To explicate:
blue galaxies
emit more
blue light (fiducial band 0.450--0.495 μm)
than
red galaxies---and
red galaxies
emit more
red light (fiducial range 0.625--0.740 μm)
than
blue galaxies.
Green valley galaxies
are just between
blue galaxies
and red galaxies
in emission.
All of these types just
emit white light???
in "absolute" true color---which
is just what psychophysical response
of the unshielded human eye would be
seeing them while the unshielded human eye
was off in outer space.
For further explication of
galaxy colors,
see file
galaxy_colors.html.
Usually, galaxy
mass
exceeds the golden mass = 10**12 M_☉
following a
galaxy merger
which randomizes the directions
and eccentricities
of star
orbits
and makes the
merged galaxy
an elliptical galaxy.
The pre-merger
galaxies
could have any
galaxy types,
including being elliptical galaxies,
of course.
Note that the sequence illustrated in the image shows NO
explicit galaxy mergers
although they would usually almost certainly have happened during the
galaxy evolution
before the sequence began.
For case of a probable galaxy merger
see Image 2 below.
What causes
galaxy quenching
(i.e., the turning off of star formation)?
Actually there are two different questions: what turns off
star formation
and what keeps it turned off.
However, it seems likely that the answers are probably much the same.
That
galaxy quenching
happens is absolutely clear:
the dichotomy
of
blue cloud
and red sequence
(shown in the figure above:
local link /
general link: galaxy_color_magnitude_diagram.html)
makes that clear.
In fact, there are many possible mechanisms for
galaxy quenching
and many of them probably actually occur separately or in combination.
Determining the quantitatively correct mix of mechanisms
(separately or in combination) is the question.
For a short review of
galaxy quenching,
see Man & Belli (2018).
We give below
The Short Story of Galaxy Quenching Circa the 2020s
The Short Story of Galaxy Quenching Circa the 2020s:
The locked up baryonic matter
is NOT available for new
star formation.
If galaxies obeyed the
closed-box model
(i.e., had NO inflow/outflow of gas),
then
star formation would
simply gradually turn off from lack of
gas to form new
stars.
But galaxies do have
inflows/outflows of gas
and there is plenty of
intergalactic medium (IGM)
(mostly primordial
hydrogen and
helium
gas from
Big Bang nucleosynthesis)
to keep star formation
going in all galaxies
at the current rate in
blue cloud galaxies
if it were available to do so.
In fact, there at
cosmic present (equal
to the age of the observable universe = 13.797(23) Gyr (Planck 2018))
there ∼ 10 times more mass
in the IGM
than mass in
stars
(see cosmos_energy_pie_chart.html or
cosmos_energy_pie_chart_images.html).
Inflows are just due gravity
and outflows mainly due
relativistic bipolar jets
from the central
supermassive black hole
(an effect which is one type of
AGN feedback)
and supernovae.
The expansion of the universe
is taking some of it away faster than it can fall into
galaxies attracted by the
gravity of said
galaxies.
Recall this gravity is mainly due to
the dark matter halos
which have > ∼ 30 times more
mass
than the mass in
stars
(see Dekel et al. (2019);
Wikipedia:
Dark matter halo: Milky Way dark matter halo).
Yours truly thinks---but CANNOT find a reference to
say so explicitly---that the
expansion of the universe
(which recall is currently in an
acceleration of the universe phase)
is the main or at least a major reason for the fall in univeral
star formation rate (SFR).
To explicate, the
universal SFR
peaked at
cosmic time
(time since the Big Bang) 3.5 Gyr
(i.e., lookback time 10.3 Gyr,
cosmological redshift z ≅ 1.9)
and has been falling exponentially ever since with
e-folding time 3.9 Gyr.
This time of peak is now called
cosmic noon (t ∼ 3.5 Gyr (z ≅ 1.9).
At
cosmic present (equal
to the age of the observable universe = 13.797(23) Gyr (Planck 2018)),
the universal SFR
is 0.07 = 7 % ≅ 1/14 of its peak value
(Madau & Dickinson 2014).
We are at "cosmic afternoon".
This decline must be mainly due to the
expansion of the universe???.
For more discussion of Image 3,
see Cosmology file:
cosmos_history.html.
This scenario though depends on highly speculative extrapolation of
the Λ-CDM model to the far future.
This may be valid, but we do NOT know that.
As aforesaid, there are many possible mechanisms for
galaxy quenching.
Here will discuss only the two mechanisms that are as currently considered as the main ones:
1)
golden-mass quenching
(see Dekel et al. 2019;
also
Bower et al 2016;
Man & Belli 2018, p. 1;
esp. Scharre et al 2024, p. 20)
and 2)
galaxy ram-pressure stripping.
Golden-mass quenching
is for galaxies larger than
∼ 10**12 M_☉
(∼ 3*10**10 M_☉
in stars):
see Dekel et al. (2019).
