Yet another supernova

Just when you’d given up hope of another starburst, a third type comes along unannounced! This third class of previously unidentified supernova could help explain some anomalous observations in the night sky and even how our bodies come to contain so much calcium.

Until recently, astronomers had assumed there were just two types of supernovae. The first two types of supernova are either hot, young giants that explode on to the scene violently as they collapse under their own weight, or old, dense white dwarves (type a1) that undergo a thermonuclear explosion to briefly add their light to the night sky.

However, a third class appeared in telescope images in early January, 2005 and scientists, seeing that it had recently begun the process of exploding, started collecting and combining data from different telescope sites around the world, measuring both the amount of material thrown off in the explosion and its chemical composition.

Avishay Gal-Yam and colleagues at the Weizmann Institute in Israel and teams in Canada, Chile, Italy, UK, and USA, soon realised that the new supernova was neither old and dense nor young and hot.

There was too little material being ejected by the 2005 supernova for it to be an exploding giant, but its remote location from stellar nurseries suggested it was old. Moreover, its chemical makeup did not match the second type of supernova. The scientists turned to a computer simulation to see if they could figure out what kind of stellar processes could give rise to this anomalous kind of starburst.

Type Ia supernovae are primarily composed of carbon and oxygen as seen in their spectra, but the newly discovered supernova has unusually high levels of calcium and titanium which derive from nuclear reactions of helium not carbon and oxygen. However, the astronomers were initially at a loss to explain the source of the helium. Their simulations suggested that a pair of white dwarves might have been involved, with one assimilating helium from the other. When the thief star’s helium load rises past a certain point, the explosion occurs. “The donor star is probably completely destroyed in the process, but we’re not quite sure about the fate of the thief star,” says Gal-Yam.

Helium theft may have led to a third class of supernova that gives rise to the calcium in your bones and the titanium in a replacement hip! (Credit: Gal-Yam, Weizmann Institute of Science.

These new supernovae are relatively dim, so may not be as rare as they at first seem. This might explain why calcium is so prevalent in the universe and so in life on earth. The existence of radioactive titanium from these supernovae might also preclude the need for exotic explanations, such as invoking dark matter, of positrons at the heart of our galaxy. “Dark matter may or may not exist,” says Gal-Yam, “but these positrons are perhaps just as easily accounted for by the third type of supernova.”

Links

Avishay Gal-Yam homepage

Black hole

The European Southern Observatory’s Very Large Telescope (VLT) has helped an international team of astronomers to detect a stellar mass black hole that lies at a much greater distance from Earth than any observed before. The black hole is in the spiral galaxy NGC 300, about six million light years away in the constellation Sculptor.

The spiral galaxy NGC 300 lying in the constellation Sculptor (Credit: Galex/NASA)
The spiral galaxy NGC 300 lying in the constellation Sculptor (Credit: Galex/NASA)

Paul Crowther and Vik Dhillon, of the University of Sheffield, UK, Robin Barnard and Simon Clark of the The Open University, Milton Keynes, UK, and Stefania Carpano and Andy Pollock of ESAC, in Madrid, Spain report the black hole which has a mass of about twenty times that of the Sun in the Monthly Notices of the Royal Astronomical Society.

The stellar-mass black holes found in our Milky Way galaxy commonly weigh up to ten times the mass of the Sun. The newly discovered black hole is not only the most distant, but the second most massive stellar-mass black hole ever found. It is also entwined with a star that will soon become a black hole itself.

Lead author Crowther, explains: “This is the most distant stellar-mass black hole ever weighed, and it’s the first one we’ve seen outside our own galactic neighbourhood, the Local Group. The black hole’s curious partner is a Wolf-Rayet star, which also has a mass of about twenty times as much as the Sun. Wolf-Rayet stars are near the end of their lives and expel most of their outer layers into their surroundings before exploding as supernovae, with their cores imploding to form black holes.

