Lecture 39: Dark Matter

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

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

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

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

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?

The center of our Galaxy visible Infrared

Sagittarius A* Radio

evidence for a supermassive black hole in our Galaxy

dark matter: do we need it? motions of stars/gas within galaxies show that there is ‘dark matter’ within galaxies 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 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 gravitational lensing also provides a different sort of evidence for the existence of dark matter

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

Evidence for dark matter in galaxies

rotation curves of four typical spiral galaxies

elliptical galaxies: absorption line broadening

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 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 9x10 10 M sun, and the luminosity is 1.5x10 10 L sun for example: the mass of the Milky Way within the Solar radius is about 9x10 10 M sun, and the luminosity is 1.5x10 10 L sun  the mass-to-light ratio is 6 M sun /L sun.

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 M sun. the motions of satellite galaxies around the Milky Way show that the mass within 100 kpc is about M sun. the total luminosity within this radius is about 2x10 10 L sun, so the mass-to-light ratio is about 50 M sun /L sun ! the total luminosity within this radius is about 2x10 10 L sun, so the mass-to-light ratio is about 50 M sun /L sun ! about 90% of the mass within 100 kpc is dark matter. about 90% of the mass within 100 kpc is dark matter.

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

Fritz Zwicky

clusters are full of hot gas

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 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 v = (0.1 km/s) x (T/Kelvin) 1/2 then use v in the usual formula then use v in the usual formula M = (v 2 x r)/G M = (v 2 x r)/G

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 10 8 K. Find the mass of the Coma cluster using both methods. Do they agree?

a third way: gravitational lensing

Abell 2218

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 M sun /L sun ! all three methods (galaxy velocities, hot gas temperatures, and gravitational lensing) show that clusters have mass- to-light ratios of M sun /L sun !

dark matter: what is it? there are two basic possibilities: there are two basic possibilities: 1. baryonic dark matter – ‘ordinary matter’ (i.e. protons, neutrons, electrons, etc.) perhaps faint stars, brown dwarfs, planets, gas? 2. non-baryonic dark matter – a new kind of particle that we have never seen directly!

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

exotic dark matter: WIMPs WIMPs stands for Weakly Interacting Massive Particles 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) 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)

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

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

Tune in next time to find out!