ASTR 1514 1st Edition Lecture 33Dark matter - Most matter in galaxies is dark matter- However, dark matter does not emit any lightDark matter in spiral Galaxies – - Dark matter is matter that does not emit any radiation- It is only detectable by its mass- In spiral galaxies we detect dark matter through the galaxies rotation curve- Rotation Curve – Examine the speed of rotation as a function of distance from the center- Kepler’s Third Law applied to Galaxies:- We measure the rotation speed of a galaxy at different distances from the center- We measure Doppler Effect- Apply to stars or gas (absorption lines)- Most stars are located towards the center of a galaxy- Instead of expected nearly Keplerian motion, we observe flat rotation curves (velocity is the same at every radius (except very close to the center)).- So there has to be more mass than we can observe spread out through the galaxy. - There must be additional source of gravity that does not make light called dark matter. - We can infer the amount of mass interior to any radius from the rotation curve- The flat rotation curve means there is a large extended amount of dark matterDark Matter in spiral galaxies continued- About 95% of the mass of a spiral galaxy is dark matter- It is located in a large dark matter halo around the galaxy – much larger than the galaxy itself. Dark Matter in Elliptical galaxies- Elliptical galaxies are too far away to follow the motion of individual stars so stellar motion will not help in finding dark matter in elliptical galaxies- Elliptical galaxies have hot gas that emits X-rays- High temperature means high velocities for atoms in the hot gas- So there is a high escape velocity- So in turn there must be a large amount of mass- There is more mass than can be accounted for in optical light- So elliptical galaxies also have a lot of dark matter and the evidence is their X-ray emission. Mass to Light Ratio- Measure dark matter content using M/L- M/L = 1 means there is only luminous matter- The larger the value of M/L the larger the fraction of dark matter- Common values for spiral and elliptical galaxies are M/L = 20.- So 95% of the mass of the galaxy emits no light and cannot be seen. - The Milky Way galaxy has dwarf galaxies orbiting it that have extremely large M/L values. (hardly any stars but a lot of dark matter)Dark Matter in Cluster Galaxies- Individual galaxies are bound together by gravity and orbit around one another - Group of galaxies – fewer than 50 members- Clusters of galaxies – can have thousands of members- Can span 3-5 million parsecs they are the largest objects in the universe. - The Milky Way is in a galaxy group called the Local Group, it has 2 giant spirals and 30 smaller dwarf galaxies. - There is a lot of dark matter in galaxy clusters- Three ways to to know dark matter is present in clusters1. Galaxy speeds2. Hot gas3. Gravitational Lensing - Galaxy Speeds -The average speed of galaxies within a cluster depends on the total massof the cluster since each galaxy is attracted by the gravity of all the others. - Hot Gas in Galaxies - Galaxies clusters have hot gas (1 billion degrees) that emits X-Rays which have atoms with high velocities. The mass holding onto the hot gas is dark matter.- Gravitational Lensing – Clusters have a lot of mass so they bend space time near it- Light from galaxies behind the cluster can be bent and focused toward the earth. - Images from these galaxies will be distorted – appearing as arcs- The distribution and shape of arcs tells us that there must be more matter present than observed, Dark Matter. - Most of a clusters mass is Dark Matter (M/L = 100 or more )What is Dark Matter?1.) MACHOS- Massive Dark Halo Objects- Small stars, planets, cold white dwarfs and neutron stars, brown dwarfs and small black holes in an extended halo of a galaxy- This theory is flawed because there should be a lot fo MACHOS between us and one of the stars in the nearby galaxies. 2.) WIMPS- Weakly Interacting massive particles- Something like neutrinos – barely interact with ordinary matter- Some theories predict faint X-ray emission when two WIMP particles interact- Other theories predict particles that can be detected in giant vats of Xenon. 3.) MOND - Modified Newtonian Dynamics - Postulates that there is no dark matter – gravity acts differently at very large distances- Possible disproved by the ‘bullet cluster’Bullet Cluster- Two interacting clusters of galaxies that have passed through one another- Hot gas strongly interacted and stayed in the middle- Dark matter stayed within the clustersThere is no certainty of what Dark Matter actually is, it is a topic of current research. Dark Matter – The cosmic Web- Mapping the locations of galaxies identifies clusters and super-clusters- Often extended and filamentary with voids in between. This is called large-scale structure, or the cosmic web. Redshift of 0.14 corresponds to a universe already 11.9 billion years old (age of the universe is 13.77 billion years.)- Gravitational instabilities in the early universe collapsed to form galaxies- Calculate structure formation with models- Dark matter seems to be integral to galaxy