An artist's impression of the newly discovered black hole and its stellar companion (Credit: ESO/L. Calçada)
An artist's impression of the newly discovered black hole and its stellar companion (Credit: ESO/L. Calçada)

In less than a million years, a blink of the eye cosmologically speaking, the Wolf-Rayet star will explode as a supernova and its remnants collapse into a black hole. Only one other system of this type has previously been seen, but other systems comprising a black hole and a companion star are not unknown to astronomy. The existence of such systems hints at an underlying galactic chemistry. Astronomers believe that a higher concentration of heavy chemical elements influences how a massive star evolves, increasing how much matter it sheds, resulting in a smaller black hole when the remnant finally collapses.

LINKS

Monthly Notices Royal Astronom Soc, 2010, in press
http://www.eso.org/public/archives/releases/sciencepapers/eso1004/eso1004.pdf

Paul Crowther
http://pacrowther.staff.shef.ac.uk/main.html

Musing on supermassive black holes

New observations from a collection of powerful telescopes have allowed astronomers from Germany and the US to settle a paradox regarding the behaviour of merging elliptical galaxies. The team has revealed evidence that the largest, most massive galaxies in the universe and the supermassive black holes at their cores grow together rather than one leading to the other, which explains the fluffy nature of their central regions.

Astronomers have known for many years that galaxies, containing billions of stars can grow as they absorb and merge with their neighbours. What was unclear though was the relationship between the supermassive black hole at the core of elliptical galaxies, and the growth of such a galaxy.

The elliptical galaxy NGC 4621 (Credit: WikiSky/SDSS)

The elliptical galaxy NGC 4621 (Credit: WikiSky/SDSS)

Initially, astronomers assumed that the huge gravitational fields of such black holes would greedily pull all galactic matter in towards them creating a relatively small, dense cluster at the centre. In the 1980s observations revealed the opposite. The biggest galaxies have huge fluffy, low-density centres.

The best theory to explain this contrary behaviour of supermassive binary black holes was the slingshot effect now observed by Kormendy and Bender. The popular theory of such galactic mergers and acquisitions has it that as two supermassive galaxies collide, their central black holes begin orbiting each other, whisking up the cores of the merged galaxy and flinging stars out of the central region. As the black hole pair sinks to the centre of the merger to form an even more supermassive black hole, the local region should appear depleted of stars.

John Kormendy

John Kormendy

Now, Ralf Bender of Germany’s Max-Planck-Institute for Extraterrestrial Physics and Ludwig Maximilians University Observatory together with John Kormendy of the University of Texas at Austin, have published details of their findings in the latest issue of Astrophysical Journal Letters. The team analysed data from Austin’s McDonald Observatory, the Hubble Space Telescope and many other telescopes around the world for 11 supermassive elliptical galaxies in the Virgo Cluster. They measured the dimming of the galactic core due to the stellar depletion, the so-called light deficit.

Ralf Bender

Ralf Bender

Finding evidence for light deficits in galactic cores is quite surprising, despite being founded on recent decades of new theory and observation by many astronomers, including Kormendy and Bender. The biggest elliptical galaxies contain enormous black holes with masses a billion or more times the mass of our Sun.

Our new observations are a strong and direct link between black holes and galaxy central properties, Kormendy explains, They are a ‘smoking gun’ that connects black holes with the formation of the surprisingly fluffy centres of giant elliptical galaxies.

The fluffiness also increases in lockstep with another galaxy property that is known to be tied directly to black holes, namely the speeds at which stars move far out in the galaxy where they cannot feel the black hole’s gravity.

Astronomers love tight correlations, Bender says. They tell us what is connected with what. The new observations give us much stronger evidence that black holes control galaxy formation, at least at their centres.

Further reading

Astrophys. J. Lett., 2009, in press
http://dx.doi.org/10.1088/0004-637X/691/2/L142

John Kormendy homepage
http://chandra.as.utexas.edu/~kormendy/kormendy.html

McDonald Observatory
http://mcdonaldobservatory.org/

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