Although it was not so thick as to block my view, the spray of dense clouds gave the interstellar space of M87 a variegated texture. I traveled onward, extrapolating what I had seen so far into the regions ahead. The clouds must continue to settle toward the galaxy’s nucleus, converging until they had no choice but to coalesce. I pictured familiar terrain, thick cloud decks and star-forming regions, like the Milky Way. But I had forgotten about the dramatic events going on in M87’s center.
About 100,000 light-years from the center, I began to encounter bubbles of gas so hot that the particles inside were dashing about at speeds indistinguishable from the speed of light. I had not encountered matter in this form since my visit to the latter-day Crab Nebula, and I immediately suspected that some powerful generator lay below, bristling with electromagnetic fields. In such situations, one has a good chance of seeing radiation emitted by the synchrotron process, the radiation of electrons gyrating under the influence of magnetic forces. This was the mechanism that had been responsible for the garish blue light of the Crab, but here, with the magnetic field much weaker and the hot gaseous atmosphere producing its own glow, only the radio waves generated by the gyrating electrons stood out. To radio-sensitive eyes, I was entering a region of wispy cirrus clouds, their fine strands sketching out the directions of the magnetic lines of force.
At first the ultrahot filaments were juxtaposed with much larger concentrations of (now) lukewarm gas settling in from the cluster. Most of the time I was still immersed in the matter that was slowly cooling and drifting inward. But the closer I got to the center, the more space was taken up by the ultrahot cells. Instead of occupying isolated pockets in the cooler substrate, the bubbles linked up to form interconnected channels, surrounding the regions of cooling gas and squeezing them. This only accentuated their cooling, and it wasn’t long before the pockets of lukewarm gas coalesced to form great cool sheets.
About 10,000 or 20,000 light-years out, the ride began to get bumpy. The sky was aglow with radio cirrus. They formed great swirling eddies, reprising the tumult that had characterized M87’s outer halo, except that now the turbulence was being driven from below by the activity in the galaxy’s nucleus, rather than from above by close encounters with the neighboring galaxies. Intermixed with the cirrus were the sheets of lukewarm gas, also stretched out along the lines of magnetic force. (Cold molecular clumps still peppered the medium sparsely but had never fulfilled their promise of merging to form a dense cloud layer.) A series of sharp jolts informed me that I was passing through a network of shock waves. As though to confirm this, the sheets of cool gas glowed bright, mostly in the pink light of hydrogen, but also with an array of other colors—the reds of nitrogen, the greens of oxygen—that showed the gas was being jostled violently. I passed through one sheet after another. The sheets of cool gas seemed to be lying closer together now, as though they were backing up against an obstacle. And so they were, but it was a curious sort of obstacle: a pressurized cavity so hot and tenuous that it was nearly indistinguishable from a perfect vacuum. I crossed the boundary of the cavity and sailed into open space, a sudden calm surrounding my craft. A shaft of light stretched across the cavity in front of me—I was approaching the jets.
33
Reprise
Despite the pressure of the Virgo Cluster’s atmosphere, despite the tremendous gravity of this enormous galaxy, the center of M87 had been cleared of gas out to a distance of 6000 light-years. The jets, which shot outward in opposite directions from the nucleus, had obviously pushed the gas aside. The cavity was not spherical; it was elongated in the same direction as the jets (like the cavity created by the jets of SS 433, I thought). As I scanned the cavity from one end to the other, I could see the impact points where the jets finally slammed into the material they were so effectively holding at bay. I recalled the hot cavity that had welcomed me to the center of the Milky Way. That region had been only 10 or 20 light-years across and could not have been blown open by any sort of emanation from the wimpy central black hole. The Milky Way’s cavity had been the product of the hot massive stars that clustered around the black hole, and their winds. Here in M87, the stars took a back seat to a giant black hole, which apparently ruled over the entire central zone of this galaxy.
I had entered the cavity near its middle, halfway between the two impact points, and could see how closely the jets resembled each other. This was no surprise. I had had considerable experience with jets by now, and I assumed that these had emerged from the two sides of a disk that swirled around the black hole. Symmetry is inherent in such an arrangement, and there was no reason why the opposing directions should differ in any fundamental way. What was surprising was the way the jets changed in appearance as I guided my craft along the wall of the cavity and headed toward one of the impact points.
Almost immediately, the jet on the opposite side of the cavity from my current position faded, and by the time I had made it two-thirds of the way to the impact point, the opposite jet had effectively become invisible. Meanwhile, the light from the nearby jet had intensified. It shone brightly at all wavelengths from radio waves to X-rays and dominated all other forms of radiation in my vicinity.
This behavior had to be illusory, but the illusion was significant. The matter in these jets was apparently moving at very close to the speed of light, and I was again seeing the famous “headlight effect” that I had read about so often and had witnessed, firsthand, at Cygnus X-1. According to the theory of relativity, a light bulb, traveling at a shade below the speed of light, would not be seen to emit radiation equally in all directions. The light would instead be beamed along its direction of motion, like the headlight of a car. By mapping out the changing asymmetry of the jets’ appearance as I moved through the cavity, I estimated the speed of the matter in the jets to differ from that of light by no more than 1 or 2 percent.
