Spiral galaxies' outskirts host massive stellar black holes

Discovery will help scientists pinpoint more potential sources of gravitational waves.
By Laurel Kornfeld | Jul 27, 2018
The outer regions of spiral galaxies can host massive black holes formed when giant stars ended their lives in supernova explosions, according to a new study led by Sukanya Chakrabarti of the Rochester Institute of Technology (RIT) School of Physics and Astronomy.

Chakrabarti's team analyzed data from the Lick Observatory Supernova Search, which compared the rates of supernova explosions in the outskirts of spiral galaxies with the rates of those that occur in smaller dwarf or satellite galaxies.

According to the Lick data, the supernova rates are comparable for the outskirts of both types of galaxies, with each having an average of two every millennium.

Knowing that these locations host black holes created by the core collapse of massive stars is a boon to scientists who study gravitational waves, which are produced by black hole collisions.

Dwarf and satellite galaxies are known to have low levels of elements heavier than hydrogen and helium, a condition ideal for both the formation and merger of stellar mass black holes. The new study, about which a paper will be published in Astrophysical Journal Letters, reveals similar favorable conditions for black holes can also be found in the outer regions of spiral galaxies.

"If these core collapse supernovae are the predecessors to the binary black holes detected by LIGO (Laser Interferometer Gravitational-wave Observatory), then what we've found is a reliable method of identifying the host galaxies of LIGO sources," Chakrabarti noted.

"Because these black holes have an electromagnetic counterpart at an earlier stage in their life, we can pinpoint their location in the sky and watch for massive black holes."

The electromagnetic counterparts to which Chakrabarti referred are the dying massive stars whose cores collapse before they explode as supernovae. As these stars die, they produce bright signatures in the electromagnetic spectrum.

Additional surveys of the outskirts of both dwarf and spiral galaxies will likely help scientists detect more LIGO events, Chakrabarti said.

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