A starless galaxy?

CUTTING EDGE

A mysterious blob of gas found in the vicinity of the spiral galaxy M94 is exciting astronomers. It might be the first example of a type of object that has long been predicted by our leading cosmological theory: a small galaxy with gas, dark matter… and no stars.

Our understanding of how gravity sculpts structure in the Universe says that from the tiny initial fluctuations in density that we see in the cosmic microwave background, objects of all masses will form. But only in the largest – those with a mass above a critical limit, which changes over time – is gravity expected to be strong enough to reach the density required for star formation to happen.

As a result, scattered all over the Universe there should be smaller, failed systems that didn’t reach this limit, which have been given the convoluted but more positive name of REionization-Limited-HI Clouds (RELHICs for short) by the authors of this month’s paper, Alejandro Benítez-Llambay and Julio F Navarro. Has the first of these failed, starless galaxies been found?

Maybe. A team of Chinese astronomers using the new Five-hundred-meter Aperture Spherical Radio Telescope (FAST) in southwest China – think a larger and more modern version of the recently collapsed famous Arecibo dish in Puerto Rico – detected radio waves at a wavelength of 21cm. These are associated with hydrogen, coming from a point in the sky just slightly less than a degree from M94, in the constellation of Canes Venatici. If the emission is indeed from hydrogen, it seems to be receding from us at about the same speed as the more massive galaxy, making it very likely to be part of the same cosmic group.

Gas rich, star poor

This new discovery, charmingly called ‘Cloud-9’ by the discovery team, could be a satellite galaxy of M94, but if so it’s a strange one. The deepest imaging we have so far shows no signs of any stellar light at a position corresponding to the radio source, though given the data we have in hand a small luminous