August 2012: A Ring To It

In the show this time, Dr Fergus Simpson tells us about mapping dark matter using weak gravitational lensing and Professor Carlos Frenk talks about exploring the nature of dark matter through simulations. Megan summarises the latest astronomy news and we find out about the August night sky from Ian Morison and John Field.

The News

In the news this month:

• To stargazers on the Earth, the familiar planets all appear to follow roughly the same path across the sky as the Sun and the Moon. This is because the major planets in our solar system, including our own, orbit the Sun in the same plane. This is one aspect of Pluto's nature which distinguishes it from the larger planets: its orbit is tilted compared to the others, making it appear to wander further around the night sky during its long orbit. The orbital plane of the planets lines up with the equator of the Sun, reflecting the way our solar system likely formed from a single rotating disc of gas some five billion years ago. However, many of the planetary systems discovered around other stars in recent years show no such regularity in their orbits, with exoplanets often having misaligned or retrograde orbits. But now, one planetary system has been found which suggests that our solar system is not so unusual after all.

  The star Kepler-30 is similar in mass and size to the Sun, but rotates faster and has more sunspot activity. It is these sunspots which are key to measuring the inclination of the orbits of the planets associated with it. Led by Roberto Sanchis-Ojeda of the Massachusetts Institute of Technology in the US, a team of astronomers used two and a half years of data from the Kepler spacecraft to identify a total of 44 transits of the three known planets in the Kepler-30 system. Hidden within the slight dips in brightness, caused by the planets passing in front of the star, the team also found signatures of the planets passing in front of starspots - regions of the star's outer layers which are cooler and darker than the surrounding surface. Crucially, they found that all three planets repeatedly transited across the same starspot. For this to happen, the orbits of the three planets must be very closely aligned with the equator of the star, and with each other, implying that Kepler-30's planetary system lies in an equatorial plane just like our own, and that the system originated in a spinning disc of gas.

  The results have implications for our understanding of the formation of planetary systems. The many known so-called 'hot Jupiters' - large gas giants orbiting close to their parent stars - are often in highly tilted orbits. It is thought that the irregular nature of these systems may be due either to a misalignment of the the star and the disc from which the planets formed in the first place, or from dynamical interactions which perturbed their orbits after the planet's formation. According to the authors of this study, published in Nature on the 26th of July, the second hypothesis is more likely, implicating dynamical interactions in the creation of hot Jupiter systems.

• In the local universe, spiral galaxies are fairly common. Some of the most well-known (and often-photographed) galaxies are grand design spirals, those with well-defined bright spiral arm structures - the Whirlpool Galaxy (M51) being one famous example. While such spectacular galaxies are common today, the further out in space we look, and the further back in the history of the Universe we observe, the fewer spiral structures we see. Up to now, only one such spiral has been identified at a redshift greater than two, corresponding to looking back in time some 10.7 billion years. Now, astronomers using the Hubble Space Telescope to survey 300 galaxies in the early Universe have found a second example of such a grand design spiral galaxy which existed just three billion years after the Big Bang.

  Led by David Law of the University of Toronto in Canada, the team surveyed a large number of distant galaxies with Hubble. They then carried out further detailed observations with a number of instruments at the Keck Observatory on Hawaii and found evidence that one galaxy in their sample has a rotating disc with prominent spiral arms. By using spectroscopy to measure the motion of gas in the galaxy, they found that the structure is an actual spiral disc, rather than two galaxies aligned to give the appearance of spiral structure.

  Most galaxies at this epoch look clumpy and irregular, so this new find (labelled BX442) is very unusual. Out of their sample of more than three hundred at a similar redshift, BX442 is the only one to show any signs of spiral structure, despite 27 galaxies having similar masses, and ten of those closely matching other properties of BX442.

