Intermediate-mass black hole

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An intermediate-mass black hole (IMBH) is a hypothetical class of black hole with mass in the range 100 to one million solar masses: significantly more than stellar black holes but less than supermassive black holes. There is as yet no unambiguous detection of an IMBH, but the indirect evidence from various directions is promising.

HST G1 (Mayall II)
Globular cluster Mayall II (M31 G1) is a possible candidate for hosting an intermediate-mass black hole at its center[1]

Observational evidence

The strongest evidence for IMBHs comes from a few low-luminosity active galactic nuclei.[2] Due to their activity, these galaxies almost certainly contain accreting black holes, and in some case the black hole masses can be estimated using the technique of reverberation mapping. For instance, the spiral galaxy NGC 4395 at a distance of about 4 Mpc appears to contain a black hole with mass of about 3.6×105 solar masses.[3]

Some ultra-luminous X ray sources (ULXs) in nearby galaxies are suspected to be IMBHs, with masses of a hundred to a thousand solar masses.[4] The ULXs are observed in star-forming regions (e.g., in starburst galaxy M82[5]), and are seemingly associated with young star clusters which are also observed in these regions. However, only a dynamical mass measurement from the analysis of the optical spectrum of the companion star can unveil the presence of an IMBH as the compact accretor of the ULX.

A few globular clusters have been claimed to contain IMBHs, based on measurements of the velocities of stars near their centers; the figure shows one candidate object. However none of the claimed detections has stood up to scrutiny.[2] For instance, the data for M31 G1, the object shown in the figure, can be fit equally well without a massive central object.[6]

Additional evidence for the existence of IMBHs can be obtained from observation of gravitational radiation, emitted from a binary containing an IMBH and a compact remnant or another IMBH.[7][8]

Finally, the M-sigma relation predicts the existence of black holes with masses of 104 to 106 solar masses in low-luminosity galaxies. There are three main types of black hole: stellar, intermediate, and supermassive.

Potential discoveries

A black hole of puzzling lightness
RX J1140.1+0307 is a spiral galaxy, centered on a lighter, intermediate-mass black hole.[9]

In November 2004 a team of astronomers reported the discovery of GCIRS 13E, the first intermediate-mass black hole in our galaxy, orbiting three light-years from Sagittarius A*.[10] This medium black hole of 1,300 solar masses is within a cluster of seven stars, possibly the remnant of a massive star cluster that has been stripped down by the Galactic Center. This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars. However, in 2005, a German research group claimed that the presence of an IMBH near the galactic center is doubtful, based on a dynamical study of the star cluster in which the IMBH was said to reside.[11] An IMBH near the galactic center could also be detected via its perturbations on stars orbiting around the supermassive black hole.[12]

In January 2006 a team led by Philip Kaaret of the University of Iowa announced the discovery of a quasiperiodic oscillation from an intermediate-mass black hole candidate located using NASA's Rossi X-ray Timing Explorer. The candidate, M82 X-1, is orbited by a red giant star that is shedding its atmosphere into the black hole.[13] Neither the existence of the oscillation nor its interpretation as the orbital period of the system are fully accepted by the rest of the scientific community. While the interpretation is quite reasonable, the periodicity claimed is based on only about 4 cycles, meaning that it is quite possible for this to be random variation. If the period is real, it could be either the orbital period, as suggested, or a super-orbital period in the accretion disk, as is seen in many other systems.

In 2009, a team of astronomers led by Sean Farrell discovered HLX-1, an intermediate-mass black hole with a smaller cluster of stars around it, in the galaxy ESO 243-49. This evidence suggested that ESO 243-49 had a galactic collision with HLX-1's galaxy and absorbed the majority of the smaller galaxy's matter.[14]

A team at the CSIRO radio telescope in Australia announced on 9 July 2012 that it had discovered the first intermediate-mass black hole.[15]

In 2015 a team at Keio University in Japan found a gas cloud (CO-0.40-0.22) with very wide velocity dispersion.[16] They performed simulations and concluded that a model with a black hole of circa 100,000 solar masses would be the best fit for the velocity distribution.[17] However, a later work pointed out some difficulties with the association of high velocity dispersion clouds with intermediate mass black holes, and proposed that such clouds might be generated by supernovae.[18]

In 2017, it was announced that a black hole of a few thousand solar masses may be located in the globular cluster 47 Tucanae. This was based on the accelerations and distributions of pulsars in the cluster,[19] however later analysis of an updated and more complete data set on these pulsars finds no positive evidence for this.[20]


Intermediate-mass black holes are too massive to be formed by the collapse of a single star, which is how stellar black holes are thought to form. Their environments lack the extreme conditions—i.e., high density and velocities observed at the centers of galaxies—which seemingly lead to the formation of supermassive black holes. There are three postulated formation scenarios for IMBHs. The first is the merging of stellar mass black holes and other compact objects by means of accretion. The second one is the runaway collision of massive stars in dense stellar clusters and the collapse of the collision product into an IMBH. The third is that they are primordial black holes formed in the Big Bang.


