Virgo A Galaxy (M87)
Virgo A Galaxy — Discovered 1781 by Charles Messier.
The giant elliptical galaxy M87, also called Virgo A, is one of the most remarkable objects in the sky. It is perhaps the dominant galaxy in the closest big cluster to us, the famous Virgo Cluster of galaxies (sometimes also called “Coma-Virgo cluster” which is more acurate, as it extends into constellation Coma), and lies at the distance of this cluster (about 60 million light-years). M87 lies well in the heart of the Virgo cluster (together with a lot of galaxies including M84 and M86).
M87 has been discovered and cataloged by Charles Messier on March 18, 1781 when he also cataloged 7 more nebulous objects in the same region, and actually are all member galaxies of the Virgo Cluster, as well as globular cluster M92.
M87’s diameter of apparently about 7′ corresponds to a linear extension of 120,000 light years, more than the diameter of our Milky Way’s disk. However, as M87 is of type E1 or E0, it fills a much larger volume, and thus contains much more stars (and mass) than our galaxy, certainly several trillion (10^12) solar masses (J.C. Brandt and R.G. Roosen have estimated 2.7 trillion). This galaxy is also of extreme luminosity, with an absolute magnitude of about -22.
This giant is situated between a number of apparently close neighbors, some of them are probably physical satellites, while others may be apparent neighbors because of projection effects only. The brightest of these dwarf galaxies are: NGC 4476, NGC 4478, NGC 4486A, and NGC 4486B.
On very deep photographic plates, David Malin of the Anglo-Australian Observatory has found that M87 extends out even further; its outer parts are no more circular but significantly elongated, and the galaxy is over half a degree long in these exposures, more than the diameter of the Full Moon.
At M87’s distance of 60 million lightyears, this corresponds to a linear extension of more than half a million lightyears. Its outlayers appear notably distorted, very probably because of gravitational interactions with other Virgo Cluster member galaxies, and because they contain material of disrupted galaxies which have been incorporated in M87 during close encounters.
M87 is famous for two peculiar and perhaps unique features, a huge globular cluster system found on long exposures (as our image), and a spectacular jet which is better seen on short exposure photographs. We also have a montage of an image showing the globular clusters in the big image, and the jet in a small inset.
This magnificient galaxy is perhaps the one with most known globular clusters. While our Milky Way has the modest number of roughly 150 to 200 globulars, M87 possesses a remarkable system of several thousands of these objects: Moderate estimates such as the 1976 work quoted by Burnham give numbers of at least 4,000, while more modern values go up to 15,000, e.g. 13,000 in W.E. Harris’ Globular Cluster Systems list), surrounding this giant galaxy in a conspicuous halo.
The many satellite globular clusters of M87 can be viewed in this image, and in the AAT images of M87. The photograph in this page was also obtained by David Malin with this telescope, and is copyrighted by the Anglo-Australian Observatory; more information on this image is available.
The luminosity function of globular clusters (which describes the percentage of globulars in certain luminosity intervals) was used repeatedly in the past to estimate the distance of M87; most recently, such a distance estimate was performed by B.C. Whitmore, W.B. Sparks, R.A. Lucas, F.D. Macchetto, and J.A. Biretta (ApJ 454, L73 [Dec 1, 1995]) using HST observations. They estimated a distance of roughly 55 million light years, in very good accordance with the 56 million light years obtained for M100 by Cepheid observations – corrected for the 1997 Hipparcos results, one arrives about at our 60 million light years. The basic problem of this measurement, however, is perhaps that we do not know if the actual GC luminosity function for M87 is the same as for spirals like our Milky Way or the Andromeda galaxy M31.
The giant jet was discovered by H.D. Curtis of the Lick Observatory in 1918. This phenomenon extends thousands of light years (sources give 5,000, but maybe it is more as they are often based on too small distances of this galaxy; the present author would estimate 7-8,000 might be more likely).
The jet is consisted of ejected gaseous material from the core of the galaxy. Polarimetric exposures of this jet have shown that its light is strongly polarized in a way which is typical for synchrotron radiation.
It exhibits a continuous spectrum, and appears blue in (short exposure) color photos (as the one in J.D. Wray’s Color Atlas of Galaxies). It is in violent turbulence; observations have shown apparent superluminal motion of gas clouds in this object – probably an illusion caused by the fact that the jet is pointing towards us.
Gorgeous detail of the Jet of M87 can be seen in this HST image which was processed by R. Mark Elowitz. Obviously it can be resolved into a string of small knots and clouds, a fact which was discovered in 1977 by Halton C. Arp of Mt. Palomar and J. Lorre of JPL (according to Burnham). Earlier, in 1966, Arp had discovered a second jet, pointing to the opposite direction, which is significantly less conspicuous. Because of its peculiar jet, H.C. Arp has included M87 as No. 152 in his Catalogue of Peculiar Galaxies.
M87 was also identified with the strong radio source Virgo A (for the brightest radio source in the constellation Virgo), by W. Baade and R. Minkowski in 1954. In 1956, a weaker radio halo was found by J.E. Baldwin and F.G. Smith of Cambridge.
It was also identified as a strong source of X-rays, and sits near the center of a hot, X-ray emitting cloud extending far over the Virgo cluster. Compare the optical and X-ray image of M87 and its surroundings in the Virgo cluster, or the whole central part of the Virgo cluster.
It is on hand that such an interesting objects as M87 is under intensive investigation with the Hubble Space Telescope. Even before its refurbishment in December 1993, early HST observations brought to light new interesting detail, especially in the jet of M87.
In the new Hubble Space Telescope views of M87 after the repair mission, the violent active nucleus of this galaxy could be glimpsed significantly closer, to reveal a massive central object of about 2-3 billion solar masses, concentrated within the innermost sphere with a radius of 60 light years.
This object is surrounded by a rapidly rotating gaseous accretion disk. The gas may be part of a more extended system of interstellar matter, which was detected by Fabry-Perot interferometry by astronomers from the Calar Alto observatory around 1990.
The only supernova recorded for M87 appeared in February 1919, but was not detected before 1922 on photographic plates, by I. Balanowski, who estimated its maximum brightness as 11.5. At M87’s distance, this corresponds to an absolute magnitude of nearly -20 mag.