Light travels through open space in a straight line, and I would have expected matter traveling at nearly light’s speed to do much the same. But to my surprise, the jets were not perfectly straight. In places they jogged and seemed to be a bit unsteady, their directions shifting slightly as I watched. At first I assumed that the jets were being wiggled at their source, but that would imply a certain symmetry in the jogs: Whatever sideways excursion one jet took, the other would take in reverse. However, before I lost sight of the distant jet, I had compared the jets’ paths, wiggle for wiggle, and had found an element of randomness that proved they were not merely mimicking one another. What could make such a fast stream change course? The only candidate seemed to be whatever it was that filled the cavity and surrounded the jets. Despite first impressions, the cavity was not a perfect vacuum, and the jets did not treat it as though it were.
I learned this the hard way. Despite its uncanny resemblance to a shaft of light, the jet was made of real gas—just like the jet coming out of a newly formed star or emerging from the disk of the X-ray binary SS 433—and it packed a punch. But it was a very light—that is, a low-density—gas, which is why I had originally ascribed to it (and to the cavity) the properties of a vacuum. When the jet hit the impact point, ramming against the much heavier (that is, denser) atmosphere of the outer galaxy, it was like a high-powered water jet running into a brick wall. The gas of the jet splashed back violently, thus becoming the material that filled the cavity. Thus the jet and the cavity were not so very different in density, pressure, and composition, and it is hardly surprising that they interacted strongly.
The ride first became bumpy as I approached the jet’s impact point. This was an unpleasant surprise, because I had gotten used to the smooth conditions that had prevailed nearly the whole way along the wall of the cavity. Before I could reverse course, Rocinante was caught up in the full turbulent backwash from the impact and bounced around vigorously. I pulled up short of the impact point itself and headed toward M87’s nucleus, skimming along the outside of the jet. This meant that I experienced the same environment the jet was experiencing and was ab
le to follow any events that befell the matter flowing along the jet, though in reverse order. Still close to the impact point, I braced for further turbulence. I could see that the jet was being buffeted harshly and was responding in rather violent jogs from side to side. With all this pounding, it was not surprising that the jet had spread out considerably and was no longer the narrow stream that I had traced outward from the nucleus when I first entered the cavity. The turbulence died down as I moved further away from the impact, only to be replaced by more regular—but still nauseating—thumps. It seems that the jet and the surrounding cavity had conspired to generate periodic impulses of pressure, much as the wind generates periodic swells on the sea. With the jet appearing foreshortened and pointing almost directly at me, I could see that even the smallest shifts in the jet’s speed and direction, caused by these pressure waves, could have dramatic effects on its appearance. One after another, a string of bright knots lined up along the jet, each one a shock wave corresponding to a sudden though slight change in the jet’s direction. I began to regard it as a minor miracle that the jet stayed as straight as it did.
Several hundred light-years from the nucleus, I began to encounter scraps of cool gas once again. Since entering the cavity, I had encountered only the superheated matter left behind by the jets. Everything else seemed to have been pushed out of the way. After a while this had become a source of worry. I knew that if the jets were being created by the interaction of the black hole’s gravity with the mass and motion of surrounding matter, then there had better be more material flowing toward the black hole than just the material left behind by the jets themselves. I therefore looked to these cool filaments as possibly the key to the jets’ fuel supply. As I moved toward the center, increasing amounts of this gas came together, swirling with angular momentum, and settled into a disk.
Where did this gas come from? There didn’t seem to be any continuous flow from the galaxy’s outer atmosphere through the nearly empty cavity. But there were the sheets of cool gas that lay just beyond. Did the cavity occasionally let down its guard, allowing some of this matter to fall in and embrace the hole? Or was this a fossil disk, the relic of some ancient era when matter had last been able to collect unimpeded in the center of M87? When this gas has all been used, will the jets turn off and the black hole go to sleep?
The story told by the gaseous motion was unmistakable. To the first level of approximation, that motion was circular, with a velocity that increased the closer the gas lay to the nucleus. It indicated that a single massive object, located at the exact center of the disk, dominated all other forms of mass that might be distributed throughout the disk—stars, gas clouds, whatever. It confirmed that the mass at the center weighed 3 billion times as much as the Sun. To the second level of approximation, I saw that the gas had found a way to give up its angular momentumso that it might flow inward to feed the black hole. The circular motion was not steady but came in fits and starts. A three-armed pinwheel pattern, outlined in the light of shocked gas—pink, reds, and greens—splayed outward, and the gas, circulating through the spiral, gradually approached the hole.
The spiral-incised platter of swirling gas did not extend all the way to M87’s nucleus. Gradually, the gas in the disk became hotter and puffed up into a thick atmosphere enveloping its orbital plane. The pink hydrogen glow excited by the spiral shock waves faded out, as the remaining atoms were dashed to pieces, to be replaced by a glow both harsher and bluer, and the gas itself became much more transparent and harder to see. Flares began to erupt from the turbulent flow, arcing out along paths that clearly traced a strengthening magnetic field. As I closed in, the jet seemed to be disassembling itself into dancing strands of magnetized fluid, which wrapped themselves around a void. Then there was the void! I was facing a black hole once more.
