Megan's Blog

In the News this month... surprising star formation from primordial clouds

Orbiting around large galaxies are collections of dwarf galaxies, relatively small groups of stars gravitationally bound to a larger companion. New results from the Galaxy Evolution Explorer, a satellite operating in the ultra-violet part of the spectrum, has found a collection of very unusual dwarf galaxies forming in a way not seen before.

Most galaxies in the nearby universe contain large amounts of dark matter, and their stars formed from gas that had already been processed by a previous generation of stars and so contains a significant proportion of elements heavier than helium. Large galaxies, especially those involved in mergers, are often surrounded by tidal dwarf galaxies, formed from the gas stripped out in the interaction. These tidal dwarfs have very little dark matter but a high proportion of heavy elements since the gas used to form them comes from previous populations of stars in the nearby larger galaxy. But a study of gas in a region known as the Leo Ring, led by David Thilker of Johns Hopkins University in the US, has found a collection of dwarf galaxies forming from gas which lacks both dark matter and heavy elements.

The Leo Ring is a huge cloud of mainly hydrogen and helium gas surrounding two massive galaxies, M105 and NGC3384, located in the constellation Leo. The cloud contains a mass of hydrogen almost 2 million times the mass of the Sun and is 200 kiloparsecs or more than 650 thousand light years in diameter. It was first discovered 25 years ago but is only visible at radio wavelengths where emission from hydrogen atoms is detectable. No optical emission has been detected from the ring, and no stars have been found to be associated with it. It is thought that the cloud is a primordial object, leftover gas from the early universe which has remained unchanged over billions of years, lacking the heavy elements created by stellar populations. The new observations with the GALEX satellite, however, have discovered evidence that star formation has been happening recently in the ring. Young stars burn bright and hot, emitting large numbers of ultra-violet photons which GALEX is sensitive to.


What the team discovered was ultraviolet signatures of young stars in several clumps of gas within the southern half of the ring, and they suggest that this is due to dwarf galaxies forming from the primordial gas. Normal galaxies are dominated by dark matter, so these star forming regions in the Leo Ring where dark matter is absent are somewhat unusual and possibly demonstrates a new mode of dwarf galaxy formation. The study was published in Nature on the 19th of February, and the authors suggest that clouds similar to the Leo Ring would have been more common in the early universe, and so many more dark-matter deficient dwarf galaxies may be out there waiting to be discovered.