  1. ^ Gebhardt, Karl; Rich, R. M.; Ho, Luis C. (December 2005), "An Intermediate-Mass Black Hole in the Globular Cluster G1: Improved Significance from New Keck and Hubble Space Telescope Observations", The Astrophysical Journal, 634 (2): 1093–1102, arXiv:astro-ph/0508251Freely accessible, Bibcode:2005ApJ...634.1093G, doi:10.1086/497023
  2. ^ a b Merritt, David (2013). Dynamics and Evolution of Galactic Nuclei. Princeton, NJ: Princeton University Press. ISBN 9781400846122.
  3. ^ Peterson, Bradley; et al. (2005). "Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. I. A Reverberation-based Measurement of the Black Hole Mass". The Astrophysical Journal. 632: 799–808. arXiv:astro-ph/0506665Freely accessible. Bibcode:2005ApJ...632..799P. doi:10.1086/444494.
  4. ^ Maccarone, T.J.; Kundu, A; Zepf, SE; Rhode, KL (2007). "A black hole in a globular cluster". Nature. 445 (7124): 183–185. arXiv:astro-ph/0701310Freely accessible. Bibcode:2007Natur.445..183M. doi:10.1038/nature05434. PMID 17203062.
  5. ^ Patruno, A.; Portegies Zwart, S.; Dewi, J.; Hopman, C. (2006). "The ultraluminous X-ray source in M82: an intermediate-mass black hole with a giant companion". Monthly Notices of the Royal Astronomical Society: Letters. 370 (1): L6–L9. arXiv:astro-ph/0506275Freely accessible. Bibcode:2006MNRAS.370L...6P. doi:10.1111/j.1745-3933.2006.00176.x.
  6. ^ Baumgardt, H.; et al. (2003). "A Dynamical Model for the Globular Cluster G1". The Astrophysical Journal. 589: L25–L28. arXiv:astro-ph/0301469Freely accessible. Bibcode:2003ApJ...589L..25B. doi:10.1086/375802.
  7. ^ Hopman, Clovis; Simon Portegies Zwart (2005). "Gravitational waves from remnants of ultraluminous X-ray sources". Mon. Not. R. Astron. Soc. Lett. 363 (1): L56–L60. arXiv:astro-ph/0506181Freely accessible. Bibcode:2005MNRAS.363L..56H. doi:10.1111/j.1745-3933.2005.00083.x.
  8. ^ "Measuring Intermediate-Mass Black-Hole Binaries with Advanced Gravitational Wave Detectors". Gravitational Wave Group. University of Birmingham. Retrieved 28 November 2015.
  9. ^ "A black hole of puzzling lightness". Retrieved 9 January 2017.
  10. ^ S2 and Central Black Hole
  11. ^ Schoedel, R.; A. Eckart; C. Iserlohe; R. Genzel; T. Ott (2005). "A Black Hole in the Galactic Center Complex IRS 13E?". Astrophys. J. 625 (2): L111–L114. arXiv:astro-ph/0504474Freely accessible. Bibcode:2005ApJ...625L.111S. doi:10.1086/431307.
  12. ^ Gualandris, A.; Merritt, D. (2009). "Perturbations of Intermediate-mass Black Holes on Stellar Orbits in the Galactic Center". Astrophys. J. 705 (1): 361–371. arXiv:0905.4514Freely accessible. Bibcode:2009ApJ...705..361G. doi:10.1088/0004-637X/705/1/361.
  13. ^ Dying Star Reveals More Evidence for New Kind of Black Hole | Science Blog
  14. ^ Waugh, Rob (2012-02-15). "Star wars: Hubble captures remains of shredded galaxy around rare new kind of black hole". Daily Mail.
  15. ^ Nease, Eric (9 July 2012). "Astronomers spot the very first intermediate-mass black hole". The Bunsen Burner. Phillips Cronkite Media Group. Retrieved 9 July 2012.
  16. ^ Oka, Tomoharu; Mizuno, Reiko; Miura, Kodai; Takekawa, Shunya (December 28, 2015). "Signature of an Intermediate-Mass Black Hole in the Central Molecular Zone of Our Galaxy". Astrophysical Journal. 816 (1): L7. arXiv:1512.04661Freely accessible. Bibcode:2016ApJ...816L...7O. doi:10.3847/2041-8205/816/1/L7.
  17. ^ "Signs of Second Largest Black Hole in the Milky Way - Possible Missing Link in Black Hole Evolution". National Astronomical Observatory of Japan. January 15, 2016.
  18. ^ Yalinewich, Almog; Beniamini, Paz (September 2017), Supernovae generated High Velocity Compact Clouds, arXiv:1709.05738Freely accessible, Bibcode:2017arXiv170905738Y
  19. ^ Kızıltan, Bülent; Baumgardt, Holger; Loeb, Abraham (2017). "An intermediate-mass black hole in the centre of the globular cluster 47 Tucanae". Nature. 542: 203–205. arXiv:1702.02149Freely accessible. Bibcode:2017arXiv170202149K. doi:10.1038/nature21361.
  20. ^ Freire, Paulo; Ridolfi, Alessandro; Kramer, Michael (2017). "Long-term observations of the pulsars in 47 Tucanae - II. Proper motions, accelerations and jerks". Monthly Notices of the Royal Astronomical Society. 471 (7640): 857–876. arXiv:1706.04908Freely accessible. Bibcode:2017MNRAS.471..857F. doi:10.1093/mnras/stx1533.

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