The ancestors of supermassive black holes may have formed in the young Universe under conditions far more common than standard scenarios predict.
Very distant quasars harbor enormous black holes, even though the Universe still appears too young for this: the speed of their formation is a mystery for astrophysicists. To reach such a mass quickly, these objects may have begun growing from already very massive “seeds.”
Artist’s impression of several primordial stars.
Credit: NASA/WMAP Science Team
One possibility involves oversized primordial stars. For them to appear, a large amount of gas must rapidly fall toward the center of a young matter halo. This gas can then fuel a star reaching tens of thousands of solar masses, before it rapidly gives birth to a massive black hole—the massive seed researchers have been seeking.
But it is not that simple: when hydrogen molecules form, they intensify overall cooling and favor the birth of many smaller stars, preventing the formation of these giant stars. However, a particular ultraviolet radiation can break apart these molecules: the gas then remains hot and continues flowing toward the center of the matter halo.
The researchers carried out 65 detailed simulations of 15 primordial halos. They varied the intensity of this radiation as well as the growth history of each halo. Their aim was to determine the conditions capable of maintaining a gas inflow rapid enough to produce a supermassive star.
The simulations reveal a transition when the radiation becomes moderate to strong. Above a certain level, the gas flow becomes sufficient, and the estimated stellar masses can then reach around 100,000 times that of the Sun.
This threshold is important for estimating the number of possible seeds within a given volume of space. Environments exposed to intermediate ultraviolet radiation are naturally far more numerous than regions receiving high intensities. A sufficiently low threshold could considerably increase the number of seeds formed.