 solar masses will leave a neutron star behind after the supernova event, while even more massive stars will produce black holes as their stellar remnants.  The initial stellar mass that separates neutron stars and black holes is not very well constrained.

Like “normal” neutron stars, magnetars are tiny (about 20 km in diameter) and extraordinarily dense, with a teaspoon of magnetar material weighing around a billion tonnes. What sets magnetars apart from other pulsars is their incredibly high magnetic fields, which are of order 1015 G (which is 100 to 1,000 times stronger than radio pulsars). They rotation every few seconds, but it is thought that they must be born with rotation rates of 100 to 1,000 times per second to achieve such strong magnetic fields.

Exactly how magnetars form is unclear.  Simon Clark from the Open University and collaborators determined that one of the Milky Way’s 21 known magnetars must have formed from a star that was initially at least 40 times the mass of the Sun. The magnetar CXOU J164710.2-455216 (aka J1647-45) resides in the stellar cluster Westerlund 1, which is the closest ‘super star cluster’ to us. At a distance of 16,000 light years, Westerlund 1 contains hundreds of massive stars that are at least 30-40 times the mass of our Sun.

The problem, however, is that stars initially 40 times the mass of the Sun or more should end their lives as a black hole rather than a neutron star. Clark and collaborators have recently identified a small, bright, carbon-rich star that is hurtling away from J1647-45. They suggests that this ‘runaway star’, discovered with the Very Large Telescope in Chile’s Atacama Desert, was once locked in a binary system (two gravitationally bound stars closely orbiting each other) with the stellar progenitor of the magnetar. When the supernovae occurred there was sufficient force in the explosion to kick the companion star, Westerlund 1-5, out across the Westerlund 1 cluster.

The European Southern Observatory’s Very Large Telescope (VLT) in Chile. The world’s most advanced optical instrument, consisting of four Unit Telescopes with main mirrors of 8.2m diameter and four movable 1.8m diameter Auxiliary Telescopes. (Credit: ESO/S. Brunier)

This finding allows astronomers to recreate the evolutionary path taken by the binary system that allowed it to form a magnetar in place of a black hole. In the scenario proposed, the larger of the two stars (Westerlund 1-5) begins to shed its outer envelope, transferring some of its mass to the smaller companion star – fated to form the magnetar (J1647-45). As a result the progenitor of J1647-45 rotates faster and faster as it accrete gas shed by Westerlund 1-5, which amplifies its magnetic field to colossal strengths. Once a critical mass is reached, the progenitor of J1647-45 ejects its outer layers, a portion of which is transferred back to Westerlund 1-5. This volatile swapping of material provides the unique mix and enrichment of elements seen in the runaway star Westerlund 1-5. Crucially, it also reduces mass of the progenitor of J1647-45, slimming the star down sufficiently to produce a magnetar instead of a black hole at the moment of its death following the supernova explosion.

The unique combination of properties of the runaway star – it high velocity (expected from a star ejected by a supernova explosion), low mass, high luminosity and carbon-rich composition – is almost impossible to understand in a single star. These properties all support the idea that the runaway star must have originally been part of a binary system.

The new results suggest that being part of a binary star system may be an essential ingredient in forming at least some magnetars. The mass transfer between the stars produces that rapid rotation rate that is needed to produce the strong magnetic field of the magnetar, and the subsequent mass loss allows the progenitor to be in the right mass range to collapse into a magnetar rather than a black hole

For more information, see

[Toby Brown & Sarah Maddison]

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Coldest brown dwarf ever discovered just around the corner

Astronomers are constantly pushing our knowledge of galaxies farther toward the edge of the visible Universe, and yet we tend to forget that our local neighbourhood has not yet been fully mapped.  Just last month astronomers discovered the sixth-closest system to us, the brown dwarf WISE J085510.83-071442.5, which is also the record holder for the coolest star around.

Brown dwarfs are sub-stellar objects with masses between those of stars and planets (roughly between 10 and 80 times the mass of Jupiter). Like stars, brown dwarfs are thought to form from the collapse of molecular clouds, but their low mass prevents them from igniting nuclear reactions in their cores and shining brightly like our own Sun. Because they are much cooler than stars, the atmospheres of brown dwarfs contain  complex molecules like methane, which is seen in the giant gas planets of our Solar System. This makes brown dwarfs unique tools to investigate both the low-mass end of star formation and the chemistry of planetary atmospheres.

