10 - Dark Matter

How do you weigh a galaxy?

Either measure it’s brightness, assuming galaxies are mostly sun-like stars, and convert total brightness to equivalent brightness, or measure the gravity and calculate the mass.

How do you measure the orbital velocity of a galaxy?

Measure spectrum at each point, find redshift, convert to velocities, compare spectrum at 3 points, and get redshifts.

What are the issues with the 2 methods to weigh a galaxy?

More careful measurements, including a range of star masses, gas and dust mass lead to different estimates.

What are the 3 rotation curve types?

Solid (constant orbital period), Keplerian (orbiting enclosed mass), differential (orbiting inside constant density field)

Rotation curves are always ____, implying mass inside radius keeps ________ well beyond visible radius of galaxy.

Flat, increasing

Dark matter dominates the ___________ mass in the outer regions.

Gravitational

What % of the total mass of a galaxy is dark?

Approximately 90%

Galaxies are spinning so fast, they would fly apart if the only _______ holding them together was from _____ and ___ (Rotation Curves)

Gravity, stars, gas

How many galaxies do clusters contain?

100s to 1000s

How can you estimate a cluster’s total mass?

Random motions of its galaxies

___ of Coma cluster is Dark Matter vs ___ of individual galaxies.

95%, 90%

Dark matter is either _____ or _______.

Exotic, Baryonic

What is the main exotic candidate for DM?

Weakly Interacting Massive Particles (WIMPs)

What is the main baryonic candidate for DM?

Massive Compact Halo Objects (MACHOs)

What are MACHOs?

Large number of massive but small objects surrounding galaxies that are too faint to individually, but collectively form dark matter

MACHOs would cause frequent __________ events. These were detected but not in large enough numbers to explain Dark Matter, so DM is not primarily MACHOs.

Micro-lensing

What is an example of an other kind of WIMP (too small and fast to be Dark Matter)?

Neutrino

Are WIMPs or MACHOs the current leading explanation for DM?

WIMPs

Give 2 examples of a detection method for DM?

Solid detectors made of Germanium, detector made with liquid noble gases (Argon or Xenon)

What is the problem with astrophysical detection?

Dark matter presence is hard to separate from other phenomena

What is an alternative for Dark Matter (doesn’t work when seriously challeged)?

MOND (Modified Newtonian Dynamics)

What is another explanation for where dark matter comes from?

Primordial Black Holes (formed in the early universe)