H1821+643



H1821+643 is an extraordinarily luminous, radio-quiet quasar in the constellation of Draco. The associated Active Galactic Nucleus (AGN) is situated in the Brightest Central Galaxy (BCG) of a massive ($$\sim 6.3 \times 10^{14} M_\odot$$), strong cooling flow cluster, CL 1821+64. Russel et al (2010) spatially isolated its X-ray signal from the surrounding cluster in Chandra X-ray observatory observations and computed $$L_\odot = 10^{47} erg/s$$ from the observed X-ray luminosity.

Supermassive Black Hole
The SMBH centred in CL 1821+64 is believed to be among the most massive in the known Universe. A variety of techniques have found different values for the mass. 5 studies found values $$M_{BH} \sim 10^9 M_\odot$$. Kim et al (2004) and Floyd et al (2008) used galactic bulge luminosity fits derived from Hubble data to find $$10^9 M_\odot$$ and $$3 \times 10^9 M_\odot$$ respectively. Russell et al (2010) provided a rough estimate of $$M_{BH} \sim 3 \times 10^9$$Solar mass. This was an underestimate with $$\log(\Delta M_{BH}/M_\odot) \geq 1$$. Kolman et al (1991) and Shapovalova (2016) independently modelled the quasar UV spectrum to find $$M_{BH} \sim 3 \times 10^9 M_\odot$$. Capellupo et al (2017) found $$M_{BH} \sim 3 \times 10^9$$ using $$H\beta$$ line emissions. 2 independent X-ray studies found significantly higher values. Reynolds et al (2014) found $$6\times10^9 M_\odot$$ by modelling reflection from the accretion disc and Walker et al found $$3\times10^{10} M_\odot$$ by modelling the interaction of the black hole with the Intracluster medim (ICM) as a Compton-cooled feeding cycle. $$M_{BH}$$ is in the range $$\log(M_{BH}/M_\odot) \sim 9.2 - 10.5$$.

The Schwarzschild diameter of this black hole is between 9.4 Tm and 188 Tm, which is about 16 times the diameter of Pluto's orbit. If the hole were a Euclidean sphere, the average density would be 18 g/m3, $$\sim 1 \%$$ the density of air at sea level on Earth.