7. Dark Matter Flashcards
How is the scale factor and the matter energy density related?
Energy density for matter is proportional to a^-3
What is the critical density?
The density of matter required to make the universe flat
Describe the properties of an elliptical galaxy
Old, little star formation
High M/L ~ 8 M solar / L solar visible
Describe the properties of a spiral galaxy
Young, lots of star formation
Low M/L ~ 0.3M solar / L solar visible
What is the density parameter of stars roughly
0.005 of the critical density
How much of the matter in the Universe is composed by stars?
A very small amount
- Most is between galaxies and baryonic matter is not visible at optical wavelengths
What is the density paramter for baryonic matter?
0.05 of the critical density
Why was dark matter hypothesised
There was not nearly enough matter to keep the universe flat from the critical density
What is dark matter?
Matter that has a gravitational influence, but does not react with light (emit/absorb/scatter)
What are the 3 pieces of evidence for dark matter?
Galaxy rotation curves
Galaxy cluser velocities
Gravitational lensing
What is the relationship of the rotational velocity and the distance from the centre of the galaxy?
Negative parabola
- As radius increases, enclose more mass until the edge of the galaxy
v = GM(R) / R
What does the data show about the relationship between the rotatational velocity and the radius from the centre of the galaxy and what does this infer?
Follows the negative parabola, but then stays constant afterwards
- Implies mass increases with radius beyond where it is visible as stars or M(R) is prop. to R
What is the mass/light ratio for dark matter?
60 M solar/ L solar (R_halo / 100 kpc)
- Considerably higher than the estimate for baryonic matter
How was dark matter hypothesised from the data of the velocity of galaxy clusters?
The velocities were way too high and they should fly apart unless there was extra matter
What is the Virial theorem?
For gravitationally bound objects in a cluster of size R_cl, W + 2K = 0
What is R_cl?
The estimated size of the cluster radius
e.g. half light radius
What is the half light radius?
Add up all total light
Keep drawing bigger and bigger circles unti half the light is enclosed
How can the mass be calculated from the Virial theorem, and what sort of result does it give?
M ~ < v > ^2 R_cl / G
- Mass is summed mass over cluster
- Includes mass in gas and DM between galaxies so is significantly higher
- Density parameter ~ 0.2
What is gravitational lensing, and how does it affect what we can see?
DM exerts a gravitational force and bends light
- Background objects are magnified by lensing
- Can detect very distant and old objects
Describe weak and strong gravitational lensing
Weak - Distortion of images
Strong - Multiple images
Is dark matter relativistic?
No it is “cold”
- Not moving fast enough to get caught into baryonic structure
What are the two classes of DM?
MACHO - Massive compact halo objects
WIMP - Weakly interacting massive particle
What are MACHOs?
Things that we know exist but can’t detect directly
- Hypothesised to populate the halo of galaxies and explain DM
- Bending causes magnification as light can take more than one path around the macho
How does the macho change the observations of a background star
MACHO moves relative to background star
- brightness goes up and down meaning magnification is time dependent
What is microlensing?
Small scale gravitational lensing
- Need LMC (large magellanic cloud) as its v. small
How does the flux of an LMC subject to a MACHO change over time, and what can be concluded?
Bell curve shape for a flux-time graph
- MACHOs been detected, but only 10% of the number needed to explain DM in our halo
- DM more evenly distributed
Have WIMPs been detected?
No
