Which came first: black holes or galaxies? In other words, did supermassive black holes form initially and then accumulate around them matter that eventually became genuine galaxies, or did the galaxies form much as we see them now, after which they gave birth to holes at their cores as matter migrated toward their centers? This is the first of several chicken-or-the-egg conundrums encountered in cosmic evolution, most of them unresolved, or at least not solved satisfactorily to date. That’s probably because nothing in Nature is black or white, few solutions are clean and clear; rather, reality, and especially our models of it, resemble shades of gray throughout.

Favoring the “inside-out” idea, whereby black holes form first, is the notion that in any gravitationally bound system the densest things tend to infall early on, followed by the host galaxy taking shape around the central hole. Some data bolster this idea demographically: Given that quasars are more abundant than galaxies as we probe farther back in time—and where there are quasars, there are likely supermassive black holes—it would seem that black holes led the way.

By contrast, the radiative effects of the really big black holes might have actually hindered the formation of host galaxies, implying that the galaxies probably formed first. If so, then the process was more “outside-in,” whereby the galaxies came first, at least in rough form, after which the stars, gas, and dust later trickled toward the cores to create the huge black-hole engines. Computer simulations do imply that powerful jets associated with young, massive holes would have blown away surrounding material (a process known as "negative feedback"), possibly preventing the formation of galaxies at all. Furthermore, many supermassive black holes are still actively accreting matter, implying that the process of creating them is actually quite slow, perhaps taking many billions of years to settle at the cores of already formed galaxies.

The answer, citing those shades of gray again, likely mixes aspects of both models—that is, massive black holes and enveloping galaxies may have formed together. Astronomers have discovered recently that the masses of the central black holes are proportional to that of the bulges of their host galaxies, so the construction of both might well have been tightly wedded and coeval in Nature.

We can pose this unsolved riddle in another related way: Did the galaxies precede the stars or was it the other way around? The answer is important for the cosmic-evolutionary scenario since, as told here, the GALACTIC EPOCH precedes the STELLAR EPOCH. Is this justified? Most modern arguments do favor early origins for galaxies, followed by later formation of stars and planets within those galaxies. But the latest data are beginning to soften that view, or at least to muddy the waters a bit.

Recent findings suggest that some star formation must have occurred early in the GALACTIC EPOCH, since traces of heavy elements, such as carbon, silicon, magnesium and iron, are observed to have been present ~10 billion years ago. We know this because quasars, being ~100 times intrinsically brighter than normal galaxies, act like thin-beam cosmic flashlights, illuminating that part of intergalactic space between the quasars and Earth. And in the quasars’ spectra—when their light is split into its component colors—is clear evidence for minute amounts of heavy elements (~100 times less than those in the Sun), implying that at least some stars lived and died back then, for stars are the only places known where heavy elements are made. (Astronomers take the “heavies”—sometimes also called metals, to the dismay of chemists—to mean any element more massive than helium.)

The idea that some massive stars preceded the galaxies is bolstered by evidence (from quasar spectra) that early in the Matter Era the Universe was reionized, separating atoms everywhere into ions and electrons, much as had been the case in the plasma-rich, even earlier Radiation Era. This would have been a relatively brief period, probably less than a billion years following the “cosmic dark ages” when no luminous objects anywhere—no quasars, stars, or any other kind of light-emitting bodies whatever—had yet graced the cosmos. All was completely and totally dark, from the first 300,000 years when the Universe became neutralized and cosmic expansion redshifted the background radiation out of the visible and into the infrared part of the spectrum, to ~500 million years after the bang when gravity finally but only locally overcame expansion enough to begin clumping matter into big spherical structures. As the first glowing objects—almost surely the building blocks of fledgling galaxies—began emerging from those dark ages, a renaissance of light began to flow through the Universe. The details are murky but a consensus has emerged:

Quasars and possibly massive stars formed in the young Universe, starting no more than a billion years into the arrow of time. Objects smaller than 10⁶ solar masses would not likely have clumped given the rapidly expansive conditions at the time; the thermodynamics in that warm environment would tend to dissipate smaller clumps. The quasars largely lit up (and ionized) matter near the start of the GALACTIC EPOCH, possibly aided by ultraviolet radiation from the earliest stars. In addition, those “first stars” quickly created some heavy elements by the same kind of nuclear fusion events that occur in stars today, as later explained in the STELLAR EPOCH—so quickly that all these first massive stars are now long gone, having dramatically expired as supernovae (or been eaten by black holes—it’s possible!) within those first few billion years. Although we do see plenty of quasars in the earlier Universe, no observational evidence exists for those first stars—which means either that they did all disappear somehow (if theory is right), or that they never existed (if theory is wrong). Nor have astronomers ever found any stars with zero heavy-element content within them, as would be the case for any celestial objects among the first genuine stars. Perhaps the quasars themselves did all the reionizing, without the need for any early stars.

