Chapter 20: Galaxies So far we have talked about “small” things like stars, nebulae and star clusters. Now it’s time to get big!

What are the “Spiral Nebulae”? The Shapley – Curtis Debate April 1920 Harlow Shapley…The Milky Way is huge so the spiral nebulae must be located within it Heber Curtis…The spiral nebulae are island universes like our Milky Way and thus are not part of it No definitive conclusion was reached because no one had any data on the distance to the spiral nebulae or their size

Edwin Hubble: late 1924 Hubble looked for Cepheid variable stars in the closest of the spiral nebulae, the Andromeda nebulae. He found it is over 2 million lightyears away and so must be at least 200,000 light- years in diameter. It cannot be part of the Milky Way. The Hubble Space Telescope still measures Cepheid variables in distant galaxies

The Hubble Galaxy Classification Scheme After observing lots of other galaxies, Hubble came up with a classification scheme based on shape Elliptical Galaxies: spherical to elliptical in shape Spiral Galaxies: Normal Spirals and Barred Spirals Irregular Galaxies: No distinct shape

The Hubble Tuning Fork diagram The diagram shows the Hubble classification scheme. When he first devised it, Hubble thought it may be related to galaxy evolution. We now know it does not show galactic evolution!

S0 Spiral Galaxies have dust lanes but no arms

Sa Spirals have tightly wound arms and a large nucleus

Sb Spirals have looser arms and a smaller nucleus

Sc Spirals have very loose arms and a small nucleus

Barred Spirals are like normal spirals except with a bar across their center

Barred Spirals are classified SBa, SBb, and SBc SBaSBbSBc

Elliptical Galaxies contain no gas or dust, they’re just big balls of stars

Ellipticals are classified according to how stretched out they are…E0 – E7 E0 E3 E6

Irr Galaxies are irregular galaxies The Large and Small Magellenic Clouds

What are the Spiral Arms? If the spiral arms are due to the rotation of stars and nebulae around the center of the galaxy, they should “wind up”.

Observed spiral arms don’t wind up We can see lots of spiral galaxies and none of them show evidence of wind-up

The Spiral Density Wave Model

Stars exist in all areas of disk but brightest ones are in arms

How fast do the stars and nebulae move around spiral galaxy?

Possible Velocity distributions

Expected Rotation Curve Predicted velocities are based on the amount of matter we can see: stars, gas & dust

Observed velocity distributions in galaxies The stars in spiral galaxies orbit too fast far from the center

The velocity curves imply “Dark Matter” must exist Since the velocity doesn’t fall off at larger distance as predicted, there must be a lot of mass “out there” that we can’t see. Since we can’t see it at any wavelength, we call it Dark Matter. It seems that dark matter makes up about 90% of the mass of galaxies.

The dark matter surrounds galaxies While the luminous matter of the disk may go out 50,000 to 75,000 lightyears from the center, the spherical halo of dark matter may go out several hundred thousand lightyears from the center.

What is Dark Matter? MACHOs: Massive Compact Halo Objects : old lone white dwarfs, old lone neutron stars (non-pulsar), non-accreting, lone black holes, brown dwarfs and free floating planets WIMPs: Weakly Interacting Massive Particles: the neutrino is a weakly interacting particle but it isn’t massive. It must be some other types of exotic elementary particles Most theories tell us only a small fraction of the dark matter can be MACHOs. Most of it is non-neutrino WIMPs.

“Seeing” MACHOs by gravitational lensing A number of searches have been conducted and they find about as many as theory predicts. Only enough to account for a few percent of the dark matter

Looking for WIMP’s

Quasars and Active Galaxies Centaurus A Galaxy X-ray over Visible

Quasars: QUAsi StellAr Radio Source Discovered in the 1950’s and 60’s, these objects are star-like in visible light but also bright radio sources.

In visible light, quasars look like stars Diffraction spikes come from the arms that hold the secondary mirror. They only appear around bright point sources of light.

Quasars have LARGE redshifts A large redshift means they are moving away at high speed. Edwin Hubble had discovered the universe is expanding so this meant they were very far away

The HST has shown that quasars are associated with distant galaxies

The Missing Link to Quasars: Seyfert Galaxies There are no quasars nearby but there are active galaxies

Some large elliptical galaxies have enormous jets

When Jets Collide with intergalactic gas they produce radio lobes

Blazars (BL Lacertae objects) are extremely luminous galaxy cores with no spectral features NGC 3185

Blazars can change brightness in less than a day

How quickly something changes brightness gives clues to its size If a blazar can change its brightness in less than a day, it must be less than a lightday across.

The energy source of blazars, quasars and Seyfert galaxies is a supermassive black hole

Difference between quasar and blazar is the view angle

Galaxy collisions can feed the monster black hole at the center of a galaxy

Galactic Collisions also lead to Starburst Galaxies