December 2012: Argo

In the show this time, Dr Phil Marshall talks to Libby about how and why he weighs galaxies, Ian Morison and John Field bring us the December night sky and Megan gives her last-ever round-up of the astronomy news.

The News

• There is convincing evidence that most, if not all, galaxies contain a central supermassive black hole with a mass between hundreds of thousands and billions of times that of the Sun. The best-studied of these black holes is also the closest: the one located in our own galaxy, known as Sagittarius A*. The most massive black holes known are predominantly located in giant elliptical galaxies, whilst smaller ones are generally found inside the central bulge of disk galaxies similar to the Milky Way. Much of the observational evidence collected to date suggests that the properties of a galaxy's central black hole correlate with those of its host galaxy's bulge, but the physical reason why this should be is not clear and many different theoretical models have been proposed to explain the observed results. Now, a team of astronomers have discovered a black hole which is abnormally massive compared to its host galaxy, a discovery which could potentially challenge models of galaxy formation.

  As massive as they are, these central compact objects make up only a small fraction of the total mass of a galaxy - typically about a tenth of one per cent - the current record holder being NGC 4486B, with 11 per cent of its mass accounted for by a central black hole. But now, publishing in Nature in November, a group led by Remco van den Bosch of the Max Planck Institute for Astronomy in Germany have discovered an unusual compact galaxy where the central black hole makes up more than half (59%) of the total mass of the galaxy's central bulge.

  Black holes, as the name suggests, are not easy to detect directly, but their presence is felt by the surrounding material and these effects are easier to see. One way the mass of a black hole can be calculated is by measuring the velocity dispersion - the distribution of velocities seen in the surrounding material. Using the Hobby-Eberly telescope in the USA, the team observed nearly 700 nearby galaxies and examined their spectra, looking for the tell-tale broadening of spectral lines caused by the motion of stars in the vicinity of the black hole. In a similar way to a siren changing in pitch as an ambulance races past, the motion of the stars alters slightly the colour of the light we see. Measuring this broadening of spectral lines gives us an idea of how the velocities of the stars in the bulge deviate from the average: the larger the deviation, the greater the velocity dispersion and the more massive the black hole must be. Despite the small size of the host galaxy, the spectrum of NGC1277 shows that its central black hole is one of the most massive yet discovered - the team's results show that it has a mass of roughly 17 billion times that of the Sun. In total, the mass of NGC1277 is around 120 billion solar masses, making the black hole accountable for an exceptional 14 per cent of the galaxy's total mass. Is this peculiarly dense galaxy an exceptional one-off, or is it the first example of a new population? From their survey the team found five other small galaxies with similarly large velocity dispersions, indicating that the central 200 parsecs contain more than 10 billion solar masses of material, some 100 times larger than typical galaxies of the same size.

  It is so far unclear whether these six galaxies with strangely dense central regions are simply outliers in the distribution of black hole/galaxy properties, or evidence of a different population which formed in a different way, but the astronomers conclude that the properties of these six galaxies (small, red, containing mainly old stars with no evidence of recent star formation) are similar to those of typical red, passive galaxies seen in the early universe, suggesting that these compact nearby systems may be present-day analogues of high-redshift galaxies.

• Supernovae come in many flavours and, until recently, there were thought to be just two main mechanisms. Now, a third class of supernova, only discovered a few years ago, is opening a new window on the early Universe.

  Supernovae lacking evidence for hydrogen and helium in their spectra are known as type Ia supernovae and are caused by material from a companion star building up on an evolved white dwarf in a binary stellar system; when the white dwarf reaches critical mass it undergoes a thermonuclear explosion. The second class of stellar explosion is the core-collapse supernova, where a massive star (greater than about eight times the mass of the Sun) fuses increasingly heavier elements in its core. This process results in greater and greater quantities of iron in the star's core until, eventually, the core collapses under gravity releasing huge amounts of energy which rip the star apart. Both of these classes of supernova are incredibly bright, and type Ia supernovae are so luminous that they can be used to probe the Universe back to a time when it was less than half its present age. It is studies of this class of explosion which led to the discovery of dark energy and the award of last year's Nobel prize in physics.

