Recent deep space observations strengthen the hypothesis that the object Cloud-9 is a starless galaxy. Despite using higher sensitivity images compared to previous ones, astronomers have found no signs of starlight in this region.
Cloud-9 was discovered at the beginning of this year and is a hydrogen cloud spanning about 4.9 thousand light-years. This object is considered the first representative of a new class of structures known as Restricted Reionization Hydrogen Clouds, or RELHICs.
The main objective was to determine whether Cloud-9 truly contains no stars. To conduct the study, researchers utilized the HiPERCAM instrument installed in the Gran Telescopio Canarias (GTC) on the Canary Islands, Spain. The new images were approximately ten times more sensitive than previous ones, but starlight was still not identified.
Based on the data obtained, the team was able to establish an upper limit on the number of stars that could exist in Cloud-9. Considering the possibility of a primordial stellar population, scientists estimate the object's stellar mass to be no more than 16 thousand solar masses. For comparison, the volume occupied by Cloud-9 near the Sun could hold tens of millions of stars.
The authors of the article noted that 'this new upper limit on stellar mass, based solely on ultra-deep images, supports the idea that Cloud-9 is an excellent candidate for a starless galaxy.'
The radio observatories that led to the discovery indicate that Cloud-9 contains a amount of hydrogen equivalent to about one million solar masses. If this object is indeed a dark halo, the ratio between dark matter and ordinary matter will be extremely high—approximately five thousand to one, whereas in most galaxies this ratio is about five to one.
The mass of Cloud-9 can also explain the absence of star formation. As explained by Andrew Fox from AURA/STScI for the European Space Agency (ESA), 'if Cloud-9 were more massive, it would collapse and form stars, becoming a galaxy. If it were less massive, the gas would disperse and be ejected. This cloud is in an 'ideal spot', not too massive and not too small.'
Cloud-9 is not the first potential starless galaxy; before it was J0613+52, a hydrogen cloud the size of the Milky Way located about 270 million light-years from Earth. Nevertheless, this object differs significantly from Cloud-9. Thanks to the new observations, Cloud-9 has become considered the most promising candidate for a starless galaxy.