The Origin of Galaxies

THE FIRST GALAXIES

ABOUT 13 JANUARY; ABOUT 36,500 METERS UP THE LINE

The first stars, and their successor populations, were essential to the synthesis of the elements out of which the Earth and its living inhabitants would ultimately be composed. And throughout the Universe huge physical processes emerged that created birthplaces of stars—the galaxies. A galaxy is simply a collection of stars, dust, and gas bound together by gravity and surrounded by a halo of dark matter (which has its own gravitational field). Galaxies are self-contained objects, although galaxies can and do collide and integrate with one another. They are among the largest structures in existence. (Clusters of galaxies form the very largest of all structures, although structure is used here in the broadest sense.) There is some question about how soon after the Big Bang gravitational force, much of it generated by dark matter, pulled together the first proto-galaxies and then the first true galaxies. Scientists peering back in time with the newest and most powerful instruments have identified numerous galaxies at 900,000,000 years after the unified force broke down. Looking back to 700,000,000 years after the Big Bang, they see far fewer, but nonetheless they do detect some. In 2011, researchers discovered a galaxy believed to have formed 500 million years after the Big Bang, the earliest yet detected.¹

The Milky Way galaxy, our Sun’s home, is not among the very oldest galaxies, but it is indeed still ancient. Recent evidence (see below) indicates that it formed between 11 billion and 12 billion years before the present, the latter estimate being only 1.8 billion years after the Big Bang itself. On our condensed calendar of the Universe’s history, the estimate of a 12,000,000,000 year-old Milky Way Galaxy would put its formation on about 14 February. If the age is closer to 11,000,000,000 years, that puts its formation at around 12 March.

Hypotheses Concerning Galaxy Formation

As is the case with star formation, there are aspects of galaxy formation that are still not well understood. Galaxy formation is even more complex than stellar development, and there is a much wider range of variables that seem to be involved in it. But a great deal is known.

Galaxy formation is an ongoing process that has taken place throughout most of the Universe’s history. The formation of galaxies is deeply connected with the presence of cold dark matter. (One recent estimate from research on galaxy formation is that the Universe is almost 73% dark energy, almost 23% dark matter, and a little more than 4% baryonic, i.e., visible, matter.)² As is the case with all structure formation in the Universe, very small perturbations in matter will trigger gravitational changes. In the case of dark matter, these small disturbances will cause gravitational collapses, which makes the perturbations grow even more. As a consequence, dark matter will develop dark matter halos. Discussing the formation of dark matter halos, Loeb puts it this way:

First a region collapses along one axis, making a two-dimensional sheet. Then the sheet collapses along the second axis, making a one-dimensional filament. Finally, the filament collapses along the third axis into a virialized [possessing interacting particles and yet gravitationally stable] halo…the resulting network of structures delineates the so-called “cosmic web”. ³  [The cosmic web is thought to be the overarching superstructure of the Universe.]

Astrophysicist Andrew Benson has described a dark matter halo as being in “an approximately stable, near-equilibrium state supported against its own self-gravity by the random motions of its constituent particles.” These halos create deep gravity wells, which draw in baryonic matter in the form of hydrogen and helium gas. It is this enormous pocket of gas that will be enfolded in the dark matter halo, and it is in this environment that the gas will start to form masses of stars—a galaxy. ⁴

The cooling of the gas within a dark matter halo will affect the characteristics of the galaxy. Numerous factors can affect the gas’s cooling, including the gas’s metallicity, the density of photons interacting with it, and the formation and cooling within it of molecular hydrogen, or ²H. But the gas must cool in order for a galaxy to form.⁵ Dark matter halos themselves build up hierarchically, as halos merge with each other, forming substructures which maintain themselves as subhalos within the larger, overall halo.⁶

Galactic disks form when a system, collapsing under gravity, maintains its angular momentum, The regions of dark matter closest to the gas formations align themselves with this momentum. Since galaxies also tend to form hierarchically, a galactic disk will gather into itself various star systems that have already formed. Once a galaxy has been formed within the embrace of dark matter, star formation can proceed, but there is a great deal about this process that has still not been discovered. We do know that dark matter halos are not 100% efficient in forming stars; quite the opposite. Only a fraction of the baryonic matter they trap will do so.⁷

Since the metallicity of gas affects its rate of cooling, the nucleosynthesis of heavier elements in Population III stars, and the spread of these elements through supernova events and stellar winds, greatly affected the generations of galaxies that emerged after the first galactic formations came together. There is, however, still a great deal about the transition from Population III stars to the first galaxies that remains uncertain.⁸

