Lecture 25 Flashcards
what does SHMR stand for
stellar to halo mass relation
at any cosmic epoch, how long does it take for gas to fall into a galaxy from the halo
20% of the age of the universe at that time - expect 80% to have fallen in
how long does it take to turn gas into stars according to kennicutt-schmidt law
10% of cosmic time
at any epoch how much gas that reached a galaxy to be in stars
80%
what is different with very massive halos for star formation
the gas cannot cool fast enough
happens for halos of 10^(13-14) solar masses
is SHMR independent of the mass of the halo
mostly
for the biggest halo masses there is some dependance
what do we expect the SHMR to be with only cosmology and cooling and what does this mean for efficiency
expect it to be 0.1, a little lower for very largest halos
expect 60%+ conversion efficiency of halo baryons into stars in the central galaxy
how to find stellar mass
mass-to-light ratio based on galaxies colour, convert to luminosity into solar mass
4 ways to get the mass of a halo ( direct, 1 indirect)
motions of satellites
gravitational lensing
virial temp equation
can get it indirectly by abundance matching - put largest galaxy in a survey into largest halo in a mock survey
expected efficiency vs reality for SHMR
expect it to be 60% or baryons but 20% in reality
expect mass dependance vs reality for SHMR
expected to have very little dependance on the mass of the halo
in reality see a strong mass dependance with a peak and Milky Way sized halos
what does the peak of the SHMR line up with
peak where we see a dramatic shift in galaxy population
below peak, star forming disks
above , quenched ellipticals
why do we need something additional in terms of overcooling
since gravity and cooling is happening, we need something additional to hold gas up against gravity or heat gas to counteract cooling requiring energy
this accounts for discrepancies
what is galactic feedback
when the process of forming galaxies releases energy
what is the overcooling problem
lambda CDM grossly over predicts numbers of stars in halos
how do we solve the overcooling problem
lower amount of cold gas that fuels star formation
what are the two ways to lower amount of cool gas that fuels star formation
ejective feedback or preventive feedback
what is ejective feedback
cool gas is ejected from a galaxy before it can form stars
what is preventive feedback
gas is kept hot by continual injection of heat so it never reaches the galaxy
what must galactic feedback act to
lower overall conversion efficiency
increasingly make efficiency even lower towards small and large halo masses
