How do galaxies form?
One theoretical possibility involves dark galaxies, which are haloes of gas having masses comparable to small galaxies.
When dark galaxies collide and combine with other newly forming galaxies, they provide interstellar gas that is the raw material for star formation.
The gas in these haloes does not, however, form stars very efficiently until the dark galaxies combine with other galaxies.
Without stars to provide illumination, these theoretical galaxies are dark and difficult to observe.
Do dark galaxies actually exist?
Searching for Dark Galaxies
A team of astronomers using the Very Large Telescope (VLT) at the European Southern Observatory (ESO) has, for the first time, observed these predicted dark galaxies. The observations by Sebastiano Cantalupo, Simon Lilly, and Martin Haehnelt are described in a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society in June 2012, and available online July 11, 2012.
Because nearly starless gas haloes emit little light directly, the team searched for indirectly illuminated dark galaxies. Quasars, extremely bright nuclei of very energetic early galaxies, are among the most luminous objects in the universe. Therefore, quasars should illuminate dark gas haloes in a fairly large volume of space around the quasars. Cantalupo’s team used the very sensitive VLT to search a volume of space around the quasar HE 0109-3518. (For the curious, the numbers in the quasar’s name refer to its position in the sky. They are the approximate right ascension and declination, which are similar to longitude and latitude, projected onto the sky.)
Illuminating Dark Galaxies
Light from the quasar illuminates the hydrogen gas in the dark galaxies, by a process called fluorescence. In detail, ultraviolet light from the quasar is energetic enough to kick electrons in hydrogen atoms up to higher energy levels. When the electrons drop back down to the lowest energy level, they emit ultraviolet light. Cantalupo’s team searched for a specific wavelength of ultraviolet light emitted when electrons jump from the second to the first energy level in hydrogen atoms. Astronomers call this spectral line the Lyman alpha line. Because the universe is expanding, quasars and the nearby dark galaxies have a significant cosmological redshift. For HE 0109-3518 and nearby dark galaxies the Lyman alpha line is actually visible as a violet, rather than ultraviolet, wavelength.
The astronomers used the VLT to observe the region within a few million light years of HE 0109-3518 at the predicted wavelength. They used a carefully selected narrow band filter to find gas regions emitting Lyman alpha light. If these regions are also faint at optical wavelengths, they contain no stars and are dark galaxies.