3C75 (a.k.a. 3C 75) is a binary black hole system in the Abell 400 cluster of galaxies. It has four radio jets (two from each accreting black hole). It is travelling at 1200 kilometers per second through the cluster plasma, causing the jets to be swept back.
The black holes involved in the binary black hole merger that produced the gravitational wave signal GW150914. Note that this detection confirmed the (former) existence of 3 black holes: the two original black holes with masses of 29 and 36 solar masses that merged to form one heavier black hole with a mass of 62 solar masses.
APM 08279+5255 was initially identified as a quasar in 1998 during an Automatic Plate Measuring Facility (APM) survey to find carbon stars in the galactic halo. The combination of its high redshift (z=3.87) and brightness (particularly in the infrared) made it the most luminous object yet seen in the universe.
Arp 220 is the 220th galaxy in Arp's Atlas of Peculiar Galaxies. This image is part of a large collection of 59 images of merging galaxies taken by the Hubble Space Telescope and released on the occasion of its 18th anniversary on 24th April 2008.
The outer horizon is the event horizon of the black hole in the usual sense of the term. The outer horizon is the boundary of the region from which null curves (the paths of light rays) do not escape to infinity. In other words, if you fall past the outer horizon, you can't come back out.
The host galaxy of S5 0014+81 is an FSRQ blazar, a giant elliptical galaxy that hosts a supermassive black hole in its center, which may be responsible for the intense activity of this blazar. In 2009, a team of astronomers using the Swift Spacecraft used the luminosity output of S5 0014+81 to measure the mass of the central black hole.