Memories of familiar sights flashed though my brain. Had I been here before? It looked eerily similar to the center of the Milky Way, except that everything here was gigantic. The black hole was scaled up in mass by a factor of 1000; it was 3 billion times heavier than the Sun instead of 3 million. It was swallowing matter at a much more furious rate than the Milky Way’s black hole, yet there was something similar about the mode in which it did so. There was no dense, opaque accretion disk here, of the sort I had seen in Cygnus X-1 and SS 433. True, I had seen no jet in the center of the Milky Way, but had it really been absent or just too faint to see? Magnetic flares crackled, here as there, and I could see the loops of magnetic field twisting about the black hole’s axis of rotation, leaping upward and eventually joining into the flow that was to become the jet. In the center of the Milky Way, it had looked much the same. Everything had been so tenuous there, but no more so than it was here, near the huge black hole of M87, the environs of which strafed me with the same kind of harsh, transparent glow.
34
On the Brink
All the questions I had repressed, when I stood opposite the Milky Way’s great black hole on that first, disappointing leg of my journey, came rushing back into my head. How did this black hole come to rest here? Did it grow from a much smaller seed by swallowing stars and clouds of gas? Could it have started out as the collapsed remnant of a single star of only a few times the mass of the Sun, like the one that had collapsed to become the black hole Cygnus X-1? I remembered daydreaming about a star being torn apart and swallowed by the Milky Way’s black hole. Had I waited 10,000 years or so, I really would have seen it happen. But even if the Milky Way’s black hole were fed in this way every few thousand years, it is doubtful whether the hole could have grown to its 2.5 million solar masses during the time since the Milky Way formed.
The problem was even more acute in M87. Here, 3 billion stars like the Sun would have been required to bring the black hole up to its current mass. The central star cluster contained nowhere near that much matter, and the disk of gas I had seen on my way to the nucleus was dribbling in much too slowly to take up the slack.
Even if the requisite amount of matter were available near the hole, that doesn’t mean the hole would grab it. I had seen how many impediments there were to the growth of black holes, the most important being the motion—the angular momentum—of the matter that might be swallowed. The ability of a black hole to grab and swallow stars does not increase very rapidly as the mass of the hole increases, and it starts out shaky when the hole has little mass. If the M87 or Milky Way black hole had started out small, how many stars would have been within its reach? Stars would have had to venture much closer to the hole before being torn apart. Instead of doubling its mass in 5 billion years, say, might it not have taken the hole 10 billion, 15 billion, 100 billion years or more, to swallow enough stars—a time so long that it would have exceeded the age of the Universe, to say nothing of that of the Milky Way or the Virgo Cluster? And even if the black hole could swallow stars at an adequate rate, would the supply of stars have remained adequate over the entire lifetime of the galaxy? As stars were depleted from the danger zone near the black hole, would they have been replaced quickly enough to keep the black hole growing apace?
My natural optimism soon asserted itself. Of course, I had been assuming that the environment at the Milky Way’s center or the nucleus of M87 was always similar to its present state. But who was to say that these places had always been so sparse, so relatively gas-free? Hadn’t there been a time when the tumbling bar in the Milky Way had not yet assumed such proportions as a gatekeeper against too much inbound gas? During an earlier epoch in the Virgo Cluster, mightn’t the flow of the cluster atmosphere into M87 have supplied 100 or 1000 solar masses of gas each year instead of just 10? Perhaps the central black hole, here and in the center of virtually every other galaxy, is merely the dumping ground for much of the debris left over from the galaxy’s creation. Or do external events routinely overwhelm the ability of a bar, or a cavity blown by a pair of jets, to keep abundant gas from reaching the black hole. The crucial event might have occurred long after the galaxy formed—the collisio
n of the Milky Way with another galaxy, perhaps, or some hapless (and nameless) spiral torn apart and absorbed by M87. In either case, the nucleus could have become a dramatic beacon indeed—ultimately as bright as a million or a billion Cygnus X-1’s—and the black hole could have grown to its present size in merely a few hundred million years.
I could not be sure that this black hole—or the Milky Way’s black hole, for that matter—was homegrown. My visit to Virgo, following on the heels of my stay in the doomed Magellanic Clouds, had shown me that whole galaxies do collide and merge from time to time. If the cluster atmosphere, which, after all, contained much more matter than all of Virgo’s galaxies combined, was flowing into M87, then why not spice it up with the occasional nucleus of some small galaxy that happened to contain a black hole. Maybe the featured black hole of M87 or the Milky Way came originally from a galaxy where conditions for growth were more favorable. An alien black hole captured by the Milky Way, like a pebble thrown into a whirlpool, would spin with the swirling disk for a little while but then quickly sink into the center. And if a modest black hole had already been waiting there, then repeated black-hole mergers could have built up the monster I saw, more rapidly than any steady feeding by absorption of gas clouds or stars.
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