UNDER RECONSTRUCTION BELOW
A major problem is the dark matter must have played major role in formation since it is overwhelmingly most of the mass. But that role is hard to determine exactly.
What the dark energy does is another rather open question.
Dark matter is matter we only notice through its gravitational effect and it seems to be overwhelmingly most of the matter of the observable universe.
About 20 % or so is baryonic dark matter. Most of this baryonic dark matter is hot and nearly invisible dilute hydrogen/helium gas between galaxies and galaxy clusters---the intergalactic medium which is mainly warm-hot intergalactic medium (WHIM)) and nearly-invisible intergalactic baryonic matter. Both these invisible matter types (which are mainly ionized hydrogen and helium with maybe a little enrichment of metals) radiate in the X-ray band, but with low detectability---we do observe it now a bit.
However, when we (and everyone else) says dark matter without qualification, we (and everyone else) mean NOT the baryonic dark matter but exotic dark matter.
The (exotic) dark matter may be some kind of very unreactive exotic particles that are spread through space, but clumped somehow in the dark matter halos of galaxies. People are trying to detect the exotic particles and there have been a few hints of detections, but nothing solid so far. On the other hand, the dark matter may be primordial black holes created in the Big Bang era before Big Bang nucleosynthesis (BBN).
Dark energy is an unknown energy that seems to be accelerating the expansion of the universe. Dark energy is discussed in IAL 30: Cosmology.
For the currently determined amounts of dark matter and dark energy (given that the Λ-CDM model is true), see the figure below (local link / general link: pie_chart_cosmic_energy.html).
Both the concepts of dark matter and dark energy may be greatly modified or even dispensed with if the relativistic MOND theory turns out to real to some degree or other (Bekenstein, J. D. 2004, An Alternative to the Dark Matter Paradigm: Relativistic MOND Gravitation, astro-ph/0412652). See IAL 25: Black Holes for more on the relativistic MOND theory.
We will only give a brief sketch of the current theory
of galaxy formation:
it is still a somewhat uncertain theory---and
yours truly does NOT know as much
as yours truly should know.
The Big Bang
(see IAL 30: Cosmology)
left fluctuations in the density of the early universe gas as
we mentioned above in connection with the
large-scale structure.
These fluctuations led to gravitational runaways to PROTOGALAXIES
and these merged to make galaxies.
The galaxies formed from the "the bottom up":
(i.e., from smaller objects) rather from "top down" (i.e., out of galaxy-sized clouds)
as was once theorized.
These objects are NOT around in the modern universe,
and so must have merged to make modern
galaxies.
It may be that SOME
ellipticals
formed when
the early star formation was very fast and occurred
before a disk of gas could form.
There is then no energy dissipation mechanism for stars
which are point-like and super-rarely collide in a hard sense:
gravitational interactions are common but these tend NOT
to dissipate kinetic and gravitational potential energy to heat.
Thus, the streaming and dissipation mechanism that leads to disk
formation for gas does NOT occur for stars.
Consequently, the stars in
ellipticals
stayed in a swarm.
However, very likely most
ellipticals form by
galaxy mergers.
Maybe they almost all do.
The process of galaxy merger
randomizes the orbits
of the stars and often strips of the gas.
The stars go right through each other in
galaxy collisions,
but the gas runs into
gas clumps and becomes physically
separated and tends to be lost which tends to turn off
star formation.
Or the gas becomes too hot
for star formation.
Its pressure is too high.
For a galaxy collisions
see the video
When galaxies collide! | 1:36
in
Galaxy videos
below
(local link /
general link: galaxy_videos.html):
The merged galaxy
(likely a new elliptical)
tends to be
quenched galaxy
in modern astro-jargon.
Why do quenched galaxies
NOT revive by inflow of new gas and
gas cooling?
They probably do or did in the past in some cases.
But it seems that dequenching is rare.
The fact that a
galaxy merger occurred
probably means crowded ????
Also star formation
is slowing down generally as the universe expands and there is less
gas inflow from the intergalactic medium (IGM).
Ellipticals
are more likely in regions of greater density of galaxies.
Ellipticals
can also be made by the collisions of spirals which
strip the gas and randomize the star orbits and strip away
the interstellar medium (ISM).
in the figure below
(local link /
general link: galaxy_mice.html).
In the universe, to look far away is to look long ago.
