Galaxy Quenching

Introduction:
Galaxy quenching is the process of star formation turning off in a galaxy (see Wikipedia: Quenching; Wikipedia: Galaxy formation and evolution: Galaxy quenching).
A quenched galaxy is usually defined as one where the star formation rate (SFR) ⪅ 1/10 of that of unquenched galaxies which are usually spiral galaxies with SFR 0.1--tens of M_☉/year (Ci-54--55). A fiducial rule for quenched galaxies is SFR per mass in stars (i.e., specific star formation rate (sSFR)) ≤ 10**(-11) M_☉/(year*mass-in-stars). The sSFR can go down to vanishing.
In fact, most elliptical galaxies and lenticular galaxies (S0 galaxies) are quenched galaxies (Ci-54).
Most spiral galaxies are unquenched galaxies, but smaller spiral galaxies in galaxy clusters are often quenched galaxies.
Dwarf galaxies have a wide range of quenching status.
Besides Galaxy Quenching, There Is a Universal Decline in Star Formation Rate with Cosmic Time:
Besides galaxy quenching, there is a universal decline in star formation rate with cosmic time since cosmic noon (z∼1.5--2.5, cosmic time ∼ 3--4 Gyr, lookback time ∼ 10--11 Gyr) (see also Wikipedia: Quenching: Quenching threshold; Pilipenko 2013, p.~2). BEFORE cosmic noon (z∼1.5--2.5, cosmic time ∼ 3--4 Gyr, lookback time ∼ 10--11 Gyr), there was a universal INCREASE in star formation rate, but that is a story for another day sine die.
Overall cosmic star formation history is illustrated in the figure below (local link / general link: cosmic_star_formation_history.html).
To explicate the DECLINE in
star formation rate AFTER cosmic noon (z∼1.5--2.5, cosmic time ∼ 3--4 Gyr, lookback time ∼ 10--11 Gyr):
1. Every generation of stars causes some baryonic matter to be quasi-eternally locked up in compact stars (white dwarfs, neutron stars, black holes), supermassive black holes, and probably a cosmically negligible amount in brown dwarfs planets, moons, and smaller astro-bodies. Also, red dwarf stars live tens to thousands of gigayears, and so on the time scale of the age of the observable universe = 13.797(23) Gyr (Planck 2018) they have been quasi-eternally locked up baryonic matter.
  The quasi-eternally 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.
2. 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 most spiral galaxies (i.e., blue cloud galaxies: see Image 2 below) if it were available to do so for a long time: many gigayears.??? In fact, at cosmic present (equal to the age of the observable universe = 13.797(23) Gyr (Planck 2018)) there is ∼ 10 times more mass in the IGM than mass in stars (see Cosmology file: pie_chart_cosmic_energy.html).
  Inflows are just due gravity and outflows are feedback mainly from relativistic bipolar jets from the central supermassive black holes (an effect which is one type of AGN feedback) and supernovae both of which launch gas out of the galaxies (which by mass are mainly dark matter halos).
  Recall gravity of galaxies is mainly due to the dark matter halos which have ⪆ 30 times more mass than the mass in stars (see Ci-404; Dekel et al. (2019); Wikipedia: Dark matter halo: Milky Way dark matter halo).
  In detail, gas inflows and outflows are complex and various, and are NOT fully understood.
  For an illustration of gas inflows and outflows, see the figure below (local link / general link: gas_inflow_outflow.html).
3. But all the IGM is NOT available inflows. Why NOT?
  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. ???
  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 inflow of IGM.
4. Now the declining inflow of IGM causes a decline in outflows. This is very plausbile, but it is only proven by computer simulations.???
  What causes the decline in outflows?
  There is a decline in SFR with declining inflows and that causes a decrease in rate of core collapse supernovae whose rate follows the SFR since they originate from stars with ⪆ 8 M_☉, and so have lifetimes ⪅ 30 Myr (see Star file: star_lifetimes.html)---which almost an instant in cosmic time. Then the decline in the rate of core collapse supernovae causes a decline in outflows to the IGM. Also a decline in inflow of IGM to the central supermassive black holes causes a decline in outflow from their relativistic bipolar jets which are fed directly or indirectly by the inflow. So declining inflow from the IGM leads to declining outflow to the IGM.
5. But what causes the SFR in galaxies to decline?
  The average amount of interstellar medium (ISM) declines with declining inflow and outflow. This is plausbile, but it is only proven by observations and computer simulations. At cosmic noon (z∼1.5--2.5, cosmic time ∼ 3--4 Gyr, lookback time ∼ 10--11 Gyr) ∼ 50 % of the baryonic matter was ISM ??? and at cosmic present = to the age of the observable universe = 13.797(23) Gyr (Planck 2018) is only ∼ 10 %.???
  Of course, the galaxy quenching of individual galaxies causes a relatively quick decline in the SFR and also contributes to the universal decline the SFR (since cosmic noon (z∼1.5--2.5, cosmic time ∼ 3--4 Gyr, lookback time ∼ 10--11 Gyr)). But yours truly believes that the galaxy quenching of individual galaxies is less important for the universal decline the SFR than declining inflows from the IGM.
6. What has been the decline in universal SFR?
  The universal SFR (measured by what is called comoving star formation rate density (SFRD, 𝜓)) peaked at cosmic noon (z∼1.5--2.5, cosmic time ∼ 3--4 Gyr, lookback time ∼ 10--11 Gyr).
  At cosmic present (equal to the age of the observable universe = 13.797(23) Gyr (Planck 2018)), comoving star formation rate density (SFRD, 𝜓) is 0.11 = 11 % ≅ 1/9 of its peak value (Madau & Dickinson 2014). The observable universe is in cosmic afternoon. For further explication of comoving star formation rate density (SFRD, 𝜓) over cosmic time see the figure above (local link / general link: cosmic_star_formation_history.html).
  
