(2014) Anti-hierarchical evolution of the active galactic nucleus
space density in a hierarchical universe.
Comerford and her colleagues observed the waltzing black holes that have gas collapsing onto them, and this gas releases energy and powers each black hole as an active galactic nucleus
Astronomers soon named it Hanny's Voorwerp, from the Dutch for "object." It seemed to be a starless blob of fluorescing intergalactic gas, most likely lit by a quasar-like active galactic nucleus
in IC 2497 (see image at right).
The spectrum attests to X-rays being scattered or reprocessed by the giant molecular cloud, and it suggests that Sgr A* once was as luminous as a typical active galactic nucleus
NGC 4438 may also have an active galactic nucleus
, wreaking further havoc.
M87 has the only very active galactic nucleus
within this distance.
Veteran variable-star observers long have known that the nominally 13th-magnitude galaxy Markarian 421 contains the brightest and most variable active galactic nucleus
(AGN) visible in backyard telescopes (S&T: May 1993, page 75).
If some cosmic event were to dump a new load of gaseous fuel into our galaxy's core, the central black hole would almost certainly flare back up, shining brightly as an active galactic nucleus
for millions of years.
Echoing this view, some researchers use the term "active galactic nucleus
" in place of quasar or QSO.
Supergamma rays of |10.sup.12~ electron volts energy (|10.sup.-16~ centimeter wavelength) have been discovered coming from a few objects including pulsars and the active galactic nucleus
Markarian 421 (May issue, page 75).
Based on theoretical studies and observations in the radio, X-ray, and gamma-ray spectral regions, astronomers had come to think of Sgr A* as a miniature version of an active galactic nucleus
-- a massive black hole encircle by a hot accretion disk.