Dark Galaxies Discovered: Early Stage in Galaxy Formation


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Low-Mass Galaxies

Dark galaxies near the quasar HE 0109-3518 imaged with the VLT and the Digitized Sky Survey 2. The quasar is marked with a red circle and the dark galaxies are marked with blue circles. Image Credit: ESO, Digitized Sky Survey 2, and S. Cantalupo (UCSC)

In this volume of space, Cantalupo’s team found 98 objects that are bright at the Lyman alpha wavelength and therefore contain large amounts of hydrogen gas.

They eliminated gaseous haloes that are self-illuminated by stars, and were therefore visible at optical wavelengths.

The remaining dozen objects are the sought after dark galaxies.

After discovering the dark galaxies, the team of astronomers started to study their properties.

They found that these dark galaxies typically have a total mass of gas that is about a billion (109) times the mass of the Sun.

By comparison, the Milky Way galaxy has a few hundred billion times the mass of the Sun. Star formation is so inefficient in these dark galaxies that they only form one star every hundred years, which is a rate that is about 200 times lower than typical galaxies.

At this rate it would take about 100 billion years, about seven times the current age of the universe, for the galaxy to turn all its gas into stars.

Understanding Galaxy Formation

As these dark galaxies collide and merge, they form normal galaxies. By studying newly discovered dark galaxies, astronomers can further understand how galaxies form.


Cantalupo, S., Lilly, S., Haehnelt, M. Detection of dark galaxies and circum-galactic filaments fluorescently illuminated by a quasar at z=2.4. (2012). Monthly Notices of the Royal Astronomical Society. Accessed July 16, 2012.

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