The Formation of Galaxies

PHYSICS 105

Lucas Talavan-Becker 4/27/2011

Image 1. Bright knots of glowing gas light up the arms of spiral galaxy Messier 74, indicating a rich environment of star formation. Introduction 2

Astronomers are still unsure as to how the galaxies form because evidence is scarce and hard to obtain due to the fact that early galaxies are billions of light years away from us. However, the advent of stronger telescopes is allowing us to observe galaxies formed during the early years of the universe. For example, in Image 2. Located in Hawaii near the 2007, the Keck Telescope, made by a team from the summit of Mauna Kea, these twin telescopes which are part of the Keck California Institute of Technology found six stars that observatory are providing optical spectroscopy of the faint Coma cluster dated back 13.2 billion years ago and therefore created galaxies. when the universe was only 500 million years old. Despite the universe’s enigmatic properties, astronomers have agreed on a general idea of how galaxies formed and clustered into their present states. But before we address how galaxies formed, we need to define what they are.

3 A galaxy is a massive

gravitationally bound system

composed of stars, star remnants,

gas dust, and an important but

poorly understood component

called dark matter (poorly

understood due to the fact that we

cannot observe anything beyond the Graph 1. Due to dark matter’s strong presence in the universe relative to stars and intergalactic gas, it plays an Schwarzschild radius and therefore the integral part in the formation and existence of all galaxies. collapse of a massive star which in

1 turn creates dark matter). The word galaxy comes from the Greek word galaxias, literally meaning

“milky”, which is why our galaxy is called the Milky Way. Galaxies range from dwarfs with as little as ten million stars to giants with as many as one hundred trillion stars. Everything in a galaxy orbits about the galaxies center of mass which is why dark matter is so important to the structure of a galaxy; dark matter is extremely massive and the accretion of dark matter alters the location of the center of mass.

Galaxies may contain star systems, star clusters, and various interstellar clouds. The Sun is an example of a star in the Milky Way galaxy and the Solar System, which includes the Earth and everything else that orbits the Sun, is an example of a star system.

Historically, galaxies have been categorized by their shape. 4 A common form is an elliptical galaxy which has an ellipse- shaped light profile. Another common galaxy is a spiral galaxy. Galaxies with incoherent shapes are called irregular galaxies. Usually irregular galaxies form by the gravitational interactions of two or more galaxies. Such interactions between galaxies may ultimately result in the merging of galaxies which induces episodes of significant Image 3. This is a picture of Zwicky star formation. Consequently, often times merging galaxies 18 (lower left portion of the image) a dwarf irregular galaxy are called starburst galaxies. Small, newly formed galaxies located 59 million light years away in the constellation Ursa Major. that have not yet assumed either the spiral or elliptical formation are called irregular galaxies as well.

There are probably more than one hundred and seventy billion galaxies in the observable universe.

Most galaxies range from one thousand to one hundred thousand parsecs in diameter. The distance

2 between galaxies is on the order of millions of parsecs. Intergalactic space (the space between galaxies) is made of extremely thin gas of an average density of one atom per cubic meter.

Galaxies usually form into clusters which make up super

clusters which are generally arranged into sheets or

filaments and surround immense voids in the universe.

The Great Wall is one of the largest known

superstructures in the universe. It is a massive cluster of

galaxies approximately two hundred million light years

away and its observable dimensions are five hundred

million light years long, three hundred million light years Image 4. The prominent concentration of galaxies running diagonally across wide, and fifteen million light years thick. It is not known the northern (that is, upper) portion of the image above has been termed the how much further the Great Wall extends because Great Wall. intergalactic dust in the Milky Way obscures the view and makes it impossible to see beyond what we know. Such structures like the Great Wall form along and follow web-like strings of dark matter. It is hypothesized that dark matter dictates the structure of the universe on the grandest scale.

Dark matter can account for ninety percent of all galaxies and usually, if not always, exists at the center of galaxies. For example, the Milky Way is hypothesized to harbor a super massive black hole at its center.

