Ay 155								Spring'89
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Galaxy Formation:  A Brief Synopsis of Important Ideas


1. Density Perturbations		(Peebles, Zel'dovich,...)

It is assumed that the observed structures (galaxies, clusters...)
originate from a gravitational collapse of positive density fluctuations
in the early universe.

The density field is described by its Fourier power spectrum.  Its origins
are not clear (quantum fluctuations, explosions, ...)  Historical favorites
include the white noise, the 1/f noise (a.k.a. the Zel'dovich-Harrison
spectrum, also suggested by the inflationary models), and power-laws in 
general.  The amplitude of density perturbations is usually assumed to be 
a gaussian variable, with an unknown normalization, and the phases to be 
random (uncorrelated).  All this is completely arbitrary. 

Prior to the recombination, the initial density spectrum is modified by a 
variety of damping processes.  After the recombination, the perturbations
evolve primarly gravitationally (MHD/hydro may be important in some cases).

Note also that this is independent of the Omega.


2.  Cooling and the Origin of Galaxies		(Ostriker, Rees, White)

Galaxies are much denser than what an extrapolation of the 2-pt. correlation
function would suggest.  This is easiest to understand if they formed via
dissipation.  That is, the binding energy has to be released somehow (e.g.,
via inverse Compton cooling on the CMBR).  There are two basic possibilities:
(i) objects which cannot cool in a Hubble time, e.g., clusters and groups of 
galaxies, and (ii) objects which can and do, e.g., galaxies.  They are 
separated in the temperature-density diagram by the "cooling curve".

If there are dark halos, they can form the "containers" in which the baryons
can accumulate.  The halos can subsequently merge away.


3.  Dissipation and Mergers		(Larson, Silk, Norman,...)

Dissipation and merging are the basic processes of galaxy building.  Mergers
can cause starbursts in protogalaxies.  Ellipticals and bulges are built by
merging (both dissipative and dissipationless), whereas disks require a
more gradual infall.  The characteristic time scale of galaxy formation
spans the range btw. the free-fall time (~ 10**8 yr) and the cluster-crossing
time scale (~ 10**9 yr).


4.  The Origin of Angular Momenta		(Fall, Efstathiou,...)

Protogalaxies acquire their angular momenta via tidal torquing.  If the baryons
collapse after the halos are spun up (e.g., during the relatively slow formation
of disks), they acquire a faster rotation.  Ellipticals collapse faster, and
are subject to mergers, which scramble any ordered rotation.

Ay 155								Spring'89
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Galaxy Formation:  A Brief Synopsis, continued



5.  Cooling and Morphology	(Silk, Faber, Blumenthal, Primack, Rees,...)

Galaxy morphology is primarly determined by the degree and pace of dissipation.
Highest density peaks (3 sigma?) dissipate most and fastest, intermediate
density peaks (2 sigma?) become spirals, 1-sigma peaks become diffuse dwarfs,
and half-sigma peaks become Ly-alpha clouds.  If the density field is gaussian,
this can also account for the morphology-density relation.

Galactic winds can sweep low-mass galaxies, which then expand adiabatically,
and become diffuse dwarfs.


6.  Biased Galaxy Formation		(Kaiser, Rees, Dekel,...)

There are several types of biases.  The first general possibility is that 
the statistical properties of the density field can account for the differences
in galaxy-galaxy and cluster-cluster correlations, and the morphology-density
relation (Kaiser).  The second class of biases contents that there is some
astrophysical interaction between the protogalaxies and their large-scale
environment, e.g., luminous protogalaxies can prevent star formation in their
vicinity by ionizing the gas; the frequency of mergers depends on the local
density; etc.

Some biasing is necessary to reconcile the N-body models (as done so far)
with observational constraints (clustering, timing of formation, etc.)


7.  Gradual Galaxy Formation		(Silk, White, and collaborators)

If the mergers and merger-induced starbursts are the dominant processes
of galaxy formation, then the epoch of galaxy formation extends over a
large range of redshifts (a broad peak at z ~ 1 - 3?).  Moreover, in the
CDM scenario, it is difficult to form any sizable structures before that.