The purpose of this review is to provide the astrophysicist who specializes in AGN, and in M87 in particular, with some astronomical background on the physical environment of M87, which is of course the well-known Virgo cluster of galaxies. I try to convince the reader that such knowledge is not only of cosmographical interest but may be essential for an understanding of certain features of the central engine of M87. For some, still unknown reason there is a remarkable coherence between the orientation of the jet axis of M87, which is probably defined by the spin axis of the central black hole on a pc scale, and the orientation of the Virgo cluster on a Mpc scale.
For an astrologer, this micro-macro connection would come as no surprise.
Consider the figure of Virgo from Hevel's beautiful Uranographia
(1690), reproduced here in Fig. 1. The position
of M87, which can be identified with respect to
the stars, happens to coincide with the ellbow joint
of Virgo's left arm, which is hidden behind her left wing.
It has always been a mystery what Virgo is pointing at with her left hand.
Now we know: this is the direction of the jet
of M87, to within 30 degrees.
So much for astrology in this contribution to the present volume.
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Figure 1. The figure of Virgo from Hevel's Uranographia (1690). North is up, West to the right, as in a modern representation. Everything appears mirrored because the entire sky map was drawn on a globe, to be viewed from the outside. The cross is indicating the position of M87 and the arrow is the direction of its jet. |
A mere hundred years after Hevel we find the first mention of the phenomenon of the Virgo cluster, still way before its extragalactic nature was known, by Charles Messier in Connaissance des Temps pour 1784 (see Tammann 1985 for the original passage). Messier noticed an unusual concentration of nebulae in the constellation of Virgo. Fifteen out of the 109 ``Messier'' objects are, in fact, Virgo cluster members. By identifying them on a conventional sky atlas, one can notice that the Messier's alone nicely trace out the direction of the jet of M87!
From the rich 20th-century history of the Virgo cluster I mention only the landmark studies of Harlow Shapley and Adelaide Ames in the 30-ties (Shapley, who, ironically, in the ``Great Debate'' of 1920 had been the opponent of Heber Curtis, discoverer of the jet of M87) and Gérard de Vaucouleurs and collaborators in the 60-ties and 70-ties (for more history see Tammann 1985).
The modern view of the Virgo cluster presented in the following is essentially based on the Las Campanas photographic survey of the Virgo cluster by Allan Sandage and collaborators (involving the writer), the galaxy redhifts measured by John Huchra, Lyle Hoffman and many others, and the ROSAT imaging of Virgo by Hans Böhringer and colleagues.