Galaxy ram-pressure stripping
is for
galaxies smaller than ∼ 10**11.3
M_☉
that
fall into rich
galaxy clusters with
greater than ∼ 10**14
M_☉
and then
galaxy quenched
on a time scale of 1 Gyr
(see Lotz et al. 2018).
But first we will preview the two main mechanisms with
Galaxy quenching videos
below
(local link /
general link: galaxy_quenching_videos.html).
In class, we just look at the
videos while
yours truly explicates them.
Observationally, galaxies
>∼ 10**12 M_☉
are almost all
galaxy quenched
(see Dekel et al. 2019)
and this is theoretically understood from
computer simulations
(Scharre et al 2024, p. 20).
The mass 10**12 M_☉
has been called the golden mass
(see Dekel et al. 2019).
UNDER RECONSTRUCTION BELOW
Note:
In fact spiral galaxies
with classical bulges
are probably inside-out
galaxy quenching
since in this case
the galactic bulge
is much like an
elliptical galaxy
superimposed on a
galaxy disk.
How do galaxies get so large
for the
golden-mass quenching
to occur?
Large galaxies form by
galaxy mergers as
we see observationally and
as shown by
computer simulations of
large-scale-structure formation.
Such galaxy mergers
randomize the
orbital planes
of the stars,
and so galactic disk
of the precursor galaxies are totally lost.
A galaxy merger
can also initiative a short phase of intense
star formation
(i.e., a phase with a
star formation rate (SFR))
if the pre-merger galaxies
had a lot of cool
interstellar medium (ISM)
including interstellar dust.
This phase is called a
starburst
and galaxies with
starbursts are called
starburst galaxies.
The starburst is triggered
by the massive collision of the two
ISM components and uses up
a significant amount of this
original ISM.
But
even starbursts must
leave a lot of
ISM since
star formation is
NOT very efficient---only a few percent of
ISM goes into
new stars in
a star forming region????.
Now the hot gas in
galaxies
>∼ 10**12 M_☉
may clump into a new galactic disk
eventually, but since
star formation has turned off
there is NO way for
a new disk of
stars to come into existence.
The upshot is that
the new galaxy of
>∼ 10**12 M_☉
is a
quenched
elliptical galaxy
and stays that way.
A new galaxy <
∼ 10**12 M_☉ might
evolve back to being a
disk galaxy unless other
processes cause it to be a
quenched galaxy.
Note the time scale for
galaxy mergers
and
galaxy quenching
by total mass
exceeding the golden mass
is typically of order a few
gigayears.
For example, the Milky Way
(mass ∼ 10**12
M_☉)
and the Andromeda galaxy (M31, NGC 224)
(mass ∼ 10**12
M_☉)
are expected to collide and undergo
a galaxy merger on the time
scale of 4 Gyr
(see Wikipedia:
Andromeda-Milky Way collision: Certainty).
Since their combined mass certainly well exceeds
10**12
M_☉, the
merged galaxy will very probably
settle into being a
quenched
elliptical galaxy.
For another example of a
galaxy merger
leading to
galaxy quenching,
see the figure below
(local link /
general link: galaxy_quenching_golden_mass.html).
The
galaxy quenching
mechanism
for
galaxies smaller than ∼ 10**11.3
M_☉
(hereafter small galaxies)
that
fall into rich
galaxy clusters with
greater than ∼ 10**14 M_☉
is
galaxy ram-pressure stripping
Rich galaxy clusters
have a
intracluster medium heated
by infall into the galaxy cluster
and by
relativistic bipolar jets
from the cluster
galaxies.
The smaller galaxies that fall into
a galaxy cluster
simply have their cold
gas continually pushed out by
hot gas that CANNOT cool
sufficiently to allow collapse to form
stars under
self-gravity.
So those small galaxies become
quenched.
The time scale for
galaxy quenching
is ∼ 1 Gyr
(see Lotz et al. 2018).
Larger galaxies than
∼ 10**11.3
M_☉
are probably
galaxy quenched
on a longer time scale or
eventually by galaxy mergers with
total mass exceeding the
golden mass.
Galaxy quenching
by
galaxy ram-pressure stripping
is probably an outside-in
quenching
since the outer
cold gas gets stripped most easily.
On the time scale of tens or more
gigayears,
the galaxy clusters are expected
to merged into a super
supergiant elliptical galaxies
that are
galaxy quenched.
What is Galaxy Quenching?
What Causes Galaxy Quenching?
EOF
Image Credits:
Image links: Wikimedia Commons.
Image link: Wikimedia Commons:
File:Eso1516a.jpg.
Image link: Wikimedia Commons:
File:The Universe across space and time ESA24866637.png.
Local file: local link: galaxy_quenching.html.
File: Galaxies file:
galaxy_quenching.html.
