Headway in mapping extragalactic interstellar organic matter

The left hand side figure shows a colour image of the AM 1353-272 system. This image has been reconstructed directly from the MUSE cube, using the V (the blue channel), R (green channel) and I (red channel) filters. The right hand side figure is a zoom in the region of AM 1353-272B, onto the galaxy studied by our research team. The figure shows those parts of the galaxy where DIBs were detected. Regions shown in red are receding from us, while those in blue a approaching us.

The DIBs detected are shown above the galaxy. Note their weakness. To observe them, it was necessary to combine an extremely sensitive spectrograph with a large aperture telescope, as well as to detect other, stronger, interstellar spectral lines.

In 1922, Mary Lea Heger, then working for a Ph.D. at the Lick Observatory in the USA, highlighted the presence in interstellar spectra of a number of weak absorption lines of unknown origin.

A few years later, astronomers realized that the interstellar medium was responsible for these lines, which were then dubbed DIB, standing for “Diffuse Interstellar Bands”.

During the century which followed this discovery, astronomers discovered over 400 such bands.

Nevertheless, the molecular species which produce these bands are still unknown The most likely candidates are carbonaceous macro-molecules in a free state in the interstellar clouds.

These bands are truly interesting for astronomers, since they could be the signature of a vast reservoir of organic matter in galaxies.

Note however that the signature is extremely weak, and thus hard to find.

Till now, astronomers have only been able to map in a fragmentary way certain regions within the Milky Way; as far as the other galaxies of the Local Group are concerned, the signatures have only been found in disparate regions.

Using data furnished by the new MUSE instrument at the VLT, a team of astronomers led by Ana Monreal-Ibero, from the GEPI, and Peter Weilbacher from the Leibniz-Institut für Astrophysik Potsdam, has managed to identify one of these mysterious bands in a huge part of an interacting galaxy , part of a system nicknamed "the dentist’s chair".

This feat is due to the fact that this galaxy, 160 Mpc away, is more than two orders of magnitude away from the other galaxies for which these bands had already been observed.

The DIBs detected are indicated above the galaxy. Note how weak they are. That they could be seen is due to the use of an extremely sensitive spectrograph with a large aperture telescope, and the fact that other stronger spectral lines of the interstellar medium were observed.

From the Canary Islands, during fine weather, one can see the neighboring islands. However, this is harder during the « calima », when the wind blows sand from the Sahara. The interstellar medium has its own "calima". This is the interstellar dust, which absorbs the light from the stars, and so makes it harder to observe them. This phenomenon is called extinction. The figure shows the relation between the importance of the DIB and this extinction (and thus the dust) for many galaxies. The team of scientists has shown that the DIBs in the galaxy AM 1353-272 B are related to the dust, in the same way as had already been noted in two large spiral galaxies in the Local Group: the Milky Way and Andromeda.

With this discovery, the team has shown that it is now possible to map these signatures in distant galaxies.

Using very sensitive instruments with large telescopes, astronomers hope to be able one day to answer the following question: at which point during the evolution of galaxies, and under what conditions, emerged the still mysterious organic species at the root of the DIBs ?

Know more

Scientific contact