Centaurus A shoots radio jets longer than a million light-years
At the heart of Centaurus A sits a black hole of 55 million suns, firing a jet whose inner portion races at roughly half light speed. The radio glow from that outflow stretches past a million light-years, making this the nearest radio galaxy to Earth and a favourite laboratory for astronomers.
James Dunlop spotted the galaxy, catalogued as NGC 5128, on 29 April 1826 while surveying from the observatory at Parramatta, near his home in New South Wales. It ranks as the fifth-brightest galaxy in the sky, a fine target for amateurs, though it can only be seen from the southern hemisphere and low northern latitudes. Astronomers still argue over basics: whether it is lenticular or a giant elliptical, and whether it lies 11 or 13 million light-years away.
Its appearance puzzled observers from the start. In 1847 John Herschel described two oval halves seemingly sliced apart by a broad dark band, and Halton Arp placed it in his 1966 atlas of peculiar galaxies as a prime example of a disturbed system with a dust lane. The accepted explanation, first proposed by Walter Baade and Rudolph Minkowski five years after radio astronomers pinned it down in 1949, is a collision between a large elliptical and a smaller spiral galaxy. That merger sparked intense star birth, with more than 100 star-forming regions in the dusty disc, while the central bulge holds mostly old red stars. Because elliptical-spiral crashes are poorly understood, the galaxy draws steady attention.
Nearly every new instrument has found something. X-rays were detected by sounding rocket in 1970, gamma rays in 1975 and 1976, and the Einstein Observatory saw an X-ray jet in 1979. Hubble picked out young blue stars along the dust band a decade later, Chandra logged over 200 new point sources in 1999, and Spitzer revealed a parallelogram of dust in 2006. H.E.S.S. in Namibia caught gamma rays above 100 GeV in 2009, and the Pierre Auger Observatory tied it to the most energetic cosmic rays a year later. X-rays arise where the jet slams into surrounding gas, forming jets thousands of light-years long.
Two supernovae have appeared there. Robert Evans found SN 1986G in the dust lane on 3 May 1986; a Type Ia explosion, it showed that such blasts do not all share identical spectra or brightening patterns.
Source: Centaurus A
