Holmberg 15A

Holmberg 15A (abbreviated to Holm 15A) is a supergiant elliptical galaxy and the brightest galaxy of the Abell 85 galaxy cluster in the constellation Cetus, about 241.4 megaparsecs (787.3 million light-years) from Earth.^[2] It was discovered c. 1937 by Erik Holmberg.^[3] It became well known when it was reported to have the largest core ever observed in a galaxy, spanning some 15,000 light years,^[3] however this was subsequently refuted.^[4][5]

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[1]
Declination	−09° 18′ 11″[1]
Redshift	0.055359±0.000016[1]
Heliocentric radial velocity	16,596±5 km/s[1]
Galactocentric velocity	16,653±5 km/s[1]
Distance	787.3 ± 55.12 Mly (241.4 ± 16.9 Mpc)h−1 0.6774 (Comoving)[1] 753 Mly (230.9 Mpc)h−1 0.6774 (Light-travel)
Group or cluster	Abell 85
Apparent magnitude (V)	14.7[1]
Characteristics
Type	cD;BrClG[1]
Mass	7×1013[citation needed] M☉
Number of stars	5×1012[citation needed]
Size	390,570 ly × 218,720 ly (119.75 kpc × 67.06 kpc) (diameter; 25.0 B-mag arcsec−2)[1][a] 386,170 ly × 301,240 ly (118.40 kpc × 92.36 kpc) (diameter; "total" magnitude)[1][a]
Apparent size (V)	1.3′ × 1.2′[1]
Other designations
Abell 85-BCG,[1] PGC 2501[1]

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_☉),^[2][3] 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_☉^[2][3] 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_☉.^[3] The paper by Lopez-Cruz et al. stated:^[3] "Therefore, we conservatively suggest that Holm 15A hosts an SMBH with a mass of 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".^[3] This makes it one of the most massive black holes ever discovered, with it being classified as an ultramassive black hole.^[6]

See also

• List of galaxies

Notes

1. The quick-look major axis physical diameters given by NED of 106.19 by 59.47 kiloparsecs (346,000 by 194,000 light-years) and 105.00 by 81.90 kiloparsecs (342,000 by 267,000 light-years) were based on a distance estimate of 214.000 ± 45.016 megaparsecs (698.0 ± 146.8 million light-years). The quoted diameters in this infobox were based on NED's provided scale "Virgo + GA + Shapley" of 1.17 kpc/arcsec multiplied by the given angular diameters.