  There could be more than one reason for this lack of spirals at high redshift. It is thought that galaxies at this early point in the Universe would have been too hot and turbulent for stable spiral arm structures to form, but it could also be that such structures are extremely short-lived, that the mechanism which triggers the formation of such structures is extremely rare, or that today's telescopes are simply not sensitive enough to detect such structures at these large distances. The success of these observations depended on the large size of the Keck telescope and the use of a laser adaptive optics system to correct for distortions caused by the Earth's atmosphere in order to see this galaxy in detail at such a large distance.

  The researchers found that, in common with many grand design spiral galaxies we see in the local universe, the images of BX442 show that it is in the process of merging with another system. Since galaxies in the early universe are thought to be too turbulent for spiral arms to form, it could be the gravitational kick from this interaction which caused the spiral structure to form. Publishing their results in Nature on the 19th of July, the researchers argue that the estimated lifetime of a merger-induced spiral structure in a massive galaxy with a hot, thick disc is consistent with their observed rarity; the estimated lifetime of such structure in galaxies like BX442 is less then 100 million years, less than half the time taken for the galaxy to rotate once. So, for observers on Earth to see a spiral structure at this redshift, not only do we have to have a massive galaxy with a large extended disc, it also has to have a suitable companion to trigger the formation of spiral structure, it has to be almost face-on to our line of sight so that we can see it, and we have to observe it during the small time window when the structure is prominent.

• And finally: Going even further back in cosmic history, a study published in the Monthly Notices of the Royal Astronomical Society during July describes the first direct detection of dark galaxies, small gas-rich galaxies with few or no stars that are thought to be the building blocks of the much larger galaxies we see in the Universe today. The trouble is, as the name suggests, since they have very few stars, they don't emit much light, making them incredibly difficult to detect. Led by Sebastiano Cantalupo at the University of California in the US, a team of astronomers has detected some of these mysterious objects for the first time.

  Such low-luminosity galaxies are predicted to exist by models of galaxy formation in the early Universe but, while hints of such dark galaxies have been seen before, this is the first time that they have been directly detected. Like a white T-shirt glowing in the light of an ultraviolet lamp, these galaxies do not emit light of their own, but reflect ultraviolet light emitted by a nearby bright object such as a quasar - a supermassive black hole at the centre of a galaxy. Using the large collecting area and sensitive detectors of the European Very Large Telescope in Chile, together with a custom-built filter, the team mapped a region of sky around a particularly bright quasar, looking for the faint fluorescent glow of hydrogen in dark, and otherwise invisible, gas-rich galaxies. Using this technique, the team discovered almost 100 gaseous objects in the region of this quasar, known as HE 0109-3518. The astronomers found evidence that twelve of them are dark galaxies with extremely low rates of star formation, exactly the sort of primordial galaxy predicted to exist in the early Universe.

  From their observations, the team were able to calculate the properties of such galaxies. They have typical masses of around one billion times that of the Sun, some one hundred times less massive than our own Milky Way Galaxy, and have star formation rates two hundred times lower than typical star-forming massive galaxies found at the same time in the history of the Universe. Galaxies such as these may be the source of the gas reservoirs from which the galaxies we see today formed their stars, and these results will help refine models of the history of galaxy formation in the early Universe.

Interview with Dr Fergus Simpson

Dr Fergus Simpson, from the Royal Observatory Edinburgh, talks to us about CFHTLenS, a wide field survey done with the Canada-France-Hawaii Telescope, focusing on mapping dark matter using gravitational lensing techniques. As light from galaxies travels towards us, it is deflected by matter on its way. By doing a statistical study of the deformation in the shapes of galaxies, a team of astronomers from France, Canada, Hawaii and the UK were able for the first time to produce a large map of the distribution of dark matter in our Universe.

