Lecture 29 Flashcards
What are the main components of galaxies?
- stars
- planets
- gas and dust
- supermassive black holes
- dark matter
What are the two methods of calculating galaxy mass?
- Light
- Dynamical mass
How can galaxy mass be calculated from light?
- by using the spectral type of stars we can estimate the mass of a star. We can count the number of different stars and convert them into units of solar masses
- estimate the mass if gas and dust clouds
- not a very good method- need to correct for gas mass and be able to count all of the stars
How is dynamical mass used to calculate the mass of galaxies?
- orbital speeds of stars are used to calculate mass. M= Rv^2/G
- stars further from the main star orbit slower
How is dynamical mass different for distributed matter?
- mass inside an orbit can be treated as a point mass at the centre
- mass outside the orbit cancels out
- M(R)= v(R)^2*R/G
How can light show movement?
The motion away from an observer will cause the light to be redshifted, while motion towards the observer will cause light to be blueshifted
How do we measure the velocities of stars/gas?
We take a spectrum of the galaxy and use Doppler Shift of absorption/emission lines to infer velocity
- z= Δλ / λ = v / c
What are the two components of Doppler Shift for rotating objects?
- the centre of mass velocity (applies to all regions)
- line of sight rotation velocity (different across the disk): receding side= redshift
What are the different velocity-mass profiles?
- constant density sphere
- isothermal sphere
- proposed profile
What is the constant density profile?
- velocity is proportional to the radius of the orbit
- faster velocities moving out
- rising rotation curve (linear graph)
-v(r) ∝ r
What is the isothermal sphere profile?
- denser in the centre than farther out
- flat rotation curve- flat graph
- all the same velocity
- v(r)= constant
What is the proposed profile?
- mass is denser in the middle, less on the outside and then nothing after that
- inner stars move at about the same speed, outer stars move slower
- flat rotation curve for the inner part and falling curve in the outer part
What does the rotation curve actually look like?
- speed increases as you move away from the centre but then flattens rather than fall.
- the luminous mass of a galaxy alone would give a falling curve
What does the non-falling rotation curve tell us?
It infers that there is a huge amount of dark mass that does not emit or absorb light that extends far past the visible galaxy
Where does dark matter extent to in a galaxy?
It occupies a spheroidal halo many times larger than the luminous galaxy