  Since light takes time to travel across space, the light we see from distant objects is old and allows us to examine the Universe as it was when it was much younger - the longer light takes to reach us, the older it is, and the further back in time we are looking. Now, an international team of astronomers led by Jeff Cooke at Swinburne University of Technology in Australia, have discovered two examples of a rare third class of superluminous supernova, explosions which are ten or more times more luminous than other types of supernova. Because they are so bright, they can be seen at much larger distances than other types of supernova, allowing us to probe even further back in the history of the Universe. This class of supernova is thought to occur in extremely massive stars, those with masses between 100 and 300 times that of our Sun. One possible explanation for this type of event is known as pair instability: such massive stars, if they are massive enough, could end up with cores big enough and hot enough to create pairs of electrons and positrons. This process can reduce the pressure in the star's core so that it shrinks and heat up, causing a massive thermonuclear explosion which releases large amounts of gamma rays. Although rare, these events are typically 10 times brighter than type Ia (thermonuclear) supernovae, and 100 times as luminous as normal core-collapse supernovae.

  Using existing data from the Canada-France-Hawaii telescope (CFHT), Cooke and his team took images taken over several years and combined them in a new way to look for distant transient events. They found two transients whose properties are similar to those of superluminous supernovae seen in the nearby Universe, but which are much further away. The previous record for detection of a type Ia supernova was at a redshift of 1.55. These new detections are at redshifts of 2.05 and 3.9, so the explosions happened just 3 and 1.5 billion years after the Big Bang. Theory suggests that massive stars and such extreme explosions were much more common in the early Universe than they are today, so, while examples of these pair-instability supernovae are still extremely rare and much uncertainty remains over the underlying mechanisms, they could potentially become highly useful probes of the earliest days of star formation.

• Being the closest planet to the Sun, Mercury's surface is hot enough in places to melt lead, so it would seem an unlikely place to find water ice. But recent data sent back from the Messenger probe (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) shows evidence of abundant water ice and other volatile materials in craters near the planet's poles. The idea that ice may exist near Mercury's poles is not a new one. Unlike the Earth, which is tilted relative to the plane of its orbit around the Sun by 23.5 degrees, the tilt of Mercury's rotational axis is almost zero, so there are places on the planet's surface which never see daylight. In 1991 a radar experiment, carried out using the 305-metre Arecibo telescope in Puerto Rico, detected reflective patches at Mercury's poles, with the characteristics expected from patches of water ice. The trouble is, very similar radar signatures could have been generated by other chemicals. Now, several lines of evidence have come together to provide stronger proof that ice is hiding in the shadows.

  November saw the publication of three papers describing findings from NASA's Messenger mission to the planet Mercury. Writing in Science Express, three teams of researchers present different results which, together, substantially strengthen the case for volatile substances on the planet's surface. One team, led by Gregory Neumann at NASA's Goddard Space Flight Center, used the Mercury Laser Altimeter to measure the reflectance of the planet's surface in regions of permanent shadow near the north pole, and found patches of unusually dark and bright deposits at the same locations as the highly radar-reflective spots seen by Arecibo 20 years ago. Comparing these deposits to thermal models of the planet by a team led by David Paige of the University of California, calculated from topographic data collected by Messenger, shows that the bright patches are located in regions where the temperature is cold enough that water ice could exist at the surface, while the dark patches correspond to regions where water ice could exist in a layer just beneath the surface regolith. The third piece of evidence comes from the neutron spectrometer on board Messenger, an experiment which is able to map the concentration of hydrogen on the planet's surface. Since water is composed of hydrogen and oxygen atoms, high concentrations of hydrogen suggest the presence of water ice deposits. This team, led by David Lawrence of Johns Hopkins University, used data from Messenger's Neutron Spectrometer to map the flux of neutrons from Mercury's northern polar region and found large concentrations of hydrogen within the same radar-bright regions. Taken together, the three results show convincing evidence for large water ice deposits both at and just below the surface of Mercury in the perpetually cold and dark polar regions. The teams conclude that the most likely source of this water is cometary or asteroid impacts which occurred perhaps 50 million years ago, which, geologically-speaking, is fairly recently.

