Elliptical galaxy NGC 4889 is one of the brightest and largest galaxies in the Coma Cluster, but otherwise it has a fairly unremarkable appearance and doesn't display much activity. However, a dark secret lurks at the galaxy's center.

NGC 4889 hosts one of the largest supermassive black holes we have ever observed. It has a mass 21 billion times that of the Sun and its event horizon has a diameter of 130 billion kilometers (80 billion miles), about 15 times the diameter of Neptune’s orbit. A new photo of NGC 4889 was released by Hubble last week.

By comparison, Sagittarius A*, the supermassive black hole at the center of the Milky Way, is 4 million solar masses, and although it’s the most massive object in our galaxy, it is dwarfed in comparison to what’s inside NGC 4889. The galaxy is located 308 million light-years away at the center of the Coma Cluster, a dense group of galaxies with over 1,000 members.

Despite this, the elliptical galaxy and its remarkable core are now quietly behaving. The supermassive black hole has stopped feeding, so there is no burst of energy from the center. Astronomers have even been able to spot new stars forming around it.

Black holes can be very active. If gas, dust and other debris accumulate around a black hole, they form an accretion disc. The material is then accelerated, pulled and stretched by the tremendous gravitational pull, which heats it up to millions of degrees.    

During its active period, the accretion disc around NGC 4889’s black hole would have emitted up to a thousand times the energy emitted by the Milky Way. The black hole would have produced powerful jets that expelled material from the disc and heated up the galaxy and its surroundings.

The black hole is now dormant but the signature of its last activity can still be seen. The galaxy went through a quasar phase when the supermassive black hole was feeding, emitting powerful jets beyond the galaxy and into the cluster. Astronomers can still see the X-ray emission extending to millions of light years from NGC 4889.

Understanding what happens to quasars after they stop being bright is an important area of research in astrophysics. Quasars were very common in the early universe and they remain for the most part mysterious objects. These observations provide interesting clues about galaxy evolution through the ages of the cosmos.