  As aforesaid, this decline must be mainly due to the expansion of the universe??? which is illustrated in Image 1 below.
7. Image 1 Caption: An illustration of cosmic history showing, among other things, cosmic noon (z∼1.5--2.5, cosmic time ∼ 3--4 Gyr, lookback time ∼ 10--11 Gyr), but with a different time than adopted here probably due to a somewhat different definition of cosmic noon.
  For more discussion of Image 1, see Cosmology file: cosmos_history_4.html.
8. Now in the far Λ-CDM model cosmic future ∼ 10**5 Gyr = 10**14 years from now, star formation and nuclear burning in stars will end: this is the end of stelliferous era (star forming era) (see Wikipedia: Graphical timeline of the Stelliferous Era). This ending will be brought about by a combination of the expansion of the universe and the quasi-eternal locking up of baryonic matter in compact stars.
  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.
Unquenched and Quenched Galaxies Plotted
To return to the subject of galaxy quenching, we further explicate the observations of unquenched galaxies and quenched galaxies in the cartoon galaxy color-magnitude diagram in the figure below (local link / general link: galaxy_color_magnitude_diagram.html).
Image 2 Caption: A diagram illustrating a disk galaxy undergoing inside-out galaxy quenching: the multi-gigayear process of the turning off of star formation.
We see the disk galaxy age from left to right changing from blue to red starting from the inside and changing from being a star forming disk galaxy (usually a spiral galaxy) to becoming a quenched galaxy (usually an elliptical galaxy).
Note the colors in Image 2 (and in Image 3 below) are conventional.
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 the figure above (local link / general link: galaxy_color_magnitude_diagram.html) explicating galaxy color-magnitude diagrams.
Inside-out galaxy quenching is probably usually initiated by galaxy mass exceeding the golden mass = 10**12 M_☉ plus some effect from a starburst (see Golden Mass Quenching below).
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 3 below.
Image 3 Caption: The interacting galaxies in Image 3 have been nicknamed The Mice (catalogue name NGC 4676) because of the long tidal tails of stars and gas (which are hard to see in Image 3) emanating from each galaxy. The interacting galaxies will probably have a galaxy merger on the time scale of 1--2 Gyr. For a longer explication of The Mice (catalogue name NGC 4676), see Galaxies file: galaxy_mice.html.
What Causes Galaxy Quenching?:
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 the short story of galaxy quenching below in section The Short Story of Galaxy Quenching Circa the 2020s.
The Short Story of Galaxy Quenching Circa the 2020s:
Now for the short story of galaxy quenching.
1. First off, the reason for the decline in universal SFR (explicated above in section Besides Galaxy Quenching, There Is a Universal Decline in Star Formation Rate with Cosmic Time) does NOT explain why right now some galaxies are galaxy quenched and some are NOT.
2. As aforesaid in section What Causes Galaxy Quenching?, there are many possible mechanisms for galaxy quenching many of which probably happen separately or in combination.
  Here will discuss only the two mechanisms that are 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): golden mass quenching is for galaxies with total mass (in dark matter and baryonic matter) ⪆ 10**12 M_☉ and mass in baryonic matter alone ⪆ 3*10**10 M_☉ (Dekel et al. (2019).
