H1821+643
Chandra X-ray Observatory image of H1821+643
Observation data(Epoch J2000.0)
Constellation	Draco
Right ascension	18h 21m 57.2365s
Declination	+64° 20′ 36.226″
Redshift	0.2970
Distance	3.4 gigalight-years (1.0 Gpc)
Type	Quasar
Apparent magnitude (V)	14.24
See also:Quasar,List of quasars

H1821+643 is an extraordinarily luminous, radio-quietquasar in theconstellation ofDraco.^[1]The associated Active Galactic Nucleus (AGN) is situated in the Brightest Central Galaxy (BCG) of a massive (${\displaystyle \sim 6.3\times {10}^{14}{M}_{\odot }}$), strongcooling flow cluster, CL 1821+64.^[2] Russel et al. (2010) spatially isolated its X-ray signal from the surrounding cluster in Chandra X-ray observatory observations and computed${\displaystyle L_{\odot }={10}^{47}erg/s}$ from the observed X-ray luminosity.^[2]

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

TheSchwarzschild diameter of this black hole is between 9.4terametres (63 AU) and 188terametres (1,260 AU), which is about 16 times the diameter ofPluto's orbit. If the hole were a Euclidean sphere, the average density would be 18 g/m³,${\displaystyle \sim 1\mathrm{\%}}$ the density ofair at sea level on Earth.^[a]

• Simbad,SIMBAD
• NED, NED, NASA/IPAC Extragalactic Database

• Draco (constellation)
• Quasars
• Supermassive black holes

• Articles with short description
• Short description is different from Wikidata