The evolution of galaxies

Edwin Hubble's observations and subsequent Hubble Law led to the idea that the universe is expanding.

We can estimate the age of the universe based on the rate of expansion. Because some galaxies are billions of light years away from us, we can discern that they formed fairly soon after the Big Bang.

Right after the Big Bang, the universe was fairly 6 homogenous. That is, when the universe was young, before the formation of stars and planets, the cosmic microwave background radiation filled the universe with a uniform glow from its white- hot fog of hydrogen plasma. So how did the universe change from its homogenous origins to its clumpy heterogeneous form that we know of it Image 5. The image above is a newly formed today? As the universe cooled, clumps of dark proto-galaxy. matter began to condense and within that matter, gas condensed as well. The formation of galaxies can largely be explained due to primordial fluctuations which are variations in density of the universe. The higher density regions gravitationally attracted dark matter and gas, and therefore created the first proto-galaxies. The helium and hydrogen in these clusters began to condense and form stars; thus the first galaxies were formed. This explains how galaxies formed, but how do we explain the distribution of galaxies about the universe?

When the universe was young, galaxies formed quickly, evolving and expanding by the accretion of smaller galaxies. Galaxies aren’t uniformly distributed about the universe but rather distributed in a

4 great cosmic web of filaments, and where these filaments meet are dense clusters of galaxies that began as the small density fluctuations to the universe.

Star Formation

Stars make up an integral part of all galaxies, so it’s only appropriate that we address how stars form. A star is formed out of a cloud of cool, dense molecular gas. In order for it to become a potential star, the cloud needs to collapse and increase in density. There are two common ways this can happen: it can either collide with another dense molecular cloud or it can be near enough to encounter the pressure caused by a giant supernova. Several stars can be born at once with the collision of two galaxies. In both cases, heat is needed to fuel this reaction, which comes from the mutual gravity pulling all the material inward. Figure 1. Here is a summary of star formation. 7

5 What happens next is dependent upon the size of the newborn star; called a protostar. Very small protostars will usually not have high enough temperatures to perpetuate hydrogen burning necessary to maintain hydrostatic equilibrium in a star. The small protostar will cool slowly over billions of years to become the background temperature of the universe.

Medium to large protostars can take one of two paths depending upon their size: if they are smaller than the sun, they undergo a proton-proton chain reaction to convert hydrogen to helium. If they are larger than the sun, they undergo a carbon-nitrogen-oxygen cycle to convert hydrogen to helium. The difference is the amount of heat involved. The CNO cycle happens at a much, much higher temperature than the PP chain cycle. Whatever the route, a new star has formed.

8 How galaxies interact and Image 6. This Hubble image of what forms from these the Antennae galaxies is the sharpest yet of this merging pair interactions of galaxies. As the two galaxies smash together, billions of stars Galaxies do not act alone. The are born, mostly in groups and distances between galaxies do clusters of stars. The brightest and most compact of these are seem large, but the diameters called super star clusters. of galaxies are also large.

Compared to stars, galaxies are relatively close to one another. They can interact and, more importantly, collide. When galaxies collide, they actually pass through one another, however, the stars inside don't run into one another because of the enormous interstellar distances. But collisions do tend to distort a galaxy's shape. Gravitational interactions between colliding galaxies could cause new waves of star

6 formation, supernovae, and or stellar collapses that form the black holes or supermassive black holes in active galaxies.

Three main types of galaxies Figure 2. Summary of spiral galaxy formation.

Spiral Galaxies 9

There are three main types of galaxies: disk galaxies, which are also commonly called spiral galaxies, elliptical galaxies and lenticular galaxies. Disk galaxies have a thin, rapidly rotating spiral structure. The formation of disk galaxies is still unclear but early scientists hypothesized that the collapse of a monolithic gas cloud triggers the formation of disk galaxies. As the gas cloud collapses, the gas settles into a rapidly rotating disk. However, some astronomers claim that all processes in the universe occur bottom-up which is defined as smaller parts grouping to bigger parts, rather than top to bottom.

