Quiz #10 What would really happen to you if you were to fall all the way into the Event Horizon of a black hole? We know the gravity is unbelievably strong. What would happen to your body and why? (Note: I am not looking for a relativity answer here. Just a gravity answer)

In the original interaction, star formation can be set off in the density wave because the gas and dust are compressed. After the interaction, there is continued star formation along the density wave as molecular clouds collide with the density wave and become compressed.

When interactions are not in the plane of the large galaxy, then warps in the disk ocurr.

The result of the creation of spiral density waves. After the interaction, there will be increased star formation for quite some time, as molecular clouds continue to collide with the spiral density waves. Over time, self-sustained star formation and disruption of the density wave, causes the S- shape density wave to fragment into spurs. Then there are multiple density wave fragments.

Over time, after several orbits of material, the spiral density wave is lost. The result is a slowing of star formation rates. Eventually, the disk galaxy has virtually no recognizable spiral pattern at all.

Galaxy Simulations It takes hundreds of millions of years for interactions to occur between two galaxies. When we observe two galaxies interacting we are only seeing a snapshot of the interaction. Fortunately, there are billions of galaxies to observe, and this is a large enough sample to find galaxy interactions at many stages, many masses, and many angles of impact. It is also possible, using computers, to model the interactions of two galaxies, using Newton’s law of gravity.

Composite image on left, and X-ray image on right. X-ray glow is from extremely hot gas.

What mass should be used for the simulations? There are two general methods for estimating the mass of a galaxy. Luminous mass and dynamical mass. Luminous mass. If you measure the apparent brightness of a galaxy it is possible to determine the total luminosity of the galaxy. What other parameter would you need to determine the total luminosity of the galaxy?

What other parameter would you need to determine the total luminosity of the galaxy? 1.The total number of stars. 2.The distance to the galaxy 3.The number of blue stars

Apparent brightness is given by this equation. B = L/4πd 2 where L is the total luminosity of the galaxy. If we know the distance, we can compute the total luminosity. Suppose a galaxy has a total luminosity that is L tot = 3 x times the Sun’s Luminosity.

Estimate the amount of mass in stars in this galaxy. 1.About 3 x solar masses. 2.About 100 times the mass of the Sun. 3.Impossible to say because most of the mass is in the central super massive black hole

Although this is just an estimate, you can say, that the luminosity of the galaxy is the same as if there where 3 x Suns in this galaxy. That means it has about 3 x times the mass of the Sun. To do a better estimate, you look at the luminosity function, which tells you how many high mass stars, medium mass stars and low mass stars make up the galaxy. It is also important to determine the amount of mass that is present from gas and dust. This is typically only about 10%.

Dynamical Mass. This mass determination uses Kepler’s 3 rd Law. It doesn’t depend on the mass that is giving off light. It is a direct measure of the mass that is present. (M interior + m * ) P 2 = 4π 2 (r 3 )/G or (M interior + m * ) = r(4π 2 r 2 /P 2 )/G or (M interior + m * ) = r(v 2 )/G we can ignore m * So v = (M interior G/r) 0.5

As you move out in the disk, there are fewer and fewer stars. The mass interior should not be changing.

To compute velocities we use the Doppler shift

Rotation curve for nearby M33

Virtually all galaxies show a flat rotation curve.

Let’s look back at the rotational velocity equation: v = (M interior G/r) 0.5 The rotational velocity is constant at big r values. What must we conclude?

1.The radius in not decreasing 2.The universal gravitational constant, G, is changing 3.More mass is being added interior to the orbit

The mass interior to the orbit is still increasing. Even after the radius r has gone beyond the last of the stars. It must be increasing in order to hold v constant. This is dark matter. We don’t know what it is but it has mass. It surrounds galaxies in a huge dark matter halo. It doesn’t interact with light or we could observe it. That is true even for gas.

Other ways to detect Dark Matter Gravitational lensing. The amount that the light is bent is directly proportional to the mass bending the light.

Clusters of Galaxies. Galaxies that are in clusters have velocities that are many orders of magnitude to large to be bound by the cluster. This means that either, the galaxy clusters everywhere in the universe are flying apart, or else there is a huge amount of dark matter present.

The results from all these different observations is that around 90 to 95% of the mass in the universe is Dark Matter. When we look at a galaxy, we are seeing the luminous matter. But that matter is embedded in a much larger Dark Matter halo which contains around 90% of the mass of the galaxy. And we can’t even see it. We can only measure its presence using velocities.

So we see this….

But it is only the tip of the iceberg

In order to model galaxy interactions, it is necessary to use all the mass, not just the luminous mass. In the simulations we have seen so far, dark matter is explicitly put into the simulation. Let see what will happen some day when the Milky Way and Andromeda collide.