Turn Right At Orion (29 page)

The solution to this problem, of course, is to scoop up matter from interstellar and (lately) intergalactic space as I go along and to use this as my fuel. Fortunately, there is at least
some
matter present, however tenuous, at every known point in the Universe.
Rocinante
had been designed to scoop up enough fuel from the interstellar spaces of the Milky Way to keep me accelerating at 1g: This required a catchment area about 10,000 kilometers across, similar to the Earth's radius. A very modest requirement, given the technology of my craft! Plying the Milky Way's halo en route to the Magellanic Clouds had required the scoop radius to be expanded by nearly a factor of 100 to compensate for the much lower density of interstellar matter in these sere regions. But crossing the space between the Local Group and Virgo was a tougher challenge still. There the gas had never been mapped; it was too tenuous to detect, even using the sensitive instruments on board
Rocinante.
Stretching the scoop to several million kilometers across (20 times the distance from Earth to the Moon) was just manageable. Still, I had to monitor the surrounding density continuously to make sure I wouldn't get marooned.

Thus I was relieved when I found myself plunging through a considerably denser atmosphere than I had expected to encounter so far from the heart of the cluster. My X-ray sensors indicated that the gas was 10 times hotter than the gas that filled the Milky Way's halo. Yet, whereas the Milky Way had trouble holding on to its atmosphere (our Galaxy's hot gas was perpetually evaporating away), this even hotter corona seemed to be held in place by gravity. This, my first direct indication of the strength of the Virgo Cluster's gravitational field, was quickly confirmed as I entered the cluster's suburbs and began to measure
the speeds of individual galaxies. Each galaxy moved as a unit, under the combined influence of all the other galaxies' attractions, as well as the attraction of any other matter that happened to be present in the cluster. Like the atoms in the hot gas, the galaxies executed random orbits, their speeds regulated so as to keep them from flying away. Still, the speeds were enormous—1000 kilometers per second was typical. Any star moving that fast in the halo of the Milky Way, or any galaxy speeding across the Local Group at such a rate, would have been on a one-way trip to oblivion.

Did it make sense that the gravity in the Virgo Cluster should be so strong? There were a lot of galaxies here, 100 or more for every galaxy in the Local Group, and gravity certainly increased with the amount of matter present. But the cluster was also vastly larger than the size of the Local Group, and it was equally certain that gravity weakened over increasing distances. I had expected the two opposing trends—those due to distance and those due to mass—to cancel each other, yielding a net gravitational effect not too different from that in the Milky Way's vicinity. Yet my measurements told me that gravity was more than 10 times stronger in Virgo.

There was only one sensible explanation. A lot more matter had to be present here than met the eye. My surprise subsided; this was nothing new. The outer reaches of the Milky Way's halo, though invisible, contained more matter than all the visible parts of the Galaxy put together. I remembered how this had been established by remote observations from Earth, long before my departure. In Virgo, the discrepancy between seen and felt mass was much more extreme: There had to be nearly 9 grams of invisible matter for every gram that was visible, and the same trend was repeated in other clusters of galaxies. Were the individual galaxies abnormally heavy, considering their apparent sizes and luminosities? No, it seemed not. One could rule out that possibility by measuring the motions of the stars that made up each galaxy. It seemed instead that most of the invisible matter was not attached to the galaxies at all but formed a collective
halo that pervaded the entire cluster, allowing free passage of the galaxies through it. It was comforting to see a pattern repeated at each stage of the hierarchy: Clusters of galaxies had dark haloes, just like each of the galaxies that constituted them. Still, it irked me that I could do no more than confirm what was already known. My presence on the scene gave me no advantage in learning the composition of this elusive matter, let alone capturing a sample of it.

If galaxies could speed through the invisible matter without suffering harm, the same could not be said of their interactions with the hot gas. There was a surprising amount of hot gas, several times more mass than was contained in all the galaxies, and perhaps 10 or 20 percent of the total amount of matter needed to account for the strength of the cluster's gravitational field. I was still 3 million light-years from the nearest dominant elliptical, but the pressure was already higher than it was in the disk of the Milky Way. It was a punishing environment for a galaxy—particularly one flying through it at 1000 kilometers per second—and it took its toll.

