GalaxiesGalaxies

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 GalaxiesTypes 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

ba10 #

A classification scheme,A classification scheme,not an evolution scheme.not an evolution scheme.

Dark MatterDark 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)

c

Vr

laboratory

observed 1

• If a galaxy’s mass

were all stars & gas, would expect curve B (Kepler’s 3rd 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 GalaxiesActive 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 ly3 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 gasdepends on 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 formedfrom stellar collapse), it can guidethe boiled-off gases into the polarjets we observe.

Galaxy Clusters Galaxy Clusters

Andromeda

Small Magellanic Cloud Large 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 UniverseThe 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 = H0D, where modern estimates give H0 ~ 70 km/sec/Mpc

1/H0 = ?

NEXT TIMENEXT TIME• Cosmology

– The study of the large scale properties of the Universe

– Origin & fate of the Universe