23-9 Galaxies formed from the merger of smaller objects

How do galaxies form and how do they evolve? Astronomers can gain important clues about galactic evolution simply by looking deep into space. The more distant a galaxy is, the longer its light takes to reach us. As we examine galaxies that are at increasing distances from Earth, we are actually looking further and further back in time. By looking into the past, we can see galaxies in their earliest stages.

Building Galaxies from the “Bottom Up”

The Hubble Space Telescope images in Figure 23-35 provide a glimpse of galaxy formation in the early universe. Figure 23-35 shows a number of galaxylike objects some 13 billion light-years (3988 Mpc) away and are thus seen as they were 13 billion years ago. These objects are between one-tenth and one-half the size of our Milky Way Galaxy and have unusual, irregular shapes. Furthermore, computer simulations of these objects show that they will likely collide to form a large elliptical galaxy.

Figure 23-35: R I V U X G
The Building Blocks of Galaxies In this Hubble Space Telescope image, the objects outlined by circles are about 13 billion ly from Earth and only a fraction of the size of our Milky Way. They are the building blocks that merge to form larger galaxies and clusters. While barely visible, these are the brightest objects at this great distance and early time, and numerous objects even smaller and dimmer are expected.

(NASA, ESA, M. Trenti (University of Colorado, Boulder, and Institute of Astronomy, University of Cambridge, UK), L. Bradley (STScI), and the BoRG team)

Images of the young universe such as those in Figure 23-35 lead astronomers to conclude that galaxies formed “from the bottom up”—that is, by the merger of smaller objects to form full-size galaxies. (These same images provide evidence against an older idea that galaxies formed “from the top down”—that is, fragmenting or breaking apart from immense, cluster-sized clouds of material.) We will see evidence in Chapter 26 that the matter in the universe formed “clumps” even earlier than this. These clumps evolved into objects like those shown in Figure 23-35, which in turn merged to form the population of galaxies that we see today.

Forming Spirals, Lenticulars, and Ellipticals

Once a number of subgalactic units combine, they make an object called a protogalaxy. The rate at which stars form within a protogalaxy may determine whether this protogalaxy becomes a spiral or an elliptical. If stars form relatively slowly, the gas surrounding them has enough time to settle by collisions to form a flattened disk, much as happened on a much smaller scale in the solar nebula (see Section 8-4). Star formation continues because the disk contains an ample amount of hydrogen from which to make new stars. The result is a spiral or lenticular galaxy (Figure 23-36a). But if stars initially form in the protogalaxy at a rapid rate, virtually all of the available gas is used up to make stars before a disk can form. In this case what results is an elliptical galaxy (Figure 23-36b).

Figure 23-36: The Formation of Spiral and Elliptical Galaxies (a) If the initial star formation rate in a protogalaxy is low, it can evolve into a spiral galaxy with a disk. (b) If the initial star formation rate is rapid, no gas is left to form a disk. The result is an elliptical galaxy. (c) This graph shows how the rate of star birth (in solar masses per year) varies with age in spiral and elliptical galaxies.

Figure 23-36c compares the stellar birthrate in the two types of galaxies. This graph helps us understand some of the differences between spiral and elliptical galaxies that we described in Section 23-3. Protogalaxies are thought to have been composed almost exclusively of hydrogen and helium gas, so the first stars were Population II stars with hardly any metals (that is, heavy elements). As stars die and form planetary nebulae or supernovae, they eject gases enriched in metals into the interstellar medium. In a spiral galaxy there is ongoing star formation in the disk, so these metals are incorporated into new generations of stars, making relatively metal-rich Population I stars like the Sun. By contrast, an elliptical galaxy has a single flurry of star formation when it is young, after which star formation ceases. Elliptical galaxies therefore contain only metal-poor Population II stars.

Figure 23-36c shows that both elliptical and spiral galaxies form stars most rapidly when they are young. This idea is borne out by the observation that very distant galaxies tend to be blue, which means that galaxies were bluer in the distant past than they are today. (Note the very blue colors of the distant, gravitationally lensed galaxies shown in Figure 23-32 and Figure 23-33.) Spectroscopic studies of such galaxies demonstrate that most owe their blue color to vigorous star formation, often occurring in intense, episodic bursts. The hot, luminous, and short-lived O and B stars produced in these bursts of star formation give blue galaxies their characteristic color.

An Evolving Universe of Galaxies

In addition to changes in galaxy colors, the character of the galactic population has also changed over the past several billion years. In nearby rich clusters, only about 5% of the galaxies are spirals. But observations of rich clusters at a redshift of z = 0.4—which corresponds to looking about 4 billion years into the past—show that about 30% of their galaxies were spirals.

Why were spiral galaxies more common in rich clusters in the distant past? Galactic collisions and mergers are probably responsible. During a collision, interstellar gas in the colliding galaxies is vigorously compressed, triggering a burst of star formation (see Cosmic Connections: When Galaxies Collide in Section 23-7). A succession of collisions produces a series of star-forming episodes that create numerous bright, hot O and B stars that become dispersed along arching spiral arms by the galaxy’s rotation. Eventually, however, the gas is used up; star formation then ceases and the spiral arms become less visible. Furthermore, tidal forces tend to disrupt colliding galaxies, strewing their stars across intergalactic space until the galaxies are completely disrupted (see Figure 23-28).

A full description of galaxy formation and evolution must include the effects of dark matter. As we have seen, only about 10% of the mass of a galaxy—its stars, gas, and dust—emits electromagnetic radiation of any kind. As yet, we have no idea what the remaining 90% looks like or what it is made of. The dilemma of dark matter is one of the most challenging problems facing astronomers today.