15.3 Galaxy Evolution Our Goals for Learning How do we observe the life histories of galaxies? How did galaxies form? Why do galaxies differ?

How do we observe the life histories of galaxies?

Deep observations show us very distant galaxies as they were much earlier in time (Old light from young galaxies)

How did galaxies form?

We still can’t directly observe the earliest galaxies

Our best models for galaxy formation assume: Matter originally filled all of space almost uniformly Gravity of denser regions pulled in surrounding matter

Denser regions contracted, forming protogalactic clouds H and He gases in these clouds formed the first stars

Supernova explosions from first stars kept much of the gas from forming stars Leftover gas settled into spinning disk Conservation of angular momentum

But why do some galaxies end up looking so different? M87 NGC 4414

Why don’t all galaxies have similar disks?

Spin: Initial angular momentum of protogalactic cloud could determine size of resulting disk Density: Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk Conditions in Protogalactic Cloud?

Collisions also a factor

Collisions were much more likely early in time, because galaxies were closer together

The collisions we observe nearby trigger bursts of star formation

Blue light from young stars, orange light from old stars; Dust causes reddening of any light.

Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical

Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together

Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies

Intensity of supernova explosions in starburst galaxies can drive galactic winds

X-ray image

Angular momentum may determine size of disk Density of protogalactic cloud may determine how fast a galaxy forms Collisions shape galaxies early on –Mergers of small objects make halo & bulge –Mergers of larger objects make elliptical galaxies Relatively undisturbed galaxies can still have disks Why do galaxies differ?

What have we learned? How do we observe the life histories of galaxies? Today’s telescopes enable us to observe galaxies of many different ages because they are powerful enough to detect light from objects with lookback times almost as large as the age of the universe. We can therefore assemble “family albums” of galaxies at different distances and lookback times.

What have we learned? How did galaxies form? The most successful models of galaxy formation assume that galaxies formed as gravity pulled together regions of the universe that were ever so slightly denser than their surroundings. Gas collected in protogalactic clouds, and stars began to form as the gas cooled. Why do galaxies differ? Differences between present-day galaxies probably can arise both from conditions in their protogalactic clouds and from collisions with other galaxies. Slowly rotating or high-density protogalactic clouds may form elliptical rather than spiral galaxies. Ellipticals may also form through the collision and merger of two spiral galaxies.