• And finally... The End.

Interview with Dr Phil Marshall

Dr Philip Marshall is a Royal Society Research Fellow at the University of Oxford. In this interview, Phil tells us how he goes about weighing galaxies using gravitational lensing and how this relates to the fraction of their mass that is composed of dark matter. He also describes how to measure the expansion rate of the Universe.

The Night Sky

Northern Hemisphere

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

The constellations of Cygnus, Lyra and Aquila are in the west after sunset. Taurus, Orion, Gemini and Canis Major rise in the east as the evening progresses. To the south is the Square of Pegasus, containing around 8 visible stars if your eyes are sensitive enough. You can star-hop from the Square to the Andromeda Galaxy and may also see M33, the Triangulum Galaxy. The Pleiades and Hyades star clusters are in Taurus, with the planet Jupiter between them. The bright star Aldebaran appears to be within Hyades, but is actually nearer to us. Below Taurus is Orion, with the Orion Nebula visible as a fuzzy glow beneath the Hunter's Belt. Below the bright star Sirius, in Canis Major, is the open cluster M41. Most of its stars are young and blue, but there is a red giant at its centre.

The Planets

• Jupiter rises at sunset at the beginning of the month and is visible throughout the night as it reaches opposition (opposite the Sun in the sky) during December. Shining at magnitude -2.8, it reaches 60 degrees' elevation in Taurus in the south, helping us to see it with little atmospheric interference. It is 5 degrees to the upper left of the star Aldebaran at the start of December, moving westwards towards the Pleiades in its retrograde motion during the month. With an angular size of around 48", plenty of detail can be seen using a small telescope, especially on a night when the Earth's atmosphere is still.
• Saturn rises around 04:00 UT (Universal Time) at the beginning of the month and about 02:30 at the end. Its brightness rises from +0.7 to +0.6 during December, while its angular diameter increases from 15.7 to 16.1". Its rings now cover around twice that diameter as they have now opened out to around 18-19 degrees from the line of sight, the greatest angle for six years. With a small telescope, you can see Saturn's southern hemisphere, the gap between its brightest rings and some of its moons.
• Mercury reaches greatest western elongation (its furthest from the Sun in the sky) on the 4th, being around 20 degrees from the Sun, and on that day rises two hours before the Sun, near to Venus. It remains at magnitude -0.5 during the month, beginning December 48 percent illuminated and with an angular size of 7.4", and ending it 96 percent illuminated and 4.8" in diameter.
• Mars is low in the south-west, moving eastwards from Sagittarius into Capricornus. It has a magnitude of +1.2 and an angular size that decreases from 4.3 to 4.2" during December, making surface details difficult to see.
• Venus, at magnitude -4, is still easily visible in the pre-dawn sky. It starts the month at an elevation of 17 degrees at dawn, dropping to 10 degrees by the end. Its angular size drops from 11.6 to 10.9" as its illuminated fraction increases from 88 to 94 percent.

Highlights

• Jupiter is in a better position to observe this month than for some years. Its North Equatorial Belt has broadened, and the Great Red Spot, in the South Equatorial Belt, is currently a pale pink colour.
• The Moon occults the open cluster M27 around midnight on the night of the 3rd to the 4th, with stars disappearing and reappearing.
• Saturn, Venus, Mercury and a crescent Moon appear together before dawn on the 10th and 11th.
• The Geminid meteor shower can be best seen after midnight on the 14th and 15th, with the new Moon not spoiling the view. The meteors come from dust released by the asteroid 3200 Phaethon, rather than a comet, and the radiant is near the bright star Castor in Gemini. Remember to keep warm as you observe!
• The lesser-known Ursid meteor shower, with its radiant in Ursa Major near to the star Kochab in Ursa Minor, can be seen around 2-3am UT on the 22nd and 23rd, producing perhaps 10-15 meteors per hour.
• Around new Moon on the 10th-15th, you can see Neptune through binoculars at magnitude +7.8, close to the star Iota Aquarii.
• Jupiter is close to a nearly-full Moon in the east after sunset on the 25th, and they may even be joined in the sky by something else. Merry Christmas!

