Appearances deceive: the giant elliptical M87

Domingos Soares

The appearances are deceiving. . . The saying was never so true when one thinks about the galaxy with the number 87 in the list of the French as- tronomer Charles Messier (1730-1817). With the aid of a small telescope, or even binoculars, M87 is seen as an almost perfect luminous disk. As we shall see below, M87 is much more than this. This galaxy is an “energy plant” of the highest power, shining not only in the visible but in waves of radio frequency and in X-rays as well. M87 is also called NGC 4486, that is, it is the galaxy number 4486 in the “New General Catalog”, organized by the Danish astronomer John L.E. Dreyer (1852-1926). It is located in the , which has this name because it is in the sky area delimited by the stellar of Virgo. The cluster has more than 2000 . M87 is a giant , the third brightest in the cluster. The other two are lenticular galax- ies, namely, M84 and M86. Some data about M87: its distance is 60 million light-years, its diameter is 120,000 light-years, about 5 times as large as the diameter of the Milky Way disk. Its mass, only in stars, is estimated in 3 trillion solar masses, that is, more than 20 times the mass of the Milky Way. Having in mind that the Milky Way is a giant , one sees that M87 is a “monstrous” galaxy, just by its dimensions and mass. But it is more than this, as we shall see next. The image of part of the shown here is centered in M87. Look at the image and let us take a walk through it. Slightly upwards and well to the right of M87 one sees, first, M86 and, beside it, M84. Some edge-on spiral galaxies can be seen as well. Just below M84 and M86 is NGC 4388 and above M86 one sees NGC 4402. To the left of M86 there is a pair of interacting galaxies — NGC 4438, the larger, and NGC 4435. The group of galaxies that begins in M84, pass through M86, by the interacting

1 pair mentioned above, and continues in curve upwards, is called “Markarian chain”. The name is a homage to the Armenian astronomer B. E. Markarian (1913-1985), who observed this structure, for the first time, in the mid 1970s. Well below M84, the NGC 4371 is seen edge-on, and below, a little to the left, NGC 4429, a spiral galaxy, is also seen edge-on. On the opposite side, well to the left of M87, and slightly above, is the elliptical galaxy M89. Above M89, further to the left, is the spiral galaxy M90, which presents an imposing appearance on more detailed images. These are, so to speak, the “stars” of this area. But there are here innumerable other fainter galaxies.

The central region of the Virgo galaxy cluster. M87 is the galaxy at the image center. The Milky Way sits in the periphery of this cluster, whose central region is at a distance of 60 million light-years from us. The width of the image corresponds to about 3 million light-years (Image: “Digitized Sky Survey”, Space Telescope Science Institute, Maryland, United States).

2 Very well. After the walk through the Virgo cluster, let us go back to our case. The first surprise concerning M87 happened in 1918, when the American astronomer H. D. Curtis (1872-1942) discovered a jet emerging from the galaxy center. A very thin jet that extends to about 5 thousand light-years, “hidden” within the visible boundaries of the galaxy. The jet can only be seen on images obtained with short exposure time, to avoid that the brightness of the galaxy stars in its neighborhood impedes its detection. The presence of the jet indicates that highly energetic processes are occurring in the center of M87. How, then, is the jet formed? The best explanation for its origin begins with gas and dust from the galaxy itself falling down towards the center, where there must be a very high mass concentration — probably a black hole of mass of about 2 billion solar masses. As it falls towards the center, the material starts to rotate and forms a disk called “accretion disk”. This disk works as a collimator of the jet. The jet is constituted of electrons and other subatomic particles, besides the ions of the material itself that is falling, which are expelled from the galaxy at diametrically opposed directions perpendicular to the accretion disk. This is so because of the the brutal resistance that exists along the plane of the disk. The jet is quite narrow due to the presence of a strong magnetic field along its entire length. The visible light is mainly emitted by electrons moving in the magnetic field present in the jet. Incidentally, as suggested above, there is a “counter-jet”, precisely at the opposite direction to the jet shown here. The counter-jet is much more faint due to its location with respect to us, the observers. In other words, the spatial orientation of the line that passes through the jet and the counter-jet favors the observation of only one of the ends of this imaginary line, i.e., exactly where the jet is seen. The was used to observe the nucleus of M87, with one of its instruments — a spectrograph. With this instrument, one is able to measure the velocities of the gas present there. The results are astonishing: the gas spins, around the center of the galaxy, at extremely high speeds, of the order of 550 km per second, or, 2 million kilometers per hour! To keep the gas bound to the galaxy nucleus, it is necessary that an enormous mass exists in that small region. This would give the tremendous gravitational attraction needed to keep the gas spinning. These observations favor the hypothesis — as we saw above — that there must be a black hole at the center of the galaxy. A black hole is one of the theoretical possibilities to explain such a mass concentration. The nuclear region of M87 is of the size of the solar system, that is, very small. It cannot contain the needed

3 number of stars — 2 billion — that may give the gravitational attraction able to keep the gaseous disk at the extremely high rotation observed. Therefrom the need to find alternative explanations. And the black hole is one of them.

