Centaurus A

Lore & Background

Centaurus A was discovered on 29 April 1826 by James Dunlop during a survey at the Parramatta Observatory. In 1847 John Herschel described it as 'two semi-ovals of elliptically formed nebula appearing to be cut asunder and separated by a broad obscure band.' In 1949 John Gatenby Bolton, Bruce Slee and Gordon Stanley localized it as one of the first extragalactic radio sources. Five years later, Walter Baade and Rudolph Minkowski suggested the peculiar structure results from a merger of a giant elliptical galaxy and a small spiral galaxy. The first detection of X-ray emissions was performed in 1970, and gamma-ray emissions were observed in 1975–76. The Einstein Observatory detected an X-ray jet in 1979, and young blue stars were found along the central dust band with the Hubble Space Telescope in 1989. The Chandra X-ray Observatory identified over 200 new point sources in 1999, and the Spitzer Space Telescope found a parallelogram-shaped dust structure in 2006. Very-high-energy gamma emissions were detected by H.E.S.S. in 2009, and Centaurus A was identified as a source of highest-energy cosmic rays in 2010. In 2016, unusual high flares of energy were found in NGC 5128 and NGC 4636, hypothesized as potentially a black hole in a yet unknown process or an intermediate-mass black hole.

Reader's Guide

Centaurus A is significant as the closest radio galaxy and BL Lac object, providing a unique laboratory for studying active galactic nuclei, relativistic jets, and black hole physics. Its jet, observed moving at about half the speed of light in radio observations separated by a decade, produces X-ray emissions thousands of light-years long and radio jets over a million light-years long. The galaxy's peculiar morphology—a lenticular or elliptical galaxy with a superimposed dust lane—is recognized as the result of a merger between a large elliptical and a smaller spiral galaxy, making it a key object for understanding such collisions. It is also one of the nearest large starburst galaxies, with over 100 star formation regions identified in its dusty disk. Two supernovae have been detected: SN 1986G (Type Ia) and SN 2016adj (initially Type II, later resembling Type Ib). A luminous red nova, AT 2020nqq, was discovered in 2020. Classical novae have also been detected, including AT 2024aeql and AT 2025nok. Distance estimates range between 3–5 Mpc, with classical Cepheids yielding ~3–3.5 Mpc and other indicators favoring ~3.8 Mpc. The galaxy is at the center of one of two subgroups within the Centaurus A/M83 Group, located in the Virgo Supercluster. In July 2021, the Event Horizon Telescope released a resolved image of the jet coming from its central black hole.

Did You Know?

• Centaurus A is the fifth-brightest galaxy in the sky.
• Its supermassive black hole has a mass of 55 million solar masses.
• The galaxy's X-ray jets are thousands of light-years long, while its radio jets are over a million light-years long.
• Two supernovae have been detected in Centaurus A: SN 1986G and SN 2016adj.