chapter 28.2 galaxy mergers and AGNs & 28.3 distribution of galaxies in space & 28.4 the challenge of dark matter & the formation and evolution of galaxies and the structure in the universe

merger

• when 2 galaxies of equal size are invovled in a collision

galactic cannibalism

• when the smaller of 2 colliding galaxies is swallowed by the larger one

• very large elliptical galaxies are most likely formed by cannabalising a variety of smaller galaxies in their clusters

  • multiple nuclei

starburst

• when galaxies collide, if either galaxy has interstellar matter, the collision can compress the gas and trigger and increase in the rate which stars being formed

• by as much as a factor of 100

cosmological principle

• the universe is both isotropic and homogenenous

  • looks the same in all directions, and a large volume of space at any given redshift or distance is much like any other volume at that redshift

local group

• neighbourhood the milky way is a part of

• small group of galaxies

• 3 million light-years

• approx. 60 galaxies

• 3 large spiral galaxies, 2 intermediate ellipticals, many dwarf ellipticals and irregular galaxies

• mass is around 4 × 10^12 Msun (around ½ contained within Milky Way and Andromeda)

superclusters

• arrangements of clusters of galaxies

voids

• separating filaments and sheets in a superclusters

• look like huge empty bubbles walled in by the galaxies

• typical diameters of 150 million light-years, with clusters of galaxies concentrated along their walls

hot dark matter

• in the early universe, if dark matter particles easily moved fast and far compared to the lumps and bumps of ordinary matter that eventually became galaxies and larger structures, we call those particles hot dark matter

• smaller lumps and bumps would be smeared out by the particle motions, meaning fewer small galaxies would get made

cold dark matter

• if the dark matter particles move slowly and covered only small distances compared to the sizes of the lumps in the early universe, we call that cold dark matter

• they slow speeds and energy would mean that even the smaller lumps of ordinary matter would survive to grow into small galaxies

dark energy

• entire universe contains some mysterious energy that pushes spacetime apart, taking galaxies and larger structures made of galaxies along with it

• observations show that dark energy becomes more and more important relative to gravity as the universe ages

• as a result, the expansion of the universe is accelerating, and this acceleration seems to be happening mostly since the universe was about half its current age

how do galaxies form and grow?

• galaxies are more numerous, but smaller, bluer and clumpier, in the distant past than they are today, and that galaxy mergers play a significant role in their evolution

• some of the first large concentrations of matter must have evolved into elliptical galaxies that we see in today’s universe

• more massive a galaxy is, the more massive its central black hole is

• top-down scenario: massive elliptical galaxies formed in a single, rapid collapse of gas and dark matter, during which virtually all the gas was turned quickly into stars, afterward the galaxies changed only slowly as the stars evolved

• bottom-up scenario: todays ellipticals were formed mostly through mergers of smaller galaxies that had already converted at least some of their gas into stars

• elliptical: combination of both scenarios

• spiral: mostly bottom up