The conventional view of galaxy formation has focused on how galaxies grow and develop by gathering material through merger and accretion. Yale astronomer Richard Larson makes the conventional case in a 1992 paper:

We have seen that the formation of at least the larger galaxies is likely to be a complex process involving interactions and mergers between subunits, and perhaps also the continuing accretion of smaller galaxies and diffuse matter. Elliptical galaxies and the bulges of early-type spirals are probably formed first in the densest parts of the universe, perhaps by a sequence of mergers of smaller subsystems. The disks of spiral galaxies probably form later from gas that remains relatively diffuse during the initial chaotic period of spheroid formation and then settles more gradually into a disk.⁹

It has been the consensus among most astronomers, therefore, that galaxies build themselves through the joining together of small bodies into huge aggregations, and by the gathering of gas from the inter-galactic medium. The merging together of groups of smaller galaxies is known as hierarchical formation. This means that established galaxies can grow even larger. Scientists in fact have witnessed this occurring. It now seems that galaxies form most of their stars when they are relatively small, but gain additional mass by the merger process. However, there does appear to be an upper limit on the size that these huge, gravitationally-assembled islands of stars can attain. It may also be possible that the large halos of dark matter that surround galaxies help pull merging galaxies into orderly shapes after a period of disorder following a merger.¹⁰

Galaxies have a variety of shapes, but the two most common ones are elliptical and spiral. Elliptical galaxies tend to be round and their structure is maintained by interior movements that are somewhat random, while spiral galaxies rotate. Spiral galaxies are not necessarily uniformly flat; most of them tend to have some sphere-like properties.¹¹ One such spiral with both disk-like and spherical properties is of particular interest to us: our own.

Characteristics of the Milky Way Galaxy

Our home galaxy is a spiral galaxy, more precisely, a barred spiral galaxy. By this we mean that the Milky Way is a galaxy in which a number of stars in its inner region have developed unstable orbits, and these orbits have gotten locked into place, creating an elongated, bar-like structure of stars which in turn tends to draw more stars into it, yet another example of a synergistic process.¹² The Milky Way has six spirals, which astronomers call arms, in all. From the galactic center outward, their common names are Norma, Scutum-Crux, Sagittarius, Orion, Perseus, and Cygnus. Our solar system is in Orion, which is not considered a major spiral.¹³ The galaxy is about 1,000 light years thick in most places, but in the center it bulges out (according to varying estimates) anywhere from 9,000 light years to 12,000 light years in thickness.¹⁴

As we have noted, there is an enormous halo of dark matter that surrounds the Milky Way galaxy. The galaxy itself is, as we have seen already, approximately 100,000 light years in diameter. (And as a reminder, just multiply the number of light years by 5.878 trillion miles to grasp the true enormity of our galactic home.) A circle drawn around the outer spiral of the galaxy would have a circumference of more than 300,000 light years. The Milky Way is estimated to have, as we have also seen, anywhere from 100 billion to 400 billion stars. The estimate varies because of the unknown number of very low-mass stars in the galaxy. In 1997 two astronomers estimated the total mass of the Milky Way to be between 590 billion and 700 billion solar masses.¹⁵

And, remarkably, many galaxies have dwarf galaxies of very low luminosity and mass which orbit them. Our home galaxy is estimated to have several hundred such bodies surrounding it.¹⁶ Another source puts the figure at as many as a thousand.¹⁷

In 2003, astronomers announced that there is a ring of billions of stars that encircles the galaxy, and if considered part of the galaxy itself, it would make our galactic home 120,000 light years in diameter.¹⁸ We toss such figures about so casually that it is worth reminding ourselves: if a beam of light had started from one side of the ring at about the time that modern sapiens were still confined to Africa, it just arrived on the other side of the ring now.

In 2000, definite evidence of a black hole at our galaxy’s center was discovered. This black hole is estimated by some scientists to have a content equal to 2.6 million solar masses, an enormous amount. But no one really knows how the black hole got there. Known as Sagittarius A, or Sgr A for short, it is not “devouring” large amounts of matter owing to a lack of stars in its immediate vicinity. It “feeds” off of material blown off the surfaces of stars well beyond its event horizon (the point at which no light can escape).¹⁹

