Galaxies Types Dark Matter Active Galaxies Galaxy Clusters & Gravitational Lensing

What is a galaxy? A vast, self-organized collection of: –Stars –Planets –Interstellar dust, gases & ices –Dark Matter

Types of Galaxies Spiral Elliptical Irregular (maybe a combination of the two)

Spiral Galaxies

Barred-Spiral Galaxies

Elliptical Galaxies

Irregular Galaxies

Why do we have different types of galaxies? –mostly due to collisions/captures

Summary: The Hubble Fork A classification scheme, not an evolution scheme.

Dark Matter Stated that galaxies are embedded in this giant cloud of dark matter –How do we know? Rotation of the galaxies.

How to measure galactic rotation speeds –Doppler Shift (again)! –Use any convenient spectral line Hα line in visible spectrum (hot Hydrogen: tracks rotation of stars) 21 cm Hydrogen line in radio spectrum (cold Hydrogen: tracks rotation of IS clouds)

If a galaxy’s mass were all stars & gas, would expect curve B (Kepler’s 3 rd Law) But, when we measure the rotation of almost every galaxy, we observe profiles like curve A

Conclusion: the galaxy has some “unseen” mass that we cannot directly observe –Planets & planet-like bodies –Brown dwarfs –Cool white dwarfs and/or neutron stars –Black holes –Neutrinos or some other exotic matter No evidence for dark matter on solar system scales (planets’ orbital speeds are Keplerian)

Active Galaxies Radio Galaxies Seyfert Galaxies Quasars Starburst Galaxies

Radio Galaxies –Generally elliptical galaxies –Have “radio lobes” up to millions of ly long High-speed electron gas flowing from poles Similar to pulsars Synchrotron radiation: intense galactic magnetic fields

Radio Galaxies emit millions of times more radio energy than a “normal” spiral or elliptical galaxy, like M81

Seyfert Galaxies –Generally spiral galaxies –Incredibly bright nucleus The central 1 ly 3 are typically brighter than other (entire) galaxies –Broadband: radiates at many frequencies across the EM spectrum

Interacting Seyfert galaxies

Quasars –“Quasi-stellar radio source” –Some of the farthest objects ever seen –Very bright –High redshifts = large distance (Hubble’s Law) –Ultra-fast relativistic jets

1973: Quasar 3C279 appears to have a jet that is moving at least 9 times faster than the speed of light! This is an “optical illusion” of motion at near light-speed –Subsequent studies have shown the beam expanding at 99.9% light-speed

What causes the activity? –Supermassive black holes! –Feeding on galactic-center material Recall accretion and frictional heating Recall role of magnetic field

Recall an atom’s speed in a gas dependson temperature. At the high temperatures in the inner parts of the accretion disk, some gases can be “boiled away” and escape the pull of gravity. If there is a magnetic field present (which there is if the BH formed from stellar collapse), it can guide the boiled-off gases into the polar jets we observe.

Galaxy Clusters

Andromeda Small Magellanic CloudLarge Magellanic Cloud

Clusters of galaxy clusters

The Einstein Cross: 4 images of the same quasar

The first-ever determination of the distribution of dark matter made from lensing studies

The Expansion of the Universe Hubble’s Law –The more distant a galaxy is, the faster it is moving away from us

Hubble’s Law: V = H 0 D, where modern estimates give H 0 ~ 70 km/sec/Mpc 1/H 0 = ?

NEXT TIME Cosmology –The study of the large scale properties of the Universe –Origin & fate of the Universe