Lecture 20 Flashcards
What portion do we expect a dark matter halo to be made up of baryonic mass and what elements
1/6 baryonic in Hydrogen and Helium
what is radiative cooling
The two step process by which hot gas emits light:
1- atoms moving around collide which kicks an electron to a higher energy level
2 - electron falls back to a lower energy level releasing a photon of light
what happens if baryons break away from the dark matter after a photon carries off energy
- loose energy as photon carried it away
- thus slow down
- since less velocity to keep suspended against gravity, they fall to the centre of the halo
what does radiative cooling depend on and where does it happen most
atoms colliding
dense gas inside cooling radius cools fast enough to fall in
what happens after cool baryons fall into the centre of a dark matter halo
since they have lost their dispersion support, only the rotation aspect of the halo is holding the baryons up
how do disk galaxies form
the baryons falling into the halo have a small net rotation
this rotation counters gravity in the plane of rotation causing a disk
what sets the size of a disk galaxy
the virial radius combined with angular momentum
what is the winding problem in spiral arms
if spiral arms were stars moving around around, the inner parts wind up very quickly compared to the other
what are serial arms actually made up of, not stars
density waves - longitudinal (like sound) waves effecting stars and gas causing patterns of compression and rarefaction
what happens when two disc galaxies merge
an elliptical galaxy is formed as this process can destroy the disc and spiral structure
what is the Hubble sequence
classification of galaxies
- ellipticals (red and dead) are classified by roundness
- spirals (blue and star forming) are classified by number and tightness of serial arms
- irregulars (mergers and starbursts) are oddballs
