Dark Matter Flashcards
How do you weigh a galaxy?
•measure their light assuming galaxies mostly sun like stars (1L=1Ms) and add up starlight to convert to star mass
•or measure their gravity and orbital velocity then use newtons law
How do you measure the orbital velocity of a spiral galaxy?
•measure spectral if two sides of a galaxy go get redshift
•lhs rotating towards us
•rhs rotating away
•use velocity difference from two sides to get rotation speed
Describe solid rotation curve?
•straight line
•merry go round or turntable
•constant orbital period
•v directly proportional to r
Describe keplerian rotation curve
•negative exponential
•orbiting enclosed mass e.g planets around sun
•v directly proportional to r^-1/2
Describe differential rotation curve
•orbiting inside constant density field
•v=constant
•more or less horizontal line
What does rotation curves being flat imply?
Implies mass inside radius keeps increasing well beyond visible radius of galaxy
What conclusion can be drawn from inconsistent rotation curves?
•dark matter dominates the gravitational mass in power regions
•galaxies embedded in a much larger invisible halo of matter
What percent of total mass is dark?
90%
What would happen if galaxies were held together only be gravity from stars and gas?
They would fly apart because they are spinning so fast
What are the most dm dominated things in the universe?
Clusters
Coma cluster is 95%
What does cluster gravity do to light?
Bends it around it(arcs background galaxies lensed around cluster mass)
What are cluster clouds dominated by?
Gas
What happens when gas clouds collide?
Feel shockwaves
Can dm particles collide?
No they move straight through each other
What is an exotic candidate for being dark matter?
Weakly interacting massive particles (WIMPs)