Interview with Prof. Carlos Frenk

Prof. Carlos Frenk is Director of the Institute for Computational Cosmology at Durham University, and works on some of the world's largest and most sophisticated simulations of the evolution of structure in the Universe. He gave a plenary talk at the National Astronomy Meeting on testing the Lambda-CDM cosmological model by observing the dwarf satellite galaxies that orbit large galaxies. Here, he explains that the observed numbers, sizes and distributions of satellites around large galaxies do not fit with those predicted by simple Lambda-CDM simulations, leading to two possibilities: either the simulations need to include more physics than just the force of gravity, or the dark matter particles are 'warm' (i.e. more energetic) rather than cold. Prof. Frenk also provides insight into how computer simulations of the Universe work, and why there can be more than one approach to solving a single set of physical equations. He talks about how simulations can now reproduce stars and galaxies, and speculates as to how far they may eventually go.

The Night Sky

Northern Hemisphere

Ian Morison tells us what we can see in the northern hemisphere night sky during August 2012.

Arcturus, in Boötes, is setting in the west in the evening. Corona Borealis and Hercules are to the south, the latter containing the globular cluster M13 on the right-hand side of the four stars of the Keystone. Overhead, the Milky Way passes through the Summer Triangle which consists of the bright stars Vega, Deneb and Altair in the constellations of Lyra, Cygnus and Aquila respectively. The Square of Pegasus is rising upside-down in the east. Following the Milky Way north from Deneb, we reach Cassiopeia, and then Perseus towards the northern horizon. Many objects, such as the Double Cluster, can be seen between these two constellations.

The Planets

• Jupiter rises around 02:00 BST (British Summer Time, one hour ahead of Universal Time) at the beginning of the month and around midnight by the end. It gets to at least 16° elevation by sunrise, shining at magnitude -2.2. It starts the month just 5° above the star Aldebaran, the eye of Taurus the Bull, and then moves towards the horns of Taurus, brightening to magnitude -2.3 as its angular diameter increases from 36 to 39", allowing plenty of surface detail, as well as its four largest moons, to be seen through a small telescope.
• Saturn is low in the west after sunset as it is coming to the end of its current apparition. It lies in Virgo at magnitude +0.8, starting the month 4.5° north of the first-magnitude star Spica. Its disc is 16.5" across, and the rings are at 14° to the line of sight and span 37". Its largest moon, Titan, lies south of Saturn on the 8th, north on the 16th and south again on the 24th, and can be seen using a small telescope.
• Mercury reaches greatest western elongation (furthest point from the Sun in the sky) on the 16th, lying 19° away. It has a magnitude of -0.1 at this time, but it is only 10° above the horizon half an hour before sunrise, so binoculars may be needed to see it (make sure you are not looking through them once the Sun comes up). Although it reaches magnitude -1 by month's end, it is closer to the horizon at dawn and so harder to see.
• Mars is close to Saturn, moving eastwards through Virgo and also reaching the end of its apparition. Its angular diameter of 5" prevents detail being seen. Its elevation is only 6° as darkness falls at the beginning of the month, getting lower as the month progresses, so you need a low western horizon to spot it.
• Venus has reappeared in the pre-dawn sky and reaches its greatest western elongation on the 15th. It then rises about three hours before the Sun and shows a crescent phase. Through the month, it moves from the horns of Taurus, past Orion's Club, and into Gemini on the 13th. Its angular size drops from 28 to 20" during August, but it also waxes, from 42 to 58%, such that its brightness decreases only slightly, from magnitude -4.6 to -4.4.

Highlights

• The Perseid meteor shower is visible from about the 11th to the 14th. The peak is from 00:30 to 03:00 BST on the 13th, when you may see 20-30 meteors per hour as the Moon is almost new and therefore does not spoil the darkness of the sky. Look to the north-east from 23:00 BST on the four nights at the peak of the shower, focusing about 15° away from the radiant, between Perseus and Cassiopeia.
• Sweeping about one third of the way from the star Altair towards the star Vega, and crossing the Cygnus Rift, where dust clouds obscure the Milky Way, you have a good chance to see Brocchi's Cluster this month. To its upper left is the beautiful double star Albireo, which forms the head of Cygnus the Swan. It is at magnitude +3, but a telescope reveals a brighter amber star and a fainter blue-green star separated by 35", the former of which is itself a double.
• The planets Saturn and Mars and the star Spica are close together in the first half of the month, low above the western horizon. The form a triangle on the 7th and a straight line on the 13th and 14th. You may need binoculars as they set not long after the Sun.
• Venus and a thin, waning crescent Moon appear together on the 14th, and Venus is the middle of a line of three planets, with Mercury and Jupiter either side of it.
• Neptune is closest to the Earth on the 24th and can be observed around new Moon on the 17th. It is located in Aquarius, and, at magnitude +7.8, can be spotted with binoculars, while a telescope may show its bluish disc of 2.4" across.
• The 31st sees a blue Moon - the second full Moon in a calendar month. The term may originally have referred to the third of four full Moons in a season.

