Holmberg 15A

Holmberg 15A (abbreviated to Holm 15A) is a supergiant elliptical galaxy and the central dominant galaxy of the Abell 85 galaxy cluster in the constellation Cetus, about 700 million light-years from Earth.^[1] It was discovered c. 1937 by Erik Holmberg.^[2] It became well known when it was reported to have the largest core ever observed in a galaxy, spanning some 15,000 light years,^[2] however this was subsequently refuted.^[3][4]

Holmberg 15A
Holmberg 15A (in the center) in X-rays by Chandra X-ray Observatory
Observation data (J2000.0 epoch)
Constellation	Cetus
Right ascension	00h 41m 50.5s
Declination	−09° 18′ 11″
Redshift	0.055672
Heliocentric radial velocity	16690 km/s
Galactocentric velocity	16747 km/s
Distance	704×10^6 ly (216 Mpc) h−1 0.678
Group or cluster	Abell 85 (cluster)Abell 85
Apparent magnitude (V)	14.7
Characteristics
Type	cD;BrClG
Mass	7×1013 M☉
Number of stars	5×1012
Size	270,000 ly (83 kpc)
Apparent size (V)	1.3 moa
Other designations
Abell 85-BCG, PGC 2501

Supermassive black hole

It has been postulated that the primary component of the galactic core is a supermassive black hole with a mass of 40 billion solar masses (M_☉),^[1][2] although no direct measurement has yet been made. Previous estimates by Lauer et al. have jointed a mass value as high as 310 billion M_☉^[1][2] using the gamma ray point break radius method. Kormendy and Bender gave a value of 260 billion M_☉ in a 2009 paper. Lower estimates were given by Kormendy and Ho et al. in 2013 at 2.1 and 9.2 billion M_☉.^[2] The paper by Lopez-Cruz et al. stated:^[2] "Therefore, we conservatively suggest that Holm 15A hosts an SMBH with a mass of ~1×10¹⁰ M_☉." Kormendy and Ho et al derived these equations using the M–sigma relation and the size of the outer bulge of the galaxy, which are indirect estimates. Rusli et al derived a value of 170 billion M_☉ using break radius methodology. In addition, Abell 85 has its velocity dispersion of dark matter halo at ~750 km/s, which could be explained only by a black hole with a mass greater than 150 billion M_☉, although Kormendy and Ho et al stated that "dark matter halos are scale-free, and the SMBH-dark matter coevolution is independent from the effects of baryons".^[2] This makes it one of the most massive black holes ever discovered, with it being classified as an ultramassive black hole.^[5]

See also

• List of galaxies

References

1. "Astronomers Just Found an Absolutely Gargantuan Black Hole The Mass of 40 Billion Suns". 6 August 2019.
2. López-Cruz, O.; Añorve, C.; Birkinshaw, M.; Worrall, D. M.; Ibarra-Medel, H. J.; Barkhouse, W. A.; Torres-Papaqui, J. P.; Motta, V. (2014). "The Brightest Cluster Galaxy in Abell 85: The Largest Core Known So Far". The Astrophysical Journal. 795 (2): L31. arXiv:1405.7758. Bibcode:2014ApJ...795L..31L. doi:10.1088/2041-8205/795/2/L31. S2CID 1140857.
3. Bonfini, Paolo; Dullo, Bililign T.; Graham, Alister W. (2015), Too Big to Be Real? No Depleted Core in Holm 15A
4. Madrid, Juan P.; Donzelli, Carlos J. (2016), The Abell 85 BCG: A Nucleated, Coreless Galaxy
5. Tangermann, Victor (August 6, 2019). "Astronomers Just Discovered One Of The Biggest Black Holes Ever". Futurism.com. Retrieved August 7, 2019.

• K. Mehrgan; J. Thomas; R. Saglia; X. Mazzalay; P. Erwin; R. Bender; M. Kluge; M. Fabricius (24 July 2019). "A 40 Billion Solar-mass Black Hole in the Extreme Core of Holm 15A, the Central Galaxy of Abell 85". The Astrophysical Journal. 887 (2): 195. arXiv:1907.10608. Bibcode:2019ApJ...887..195M. doi:10.3847/1538-4357/ab5856. S2CID 198899965. Measures SMBH mass as (40±8)×10⁹ M_☉.