At the time of their formation, brown dwarfs have a typical surface temperature around 1,000 K and will cool down as they get older.  This means that they cannot be seen at optical wavelengths and infrared telescopes are needed to detect brown dwarfs.  In the last few years, the Wide-field Infrared Survey Explorer (WISE) has nearly doubled the number of known brown dwarfs within 30 light-years from the Sun. By surveying the entire sky several times, WISE has not only been able to detect a large number of brown dwarfs, but also measure their proper motion (which is their angular velocity as they are seen to move across they sky). By following the motion of brown dwarfs over several years, astronomers can then determine their distance from the Earth using parallax.

WISE J085510.83-071442.5 is the latest brown dwarf discovered by WISE. Not only is it the closest brown dwarf to us at a distance of just 7.2 light years (compared to the closest star system alpha Centauri at a distance of about 4 light-years), but it’s also the coldest with a temperature of about 240 K, or about -30 C.

Kevin Luhman of Pennsylvania State University’s Center for Exoplanets and Habitable Worlds noticed that WISE J085510.83-071442.5 was moving pretty fast when comparing images from WISE that were six months apart.  The fast motion compared to the other stars in the images suggested that the object was very close to the Earth.  Luhman analysed images from the Spitzer Space Telescope and the Gemini North telescope in Hawaii to further track the motion of WISE J085510.83-071442.5, as well as help determine its temperature.  Combined the detections from WISE and Spitzer, taken from different positions around the Sun, enabled the measurement of its distance through parallax.

The coldest and closest brown dwarf yet known, WISE J085510.83-071442.5 was discovered through its rapid motion across the sky. It was first noticed in two infrared images taken six months apart in 2010 by NASA’s Wide-field Infrared Survey Explorer, or WISE (orange triangles). Two additional images taken with NASA’s Spitzer Space Telescope in 2013 and 2014 (green triangles) show its continued motion across the sky. The four images were used to measure a distance of 7.2 light-years using the parallax effect. (Credit: ASA/JPL-Caltech/Penn State)

Its incredibly low temperature implies that WISE J085510.83-071442.5 is very old, as it would take several billion years for a brown dwarf to cool from a thousand degrees to its current chilly temperature. The estimated mass is extremely low, between 3-10 Jupiter masses.  This makes WISE J085510.83-071442.5  the closest, coldest, oldest and lowest mass sub-stellar objects detected to date!  Such a low mass puts it in the mass range of giant planets, which opens the intriguing possibility that WISE J085510.83-071442.5 may not be a brown dwarf, but might in fact be a giant planet that originally formed around a star and was later ejected by gravitational interactions with other planets in the system. Unfortunately, there is no way to directly test this scenario and Luhman excluded the hypothesis on a statistical basis, given that there brown dwarfs are expected to be common while the frequency of ejected planets is unknown.

Whatever the real nature of WISE J085510.83-071442.5, this exciting discovery shows that, even after many decades of studying the sky, our local neighbourhood is still full of surprises.

For more information, see

[Luca Cortese and Sarah Maddison]

 

 

 

 

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First exoplanet day determined

Astronomers have determined the length of a day on an exoplanet for the first time. While over 1000 exoplanets have been discovered and characterised, none have yet had their rotation rates measured.

Most exoplanets that we know of have been detected in an indirect way, by measuring the effects the unseen planet has on the star it orbits.  Exoplanet are discovered by either measuring the  “wobble” of the star due to the gravitational perturbations of the planet, or by measuring the dimming of light as the unseen planet passes in front of the bright star and blocks some of the light. However, some exoplanets, like  β Pictoris b, have been directly detected, meaning astronomers observe the light coming from the planet itself.  It is easiest to directly detect young, massive planets that are not too close to their host star.  The further the planet is from its host star, the easier it is to distinguish the light from the planet (which is extremely faint compared to the bright star). Young planets still contain a large amount of heat left over from their formation, which makes them relatively big and puffed up.  The more massive a young planet is, the warmer and hence brighter it will be, making it easier to detect at infrared wavelengths.

