All lined up

This image features an interacting galaxy system known as Arp-Madore 2105-332, that lies about 200 million light-years from Earth in the constellation Microscopium. Like other recent Hubble Pictures of the Week, this system belongs to the Arp-Madore catalogue of peculiar galaxies. The wonderful quality of this image also reveals several further galaxies, not associated with this system but fortuitously positioned in such a way that they appear to be forming a line that approaches the leftmost (in this image) component of Arp-Madore 2105-332, which is known individually as 2MASX J21080752-3314337. The rightmost galaxy, meanwhile, is known as 2MASX J21080362-3313196. These hefty names do not lend themselves to easy memorisation, but they do actually contain valuable information: they are coordinates in the right ascension and declination system used widely by astronomers to locate astronomical objects.

Both the galaxies are of a type known as emission-line galaxies. This simply means that, when observed with spectrometers, the spectra of both galaxies exhibit characteristic bright peaks, known as emission lines. This is distinct from, for example, absorption-line galaxies whose spectra contain distinct gaps, known as absorption lines. Emission lines are produced when gases are very hot, and therefore have sufficient energy that the atoms and molecules are ‘excited’ and emit light. In other words, emission-line galaxies are highly energetic places, marking them out as likely hotbeds of star formation.

As with many galaxy types, categorising a galaxy as an emission-line galaxy does not exclude it from having other descriptions that refer to its other properties. Arp-Madore 2105-332, for example, is also a ‘peculiar’ galaxy, reflecting the atypical shapes of its two constituent galaxies.

[Image Description: A pair of interacting galaxies, one smaller than the other. Each has a bright spot at the centre and two loosely-wound spiral arms, with threads of dark dust following the arms. They appear as a broad, soft glow in which individual stars can’t be seen. A number of bright stars and smaller, background galaxies can also be seen — three such galaxies lie in a vertical line below the right-hand galaxy of the pair.]

Links

• Pan of Arp-Madore 2105-332