25 - Efficiency of a galaxy formation

What is the SHMR (Stellar to Halo mass relation)?

The ratio between the mass of stars in the central galaxy and the mass of its halo

How long does it take for gas to fall into a galaxy from the halo at any cosmic epoch?

At any cosmic epoch, it takes about 20% of the age of the universe to fall in

How long does it take to turn gas into stars according to KS law?

10% of cosmic time

At any epoch, how much gas that reached a galaxy is expected to be in stars?

80%

What about star formation is different with very massive halos?

The gas cannot cool fast enough for halos of 10^13 solar masses and upwards.

With only cosmology and cooling, we expect SHMR ~ ___, so for most halos we expect about a ___ conversion efficiency of halo baryons into stars in the central galaxy.

0.1, 60%

Name 3 direct ways of measuring the mass of a halo.

Motions of satellites, Gravitational lensing, getting the virial temperature from X-ray gas

Name a way of measuring the mass of the stars in a galaxy.

Get mass-to-light ratio based on the galaxy colour then convert luminosity into M*

We expect that SHMR would be fairly efficient, with ___ of baryons in stars, yet the reality is that it is inefficient, at most ___ of baryons in stars.

60%, 20%

What is the overcooling problem?

Lamda CDM and cooling alone over predicts the number of stars in halos.

To either hold gas up against gravity or to heat gas to counteract cooling requires energy. The process of forming galaxies must be releasing some energy. What do we call this energy?

Galactic feedback

What are the two types of galactic feedback?

Ejective and preventive feedback

What is ejective feedback?

Once the cool gas gets in the galaxy, eject it back out before it can form into stars

What is preventive feedback?

Keep gas hot by continual injection of heat, so it never reaches the galaxy

Galactic feedback must act to: (give 3 reasons)

Lower the overall conversions efficiency of converting gas into stars in halos; increasingly make the efficiency even lower towards smaller halo masses; increasingly make the efficiency even lower towards larger halo masses