I could see the price being paid by galaxies that spent most of their time near the outskirts of the Virgo Cluster but whose orbits took them on quick plunges, at high speed, through the denser regions of the atmosphere. These were mainly spirals, mixed in with raggedy-looking galaxies that reminded me of the Magellanic Clouds. As one galaxy sped by, I observed the damage wreaked by the wind that rushed past as a result of the galaxy's motion through the hot gas. First the galaxy's halo was stripped of its own atmosphere, leaving the disk open to a full assault by the wind. Where the disk's cloud deck was thin or had been punched through, the wind could penetrate right to the disk's midplane, disrupting the coagulation of clouds and, undoubtedly, affecting the normal cycles of star formation. Even where such opportunistic damage was impossible, the wind waged a war of attrition near the disk's outer edge, tearing off shreds of gas constantly. By the time the galaxy had completed its plunge and come out the other side of the cluster, only the
complexes of molecular clouds would be intact. The damaged disk, grotesquely truncated, would have less than a billion years to repair itself before the plunge was repeated. I saw another spiral moving so fast that the surrounding gas had no time to get out of the way. It rammed the gas ahead of it, squeezing it into an arc—a shock wave that propagated like a sonic boom through the cluster. A third galaxy, one of the ragged Magellanic types, seemed to be “beside itself” in the sense that nearly its entire complement of hydrogen cloud had been displaced from the disk by the force of the wind and was following behind it, like a ghost.

These effects damaged only the gas of a galaxy: The hot cluster atmosphere was able to flow between the stars with ease. Perhaps this was why, as I moved toward the dominant ellipticals that marked the central zones of the Virgo Cluster, the mix of galaxy types changed, gas-rich spirals and Magellanic types becoming less common and small ellipticals more so. I suspected that spirals simply could not survive in these crowded regions. Where the galaxies—and the hot cluster gas—were most highly concentrated, spirals were scarcest. The ellipticals, on the other hand, seemed to love a crowd. I imagined the fate of a spiral that ventured into one of these crowded zones, its gas stripped away, star formation stymied, perhaps even the stellar component of its disk fading into obscurity. If all that remained was its bulge, could I distinguish it from one of the small ellipticals that now surrounded me? Or perhaps the disk of stars, if not the gas, would remain, and the decimated spiral would take on the appearance of one of the gas-poor, bulge–disk hybrids that also populated these parts. There were other hazards, too, that could have “gotten” the spirals. These regions of concentrated galaxy population presented an increased risk of collisions between galaxies. Surely a spiral's disk would have trouble surviving such an encounter?

By now I had reached the inner sanctum of the Virgo Cluster and had to make a decision: which elliptical to explore. All of the bright ellipticals were old friends from my stargazing days
on Earth. They had figured in Charles Messier's list, that undifferentiated eighteenth-century catalogue in which the Crab Nebula featured as number 1, Orion as number 42, the Dumbbell as 27, and Andromeda as 31. The brightest galaxy in Virgo was M49, a star system that could be seen easily with binoculars from Earth, and it was there that I headed first. But my visit was brief, and I left disappointed. I had known, from distant observation, that many spirals were among the galaxies surrounding M49. I had attributed this to the vicissitudes that governed the collection of galaxies into clusters—in my naïve view, this was simply a “neighborhood” favored by spirals. But now I saw the association in a new light. For some reason, this huge elliptical had attracted only a sparse following of galaxies of any kind. Despite its huge mass and luminosity, the gaseous atmosphere that its gravity anchored was meager. Was it any surprise that spirals—a common but fragile type of galaxy—should find this a salubrious environment?

