Galaxy Formation PDF

Galaxy Formation PHY113, Dr. John Ruan  Learning Goal: Explain the different pieces of evidence for the existence of dark matter in the Universe. What role does dark matter play in galaxy formation? What do we think dark matter made up of? Structure of the Milky Way Morp...

Galaxy Formation PHY113, Dr. John Ruan  Learning Goal: Explain the different pieces of evidence for the existence of dark matter in the Universe. What role does dark matter play in galaxy formation? What do we think dark matter made up of? Structure of the Milky Way Morphology of galaxies Galaxies can a variety of morphologies, that lie on a spectrum of being bulge- dominated (called elliptical galaxies) to being disc- dominated (called spiral galaxies). Spiral galaxies are sometimes also separated on whether they have a bar. This classification scheme was originally conceived of by Edwin Hubble, and is still used today. Spiral Galaxies Morphology is dominated by the presence of a disc. Face-on and edge-on views of two spiral galaxies. The disks of spirals have substantial gas and dust. Ionized gas regions that trace new star formation. Barred Spiral Galaxies Many of the spiral galaxies have central bars. The spiral arms begin at the end of these bars. Morphology of galaxies Galaxies can a variety of morphologies, that lie on a spectrum of being bulge- dominated (called elliptical galaxies) to being disc- dominated (called spiral galaxies). Spiral galaxies are sometimes also separated on whether they have a bar. This classification scheme was originally conceived of by Edwin Hubble, and is still used today.  Elliptical Galaxies Composed of only a spheroidal (i.e., bulge) component. Giant Elliptical Dwarf Elliptical Contains a trillion (1012) stars Contains only 100 million (108) stars  Different morphologies = Different formation Spiral galaxies: Collapse of a protogalactic gas cloud, causing the formation of a rotating disc due to conservation of angular momentum. Galaxy continues to grow through smooth gravitational infall of gas into the dark matter halo. No significant mergers. Formation of spiral galaxies  Different morphologies = Different formation Elliptical galaxies: Successive mergers of galaxies that scrambles the orbits of stars, resulting in a spheroidal shape. Formation of elliptical galaxies Formation of elliptical galaxies Much of what we know about the history of the Milky Way is pieced together from observations of other galaxies Evidence of dark matter: Milky Way rotation curve The flat rotation curve of the Milky Way is evidence that there is additional mass at large distances from the Milky Way center: dark matter. By definition, dark matter interact only gravitationally, but does not emit or interact with light or other matter. Evidence of dark matter: Milky Way rotation curve The majority of the matter in the Milky Way is in the form of dark matter. The dark matter lies in a spherical halo. There is vast amounts of other evidence for dark matter. Our modern picture of structure formation in the Universe is built on the existence of dark matter. Further evidence for dark matter: galaxy cluster velocity dispersions Galaxy clusters are the largest gravitationally-bound objects in the Universe. They have hundreds to thousands of galaxies gravitationally bound together, and can weight 1015 times the mass of the Sun. These galaxies have random orbits around the center of the cluster Question: Why can’t galaxy clusters grow even larger? Further evidence for dark matter: galaxy cluster velocity dispersions Galaxy clusters are the largest gravitationally-bound objects in the Universe. They have hundreds to thousands of galaxies gravitationally bound together, and can weight 1015 times the mass of the Sun. These galaxies have random orbits around the center of the cluster Question: Why can’t galaxy clusters grow even larger? Further evidence for dark matter: galaxy cluster velocity dispersions More massive galaxy clusters have stronger gravity, so the galaxies orbit around the cluster center faster. The velocity dispersion of these galaxies in the cluster thus galaxies enable an estimate of the cluster mass.  Further evidence for dark matter: galaxy cluster velocity dispersions More massive galaxy clusters have stronger gravity, so the galaxies orbit around the cluster center faster. The velocity dispersion of these galaxies in the cluster thus enable an estimate of the cluster mass. Astronomer Fritz Zwicky noticed that galaxies the inferred masses of galaxy clusters was much more than the total mass of stars in the cluster, implying the existence of a large amount of unseen matter. Further evidence for dark matter: galaxy cluster collisions Galaxies clusters sometimes merge with each other due to gravitational attraction. Gas in clusters will collide, but galaxies won’t because they are too diffuse, and dark matter won’t because it is collisionless. Further evidence for dark matter: galaxy cluster collisions Galaxies clusters sometimes merge with each other due to gravitational attraction. Gas in clusters will collide, but galaxies won’t because they are too diffuse, and dark matter won’t because it is collisionless. The observed positional offsets of mass (mostly dark matter) and gas reveal the presence of dark matter. Further evidence for dark matter: galaxy cluster collisions The mass maps of galaxies clusters can be inferred through gravitational lensing of background galaxies. Further evidence for dark matter: galaxy cluster collisions The mass maps of galaxies clusters can be inferred through gravitational lensing of background galaxies. Lensing causes a shear in the shape of background galaxies. The mass maps of galaxy clusters are inferred statistically through this shear. Further evidence for dark matter: CMB The formation of dark matter halos drives the growth of galaxies. The amount of structure in the Universe depends on the amount of dark matter. The CMB allows us the map the amount of structure in the Universe, and thus what the Universe is composed of. The formation of the first stars The first stars formed formed at the centers of dark matter halos at redshifts of 100, approximately 400,00 years after the Big Bang (4% of the current age of the Universe). The gas that formed the first stars was pristine (metal-free) gas, and were not able to fragment, thus forming very massive stars (100x the mass of the Sun). The formation of the first galaxies Dark matter halos continue to grow over time due to gravity, through both smooth accretion and mergers. Larger dark matter halos can contain enough gas in them to form many stars; these become the first galaxies, approximately 650,000 years after the Big Bang (5% of the present age of the Universe). Cosmic reionization These first galaxies lit up the Universe from the cosmic dark ages. The light from these galaxies ionized the Universe. Thus, the Universe underwent a second transition, from a neutral state to an ionized state. This process is known as cosmic reionization. Most of the Universe today remains ionized. Cosmic reionization These first galaxies lit up the Universe from the cosmic dark ages. The light from these galaxies ionized the Universe. Thus, the Universe underwent a second transition, from a neutral state to an ionized state. This process is known as cosmic reionization. Most of the Universe today remains ionized.