M 51

Galaxy merger simulation

Discussion What do you think is going to happen if two large spiral galaxies collide and merge? What happens to the stars’ orbits. Note that stars are so far apart in galaxies that they do not collide with each other.

Discussion What is the likely result of a supersonic collision between two clouds of gas? Note that gas clouds do collide in galaxy collisions.

Major Mergers Collisions between similar massed spiral galaxies can convert gas into stars, and randomize orbits, turning a spiral galaxy into an elliptical.

Discussion If every mass in the Universe attracts every other mass, what is eventually going to happen to all the masses in the Universe?

Discussion How can you prevent this from happening?

Galaxy redshifts Slipher – 1914 began taking spectra of a large number of spiral “nebulae” and discovered that most had velocities greater than 1000 km/s.

Discussion Why do the high velocities of spiral “nebulae” help support the hypothesis that they do not belong to our galaxy?

Discussion The proof that galaxies were actually separate from the Milky Way came in 1923 when Hubble identified Cepheid variable stars in the Andromeda galaxy taken with the 100 inch telescope on Mount Wilson. Why is this important?

Discussion If you had to figure out the distances to galaxies, what are some things you might try?

Hubble’s law The farther something is from us, the faster it will appear to be moving away. velocity = H0  distance

Galaxy distances If we know the value of Hubble’s constant, we can find the distances to galaxies by simply measuring their recessional velocities using the redshift.

Discussion What do we need to know to find Hubble’s constant?

Hubble’s data

The expansion of spacetime The galaxies are not moving through spacetime. Instead, the spacetime between the galaxies is being stretched and is getting bigger.

Expansion of the Universe Hubble’s law implies that the Universe is expanding uniformly. That is, it is expanding at the same rate in all directions.

Discussion Does this mean that the Earth’s radius is getting bigger? How about the radius of Earth’s orbit, is it getting bigger as spacetime between it and the Sun expands?

Dark Matter

HI neutral hydrogen map

Discussion How can we measure the rotation curve for spiral galaxies other than the Milky Way?

Rotation curves of spiral galaxies In spiral galaxies, atomic hydrogen extends to a radius about twice that of the visible stars. By using 21 cm radio emission, we can follow the rotation curve beyond the visible disk of a galaxy.

Discussion Do elliptical galaxies have rotation curves?

Discussion It’s rather easy to measure the rotation speed of edge-on disk galaxies. How do you think we might measure the mass of an elliptical galaxy which does not rotate?

Dark matter 90% of the matter in galaxies is in a form we cannot see.

Discussion Can we explain this dark matter in galaxies by assuming that there is a lot of mass in the central black hole? Explain.

Discussion 90 % of the mass in galaxies is dark matter. Is the same true for the Solar System? How can we tell?

Discussion What are some of the objects we’ve talked about that do not emit a lot of light that might make up this dark matter in galaxies.

MACHO project Massive Compact Halo Objects If the object is compact and massive it will bend and focus background star light. Thus, if one of these happens to pass in front of a background star, that star will appear to brighten and then dim back to normal.

Discussion How would you go about looking of these things?

The results There are not enough MACHO events to explain the missing mass in our galaxy. WIMPs? Weakly Interacting Massive Particles.

Discussion Can we use the 21 cm emission for neutral hydrogen to determine the total mass of galaxies? Why or why not?

The Local Group The Local Group contains about 3 dozen galaxies within 1 Mpc. 90% of the mass is in two large spirals, the Andromeda galaxy, the Milky Way and a dwarf spiral M33. The rest of the galaxies are dwarf galaxies.

Local Group dynamics While it is easy to get radial velocities of Local Group galaxies, plotting their orbits is impossible. Proper motions are simply too small to measure.

Discussion How can we use the radial velocities of the galaxies in the Local Group to estimate the mass?

Magellanic Stream We can estimate the orbit of the Magellanic Clouds because they appear to be leaving behind a trail of HI. This gas is most likely tidally pulled from the clouds as the orbit the Milky Way.

Discussion Why do you think the neutral hydrogen gas forms a disk that is much thinner than the disk formed by the stars?

HI neutral hydrogen map

Discussion Why do you think the dark matter hasn’t become flattened into a disk?