Southern Hemisphere

John Field from the Carter Observatory in New Zealand speaks about the southern hemisphere night sky during August 2012.

Scorpius and Crux are high in the evening sky, running along the path of the Milky Way which stretches from east to west. A number of avian constellations cluster around the south celestial pole. Pavo, the Peacock, contains the bright star Alpha Pavonis and the nearby Sun-like star Delta Pavonis, as well as the barred spiral galaxy NGC 6744. The galaxy is similar in structure to the Milky Way, and, at magnitude +9, can be seen as a haze using a small telescope, while larger telescopes reveal its spiral arms. Pavo also contains the globular cluster NGC 6752, the third-brightest globular cluster in the night sky, which, at magnitude +5.4, is just visible to the naked eye under a dark sky. Binoculars or a telescope may reveal curved lines and loops of stars. Nearer to the south celestial pole is Apus, the Bird of Paradise, which contains a double star of magnitude 4.6 called Delta Apodis. It consists of a red and an orange star which may be connected but are not gravitationally bound, as they are at different distances from us. A telescope of 20 centimetres in diameter reveals the 9th magnitude globular cluster NGC 6101 in Apus. Tucana, the Toucan, sits near Pavo. It hosts a spectroscopic binary star, Alpha Tucanae, which has an orbital period of 11.5 years and a magnitude of +2.8. The star Beta Tucanae is a group of six stars bound loosely together by gravity. The dwarf galaxy known as the Small Magellanic Cloud (SMC) resides in Tucana, and looks like an indistinct cloud to the naked eye. Appearing next to it is the brightest globular cluster in the sky, 47 Tucanae. It looks like a fuzzy star to the unaided eye, but in fact contains millions of stars and makes a stunning sight in binoculars.

Highlights

• The planets Mars and Saturn move closer together during early August, and Mars passes between Saturn and the star Spica from the 13th to the 17th. A small telescope Saturn's rings and its largest moon, Titan, and possibly other moons. Mars, meanwhile, has a very small disc at the moment and displays very little detail.
• Venus and Jupiter appear in the morning sky this month. The asteroid Vesta is close to Jupiter in the sky, and on the 8th it appears, at 8th magnitude, near to Aldebaran, the brightest star in Taurus. The dwarf planet Ceres, the largest object in the asteroid belt, is not far away either. NASA's Dawn spacecraft will soon leave Vesta and head towards Ceres, arriving in February 2015 to take the first close-up images of a dwarf planet.

Odds and Ends

Astronomers using NASA's Spitzer Space Telescope have detected what they believe is a planet two-thirds the size of Earth. The exoplanet candidate, called UCF-1.01, is located a mere 33 light-years away, making it possibly the nearest world to our solar system that is smaller than our home planet.

MediaCity, in Salford, will be hosting a NASA event, streaming the landing of the Mars Rover Curiosity live onto a big screen. The event is open to the public and the screening begins at 5am on Monday the 6th of August.

The Defence Advanced Research Project Agency, a US governmental organisation, is hoping to start a project called Phoenix, which will ultimately use a number of mini-satellites to harvest useful components from defunct communication satellites and use them to create new, functioning satellites, all whilst in orbit around the Earth. As Universe Today puts it: "Frankensatellites anyone?"

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