One bottom-up galaxy formation hypothesis involves the interactions of matter that composed the early universe, dark matter, and gas. Dark matter halos and gas formed the early galaxies, and as smaller galaxies accreted with larger galaxies, the dark matter stayed mostly on the outer parts of the galaxy

7 because dark matter only reacts gravitationally and cannot dissipate. The gas, however, contracts, and as it does so it rotates faster until it forms a spirally disk.

Astronomers still do not know what stops the contraction. Some hypothesize that radiation from newly formed stars or active galactic nuclei halts the contraction. Others believe that the dark matter can pull the galaxy and thus stop disk contraction. Regardless, believers of the disk contraction process still cannot correctly predict the speed at which the disk rotates or the size of the galaxy.

Elliptical galaxies 10

An elliptical galaxy is a galaxy that has an ellipsoid shape and a smooth, nearly featureless brightness profile. Elliptical galaxies range in shape from nearly spherical to almost flat and can have as few as a hundred million to as many as a trillion stars. An elliptical galaxy is often the result of two galaxies colliding Image 7. The above image of Elliptical and merging together. Most elliptical galaxies are Galaxy M87 was taken recently by the Canada-France-Hawaii Telescope on top of composed of older, low-mass stars with sparse the dormant volcano Mauna Kea in Hawaii, USA. interstellar medium (the space between star systems in a galaxy) and minimal star formation activity. They are surrounded by large numbers of globular clusters, which are tightly bound groups of stars that orbit a galactic core as satellites. Elliptical galaxies make up approximately ten to fifteen percent of the local universe but are by no means the dominant type of galaxy in the overall universe. They are typically found near the centers of galaxy clusters and are less common in the early universe.

8 Elliptical galaxies have several properties that make them distinct from other classes of galaxies. Stars in elliptical galaxies orbit about a central point in the galaxy whereas the motion of stars in spiral galaxies is characterized by revolutions. Because star formation is minimal due to the lack of gas, dust, and space, elliptical galaxies get their glow and color from aging stars. Therefore, they tend to be yellow-red, which contrasts with the white, blue color of spiral galaxies which are much more conducive to star formation activity and have hotter, youngers stars that radiate brighter colors.

There is a wide range in size and mass for elliptical galaxies: some are as small in diameter as a tenth of a kilo parsec while others as big in diameter as one thousand kilo parsecs. Some small elliptical galaxies are as big as globular clusters but contain a considerable amount of dark matter at their centers differentiating them from globular clusters.

There are two main types of elliptical galaxies: the boxy giant elliptical galaxies which get their shape from the random motion of stars and the “disc-like” low luminosity galaxies that are also characterized by the random motion of stars but are flattened due to rotation.

Dwarf elliptical galaxies have properties that are intermediate between globular clusters and normal elliptical galaxies. Dwarf spheroid galaxies are similar in shape and composition to dwarf elliptical galaxies but generally have lower luminosity and are recognized only as satellite galaxies.

9 11 The formation of an elliptical galaxy is thought to occur due to the collision and merging of two galaxies. These major galactic mergers were thought to occur more frequently in the early universe. Nonetheless, minor galactic mergers continue to occur often. In fact, our very own Milky Galaxy is ingesting smaller galaxies right now.

Furthermore, our Milky Way galaxy is on collision course Image 8. This is Hubble image of an actual with the Andromeda galaxy. This collision is expected to take sideswipe of galaxies called The Mice. place in three to four billion years and the result of the collision of the two spiral galaxies will most likely be an elliptical galaxy.

Every bright elliptical galaxy is believed to contain a super massive black hole at its center which limits star formation and in turn the growth of elliptical galaxies.

Lenticular galaxies

Lenticular galaxies are an intermediate between elliptical galaxies and spiral galaxies. Lenticular galaxies have a disk-like shape similar to spiral galaxies but have 12 lost or consumed most of their interstellar matter essential to star formation. Therefore, they are similar to elliptical galaxies in the sense that they’re mostly comprised of aging stars and because of their ill-defined spiral arms, when inclined face-on, it is often difficult to Image 9. NGC 5866 is a lenticular distinguish between them and elliptical galaxies. galaxy discovered by Charles Messier in 1781.

