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Lecture 39:

Lecture 39:. Dark Matter. review from last time: quasars. first discovered in radio, but not all quasars are detected in the radio appear ‘star-like’ (point-like) in ground-based images huge luminosities broad, flat spectra; strong emission lines emission comes from a very small region

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Lecture 39:

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  1. Lecture 39: Dark Matter

  2. review from last time: quasars • first discovered in radio, but not all quasars are detected in the radio • appear ‘star-like’ (point-like) in ground-based images • huge luminosities • broad, flat spectra; strong emission lines • emission comes from a very small region • only at high redshift

  3. related beasts • radio galaxies • normal looking galaxies with radio lobes/jets etc • found at all redshifts • Seyfert galaxies/active galactic nuclei • ‘mini-quasars’ buried inside of galaxies • only found at low redshift (but too faint to be seen at high redshift)

  4. What do they have in common? • very large energies • fueled by accretion onto a supermassive black hole • perhaps actually the same objects

  5. Example problem: fueling a quasar How much mass, in solar masses per year, does a quasar with a luminosity of 1040 W need to accrete? Assume that the efficiency of the accretion process is ten percent.

  6. Example 2: Weighing a super-massive black hole The rotation curve of M31 peaks at about 100 km/s at a distance of 3.6 pc from the center. What is the mass enclosed within this radius?

  7. The center of our Galaxy visible Infrared

  8. Sagittarius A* Radio

  9. evidence for a supermassive black hole in our Galaxy

  10. dark matter: do we need it? • motions of stars/gas within galaxies show that there is ‘dark matter’ within galaxies • motions of galaxies within groups and clusters show that there is dark matter between galaxies as well • gravitational lensing also provides a different sort of evidence for the existence of dark matter

  11. rotation curves: review • draw a rotation curve for a ‘solid body rotator’ (like a merry-go-round) • draw the rotation curve for our Solar System • draw the rotation curve for our Galaxy

  12. Evidence for dark matter in galaxies

  13. rotation curves of four typical spiral galaxies

  14. elliptical galaxies: absorption line broadening

  15. mass-to-light ratio • the mass-to-light ratio is defined as the total mass in solar masses divided by the luminosity in solar luminosities • for example: the mass of the Milky Way within the Solar radius is about 9x1010 Msun, and the luminosity is 1.5x1010 Lsun  the mass-to-light ratio is 6 Msun/Lsun.

  16. mass-to-light ratio depends on radius • the motions of satellite galaxies around the Milky Way show that the mass within 100 kpc is about 1012 Msun. • the total luminosity within this radius is about 2x1010 Lsun, so the mass-to-light ratio is about 50 Msun/Lsun! • about 90% of the mass within 100 kpc is dark matter.

  17. dark matter in clusters • we can find the mass of a cluster using the velocities of galaxies relative to the central galaxy

  18. Fritz Zwicky

  19. clusters are full of hot gas

  20. another way to weigh a cluster • assuming that the hot gas in clusters is in gravitational equilibrium, we can use the temperature of the gas to estimate the mass of the cluster • v = (0.1 km/s) x (T/Kelvin)1/2 • then use v in the usual formula • M = (v2 x r)/G

  21. Example: the Coma cluster The galaxies in the Coma cluster have an average orbital velocity of 1200 km/s within a radius of 1.5 Mpc. The hot gas has an average temperature of 108 K. Find the mass of the Coma cluster using both methods. Do they agree?

  22. a third way: gravitational lensing

  23. Abell 2218

  24. cluster mass-to-light ratios • all three methods (galaxy velocities, hot gas temperatures, and gravitational lensing) show that clusters have mass-to-light ratios of 100-500 Msun/Lsun!

  25. dark matter: what is it? • there are two basic possibilities: • baryonic dark matter – ‘ordinary matter’ (i.e. protons, neutrons, electrons, etc.) perhaps faint stars, brown dwarfs, planets, gas? • non-baryonicdark matter – a new kind of particle that we have never seen directly!

  26. the search for MACHOs • perhaps the dark “halo” of our Galaxy is made up of normal material (like faint stars or brown dwarfs) • these are called Massive Compact Halo Objects (MACHOs). • they might be detected by microlensing

  27. exotic dark matter: WIMPs • WIMPs stands for Weakly Interacting Massive Particles • the definition of a WIMP is a kind of particle that interacts with other matter only via gravity and the weak force (not the strong force and electromagnetic force)

  28. hot and cold dark matter • hot dark matter is made of particles that move very close to the speed of light (such as neutrinos) • cold dark matter is made of particles that move much slower than the speed of light • we now think most of the dark matter must be cold

  29. the fate of the Universe • the total amount of dark matter in the Universe determines its fate… • will the Universe • expand forever • expand forever but slow down until it is expanding infinitely slowly • turn around and start collapsing?

  30. Tune in next time to find out!

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