Astronomers observe acidic Milky Way galaxies

SRON astronomer Floris van der Tak is the first to have observed acidic particulate clouds outside of our own Milky Way galaxy. He did this by focusing the James Clerk Maxwell Telescope, located on Hawaii, on two nearby Milky Way galaxies. Astronomers think that acidification inhibits the formation of stars and planets in the dust clouds. Now it is a case of waiting for precise measurements from the SRON-built HIFI space instrument that will be launched on the Herschel space telescope next year. Van der Tak: ‘I have already submitted my observation proposal’.

M82 is een melkwegstelsel dat relatief dichtbij staat, op 12 miljoen lichtjaar. Aan de sterrenhemel staat M82 in het sterrenbeeld Grote Beer. (beeld: Hubble Space Telescope, NASA/ESA)

The formation of stars and planets in the universe is a delicate process. Clouds of gas and matter rotate and draw together under the influence of gravity. Pressure and temperature then rise, which eventually leads to the kindling of a new star with planets potentially orbiting it. Yet why does this happen at some locations in the universe and not at others? What are the conditions for star and planet formation? How does this process start and when does it stop? Astronomers are fumbling in the dark.

‘The quantity of charged molecules in the dust cloud appears to have an inhibitory effect’, says Floris van der Tak. ‘These ensure that the magnetic fields can exert a greater influence on the cloud, as a result of which the entire cloud becomes agitated and the star-forming process is disrupted’. Observing these charged molecules directly is difficult. The ratio of acidic water molecules to ordinary water molecules is a measure of the quantity of charged molecules.

De James Clerk Maxwell Telescope op Hawaii. (beeld: JCMT)

However, it is difficult to observe water molecules from under an atmosphere that is itself predominantly made up of water molecules. ‘It is like looking for stars in the daylight.’ On Earth it can only be done from a high mountain where the air is rarefied. Such a spot is the 4092 metre-high top of the Hawaiian volcano Mauna Kea, where the James Clerk Maxwell Telescope is located. Van der Tak focused this telescope on the Milky Way galaxies M82 en Arp 220, where he discovered areas rich in acidic water molecules.

‘Amazingly, what causes these acid water molecules to be present in both Milky Way galaxies is completely different’, says Van der Tak. ‘In Arp 220 they develop under the influence of X-rays in the vicinity of the central supermassive black hole. In M82, the cause is the ultraviolet radiation emitted by hot young stars in the star-forming area. Therefore, in these particular galaxies the process of star formation inhibits itself, due to more and more charged molecules being created.’

HIFI wordt op dit moment gemonteerd op de ruimtetelescoop Herschel van de Europese ruimtevaartorganisatie ESA.(beeld: Astrium/ESA)

The astronomer will be able to deploy even heavier equipment for his research in the not too distant future. Next year, the European Space Agency (ESA) is launching the Herschel space telescope with the SRON-constructed Heterodyne Instrument for the Far Infrared (HIFI) attached to it. And in the 5000 metre-high and completely arid Atacama Desert in Chile, a start has been made on the construction of ALMA, 66 smart telescopes that can together produce detailed maps of the Milky Way galaxies. SRON is one of the partners involved in developing the detectors for these telescopes.

The results of Floris van der Tak’s research are published this week in the scientific journal Astronomy & Astrophysics.