ACTIVE GALAXIES

Astronomers have chartered normal galaxies out to several billion light-years. Many galaxy-like objects are also known to exist beyond this galaxy horizon, but their burry appearance makes it tough to place them into any of the normal galaxy categories. More importantly, the basic character of many of the most distant objects differs from those nearby. By and large, objects more than several billion light-years away are more “active,” to a certain extent more violent. Overall, the radiative powers of the active galaxies are much greater than those of the nearby spirals and ellipticals. Furthermore, the active galaxies emit copious amounts of different kinds of radiation—for example, x rays from interior cores of those galaxies and radio waves from their exteriors well beyond their cores.

The adjective “normal,” used to describe the elliptical, spiral, and irregular galaxies, conveys that those objects radiate the accumulated light of large numbers of stars. Much of their emitted energy is of the visible type, supplemented by lesser amounts of radio, infrared, ultraviolet, or x-ray radiation. That’s because stars, too, emit mostly in the visible part of the spectrum. But this is not true for the active galaxies. Some of them are completely invisible to us, undetectable with even the world’s largest optical telescopes. Their presence is sensed and studied by radio and infrared telescopes on Earth and by orbiting satellites capable of capturing higher-energy photons. Radiation from the active galaxies, then, is largely inconsistent with the summed emission of myriad individual stars. To be blunt about it, astrophysicists are unsure if active galaxies really have many stars.

The abnormal power and odd character of the predominantly distant astronomical objects imply that the Universe was once more robust than it is today. They confirm the idea, noted in the previous PARTICLE EPOCH, that the earlier Universe must have been a tumultuous period, quite unlike the more tranquil state surrounding us now in space and time. Since physical conditions were undoubtedly different in the first few billion years of the Universe—and recalling that probing great distances in space equals searching far back in time—it shouldn’t surprise us that remote objects seen in their youth differ from nearby, older ones. What is enigmatic—in fact, downright astounding—is the enormous amount of energy radiated by some of the most powerful active galaxies. Their total release of energy often stretches astrophysical understanding to its limits.

To gain some perspective, an average star such as our Sun emits in any one second the equivalent of about a billion 1-megaton nuclear bombs—an impressive feat in and of itself. Yet our Galaxy is ~1011 times more powerful because, after all, it contains that many more stars. By contrast, an active galaxy is generally 100-1000 times additionally energetic than that. Active galaxies can launch in one second as much radiation as the Sun emits in about a million years.

Now imagine the equivalent of a hundred normal galaxies all packed into the space usually occupied by one. This is the crux of the problem encountered while trying to fathom the monstrously active galaxies. Decades ago, it was fashionable to suggest that these objects were the sites of spectacular galaxy collisions. However, as noted above, computer simulations now show that even such collisions would not produce energy in the amount required, nor much explosiveness at all.

The fact that active galaxies often emit more invisible than visible radiation implies that these objects differ fundamentally from normal galaxies. What’s more, some of the active galaxies’ cores are extremely luminous while others sport huge external lobes that resemble wings, all of which further exacerbate their many oddities, making them among the hardest objects in the Universe to decipher. Perhaps we shouldn’t even be calling them galaxies.

Non-thermal Properties The gross emission features of some active galaxies can be explained by invoking a distinctly non-stellar mechanism. Called the “synchrotron process” after the laboratory accelerators (sometimes called synchrotrons) used to study subatomic particles, this non-thermal action describes the emission of radiation when charged elementary particles interact with magnetic fields. No stars are involved, nor is any heat per se, hence the term “non-thermal.” The radiation arises simply from fast-moving particles, especially electrons, traveling through magnetized regions of space.

Magnetism presumably pervades all things, not just the Earth, Sun, and our Solar System, but also entire galaxies. Although the magnetic forces in typically diffuse galaxies are some millions of times weaker than on Earth, magnetism can still play a significant role, especially when its effects mount across an entire galaxy. For many active galaxies, especially a subclass known as radio galaxies, the emitted radiation arises from a pair of oppositely aligned and hugely extended lobes that often span a million light-years; that’s a single object equal to ~10 times the size of our Milky Way, in fact comparable to the Local Group of galaxies. One such object is shown in the left part of Figure 2.5.

