First detection of the 670µm water transition in space

12 June 2017

We presented the first detection of the 670µm water transition in space in A&A, 601, L3. This transition can be used to trace the infrared radiation in the nucleus of galaxies providing key information on dust obscured events like the growth of supermassive black holes or extreme bursts of star-formation. This will help us to understand how galaxies evolve from star-forming spirals, like the Milky Way, to dead elliptical galaxies.

Unlike on Earth, most of the water in space is either in the form of vapour or as ice mantles stuck to interstellar dust grains. This is because the extremely low density in the interstellar space prevents the formation of liquid water. We can observe water in space thanks to the transitions occurring between the different quantum energy levels of the water molecule. However, the observation of these water transitions from the ground is challenging because the vapour in the Earth atmosphere almost completely blocks the interstellar water emission. For this reason, we targeted this transition in a nearby galaxy where it is redshifted at 676 μm and Earth’s atmosphere is more transparent.
fig_levels-crop.jpgPartial energy level diagram of water. The 670µm (or 448 GHz) transition is indicated in red. The blue transitions populate the upper level of the 670µm transition through the absorption of infrared photons.

Our study (A&A, 601, L3) reveals that the water emission originates in a nuclear rotating disc with an equivalent amount of water 30 trillion of times the water contained in all the Earth oceans and a diameter 15 millions of times the distance from Earth to the Sun. The analysis shows that this transition is excited by the absorption of infrared photons, so it can be used to investigate the infrared radiation field in the nucleus of galaxies at spatial scales much smaller than those allowed by direct infrared observations.

This infrared radiation is produced by events like the growth of supermassive black holes or extreme bursts of star-formation. These usually occur in extremely dust obscured environments where the optical light is almost completely absorbed by dust grains. The energy absorbed by the grains increases their temperature and they begin to emit thermal radiation in the infrared. Therefore, the 670µm water transition will reveal what hides behind these dust screens and help us to understand how galaxies evolve from star-forming spirals, like the Milky Way, to dead elliptical galaxies where no new stars are formed.

The work was undertaken by an international team of astronomers from Oxford (led by Miguel Pereira Santaella) Spain and Brazil. More information in the Oxford Science Blog Detecting water in space and why it matters