Conditions favorable for mergers of massive black holes exist in the outer gas disks of massive spiral galaxies, according to a study led by Rochester Institute of Technology astrophysicist Sukanya Chakrabarti.
Chakrabarti et al identify an overlooked region that may prove to be rife with orbiting black holes and the origin of gravitational-wave chirps heard by the two Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in the United States and the Virgo detector in Italy. This Hubble image shows the barred spiral galaxy NGC 1300. Image credit: NASA / ESA / Hubble Heritage Team / STScI / AURA / P. Knezek, WIYN.
Until now, small satellite or dwarf galaxies were thought to have the most suitable environment for hosting black-hole populations: a sparse population of stars, unpolluted with heavy metals like iron, gold and platinum — elements spewed in supernovae explosions — and inefficient winds that leave massive stars intact.
Dr. Chakrabarti and her colleagues realized the edges of galaxies like the Milky Wavy have similar environments to dwarf galaxies but with a major advantage — big galaxies are easier to find.
“The metal content in the outer disks of spiral galaxies is also quite low and should be rife with black holes in this large area,” she explained.
“This study shows that, when predicting or interpreting observations of black holes, we need to account not only for differences between different types of galaxies but also the range of environments that occur inside of them,” added co-author Dr. Richard O’Shaughnessy, assistant professor of mathematical sciences at Rochester Institute of Technology and a member of the LIGO Collaboration.
A deeper understanding of the Universe is possible now that scientists can combine gravitational wave astronomy with traditional measurements of bands of light.
Existing research shows that even black holes, which are too dense for light to escape, have a gravitational wave and an optical counterpart, remnants of matter from the stellar collapse from which they formed.
“If you can see the light from a black-hole merger, you can pinpoint where it is in the sky,” Dr. Chakrabarti said.
“Then you can infer the parameters that drive the life cycle of the Universe as a whole and that’s the holy grail for cosmology.”
“The reason this is important is because gravitational waves give you a completely independent way of doing it so it doesn’t rely on astrophysical approximations.”
Details of the research will be published in the Astrophysical Journal Letters and are publicly available at arXiv.org.
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Sukanya Chakrabarti et al. 2017. The Contribution of Outer HI Disks to the Merging Binary Black Hole Population. ApJL, in press; arXiv: 1710.09407
