Old supernova dust factory revealed at the Galactic centre

Astronomers have finally found evidence of dust around a relatively “old” supernova remnant near the centre of our Milky Way galaxy. This new finding can help explain the mystery of how galaxies in the early Universe, only a few billion years after the Big Bang, can contain so much dust.

The main elements from which interstellar dust is made –  carbon, silicon, magnesium and iron – are all made inside stars and expelled into the interstellar medium when stars die. This dust is then recycled in the next generation of stars (and some of it goes into forming planets like the Earth).

The cosmic dust cycle. Dust is formed in stars and expelled into the interstellar medium by slow winds from evolved low-mass stars and in the supernova explosions from high-mass stars. The dust is then ‘recycled’ through the interstellar medium, some being consumed in the next generation of stars. (Credit: Herschel)

But stars like our own Sun, with lifetimes of about 10 billion years, live too long to be able to account for the vast amounts of dust in the early Universe. Astronomers have long believed the answer to this riddle lies in supernovae explosions that result when massive stars die. The accelerated timescales on which these explosions take place can potentially explain the early dust content in the Universe.

There is one problem with this idea—supernovae explosions are some of the most energetic events in our Universe. A single supernova can briefly outshine an entire galaxy! While these conditions are necessary to fuse the heaviest elements in the Universe, supernova explosions and their resulting supernova remnants are very violent environments and it was unclear whether dust grains could survive the shocks and reverse shocks that reverberate in the surrounding medium.

Schematic of a supernova explosion. A forward and a reverse shock are created when the supernova shock wave interacts with the surrounding interstellar medium (ISM). The forward shock continues to expand into the ISM, while the reverse shock travels back into the freely expanding supernova ejecta. (Credit: Swinburne Astronomy Online)

In July of last year, researchers published new results from the Very Large Telescope which observed dust formation in real-time over a 2.5-year period following the explosion of supernova SN2010jl. Their observations showed that an accelerated dust formation process begins just a few hundred days after the explosion, producing grains as large as 1 micron in size—large enough to be resistant to destruction. These larger grains should still be observable after many years.

Using the mid-infrared camera on the Stratospheric Observatory for Infrared Astronomy (SOFIA), a modified Boeing 747 that flies at an elevation of 12 km, Ryan Lau of Cornell University and  collaborators found an “old” supernova remnant,  Sagittarius A East, near the Galactic center which contains about 6,700 Earth-masses of warm (~100 K) dust.  Sagittarius A East is about 10,000 years old which means that the dust has survived the destructive shocks and reverse shocks created from the initial supernova explosion.

False color image of the Sgr A East supernova remnant overlaid with contours of the warm dusty emission detected by SOFIA. The blue is hot X-ray gas detected by Chandra and the red is 6 cm VLA radio emission from the expanding shock, which is colliding with surrounding interstellar cloud, represented by the green 160 μm Herschel/PACS far-infrared emission. (Credit: NASA/CXO/Herschel/VLA/Lau et al.)

The results of Lau et al. supports the theory that galaxies in the early Universe can indeed be enriched with dust by supernovae, which could solve the long-standing mystery. 

For more information, see

[Mark Hutchison & Sarah Maddison]

 

 

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