Wednesday, July 30, 2014

Largest Mass of Water in the Known Universe

In July 2011, a team of astronomers reported on the discovery of the largest reservoir of water ever detected in the universe in a distant quasar identified as APM 08279+5255. A quasar is an extremely luminous object that is powered by a supermassive black hole accreting material at a stupendous rate. APM 08279+5255 sits in the core of a giant elliptical galaxy that is located at a distance of 12 billion light years, near the “edge” of the known universe. What this means is that light observed from APM 08279+5255 left it 12 billion years ago, at a time when the universe is still relatively young since the universe itself is only 13.8 billion years old.

Figure 1: Artist’s impression of a quasar, similar to APM 08279+5255. Image credit: NASA/ESA.

APM 08279+5255 is incredibly luminous. Its radiant power is estimated to be roughly equivalent to a thousand trillion Suns. At its heart is a supermassive black hole whose mass is estimated to be a whopping ~20 billion times the Sun’s mass. Observations of APM 08279+5255 were performed in the millimetre waveband using the Z-Spec instrument at the Caltech Submillimeter Observatory (CSO) and the Combined Array for Research Millimetre-Wave Astronomy (CARMA). The observations revealed an enormous mass of water vapour swirling around this cosmic monstrosity. The mass of water vapour is distributed around the supermassive black hole in a gaseous region spanning a few hundred light years across. Measurements show that the total mass of water vapour around the quasar is at least ~100,000 times the mass of the Sun. That is over ~100 trillion times the amount of water in Earth’s oceans.

In a typical galaxy like the Milky Way, most of the water is frozen as ice. In the case of APM 08279+5255, the immense amount of energy being put out by the quasar keeps the water in its gaseous phase. Water-ice sublimates into water vapour when it is heated above ~100 K. In fact, the water vapour in APM 08279+5255 is observed to have temperatures ranging from 100 to 650 K. The energised mass of water vapour is continuously cooling; generating a total observed cooling luminosity of at least ~6.5 billion times the Sun’s luminosity. This discovery shows that water is common throughout the universe and it can occur in ginormous masses even in the early universe.

Figure 2: Comparison of the water spectrum in Mrk 231 with that in APM 08279+5255, as measured with Z-Spec. Bradford et al. (2011).

- Bradford et al. (2011), “The Water Vapor Spectrum of APM 08279+5255: X-Ray Heating and Infrared Pumping over Hundreds of Parsecs”, arXiv:1106.4301 [astro-ph.CO]
- Riechers et al. (2008), “Imaging the Molecular Gas in a z=3.9 Quasar Host Galaxy at 0.3" Resolution: A Central, Sub-Kiloparsec Scale Star Formation Reservoir in APM 08279+5255”, arXiv:0809.0754 [astro-ph]