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Potential Black Hole Seeding of the Spiral Galaxy NGC 4424 via an Infalling Star Cluster

Abstract : Galaxies can grow through their mutual gravitational attraction and subsequent union. While orbiting a regular high-surface-brightness 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. From archival Hubble Space Telescope images, we have discovered a red, tidally stretched star cluster positioned ~5″ (~400 pc in projection) from, and pointing toward the center of, the post-merger spiral galaxy NGC 4424. The star cluster, which we refer to as "Nikhuli," has a near-infrared luminosity of (6.88 ± 1.85) × 106 L ⊙,F160W and likely represents the nucleus of a captured/wedded galaxy. Moreover, from our Chandra X-ray Observatory image, Nikhuli is seen to contain a high-energy X-ray point source, with ${L}_{0.5-8\,\mathrm{keV}}={6.31}_{-3.77}^{+7.50}\times {10}^{38}$ erg s-1 (90% confidence). We argue that this is more likely to be an active massive black hole than an X-ray binary. Lacking an outward-pointing comet-like appearance, the stellar structure of Nikhuli favors infall rather than the ejection from a gravitational-wave recoil event. A minor merger with a low-mass early-type galaxy may have sown a massive black hole, aided an X-shaped pseudobulge, and be sewing a small bulge. The stellar mass and the velocity dispersion of NGC 4424 predict a central black hole of (0.6-1.0) × 105 M , similar to the expected intermediate-mass black hole in Nikhuli, and suggestive of a black hole supply mechanism for bulgeless late-type galaxies. We may potentially be witnessing black hole seeding by capture and sinking, with a nuclear star cluster the delivery vehicle.
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Submitted on : Friday, July 8, 2022 - 1:54:16 PM
Last modification on : Saturday, July 9, 2022 - 3:04:59 AM

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Alister Graham, Roberto Soria, Bogdan Ciambur, Benjamin Davis, Douglas Swartz. Potential Black Hole Seeding of the Spiral Galaxy NGC 4424 via an Infalling Star Cluster. The Astrophysical Journal, 2021, 923, ⟨10.3847/1538-4357/ac235b⟩. ⟨insu-03717912⟩



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