Fortunately, images of the innermost parts of a handful of these objects—most notably one of the closest (at 3 billion light-years!) of the active galaxies and also shown in Figure 2.5—reveal a kind of Rosetta Stone: a jet of high-speed matter fired from the core of the galaxy (sometimes called a quasar, see below) out into the intergalactic medium, thus “feeding” the extended lobes farther away. The velocity of the outflowing matter in the jets typically measures 50,000 km/s, or nearly 0.2 the speed of light, and some of the most energetic ones surpass half light’s speed. The jets themselves not only point toward the huge lobes from which most of the invisible radiation arises, but, more tellingly, they also point back to the central nucleus where the energy is actually produced.

FIGURE 2.5 FIGURE 2.5 – The radio image (left) shows a typical radio galaxy, this one called 3C175 some 10 billion light-years away, with its huge jets feeding faint radio lobes spanning about a million light-years. The images (right) of another such active galaxy, 3C273, shows a starlike appearance of its core, but also a close-in jet extending ~100,000 light-years. (NRAO; AURA)

Laboratory experiments have proved that when charged particles, particularly electrons, are injected into a magnetic field, they spiral around much like the needle of a compass thrown spinning through the air. Magnetism slows the particles, causing some of their kinetic energy to be changed into radiant energy (which is why the process is technically termed “non-thermal bremsstrahlung,” or braking radiation). The amount of radiation emitted from a single encounter of an electron and magnetism is not terribly large in the laboratory. But in the case of a huge galaxy-like object, the radiation can mount fiercely because of vast numbers of electron encounters. Furthermore, the emitted radiation is observed in the lab to be of the radio variety, in accord with what is observed on the sky.

That said, the details of the emission mechanism within many active galaxies remain enigmatic, even assuming repeated injections of fast and numerous electrons into the galaxies’ lobes. Although the synchrotron process gives us an inkling of the type of abnormal events responsible for the emission of such intense radio power, active galaxies also display a kind of explosiveness that requires continual acceleration of electrons to speeds close to that of light itself. Moreover, large clumps of plasma are occasionally tracked moving outward, forming the extended lobes so characteristic of many of these active galaxies. The implication is that fast-moving matter is violently ejected in opposite directions by extraordinarily energetic events at the cores of these galaxies.

Energy Sources What might be the source of such great energy? Can any known means explain such outbursts on truly galactic scales? Somewhat ironically, black holes can—or so astronomers think. But before describing these denizens of Nature, do note that the active galaxies are still not the most energetic objects in the Universe. An additional, extraordinarily luminous class of active astronomical objects has been monitored for several decades now—objects so puzzling that they sometimes seem to defy the currently known laws of physics. These are the innocuous-looking, though very distant and inordinately powerful, quasi-stellar sources—quasars for short. Not content just to rival the energy emission problems of active galaxies, quasars exacerbate those problems. Here’s why:

Not only are the quasars the most energetic objects in the known Universe, but their radio and optical radiation is highly variable, often displaying variations from week to week, sometimes from day to day. The implication is straightforward: Galaxy-sized objects could never synchronize their front-to-back emission to produce such rapid and coherent time variations; otherwise, the intensity of those variations would be blurred and not as sharp as observed. Expressed another way, cause-and-effect arguments demand that no object can flicker more quickly than radiation can cross it. Thus, daily variations imply that quasars cannot be much larger than a light-day across, or roughly the diameter of our Solar System. The enormous power of the quasars, ranging from a hundred on up to a million times that of our Milky Way Galaxy, must then arise from a region much smaller than our Milky Way, in fact tiny by cosmic standards. All of which drives us further toward the idea of compact black holes as candidates for the quasars’ central engines.

Quasar emission mechanisms—whatever they really are—must operate, again by comparative cosmic standards, within an extremely small realm of space, conceivably well less than a single light-year. Try to imagine the equivalent of a hundred or more normal galaxies all packed into a region comparable to the Solar System. That’s an indication of the anomalous state of affairs needed to appreciate the Herculean quasars, certainly among the most baffling objects in all the Universe.


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