Caption: "This graphic shows the evolutionary sequence in the growth of massive elliptical galaxy (e.g., cD or giant elliptical galaxies) over 13 Gyr, as gleaned from space-based and ground-based telescopic observations. The growth of this class of galaxies is quickly driven by rapid star formation and galaxy mergers." (Slightly edited.)
The growth of massive ellipticals is called galactic cannibalism---they eat their own.
Actually, galaxy formation and evolution is NOT yet perfectly understood. In particular, the galaxy quenching (i.e., the turning off of star formation) is NOT well understood.
Some considerations on galaxy quenching:
- Galaxy mergers
and feedback from inflow to
central
supermassive black holes
are important ingredients.
- Galaxy mergers
randomize orbits of stars
and
interstellar medium (ISM)
and eject some of them/it into
intergalactic space.
- So
galaxy mergers
should create
ellipticals.
Certainly, in the early early formation of large-scale structure circa 1 Gyr after the Big Bang, galaxy mergers also created spiral galaxies or galaxies that evolved to being spiral galaxies. But this process may have turned off or drastically slowed over cosmic time since then.
- Why do ellipticals
NOT evolve back into being
spiral galaxies or at least have
much star formation in the modern
observable universe?
Electromagnetic radiation (EMR) from accretion disks from sufficiently large central supermassive black holes is thought to keep any ISM gas too hot and buoyant to cool, lose pressure support, and initiate star formation.
We now know that there is hot ISM gas in ellipticals. It just tends to rather invisible radiating in the X-ray band????.
There is, however, little interstellar dust in ellipticals. Without new star formation, interstellar dust is NOT much replenished since it is made from metals created and ejected from stars in strong stellar winds or when they go and supernova.
If a galaxy merger strips most interstellar dust and thereby stops star formation by removing the cooling and shielding effect of the interstellar dust, it may NOT be possible to replenish the interstellar dust sufficiently in the modern observable universe to restart star formation.
- Another factor is that the modern
observable universe
just has a lower density
of intergalactic medium (IGM)
(made of mainly of
hydrogen and
helium
gas)
compared to the epoch of
early formation
of large-scale structure (circa 1 Gyr after the
Big Bang)
just due to the
expansion of the universe.
Cosmic time
circa 1 Gyr after the
Big Bang
corresponds to
cosmological redshift
z ≅ 5.5
(see Cosmology file:
cosmos_distance_z_10000.html;
Chip Kobulnicky: Cosmology Calculator;
Chip Kobulnicky: Cosmology Calculator: z∈[5.5,6.4]).
Since then the
cosmic scale factor
has increased by a factor of
z+1 ≅ 6.5
which has caused density of
the IGM to decrease
on average in density by factor of order 270.
The decrease in
IGM
must have a strong tendency to reduce inflow to
galaxies
causing star formation
to be generally reduced.
In fact,
star formation
in the observable universe
seems to have peaked
at cosmic time ∼ 4 Gyr.
This peak epoch is called
cosmic noon (z≅2, cosmic time ∼ 4 Gyr; lookback time ∼ 10 Gyr).
- In fact, as just discussed, there are several factors turning off
star formation
in elliptical galaxies
and other galaxies too
and the exact weighting of them is NOT fully answered question.
The study of
galaxy quenching
is still in its early days.
- For a more extended discussion of
galaxy quenching,
see Galaxies file:
galaxy_quenching.html.
Image link: Wikipedia: File:Development of massive elliptical galaxies.jpg.
Local file: local link: galaxy_elliptical_evolution.html.
File: Galaxies file: galaxy_elliptical_evolution.html.