Colliding black holes could be lurking at the outskirts of spiral galaxies like the Milky Way

The outskirts of spiral galaxies like our own could be crowded with colliding black holes of massive proportions, says a study led by an Indian-origin researcher. [caption id=“attachment_4109127” align=“alignleft” width=“380”] Merging black holes. Image: LIGO.[/caption] The findings, to be published in an upcoming issue of the Astrophysical Journal Letters, point to a prime location for scientists hunting the sources of gravitational waves . Conditions favourable for black-hole mergers exist in the outer gas disks of big spiral galaxies, according to Sukanya Chakrabarti, Assistant Professor at Rochester Institute of Technology. Until now, small satellite or dwarf galaxies were thought to have the most suitable environment for hosting black-hole populations. Chakrabarti realised the edges of galaxies like the Milky Way 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,” Chakrabarti said. 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,” 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,” she added.