The upshot is that mostly big, million-to-billion solar-mass blobs likely took shape early in the GALACTIC EPOCH. These were the building blocks of galaxies, much as probably shown in Figure 2.18—almost surely quasars and their black-hole engines, and possibly massive star groupings that resembled today’s globular star clusters still lingering today in the haloes of many nearby galaxies. The quasars were clearly there then, probably thousands of times more populous than now; our telescopes spy on them in the distant past, a few billion years after the big bang, when the number of quasars peaked. (None of them resides near us in space or time; the closest quasar is more than 2 billion light-years distant, the last of a dying breed.) As best we can explore those truly ancient times, the primordial blobs are mostly gone, presumably having merged together quickly to build the galaxies. Those blobs, either lit with stars or not, must have repeatedly merged to make virtually all the galaxies within the first few billion years—which is probably why, even in our own Milky Way, most globular star clusters in the halo average 12 billion years in age and none is younger than 9 billion years. To what extent stars were already up and running in those formative blobs, or whether the stars originated mostly after the fledgling galaxies had formed, is frankly unknown.

FIGURE 2.18 – This photograph, one of the deepest ever taken of the Universe, provides suggestive “fossil evidence” for hundreds of galaxy shards and fragments, some of them up to 10 billion light-years distant. Events pictured here therefore took place ~10 billion years ago. The image, called the Ultra Deep Field, was taken by the Hubble Space Telescope, (STScI)

Astronomers are closely examining the latest data from both stars and galaxies, struggling to understand the timing and sequencing of these grand events. The task is nontrivial, for we are looking way back in time, trying to decipher events long over and done. To date, these data imply major assembly of the galaxies from smaller blobs mostly within 2-4 billion years after the bang; later formation wasn’t as robust if only because universal expansion was continuing to carry those building blocks and the young galaxies away from one another, reducing the number of interactions. By contrast, stars peaked their formation rates some 5-10 billion years after the bang; this we know by tracking back in time their ultraviolet radiation—the hallmark of newborn stars. In the main, then, the origin of most galaxies definitely preceded that of most stars. Although star production has been declining during for the past several billion years, stars still do now originate, yet galaxies do not—which means, again in the main (for these are averages over all the details), that the GALACTIC EPOCH preceded the STELLAR EPOCH.

An Ongoing Process When did galaxy formation stop, or has it? Astronomers are divided on this issue, too, which may be more semantics than astrophysics. Some contend that at a fairly well-determined time in the past—given by the age of the globular clusters in our Galaxy, for example—most galaxy formation was over. If true, then all galaxies are old, in fact nearly equally old and of order 12 billion years. Other astronomers demur, citing evidence that many galaxies seem to experience repeated mergers and accumulation of smaller satellite galaxies over extended periods of time—perhaps even up to the present day. If this is true, then galaxies might be said to be still forming today.

What constitutes an origin in contrast to evolution? Most experts have reached a tentative consensus that billion-solar-mass protogalaxies became established in some form or another relatively early on, probably within the first couple of billion years of the Matter Era. Virtually all galaxies originated contemporaneously long ago, as simply structured yet distinct objects. Their emergence was clearly the dominant feature of the GALACTIC EPOCH. In addition, ongoing mergers, interactions, and rearrangements within and among the galaxies ever since are regarded as evolution—developmental changes that further bulked up the galaxies with each successive merger. To be sure, astronomers have ample evidence that galaxies have evolved in response to external factors, indeed that evolution continues among the galaxies today.

Much of the fascination felt by workers studying the subject of galaxy formation derives from our inability to disprove many contending theories. An array of ideas remains possible, there being only meager observational data to discriminate among the details of various formation scenarios. However, this state of affairs will not likely last long when new data begin pouring in at rapid pace as telescopes of the 21^st century become powerful “time machines” designed to probe the far away and long ago. New and ambitious projects—including many international ground-based collaborations as well as robotic observatories in orbit—are expected to map accurately millions of galaxies across the sky in the next few years. Until galaxy data become demonstrably better, however, researchers familiar with the sophisticated mathematics of notoriously tough subjects such as fluid mechanics, turbulent physics, and magnetohydrodynamics will continue to justify their interests by tinkering with the problem of galaxy origins. For despite heroic efforts of the past few decades to unlock the secrets of galaxy formation, the specifics of a tested, plausible process have thus far eluded discovery.