eso8608 — Science Release
Big Radio Galaxy is Nearer than Previously Thought
13 May 1986
Detailed observations of a bright supernova in the peculiar galaxy NGC 5128 = Centaurus A, have led astronomers at the European Southern Observatory to believe that this galaxy is much closer to us than previously thought. It is the nearest, strongly radio-emitting galaxy and is as such an object of crucial importance in modern astrophysical research. The revised distance is 7 - 10 million light years or only 3 - 4 times farther away than the Andromeda Nebula. Cen A may therefore even be an outlying member of the Local Group. The total radio emission energy corresponds to the conversion to pure energy (annihilation) of a mass equal to 10.000 suns.
The supernova in Cen A, which has received the official designation 1986G by the International Astronomical Union, was discovered on May 3.5 UT by Reverend R. Evans, an amateur astronomer in Australia, who has more than a dozen earlier discoveries to his credit. It appeared as a “new star", southeast of the center of Cen A and almost in the middle of the broad dust band that girdles this unusual galaxy (see attached picture). The magnitude was estimated as 12. No supernovae have been detected in this galaxy before. This event is of particular interest, because bright supernovae are rather rare and also because of the peculiar nature of the parent galaxy. The most recent supernova of a similar magnitude was in 1980, in the northern, spiral galaxy NGC 6946 .
Observations at ESO with the 1 m and 50 cm photometric telescopes have shown that supernova 1986G was still brightening at a rate of about 0.05 mag/day on May 11.2 UT. On this date, the V-magnitude was 11.4 and colour index (B-V) was 1.1 mag. CCD images were exposed at the Danish 1.5 m telescope. Low-dispersion IDS and CCD spectra have been obtained with the ESO 1.5 m spectroscopic telescope and with the 2.2 m telescope. They show a typical Type I supernova spectrum before maximum, significantly reddened by absorption in Cen A. Of special interest are very high dispersion spectral observations, obtained with the CASPEC spectrograph at the ESO 3.6 m telescope. The Calcium and Sodium spectral lines show a complicated structure with no less than six very deep absorption components, four of which originate in rapidly moving interstellar clouds in Cen A.
These observations, and the position near the middle of the dust band, indicate that the supernova is situated well inside the galaxy and that its light is dimmed by about 4 mag due to obscuring dust. Had it been situated in an unobscured region, its magnitude would have been about 7.5, making it the brightest supernova in this century. Due to Cen A's peculiar structure (some astronomers consider it to be the result of a collision among two galaxies), it has not yet been possible to measure an accurate distance to this galaxy. However, if the intrinsic brightness of 1986G is that of a normal Type I supernova, then the distance to Cen A would be only 2 - 3 Megaparsecs (7 - 10 million light years). Assuming the upper figure, the total radio energy is at least erg, that is the equivalent of solar masses. Obviously, Cen A was the site of a most energetic event not so long ago - the velocities of the interstellar clouds may be relicts of this.
 Supernovae are believed to represent a late evolutionary stage of massive stars in which the star runs out of atomic fuel. It can no longer support its own weight and collapeses. Immediately thereafter follows a dramatic thermonuclear explosion during which the outer layers are blown into the surrounding space. A small and very compact object may remain at the centre. The best known historical supernova was seen in the year 1054, giving birth to the Crab Nebula and an associated neutron star, which was detected as a radio pulsar in 1967. Most, if not all heavy elements in the universe have been generated in the exceedingly hot interiors of stars in the supernovae phase. Suernovae are very rarely discovered before they reach their maximal brightness and little is known about the early phases. Currently, about 20-25 supernovae are detected per year in exterior galaxies; the last one in our own galaxy, the Milky Way, appears to be the one found by Kepler in the constellation Ophioucus in 1604.
The ESO observations are continuing. The following ESO staff and visiting astronomers have participated so far: I. Bues, P.R. Christensen, S. di Serego Alighieri, H. Duerbeck, G. Galletta, P. Magain, P.E. Nissen, D. Reimers, P. Schulte Ladbeck and J. Sommer-Larsen.
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