Southern Hemisphere

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

The northern sky is dominated by the consellations of Orion, Taurus, Canis Major and Canis Minor. The summer part of the Milky Way passes through them and stretches along the southern horizon, with bright regions of combined starlight and dark patches where clouds of interstellar dust and gas obscure the stars beyond. Orion's Belt is in the north, consisting of three giant blue stars. Alnitak, the easternmost of the three, is a double star system that can just about be split with a small telescope. NGC 2024, the Flame Nebula, is illuminated by Alnitak, while IC 434, which includes the dark Horsehead Nebula, is also nearby - Alnitak makes both hard to see but they show up well in long-exposure photographs. The Sword of Orion is made up of three stars aligned above the Belt, the middle of which is actually the Orion Nebula. The nebula is illuminated by a star at its centre, and forms part of a much larger cloud of mainly dark gas. Binoculars or a small telescope reveal detail in the nebula, while a photograph brings out its colours. The larger and brighter Carina Nebula sits towards the south, between the False and Southern Crosses. Betelgeuse, a red giant star marking one of the Hunter's shoulders, is below Orion's Belt. Its brightness varies between magnitudes +0.2 and +1.2 with a period of about six years. Rigel, marking one of Orion's feet, is a blue giant star above the Belt, and has a companion that is visible in a medium-sized telescope.

The V-shape of the head of Taurus is to the left of Orion in the evening sky. The planet Jupiter currently sits about halfway between the horns and the head of the Bull and is in its brightest apparition of the year. The Moon moves through Taurus and past the Pleiades star cluster just before Christmas Day, before swinging by Jupiter and the star Aldebaran. Uranus and Neptune are visible in Pisces and Aquarius, respectively, in the evening sky. Crux is low in the south-east with a dark cloud called the Coalsack Nebula beside it. Though optically dim, the nebula has bright infra-red hotspots where stars are forming as gas collapses under gravity, giving off light that is absorbed and re-emitted by the cloud. Eventually the stellar winds from the infant stars will break up the nebula, leaving a star cluster similar to the Jewel Box and Pleiades. Another visible nebula is the Large Magellanic Cloud, which is in fact a satellite galaxy of the Milky Way. Binoculars or a small telescope show that it possesses star clusters and nebulae of its own. The Small Magellanic Cloud, another satellite galaxy, and the bright, round globular cluster 47 Tucanae, are nearby. NGC 362 is another globular cluster than can be found in the vicinity using a small telescope.

Highlights

• The Phoenicid meteor shower peaks on the 6th, with a radiant in the constellation of Phoenix, near the star Achernar. It is high in the sky and a few meteors per hour may be seen.
• The Geminid meteor shower peaks on the 14th. The radiant is in Gemini, near the star Castor, but rises around 03:00 NZDT (New Zealand Daylight Time, 13 hours ahead of Universal Time) and remains low on the northern horizon. Nevertheless, peaking only one day from new Moon gives good visibility for its meteors, which can also be seen for a week either side of the peak.
• The summer solstice on the 21st marks the longest daytime hours of the year. The Sun's activity should be increasing towards solar maximum, but has instead declined over recent months, with few sunspots visible. Solar activity is weaker than usual but may pick up next year.
• A comet, C/2011 L4 (PANSTARRS), may grace the skies at a brightness of up to magnitude -4 in March 2013. Its brightness, however, will not be certain until it passes the Sun in March, when it will be low in the west after sunset.

Odds and Ends

Astronomers have used an occultation of a star by the dwarf planet Makemake to shine a light on this distant Kuiper Belt object. They used the changes in the starlight to deduce that Makemake has no measurable atmosphere and is a body of ice and rock around 1.7 times denser than water.

A plan for a Mars colony has been devised by SpaceX CEO Elon Musk. The colony would eventually support up to 80,000 people, who would pay some $500,000 for the trip. In this vision for the future, the initial settlement and infrastructure would be established in the coming decades by a small 10-person crew.

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