The Hubble Space Telescope confirmed the discovery of one new class of astronomical objects never observed until now. Named Cloud-9it is one compact cloud of gas of hydrogen in a neutral form, devoid of stars, immersed in a halo of dark matter. Initially discovered thanks to radio data from Five-hundred-meter Aperture Spherical Telescope (FAST) in China, the cloud is what scientists define as RELHIC (REionization-Limited HI Cloud), or one “failed galaxy” which was unable to accumulate enough gas and therefore create the conditions necessary for star formation. These objects had been theorized, but never observedconfirming one of the predictions of standard model of cosmologyor the existence of halos of dark matter without a galaxy inside them. Their discovery will help astronomers to better understand what the physical conditions must be for a galaxy to form and to discover more details about this elusive component, dark matter, which represents the dominant form of matter by mass in the Universe.
The first starless galaxy: details of the study
The announcement of the discovery of the first “failed galaxy” in history has happened Monday 5 January in the course of 247th meeting of the American Astronomical Society in Phoenix, Arizona. The discovery, led by Alejandro Benitez-Llambay of the Bicocca University of Milan, was obtained by combining i optical data coming from Hubble Space Telescope with those radio of Five-hundred-meter Aperture Spherical Telescope (FAST) in Guizhou, China. The latter was in fact the first to sight, in the direction of the constellation Ursa Major, a 14 million light-years from Earth, one huge non-rotating cloud of hydrogen gas in neutral form (HI according to astronomical jargon) having a mass equal to one million solar masses and a diameter of 4,900 light-years.
Looking with optical telescopes from Earth in that direction, astronomers they had not been able to see any accumulation of starswhich you would expect to see where hydrogen gas, which is the fuel for star formation, is present. Initially, astronomers hypothesized that there was a faint dwarf galaxy at the center of this cloud, meaning that the absence of stars was simply due to the poor sensitivity of ground-based telescopes. Observations conducted with the Hubble Space Telescope have instead confirmed that inside the cloudrenamed Cloud-9, there was actually no cluster of stars. The only visible things were galaxies which, although located in the star’s direction, were much further away than the object. We are therefore faced with the first example ever discovered by man of a “failed galaxy”or RELHIC (REionization-Limited HI Cloud) for scientists, one starless cloud of neutral hydrogen gas immersed in a halo of dark matter.
What are RELHICs and why are they important
According to the standard cosmological model, the Universe is dominated en masse by dark mattera form of matter that acts only gravitationally without emission or absorption of light (which is why it darkens). There dark matter gravitationally collapses to form called ellipsoidal structures dark matter halosinside which galaxies form and evolve. One of the predictions of the standard model is that dark matter halos may exist within them there is yes hydrogen gas, the fuel for star formation, but in which the physical conditions have never been such that the gas has managed to collapse and trigger nuclear fusion reactions to form stars.
To this day these objects, called RELHIC (REionization-Limited HI Cloud), existed only on a theoretical level, but had never actually been observed. Cloud-9 represents the pfirst ever observed example of these “failed galaxies”a compact structure in which the gasin conditions of hydrostatic equilibrium, balances the gravitational pull of dark matter to create an almost inert object, whose existence can only be proven through the radio emission of neutral hydrogen. It is precisely the balance between gas and dark matter that has allowed scientists to determine the mass of the latter known as that of gas via radio waves: five billion solar masses.
Cloud-9 gives us a rare look at a cloud dominated by dark matter, a fossil residue presumably formed a few hundred million years after the Big Bang and remained starless to this day. These RELHICs therefore represent an opportunity to study what the early stages of galaxy formation must have looked like and allow us to understand what the minimum limit in mass and the physical conditions for the gas inside the dark matter haloes to collapse to trigger star formation. Above all, their existence opens the hunt for new examples of RELHIC scattered across the cosmos. Finding them, however, will not be easy because the lack of stars makes these objects visible only in radio waves. Furthermore, interaction with other gas or galaxies in the cosmos can remove gas or completely destroy the cloud.
What will happen to RELHIC in the future? There cloud could form a galaxyprovided it becomes more massive, although how this will happen is still a matter of speculation. If it were much larger, more than 5 billion times the mass of our Sun, it would collapse, form stars, and become a galaxy no different than any other galaxy we see. However, if Cloud-9 were much smaller, the gas could have dispersed and ionized, not surviving as long. If left to its fate, Cloud-9 is in an ideal position where it could remain as a RELHIC virtually forever.