Three infrared observations over 7 years of the exoplanet β Pic b. The light from the central star is blocked out, but the stellar location marked. The planet has moved to the other side of the star from 2003 to 2009, and its motion is clearly seen from October 2009 to March 2010. (Credit: Bonnefoy et al. 2011, Astronomy & Astrophysics)

While these images clearly show the motion of the planet, they cannot tell us the direction of the planet’s motion.  Using near-infrared high-dispersion spectroscopy with the help of the CRIRES instrument on the Very large Telescope in Chile, Ignas Snellen and collaborators have determined the orbital and rotational velocity of the planet orbiting  β Pic. The CRIRES  spectrograph uses a diffraction grating to break the infrared light up into its component wavelengths so that astronomers can target specific spectral lines. Snellen’s team targeted molecules produced in the cool atmospheres of giant planets (which include water, carbon monoxide, methane and ammonia). The spectral lines from these molecules, which are found in the near-infrared part of the electromagnetic spectrum, are quite distinct from the relatively smooth spectrum of the star. The atmosphere of the star is much hotter than that of the planet – especially for a large, hot A-type star like  β Pic – and it contains only a few atomic spectral lines rather than molecular lines.

The spectrum of the planet is shifted in wavelength by an amount that depends on the orbital speed and inclination of the planet’s orbit, and the spectral line will also be broadened by an amount the depends on the rotation speed of the planet. As the planet rotate, half of the planet appears to be moving toward us, which shifts the spectral lines towards to blue, while the other half of the planet appears to be moving away from us, shifting the spectral lines towards to red. These two effects combine and make the spectral lines both broader and shallower.  Snellen et al. found that the CO spectral lines of the planet was broad and blueshifted. By studying the position and shape of the spectral lines and comparing them to model spectra with different planetary rotation rates and spin axis inclinations (planetary “obliquity”), they calculated that the rotation rate must be at least 25 km/s.  The diameter of β Pic b is thought to be about 1.6 times that of Jupiter, which implies that a day on β Pic b lasts for 8 hours. The blueshifted spectra indicates that the planet is currently on a part of its orbit that is moving towards the Earth.

Spectral signal of β Pic b, which is rotationally broadened by 25 km/s, and also blueshifted with respect to the velocity of the host star, indicating that the planet is currently moving towards the Earth on its orbit.  (Credit: Snellen et al. 2014, Nature, 509, 63)

Spectral signal of β Pic b, which is rotationally broadened by 25 km/s, and also blueshifted with respect to the velocity of the host star, indicating that the planet is currently moving towards the Earth on its orbit. (Credit: Snellen et al. 2014, Nature, 509, 63)

The equatorial rotation rate of 25 km/s of β Pic b fits in quite well with the “mass-spin relation” we see for the planets of our Solar System, where the rotation rate increases with planetary mass.  The exact origin of this relation is not clear, but is likely due to planet formation. While the rocky terrestrials and gas giant planets follow different formation paths, all planets grow by the accretion of impacting bodies which add their angular momentum, or spin, to the growing planet. The larger the planet, the more impacting bodies it will have accreted and this could explain why more massive planets have faster spin rates. Given that β Pic b is still a young and hot planet, as it slowly cools it will contract, which will speed up its rotation rate (due to conservation of angular momentum). Over the next few hundred million years, the length of a day on β Pic b is expected to shorten to about 3 hours.

Rotation rate versus mass for the planets in our Solar System, plus  β Pic b. Mercury and Venus do not fit thsi relation as they have been spun down due to tidal interaction with the Sun.   (Credit: Snellen et al. 2014, Nature, 509, 63)

Rotation rate versus mass for the planets in our Solar System, plus β Pic b. Mercury and Venus do not fit this relation as they have been spun down due to tidal interaction with the Sun. (Credit: Snellen et al. 2014, Nature, 509, 63)

As number of exoplants that are directly detected increases, this technique will help astronomer investigate the spin-mass relation on other planets, as well as study the weather systems on exoplanets.  The rapid rotation rates of the giant planets in our Solar System dominate their weather systems, which is most likely true on β Pic b as well.

For more information, see

[Sarah Maddison]

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