PHYSICS 105 The Formation of Galaxies Lucas Talavan-Becker 4/27/2011 1 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. 5 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. 3 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.