A rare intermediate-mass black hole may have been delivered to the barred spiral galaxy NGC 4424 by another, smaller galaxy, according to an analysis of images and data from NASA’s Chandra X-ray Observatory and the NASA/ESA Hubble Space Telescope.
The main panel of this image shows a wide-field view of NGC 4424 in optical light from Hubble. The image is about 45,000 light-years wide. The inset features a close-up view of NGC 4424 that shows Chandra X-ray data (blue) plus a version of the optical data (red) that has had light from a model of NGC 4424 subtracted from the image to show other faint features. This inset image is about 1,700 light-years across. The Nikhuli star cluster is the elongated red object. Image credit: NASA / CXC / Swinburne University of Technology / Graham et al. / ESA / STScI.
“Galaxies can grow through their mutual gravitational attraction and subsequent union,” said Dr. Alister Graham from the Centre for Astrophysics and Supercomputing at the Swinburne University of Technology and colleagues.
“While orbiting a regular galaxy, the body of a low-mass galaxy can be stripped away.”
“However, the stellar heart of the infalling galaxy, if represented by a tightly bound nuclear star cluster, is more resilient.”
Using Hubble images, the astronomers discovered a red, elongated star cluster near the center of NGC 4424, a barred spiral galaxy located about 54 million light-years away in the constellation of Virgo.
This star cluster may be the remnant of the infalling galaxy responsible for the past merger event that disturbed NGC 4424.
Named Nikhuli, the cluster is likely the center of the captured galaxy that has had most of its stars stripped away as it collided with NGC 4424.
Nikhuli has also been stretched out by gravitational forces as it falls towards the center of the larger galaxy, giving it an elongated shape.
Currently, Nikhuli is about 1,300 light-years from the center of the larger galaxy, or about 20 times closer than the Earth is to the Milky Way’s supermassive black hole.
Using Chandra data, the researchers found evidence for a massive black hole in the smaller galaxy.
“One possible explanation for the X-ray source is that matter from Nikhuli is falling rapidly into a stellar-mass black hole,” they said.
“However, because these smaller black holes are expected to be rare in a cluster the size of Nikhuli, it is more likely from material falling slowly onto a more massive black hole between about 40,000 and 150,000 solar masses.”
“This is similar to the expected size of the black hole in the center of NGC 4424.”
The results imply that Nikhuli is likely acting as a delivery system for NGC 4424’s supply of black holes, in this case bringing along a massive one.
“If the center of NGC 4424 contains a massive black hole, Nikhuli’s massive black hole should end up orbiting it,” the authors said.
“The distance separating the pair should then shrink until gravitational waves are produced and the two massive black holes merge with each other.”
The findings were published in the Astrophysical Journal.
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Alister W. Graham et al. 2021. Potential Black Hole Seeding of the Spiral Galaxy NGC 4424 via an Infalling Star Cluster. ApJ 923, 146; doi: 10.3847/1538-4357/ac235b
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