Holmberg 15A

Holmberg 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. It was discovered c. 1937 by Erik Holmberg.^[1] It briefly shot to fame when it was reported to have the largest core ever observed in a galaxy, spanning some 15,000 light years^[1], however this was subsequently refuted.^[2][3]

Supermassive black hole

It has been alleged that the primary component of the galactic core is a supermassive black hole with a mass of 10 billion M_☉^[1], although no direct measurement has yet been acquired. Previous estimates by Lauer et al have jointed a mass value as high as 310 billion M_☉^[1] 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_☉.^[1] The paper by Lopez-Cruz et al^[1] stated: "Therefore, we conservatively suggest that Holm 15A hosts an SMBH with M• ∼ 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 only be explained 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".^[1] This makes it one of the most massive black holes ever discovered.

References