Galaxies have densely populated groups of very old Population II stars that orbit the galactic center, groups known as globular clusters. As of 2007, 158 such bodies had been identified in the Milky Way.²⁰ As many as one-fourth of the globular clusters in our galaxy appear to have drifted in from other galaxies.²¹ Globular clusters were once studied because of their apparently high concentrations of stars, ranging from several thousand to more than a million, but now the study of these bodies tends to focus on their antiquity. Globular clusters may have been the first true stellar systems in the Universe; they have been characterized “[a]s fossil relics dating from the formation of the Galaxy”.²²

Despite the generally accepted view of astronomers and cosmologists that galaxies grow by merger and accretion, preliminary Hubble data would seem to indicate that several of the component parts of the Milky Way were formed at the same time (11-12 billion ybp) following the collapse of a single mass of dust and gas, and that our galaxy is not the product of mergers. The age of our galaxy’s central bulge and its outer halo seem to be the same. It is possible, however, that areas of our galaxy have grown by accretion, pulling in matter from neighbors (as is apparently the case with the Sagittarius dwarf galaxy).  It is also possible that the Milky Way underwent a very violent collision early in its history, which drove material toward its bulge. Scientists caution that these findings are still tentative.²³  These data have to be weighed in conjunction with discoveries announced in 2006 that reveal multiple instances of the Milky Way growing through incorporating streams of stars from a number of small, nearby dwarf galaxies.²⁴

In terms of our relationship to the galaxy, our solar system is about 26,000-30,000 light years from galactic center, depending on the estimate. The northern hemisphere of the Earth looks out at the galaxy’s outer arms; the southern hemisphere is aimed toward the galactic center. This means that our solar system is close to perpendicular to the galactic plane. Our main sequence local star is considered to be a very ordinary one in the context of the entire galaxy. And our planet has a great deal of company; in 2011, NASA estimated that there are at least 50 billion planets in the galaxy.²⁵

The Local Group and Beyond

The Milky Way Galaxy is part of what is known simply as the Local Group, a set of galaxies gravitationally bound to each other. Our galaxy and Andromeda, or M31, are the two most prominent members of the group. Andromeda is a spiral galaxy like our own, about 2 million light years distant. About 50 members have been identified (but presumably the discovery of more and more dwarf galaxies orbiting the Milky Way may significantly alter our estimate or cause us to change our classifications). The Local Group, about 4 million light years across, is in turn a part of what is known as the Virgo Supercluster, a collection of thousands of galaxies more than 150 million light years across. Between superclusters is a great void of space, empty except for the hydrogen gas in it.²⁶

Galaxy Distribution in the Universe

A 3-D map announced in 2011 revealed that within 380 million light years of the Earth there are approximately 45,000 galaxies.²⁷ Within the whole expanse of space-time, there are estimated to be somewhere between 100,000,000,000 and 200,000,000,000 galaxies, but given how distant many galaxies are from us, no one can really know. Estimates are made by evaluating the density of galaxies found in particular areas of space. Galaxies do not appear to be evenly distributed throughout the Universe. The average distance between galaxies is 3.2 million light years. And even in the early Universe, more than 11 billion years ago, they were already forming clusters.²⁸

Among the most distant highly luminous objects are the quasars, first detected in the 1960s and originally thought to be stars. Embedded in galaxies and believed to be powered by black holes, they are (or were) apparently huge in size and they emit(ted) gigantic amounts of energy. In 2003, the Sloan Digital Sky Survey announced the detection of a quasar 13 billion light years distant from the Earth.²⁹ And of course, the discovery by the Hubble Space Telescope of a galaxy at 13.2 billion light years in distance revealed the most distant object of any kind yet detected. ³⁰

Our solar system came to be because of the physical processes at work within our home galaxy. The stars that inhabited a particular region of it contributed the stars out of which our home star emerged. It was multiple supernova events that apparently supplied our Sun with much of its material, and its elements, the elements out of which our planet and we are composed. Our solar system orbits the galactic center, and our star is a minor player in the great drama of the galaxy’s existence. On dark nights, away from sources of artificial light, we can see its arms stretching across the sky, helping us put our own humble lives in perspective. Knowing that our “island Universe” is simply one of countless billions of others humbles us even more.

But for at least 6,400,000,000 years, and perhaps as many as 7,400,000,000 years, the Milky Way existed without our Sun. Stars in our galaxy were born and died; stars exploded in spectacular supernova events. The Milky Way continued to gather material from the regions of space closest to its outer reaches. Its immense spiral arms slowly rotated in space, taking 250,000,000 years to complete a single rotation. But there was no trace of our solar system. After 12 March (at the latest), it would take more than 5 ½ months out of our imagined one year calendar for the next major event to occur.