Joint Astronomy Centre

Tuesday 28st April, 1998

Astronomers discover a distant galaxy hidden in dust

HILO, HAWAII -- A team of scientists from Italy and The Netherlands has discovered a distant, optically almost invisible galaxy in which hundreds of new stars are being formed every year. The astronomers Andrea Cimatti (Astrophysical Observatory of Arcetri, Firenze), Huub Röttgering (Sterrewacht Leiden), Paola Andreani (University of Padova, Padova), and Remo Tilanus (NFRA, Dwingeloo and Joint Astronomy Centre, Hawaii) announced their discovery in the April 30 issue of the internationally renowned journal Nature.

The galaxy, HR10, is placed at a redshift of 1.4 which means that its light has been underway so long that the image we observe is that of the galaxy from a time when the universe was less than half its present age. The astronomers selected this galaxy for observations, because HR10 is the best known example of a class of very faint galaxies with extremely red colours. Andrea Cimatti says: "These galaxies are so red that their faint optical emission can only be detected after very long exposure times with the largest telescopes in the world. In recent years increasingly more of these mysterious galaxies have been detected and the question arose why they are so red."

Huub Röttgering adds: "On Earth we see that blue light is strongly being absorbed by dust, but that red light can pass through a dusty cloud relatively unhindered. Until now, an unproven hypothesis was that the stars of these red galaxies are deeply embedded in dust clouds. These clouds severely redden the observed colours of the stars and hence the galaxies.

The team of astronomers used the fact that dust is being heated by starlight and as a result of this radiates at wavelengths around a millimetre. For comparison, optical light has a wavelength of less than 1/1000th of a millimetre. Sub-millimetre radiation can be observed and measured by the 15 metre diameter James Clerk Maxwell Telescope (JCMT) atop the dormant volcano Mauna Kea in Hawaii. The JCMT is jointly owned and operated by the United Kingdom, Canada, and The Netherlands and is the largest telescope which can observe sub-millimetre radiation. Recently this telescope has been outfitted with a new camera called SCUBA (Submillimetre Common User Bolometer Array), which was built by the Royal Observatory in Edinburgh. The detectors in SCUBA are cooled to a tenth of a degree above absolute zero which makes it possible to obtain extremely sensitive images.

Remo Tilanus reports: "When we pointed the telescope and SCUBA at HR10 we found that this extremely faint optical galaxy emits very strongly at submillimetre wavelengths. This strong emission indicates that in the galaxy a large amount of dust is being heated by embedded hot and young stars." In addition, HR10 was also detected with the IRAM 30-m telescope in Spain which makes observations in the millimetre wavelength range.

Paola Andreani adds: "The reliability of our results on the presence of the large amount of dust obscuring the optical light is based on the combination of two different measurements, taken with different instruments. Note that only the association of the two measurements at different wavelengths allowed us to estimate the temperature of the dust."

According to Rottgering this is an exciting discovery because for the dust particles to be emitting so much radiation there must be a lot of young stars in the galaxy. The astronomers have calculated that in HR10 hundreds of new stars must form every year in order to account for the observed emission. At that rate HR10 produces tens of times more stars than our own Galaxy! As the galaxy ages, the rate of star formation must decline: the amount of dust and gas, the building blocks for new stars is being exhausted. Röttgering adds: "In HR10 we not only see the formation of new stars, but also how a massive galaxy could form".

Cimatti explains: "Our result has two main implications. First of all, it sheds new light on the enigmatic population of optically faint and extremely red galaxies, suggesting that at least a fraction of them are very dusty star-forming young galaxies. Secondly, our results show that the history of the global star formation in the Universe cannot be derived from optical observations only. The star formation rate of HR10 derived from optical observations only is at least a hundred times lower than that suggested by our sub-millimetre observations, and this occurs because of the large amount of dust which obscures the light coming from the star-forming regions."

"Our observations suggest that a, possibly substantial, fraction of the global star formation in the distant Universe could be hidden by dust obscuration and that sub-millimetre observations are crucial to unveil the population of dusty galaxies where vigorous star formation occurs. Only with telescopes like the JCMT and with new instruments like the SCUBA camera is it possible to study these red objects in more detail. This is important: most theories about the formation of galaxies do not take this class of objects into account yet. Future observations will indicate how important objects like HR10 are for our ideas about the evolution of galaxies and the creation of the Universe as we presently see."

More information:

The JCMT is operated by the Joint Astronomy Centre, on behalf of the UK Particle Physics and Astronomy Research Council, the Netherlands Organisation for Scientific Research, and the Canadian Research Council.

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