4.2-4.3 Measuring Galaxy Rotations and Rotation Spirals Flashcards
What is long slit spectroscopy?
When you take a spiral galaxy, and place a slit over it vertically
- Get 3 intercepts at the bulge and top and bottom of the arm
- One spectrum per point along the slit in the spatial direction
What is integral-field spectroscopy?
Place a grid over a spiral galaxy and get intercepts at all points
- One spectrum per pixel on the plane of the sky
How can we measure the motion in the radial direction along the line of sight for a galaxy?
Measure it via the Doppler shift of the spectrum
When might we measure a blue/red shifted arm?
When we measure the velocity of the arm moving towards/away from us
What is the bulk velocity?
The radial component of the galaxy as a whole compared to us
What is the rotational velocity of the galaxy?
The velocity at which material moves around the centre
- Subtract the bulk velocity and correct for inclination
What are the three rotational velocities that could be measured when looking at a resolved and unresolved galaxy?
Blue shifted arm, red shifted, bulk for a resolved galaxy
For an unresolved galaxy, we will see a velocity profile which is a combination of all these effects
Define velocity dispersion
The intrinsic spread in the local velocity
What do we mean by supporting a spiral galaxy?
Preventing collapse of the arms into the centre
What supports a spiral galaxy, and what is a consequence of this?
Rotation - Angular momentum of matter balancing gravity
- Naturally get disk like structures
What is meant by spiral galaxies exhibiting bulk motion?
There is a low dispersion
- Relative velocity of nearby orbits is small
Describe the shape for the line profile of galaxies:
When the spiral arms and core balance
When spiral arms are dominant
When core is dominant
Intensity wavelength graph with λ (0) at the centre
Balance: Sharp increase, flat line then sharp decrease centered about λ (0)
Spiral: Quartic M shape with minimum at λ(0) - Called a flared disk (double peaked)
Core: Bell shaped curve peaking at λ (0)
For the intensity wavelength profile, what does the width show?
Width is determined by max/min velocities but use FWHM as a proxy
For the intensity wavelength profile, what does the shape depend on ?
The balance between red and blue shifted material
Also the balance between the bulge and the disk
What are the three components of the model that fit the rotational profiles of spirals well?
Stellar, gas, halo (Plummer sphere)