There are two hypotheses that describe how lenticular galaxies form. One is that lenticular galaxies are the remnants of faded spiral galaxies whose spiral arms disappeared. Another hypothesis is that lenticular galaxies are the result of galaxies merging. The faded spiral galaxy hypothesis is supported by the fact that lenticular galaxies are characterized by the following properties: the absence of gas, the presence of dust, the lack of star formation rotational support, which are all attributes one might expect for a spiral galaxy that has consumed all of its interstellar matter. This hypothesis is also enhanced by the existence of gas poor or “anemic” spiral galaxies. If the spiral pattern disappeared, the resulting galaxy would be very similar to many lenticular galaxies.

The merging hypothesis is supported by the fact that lenticular galaxies are more luminous than spiral galaxies and have higher bulge-to-disk ratios than spiral galaxies. Merging galaxies would form a galaxy with increased stellar matter and therefore more stars to increase luminosity. Furthermore, a merger would explain the spiral, arm-less structure of lenticular galaxies.

Galaxy Morphological Classification

Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies.

11 13

The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1936. Hubble’s scheme divides galaxies into 3 broad classes based on their visual appearance. These broad classes can be extended to enable finer distinctions of appearance and to Figure 3. Summary of Hubble’s classification scheme. encompass other types of galaxy, such as irregular galaxies, which have no obvious regular structure either disk-like or ellipsoidal.

The Hubble sequence is often represented in the form of a two-pronged fork as shown above, with the ellipticals on the left with the degree of ellipticity increasing from left to right and the barred and unbarred spirals forming the two parallel prongs of the fork. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongs meet the handle.

To this day, the Hubble sequence is the most commonly used system for classifying galaxies.

The de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence.

De Vaucouleurs argued that Hubble's two-dimensional classification of spiral galaxies based on the tightness of the spiral arms and the presence or absence of a bar did not adequately describe the full range of observed galaxy morphologies. In particular, he argued that rings and lenses were important structural components of spiral galaxies.

12 Which came first: the Black Hole or the Galaxy?

Do galaxies form first and then a black hole springs up in the center, or possibly, do galaxies form around an already existing black hole?

Previous studies of galaxies and their central black holes in the nearby Universe revealed an intriguing connection between the masses of the black holes and of the central “bulges” of stars and gas in the galaxies. For central black holes from a few million to many billions of times the mass of our Sun, the black hole’s mass is about one one-thousandth of the mass of the surrounding galactic bulge. This constant ratio indicates that the black hole and the bulge affect each other’s growth in some sort of interactive relationship. The big question has been whether one grows before the other or if they grow together, maintaining their mass ratio throughout the entire process.

We finally have been able to measure black-hole and bulge masses in several galaxies seen as they were in the first billion years after the Big Bang, and the evidence suggests that the constant ratio seen nearby may not hold in the early Universe. The black holes in these young galaxies are much more massive compared to the bulges than those seen in the nearby Universe. The implication is that the black holes started growing first.

Conclusion

Less than a century ago astronomers knew only about our own galaxy, the Milky Way, which they believed held about 100 million stars. Then observers discovered that some of the fuzzy blobs in the sky weren't in our own galaxy, but were galaxies in their own right—collections of stars, gas, and dust bound

13 together by gravity. Today we know that the Milky Way contains more than 100 billion stars and that there are some 100 billion galaxies in the universe, each harboring an enormous number of stars. Our view of the universe is changing completely with the introduction of stronger telescopes, new technology, and a deeper understanding of the fundamental laws of the universe.

Hopefully future discoveries can clarify or unravel some of the enigmatic properties of the universe.

Dark matter, for example, fundamentally determines the structure of the universe but is poorly understood due to the tentative fact that we cannot observe it. Getting a better understanding of dark matter may lead to an understanding of how galaxies are distributed about the universe. The apparent incomprehensibility of the universe may at first appear negative, but the beauty is that there’s always something new to discover and therefore an infinite amount of knowledge to gain.

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