The second brightest galaxy, M87, lay 6 million light-years away. As a destination it looked promising. The combined light of its stars was slightly less than that of M49, but in every other way it reigned over the Virgo Cluster. Galaxies were grouped around it densely, a rich assortment dominated by ellipticals, including several of the other giant ellipticals that made Virgo appear so impressive from afar. The X-ray glare coming from its broad-shouldered halo showed that it had also captured the lion's share of the cluster's gas. But what really riveted my attention was the strange activity going on in its center, a luminous projection that looked like one of the jets I had seen coming out of a newly formed star in Orion, except that it spanned such an enormous distance that I could not grasp how the two phenomena could be related. That settled it: I was going to M87.

There was something alarming about the size of this galaxy. It was not just its huge diameter, or the fact that it weighed 100 times as much as the Milky Way. It was the way it merged into its surroundings gradually, without ever seeming to end. It was the way its appearance could trick you into thinking its scale was more comprehensible than it really was and then overwhelm you at the last minute. I remember seeing, from a distance, the star-like images that clustered around it and imagining that they were individual giant or supergiant stars. Only as I approached did I realize that each of these “stars” was an entire globular cluster, 100,000 to a million stars each, 10 light-years across. Whereas no more than 200 globulars hugged the inner halo and outer bulge of the Milky Way, M87 had thousands, maybe 10,000, of them spread throughout a region that extended over hundreds of thousands of light-years. And where Andromeda and the Milky Way both had two close companions, M87 was the focus of a swarm of galaxies, some of them comparable in size and brightness to the two great spirals of the Local Group.

M87 had almost certainly boasted even greater numbers of attending galaxies during the course of its history. Many of the galaxies that surrounded it now grazed or even penetrated its
immense halo and were in serious jeopardy of being gobbled up. If M87 was trying to conceal a cannibalistic past, its appearance did nothing to allay suspicion. How else could the galaxy have developed its vast, shallow halo of stars, if not by accumulating loose swarms of stars through the destruction of smaller galaxies that got too close. M87 did not look like other elliptical galaxies, which, like the bulges of spirals, seemed to have well-defined boundaries beyond which the light of their stars petered out. For M87, what might have been an entire normal, or even a giant, elliptical galaxy was merely the bright core at its center.

One of the advantages the dominant galaxy in a cluster enjoys is that it suffers little from the blasts of hot gas that can decimate the atmospheres of lesser galaxies. Spirals are most at risk, because their identities are tied so closely to the appearance of their gaseous components. Ellipticals have atmospheres, too, and a fast-moving elliptical on a plunging orbit through the cluster can suffer as much damage as any spiral, though it may not be so obvious to a casual observer. But M87 was immune to such risks. First, with a mass so enormous that it dwarfed that of any nearby galaxy, M87 didn't plunge through the cluster. Rather, it defined the frame of reference against which the plunges of other galaxies were measured. Second and more important, M87 didn't need its own atmosphere—it had appropriated that of the cluster. To a very good approximation, the Virgo Cluster's hot atmosphere was pinned by the gravity of M87 and was carried along with it. There were great eddies of hot gas toward the outer parts of M87's halo, probably stirred up by the motions of nearby galaxies. But the closer I got to the core, the more tightly this gas clung to the galaxy's stolid gravitational field. The swirling motions died away, and I grew accustomed to sailing smoothly through a vast sea of stars.

These calm conditions lasted until I had descended to a point about 300,000 light-years from M87's center. I had sped through more than 100,000 light-years of unchanging atmosphere: constant temperature, constant density, and a monotonous X-ray glow. The concentration of stars around me in the
shallow galactic halo had increased so gradually that I had scarcely noticed. But now things began to change. The temperature of the gas, which should have been going up as the gas became compressed under its own weight, began to
decline
as I continued toward the galaxy's center. Small clumps of gas, slightly cooler than the still-hot background, appeared spontaneously and quickly lost all their heat in a blaze of X-ray and ultraviolet radiation. Now cold, these clumps were kneaded and compressed by the pressure of the hotter gas that enveloped them, until they formed dense knots rich in molecules. The remaining hot gas, cooling more gradually than these clumps, began to slump inward toward the center of M87.