The VLA radio telescope accidentally discovered in the Cygnus A galaxy, the first radio galaxy in the history of astronomy, the second supermassive black hole that has been hidden from scientists for over 20 years, according to an article accepted for publication in the Astrophysical Journal.
“The first photographs of this galaxy, taken by the VLA in 1980, became the hallmark of radio astronomy and its capabilities. When the telescope was updated in 2012, we wanted new images of this object. To our surprise, we suddenly found an object in the center of the galaxy that was missing from all the old images,”says Rick Perley from the US National Radio Astronomy Observatory in Socorro.
Galaxy Swan A, or 3C 405, is the first known to mankind "radio galaxy" - a giant family of stars that we cannot see in the optical range due to the large distance between them and the Milky Way, but can be seen by their powerful radio emission. For example, Swan A generates about the same amount of energy every second as about 260 billion suns could emit.
The source of this radiation, as shown by observations of Cygnus A back in the late 1970s, are super-powerful quasars - active black holes in the centers of radio galaxies, ejecting a part of the matter "chewed" by them into the intergalactic medium in the form of thin beams of super-hot matter, accelerated to near-light speeds. These beams, the so-called jets, glow brightly in the radio range, and can be easily seen in the images of galaxies by their bright and long "tails."
Photo of two black holes in the center of the Cygnus galaxy A. Photo: Perley, et al., NRAO / AUI / NSF
While conducting routine observations of Cygnus A after the VLA update in November 2016, Perley and his colleagues noticed that the images of the 3C 405 core show not one, but two pairs of such "tails". One of them corresponded to the black hole that astronomers saw back in the 1970s and 1980s, and the second, located about 1,500 light years away, was a new and previously unknown object.
This discovery piqued the interest of scientists, and they traced the mysterious structure in Cygnus A with other telescopes that can see this galaxy in the rest of the electromagnetic spectrum. These observations confirmed that the "unidentified object" really exists, and narrowed the list of possible variants of its origin to two things - an outburst and the remains of a powerful supernova or another supermassive black hole that "woke up" recently and began to eat the gas and dust surrounding it.
As the scientists note, it is still impossible to say which of these options is closer to the truth - the luminescence strength of the Cygnus A2 object and its spectral characteristics, in principle, fit into the values acceptable for both supernovae and black holes that have begun to absorb matter.
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In general, however, Purley's team is leaning towards a black hole, as supernovae of this type, producing powerful and long-lasting gamma-ray bursts and radio beams, are extremely rare. Further observations of 3C 405, scientists hope, will help to understand whether this is so or not.
If Cygnus A2 is indeed a black hole, then scientists will have a unique opportunity to trace the "waking up" quasar and understand how their periods of activity and "hibernation" affect the evolution of galaxies, and how the appearance of a second supermassive black hole can affect the behavior of such cosmic "Heavyweights".