See the figure below for further discussion of
ellipticals,
galaxy mergers,
and
galaxy quenching
(the turning of
star formation).
Such gas is probably shocked heated during collisions that stripped it from
the parent galaxies
(FK-596).
In both
ellipticals
and
spirals
the star formation rate probably
peaked early on in the universe.
The adjacent cartoon illustrates this, but the cartoon
simplifies things by ignoring that some
spirals
have merged to make
ellipticals.
The "bottom up" process is evidenced by HST images of
abundant star groupings smaller than modern day
dwarf ellipticals
at distances of about 3400 Mpc which is
a large fraction of a
Hubble length = L_H = c/H_0 = 4.2827 Gpc/h_70 = 13.968 Gly/h_70
away and at a look back
time of about 11 Gyr
(FK-602).
The merger process never stopped: there are still mergers in the modern universe
(i.e., the local universe).
UNDER RECONSTRUCTION BELOW
Spirals
(see figure below
local link /
general link: galaxy_hubble_sequence.html)
formed from protogalaxies that were mainly still gas.
The gas collapsed into disk according to the process we
have discussed many times (see
IAL 21: Star Formation).
Then most stars formed in the disk.
Computer simulations show that BAR FORMATION is quite natural for spiral galaxies.
However, a sufficiently massive dark matter halo may inhibit BAR FORMATION---one theory anyway.
The numbers of ordinary and
barred spirals are comparable
(CK-393;
FK-583).
EOF
The merged galaxy
will strong tend to be an elliptical
without much star formation
because without much gas or at least
much cold enough gas.
UNDER RECONSTRUCTION BELOW
The distinction in formation between spirals and
ellipticals probably has to do with the richness
of the environment.
Mergers have evidently happened in rich clusters:
distant rich clusters have more spirals than local ones.
Question: Distant rich clusters are rich clusters
seen in an earlier epoch of the universe because of:
Collisions of
ellipticals
in dense environments probably also strip
any new gas from ejecta from old stars through winds.
This mostly prevents renewal of star formation.
Answer 1 is right.
Rich clusters typically have a very hot intergalactic medium
that radiates X-rays: temperatures of order 10**7 and 10**8 K.
Well let's go into the stories with lots of simplifications and omissions to keep from turning our discussion into a book.
The "rich get richer, the poor get poorer".
Gravitational runaway led to dark matter halos where the dark matter where the density was high and voids where it was low.
The dark matter halos formed gravitational wells that attracted ordinary matter almost entirely the hydrogen and helium in a 3:1 ratio.
If star formation proceeded very rapidly in a dark matter halo due to some initial condition????, most of the gas is used up to form stars in a few early generations of stars leaving little for later generations of stars.
Also there was no time for the gas to relax to galactic disk which requires pressure interactions which stars do NOT have---they pinpricks that interact gravitationally virtually only.
If there are no interactions with other galaxies, the elliptical galaxy only ages through the aging of its stars.
As cosmic time passes, progressively the less massive stars end their main-sequence lifetimes and turn into white dwarfs.
The end of the main-sequence lifetime must inject some matter back into the interstellar medium (ISM), but evidently NOT enough to promote much new star formation.
Stars keep forming up to the present in cosmic time and far into the future.
The ISM has time to relax through pressure interactions into a galactic disk where almost all the star formation happens.
Usually there are spiral arms for reasons that we go into section spiral arms.
The result is a spiral galaxy.
These are, in fact, common.
Galaxy mergers are complex events that tend to strip ISM and randomize star orbits of merging spiral galaxies or merging spiral galaxies elliptical galaxies.
So one tends to end up with elliptical galaxies.
Merging elliptical galaxies just give larger elliptical galaxies.
Both calculations and empirical evidence show that mergers yield elliptical galaxies.
The more compact the galaxy cluster, the more galaxy mergers will occur. So NOT surprisingly that compact galaxy clusters tend to have a higher fraction of ellipticals than less compact galaxy clusters.
These are the cD or giant elliptical galaxies: e.g., M87 in the Virgo Cluster in Virgo.
The growth of massive ellipticals is called galactic cannibalism---they eat their own.
This growth is illustrated in the figure below.
Form groups of 2 or 3---NOT more---and tackle Homework 29 problems 2--8 on galaxies, large scale structure, and galaxy formation.
Discuss each problem and come to a group answer.
Let's work for 5 or so minutes.
The winners get chocolates.
See Solutions 29.