WISE and galaxy evolution

Tuesday 16th February, 2016

Our speaker for the evening was Professor Andrew Blain from Leicester University who came to talk to us about "WISE and very luminous galaxies". He is part of the science team on NASA's Wide Field Infrared Survey Explorer mission that was launched in 2009. By early in 2011 it had completed a scan of the entire sky twice in infrared wavelengths, imaging three-quarters of a billion objects which included remote galaxies, stars and asteroids.

WISE is a solar-powered spacecraft with a 40 cm (16") telescope and four infrared detectors that each have one million pixels. To work at infrared wavelengths, the telescope and detectors need to be cooled and so they are sited within a cryostat which is essentially a large Thermos bottle. It is in a polar orbit around the Earth following the terminator with its solar panels pointing towards the Sun and its telescope always away. The spacecraft was designed to record in infrared light so that it could observe certain objects invisible to optical telescopes. These include distant galaxies in the young Universe that are relatively bright at infrared wavelengths, stellar nurseries surrounded by interstellar dust, interacting galaxies, proto-planetary discs, brown dwarf stars and in its new mission asteroids and comets.

Prof Blain commented that the spacecraft and its instruments performed flawlessly from January 2010 to January 2011. After its necessary hydrogen coolant ran out it then was tasked with a new three-year mission called NEOWISE to search for near-Earth objects such as comets and asteroids that could prove a threat to Earth. It was able to take on this task as two of its four infrared detectors did not need cooling to spot these closer objects. NEOWISE is currently working on its fifth mapping of the entire sky and has already found over 18,000 Solar System objects including 489 near-Earth objects and 91 comets.

Prof Blain's research focuses more on WISE's earlier mission as he is mainly interested in the formation and evolution of galaxies. He is especially interested in ELIRGs or extremely luminous infrared galaxies which have luminosities a hundred billion times that of our own Sun. It is thought that these galaxies' emissions are due to a compact region at their centre surrounding a supermassive black hole. Any gas or material circling the black hole is dragged inwards forming what is known as an accretion disc. The resulting turbulence heats the matter in the disc which then emits light across the whole of the electromagnetic spectrum. The infrared light is able to escape as it is not blocked by the galaxy's dust and gas and also the other wavelengths can be re-radiated by the gas and dust at infrared wavelengths.

The current record holder for the most luminous infrared galaxy (as of May 2015) has the very unwieldly designation of WISE J224607.57-052635.0. This galaxy is 12.5 light-years away and is smaller than our own Milky Way and yet 99% of the light it emits is in the infrared. Scientists think that this galaxy is host to a supermassive black hole at its centre which is undergoing a tremendous growth spurt as it consumes material from a surrounding accretion disc. Prof Blain is currently analysing data from the ALMA (Atacama Large Millimeter/submillimeter Array) to examine such objects in more detail.