Homework #10 is due Wednesday, April 25, 9:00 pm. Review session Wednesday, April 25, 7:15 pm.

What are quasars? Active galactic nuclei are very bright objects seen in the centers of some galaxies, and quasars are the most luminous type What is the power source for quasars and other active galactic nuclei? The only model that adequately explains the observations holds that supermassive black holes are the power source Do supermassive black holes really exist? Observations of stars and gas clouds orbiting at the centers of galaxies indicate that many galaxies, and perhaps all of them, have supermassive black holes

Dark matter and dark energy

Unseen Influences Dark Matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence on galaxies and groups of galaxies Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate

Contents of Universe “Normal” Matter: ~ 4.4% Dark Matter: ~ 23% Normal Matter inside stars: ~ 0.6% Normal Matter outside stars: ~ 3.8% Dark Matter: ~ 23% Dark Energy ~ 73%

The Evidence for Dark Matter

Galaxy Motions in a Cluster of Galaxies: The “Missing Mass” Problem Clusters of galaxies are gravitationally bound - they do not “evaporate” due to galaxies moving at greater than the escape velocity. Motions of galaxies tell us how much mass is needed to gravitationally “bind” the cluster Adding up all the visible matter seen in the cluster does not yield enough mass to bind it. The Coma Cluster 8

Although a problem, it didn’t seem serious, Motions of nearby stars in a direction perpendicular to the disk of the Milky Way also indicated more mass than is seen. The Missing Mass problem remained a curiosity for the next four decades. Although a problem, it didn’t seem serious, Then along came Vera Rubin and things became much more serious. 9

Rotation Curves: rotational (orbital) velocity versus radius “solid body” rotation Solar system’s (“Keplerian”) rotation curve declines because Sun has almost all the mass Spiral galaxy’s rotation curve becomes flat at distances from the center where little to no light is seen 10

The rotational (orbital) velocity of a spiral galaxy at a given radius gives a measure of the mass of the galaxy interior to that radius. Mass within Sun’s orbit: 1.0 x 1011 MSun Total mass: ~1012 MSun

M31 – “Flat” Rotation Curve We expected that the rotation curve of spiral galaxies would become Keplerian outside the visible range of the galaxy, since it was assumed that the visible edge marked the limit of where mass existed in these galaxies. Twasn’t so!! M31 – “Flat” Rotation Curve

Flat Rotation Curves are found in almost all spiral galaxies NGC 3198 Rotational velocities remain roughly constant far beyond where stars are visible 13

Most of the mass in spiral galaxies seems to be dark matter! CONCLUSION: Mass in spiral galaxies is spread out over a larger region than the stars Most of the mass in spiral galaxies seems to be dark matter!

The visible portion of a galaxy lies deep in the heart of a large halo of dark matter

Broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting These galaxies also have dark matter

Dark Matter in Elliptical Galaxies – Details of X-ray emissions from hot gas show how strong the gravity must be to keep this gas from escaping from the galaxy Broadening of spectral lines tells us how fast the stars are orbiting Both support the existence of dark matter in elliptical galaxies NGC 4697 optical X-ray 17

Dark matter in clusters: We can measure the velocities of galaxies in a cluster from their Doppler shifts

The mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars!

Clusters contain large amounts of X-ray emitting hot gas Temperature of hot gas (particle motions) tells us cluster mass: 85% dark matter 13% hot gas 2% stars

Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass

All methods of measuring cluster mass indicate similar amounts of dark matter

Our Options Dark matter really exists, and we are observing the effects of its gravitational attraction Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter

Our Options Dark matter really exists, and we are observing the effects of its gravitational attraction Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter Because gravity is so well tested, most astronomers prefer option #1

Some observations of the universe are very difficult to explain without dark matter

What is dark matter?

Two Basic Options: MACHOS & WIMPS Ordinary Dark Matter (MACHOS) Massive Compact Halo Objects: dead or failed stars in halos of galaxies Extraordinary Dark Matter (WIMPS) Weakly Interacting Massive Particles: mysterious neutrino-like particles

Two Basic Options Ordinary Dark Matter (MACHOS) Massive Compact Halo Objects: dead or failed stars in halos of galaxies Extraordinary Dark Matter (WIMPS) Weakly Interacting Massive Particles: mysterious neutrino-like particles The Best Bet

Why Believe in WIMPs? There’s not enough ordinary matter WIMPs could be left over from Big Bang Models involving WIMPs explain how galaxy formation works

What is the evidence for dark matter in galaxies? Rotation curves of galaxies are flat, indicating that most of their matter lies outside their visible regions What is the evidence for dark matter in clusters of galaxies? Masses measured from galaxy motions, temperature of hot gas, and gravitational lensing all indicate that the vast majority of matter in clusters is dark

Does dark matter really exist? Either dark matter exists or our understanding of our gravity must be revised What might dark matter be made of? There does not seem to be enough normal (baryonic/MACHO) matter to account for all the dark matter, so most astronomers suspect that dark matter is made of (non-baryonic) particles that have not yet been discovered

Dark matter and galaxy formation

Gravity of dark matter is what caused protogalactic clouds to begin their contract early in time, i.e., dark matter provided the seeds for galaxy formation

WIMPs can’t contract to centers of protogalaxies because they don’t radiate away their orbital energy They produce dark matter halos within which the luminous galaxy is built

Dark matter is still pulling things together After correcting for Hubble’s Law, we can see that galaxies are flowing toward the densest regions of space This produces immense structures

Maps of galaxy positions reveal extremely large structures: superclusters and voids

Time in billions of years 0.5 2.2 5.9 8.6 13.7 13 35 70 93 140 Size of expanding box in millions of lt-yrs Models show that gravity of dark matter pulls mass into denser regions – universe grows lumpier with time

Structures in galaxy maps look very similar to the ones found in models in which dark matter is WIMPs

What is the role of dark matter in galaxy formation? The gravity of dark matter seems to be what drew gas together into protogalactic clouds, initiating the process of galaxy formation What are the largest structures in the universe? Galaxies appear to be distributed in gigantic chains and sheets that surround great voids

Will the universe continue expanding forever?

Does the universe have enough kinetic energy to escape its own gravitational pull?

Fate of universe depends on the amount of dark matter

Observations indicate that the amount of dark matter is ~25% of the critical density, suggesting that the fate of the Universe is eternal expansion

But expansion appears to be speeding up! Dark Energy?

Estimated age of universe depends on both dark matter and dark energy