Supermassive stars as seeds of supermassive black holes

Quasars are distant galaxies harbouring supermassive black holes (SMBHs) of billions of solar masses. While swallowing matter such black holes generate enormous amounts of energy, much more energy than emitted by the billions of stars of their hosts. But how could the black holes grow so enormously in the short time since the big bang? One explanation anticipates that these quasar black holes had already formed supermassive from the first generation of stars in the universe. But astrophysical models so far had problems generating stars at masses required to form such SMBH.

To form a supermassive star the heat generated in a collapsing gas cloud needs to be radiated away. The less effective this process appears to be the larger a star can grow until gravity eventually takes over which, depending on the total mass enclosed, leads to a rapid accretion of matter and collapse of the gas cloud to form a SMBH. 

Radiating Hydrogen molecules were supposed to be the mayor cooling agents in the early universe. However, this process is far too effective to form stars at masses required. How do we get rid of these molecules? Possibly by ultraviolet radiation from star formation nearby which breaks down these molecules into atomic hydrogen, allowing these gas clouds to grow to an enormous 10⁷–10⁸ solar masses until they finally collapse to form a star. 

Tyrone Woods from Monash University (Australia) and colleagues, among them Lionel Haemmerle and Ralf Klessen from the Institute for Theoretical Astrophysics at ZAH, had run stellar-evolution models which simulate the birth, growth and collapse of supermassive stars incorporating effects of nuclear burning, cooling by atoms and molecules as well as hydrodynamics.

Their findings are really striking: they demonstrated that supermassive stars may have collapsed into massive black holes of 150.000 to 300.000 solar masses. Their final mass depends on the rate matter is accreted onto the center of the collapsing gas cloud and which can rise up to a catastrophic rate of 10 solar masses per year (see figure). The massive black holes generated at the end of this process thus seem to be the perfect seeds to grow into SMBHs observed in high redshift Quasars.

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