  2. galaxy ram-pressure stripping: Galaxy ram-pressure stripping is for galaxies with total mass ⪅ 10**11.3 M_☉ that fall into rich galaxy clusters with total mass ⪆ 10**14 M_☉ and then the galaxies become galaxy quenched on a time scale of order 1 Gyr (see Lotz et al. 2018).
  Before we further explicate the two main mechanisms, we will preview them in Galaxy quenching videos below (local link / general link: galaxy_videos.html).
  But in class, we just look at the videos while yours truly explicates them.
3. Golden Mass Quenching:
  Before reading the text, see video When galaxies collide! | 1:36: also shown in Galaxy quenching videos above (local link / general link: galaxy_videos.html).
  1. 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).
  2. EOF
  3. The heating process in golden mass quenching actually takes time and is thought to be usually by inside-out galaxy quenching as illustrated in Image 2 above.
    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.
  4. 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.
  5. 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????.
  6. Now the hot gas in galaxies ⪆ 10**12 M_☉ may clump into a new galactic disk eventually, but since star formation has turned off (or MOSTLY turned off) there is NO way for a new disk of stars to come into existence. The upshot is that a post-galaxy-merger galaxy of ⪆ 10**12 M_☉ is a quenched elliptical galaxy and stays that way.
  7. A post-galaxy merger galaxy ⪅ 10**12 M_☉ might evolve back to being a star forming disk galaxy unless other processes cause it to become a quenched galaxy.
  8. 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 may collide and undergo a galaxy merger on the time scale of 4 Gyr (see Wikipedia: Andromeda-Milky Way collision), but circa 2025 there is a calculation that a Andromeda-Milky Way collision and then a galaxy merger in the next 10 Gyr is only ∼ 50 % likely (Sawala et al., 2024). Since their combined mass certainly well exceeds the golden mass 10**12 M_☉, the post-galaxy-merger Andromeda-Milky Way 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).
4. Galaxy Ram-Pressure Stripping Quenching:
  Before reading the text, see video Animation of Ram Pressure Stripping ESA | 0:23: also shown in Galaxy quenching videos above (local link / general link: galaxy_videos.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.
5. Other Quenching Mechanisms:
  As mentioned above in section What Causes Galaxy Quenching?, there are other mechanisms for galaxy quenching and all or some of these mechanism can combine.
6. The Last Galaxies to Quench:
  It seems likely that isolated, smallish galaxies may be the last to galaxy quench probably just due to quasi-eternal locking up of baryonic matter in compact stars. and the expansion of the universe far in the Λ-CDM-model cosmic future if that future actually happens.
For Reference:
For reference, see Galaxy Classification Systems and Types of Galaxies below (local link / general link: galaxy_types.html).

Image Credits:

Credit/Permission: © European Space Agency (ESA) 2023 (uploaded to Wikimedia Commons by User:OptimusPrimeBot, 2023) / CC BY-SA 3.0.
Image link: Wikimedia Commons: File:The Universe across space and time ESA24866637.png.
Credit/Permission: © ESO, 2015 (uploaded to Wikimedia Commons by User:Artem Korzhimanov, 2015) / CC BY-SA 4.0.
Image link: Wikimedia Commons: File:Eso1516a.jpg.
Credit/Permission: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA, 2002 / Public domain.
Image link: Wikipedia: File:NGC4676.jpg.

Wikimedia Commons

local link: galaxy_quenching.html

Galaxies file

galaxy_quenching.html