Image of M87 and its jet obtained by the Hubble Space Telescope. The jet consists of ionized gaseous material, electrons and other subatomic particles emitted with velocities near the speed of light from the galaxy nucleus (Image: J.A. Biretta, W.B. Sparks, F.D. Macchetto, E.S. Perlman and R. Mark Elowitz, Hubble Space Telescope).

The discovery of the jet has shown that M87 is a very especial galaxy, with regard to energy production. And there is more. It is also a very strong source of radio waves, being known as the radio source Virgo A. The emission of radio waves extends to a region of approximately 200,000 light- years, therefore, larger than the region occupied by the stars of the galaxy. .

4 The radio emission of M87, known as the radio source Virgo A, one of the most powerful sources of radio of the whole sky. It is shown here in colors, which represent the different intensities of emission. The jet, shown in another figure, is at the central part of this image, in orange and with a pear shape. More precisely, the jet departs from the center of the pear to its narrower part (Image: Frazer Owen, Jean Eilek and Namir Kassim, National Radio Astronomy Observatory, New Mexico, United States).

And last, but not least spectacular, M87 is also a very powerful X-ray emitter! Such a radiation of extremely high energy comes from highly ionized gas at the temperature of more than 10 million degrees Celsius, distributed over a region yet larger than the radio emitting region exhibited above. The gas consists of iron, silicon, neon and other atoms, all produced inside the stars and “thrown” out of the galaxy. These atoms, due to the enormous temperature, lost their electrons, which mix with them, forming a vast X-

5 ray emitting halo. The electrons, like in the jet, are the ones that emit the radiation. Accelerated electric charges emit radiation. In the jet, the electrons are accelerated by the strong magnetic field present in the jet. In the X-ray halo, the electrons are accelerated when passing close to the ionized atoms, which are, therefore, electrically charged. Both the jet and the X-ray halo constitute a state of matter which is called “plasma”. Plasma is a gas of electrically charged particles. Incidentally, the universe is formed by more than 99% of plasma. M87 is also spectacular by an additional detail: its sys- tem. A globular cluster is a stellar system constituted by hundreds of thou- sands of stars, which are distributed in a spherical shape — hence the name “globular” —, and are bound to each other by their mutual gravitational at- traction. They constitute, consequently, independent subsystems of a galaxy. The stars of globular clusters are very old and represent a sample of the first stars of a galaxy. The Milky Way has between 100 and 200 globular clus- ters, the majority of them spread over an external halo, which surrounds the galaxy disk. Amongst the globular clusters of our galaxy are some of the most beautiful spectacles of the sky. For example, the cluster Omega Centauri (NGC 5139), at the Centaurus constellation, located near and to the north of the Southern Crux, provides an unforgettable view to whom observes it — in this case, at least the binoculars are needed.

6 Image of M87, obtained with the Hubble Space telescope, especially processed to emphasize the visualization of its globular clusters. They are mainly viewed throughout the periphery of the galaxy as “little balls” of light. Each of them is, in reality, a set of hundreds of thousands of stars distributed in a spherical shape (Image: R. Mark Elowitz, Hubble Space Telescope).

M87 is extraordinary in this respect for having several thousand globular clusters! Recent estimates point to the existence of up to 15,000 globular clusters. The clusters are the oldest objects of a galaxy and are intimately related to the process of galaxy formation. In the case of M87, the large number of globular clusters suggests that this galaxy may have “swallowed” smaller galaxies during its formation process, and part of the existing globular clusters may have their origin linked to these galaxies. That is, the successive mergers with other galaxies did not destroy the individuality of the globular clusters, which are strongly bound gravitational systems. It is evident in the image shown here the presence of hundreds of globular clusters, seen as little luminous disks spread around the galaxy. The majority of them cannot be

7 seen due to the intense brightness of M87. Of all that we have seen, is not it true, then, that appearances deceive? M87 is too much. . .

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