Quenching Galaxies

The death of a galaxy

Galaxies come in two flavors that you can tell apart from across the universe just by color. One kind is blue, dusty, full of gas, and busy making new stars — spirals and irregulars, lit by short-lived, hot, blue O and B stars. The other kind is red, smooth, gas-poor, and dead — ellipticals and lenticulars whose light comes entirely from old, cool, low-mass stars that have been around for billions of years. There is remarkably little in between. Plot a few hundred thousand galaxies by color, as the Sloan Digital Sky Survey first did, and you get two peaks with a valley between them: the blue cloud and the red sequence, separated by the sparsely populated green valley.

Quenching is the process that carries a galaxy from the first population to the second — the shutdown of star formation. It is one of the central problems of galaxy evolution, because a galaxy left to its own devices, with cold gas continuing to flow in and cool, would keep making stars more or less forever. Something has to actively turn it off, and that something has to act selectively: it shuts down the massive galaxies preferentially (mass quenching), and it shuts down the satellites in clusters preferentially (environmental quenching). Understanding quenching means understanding the three engines that do the killing — AGN feedback, halo and environmental processes, and mergers — and why the universe as a whole has been gradually dying since cosmic noon.

The color-mass diagram and the green valley

The natural arena for quenching is the galaxy color-magnitude diagram, or its modern cousin the color-mass diagram (color versus stellar mass M_star). Star-forming galaxies form a diagonal locus — the star-forming main sequence — where the star formation rate scales roughly linearly with stellar mass. Quenched galaxies sit far below it. The cleanest single number for "how star-forming is this galaxy" is the specific star formation rate:

sSFR = SFR / M_star          [units: 1 / year]

main sequence (blue cloud):  sSFR ≈ 10⁻⁹·⁵ to 10⁻¹⁰ /yr
green valley (transitioning): sSFR ≈ 10⁻¹⁰·⁵ /yr
red sequence (quenched):      sSFR < 10⁻¹¹ /yr

The inverse of sSFR is roughly the time it would take the galaxy to double its stellar mass at its current rate. For a main-sequence galaxy that is a few Gyr; for a red-sequence galaxy it is far longer than the age of the universe — the galaxy is, for practical purposes, finished.

The green valley is not a true gap — galaxies do live there — but it is a deficit, and the deficit is informative. Galaxies pile up where they spend their time. They spend many Gyr blue and many Gyr red, but they cross the intermediate colors fast: the green-valley transit takes under about 2 Gyr. That short crossing time, set against the multi-Gyr lifetimes on either side, is exactly why the valley is underpopulated. The existence and emptiness of the green valley is therefore direct evidence that quenching is relatively rapid, not a slow, graceful fade.

The three quenching channels

1. AGN feedback (mass quenching)

Every massive galaxy harbors a central supermassive black hole, and when that black hole accretes gas it becomes an active galactic nucleus that dumps enormous energy back into the galaxy. This AGN feedback comes in two modes. The radiative or "quasar" mode operates at high accretion rates: radiation pressure and fast nuclear winds — often exceeding 1000 km/s — drive cold gas out of the galaxy entirely. The kinetic or "radio" mode operates at low accretion rates in massive halos: relativistic jets inflate buoyant bubbles in the surrounding hot gas, depositing mechanical energy that keeps the halo from cooling, so fresh fuel never reaches the disk.

Because black hole mass and accretion energy scale steeply with galaxy mass, AGN feedback is the dominant quenching channel for massive galaxies — those above roughly M_star ≈ 10¹⁰·⁵ M☉. This is the physical origin of mass quenching: the empirical fact, robust across surveys, that the quiescent fraction climbs sharply with stellar mass and is essentially independent of environment. Above ~10¹¹ M☉, almost every galaxy is dead.

2. Halo and environmental quenching

The second channel acts on satellite galaxies — those orbiting inside a larger group or cluster halo — and it explains why dense environments are full of red galaxies. There are two sub-processes, distinguished by which gas reservoir they attack and how fast.

• Strangulation (starvation): when a galaxy falls into a bigger halo, its own extended hot-gas envelope — the reservoir that would have cooled and replenished its cold gas — is stripped or tidally removed. Cut off from fresh supply, the galaxy keeps forming stars from its existing interstellar medium until that runs out, a slow quench over several Gyr. Peng, Maiolino & Cochrane (2015) argued from stellar metallicities that strangulation is the dominant satellite mechanism for galaxies below ~10¹¹ M☉, with a characteristic timescale of about 4 Gyr.
• Ram-pressure stripping: far more violent. As a galaxy plows through the dense, hot intracluster medium at ~1000 km/s, the headwind directly strips the cold disk gas. Stripping wins wherever the ICM ram pressure exceeds the disk's gravitational restoring force, the Gunn & Gott (1972) criterion. This can quench in a few hundred Myr and produces spectacular "jellyfish" galaxies trailing tails of stripped, still-glowing gas.

3. Mergers

A major merger quenches by a paradoxical two-step. First it increases star formation: tidal torques funnel cold gas to the center, igniting a starburst that converts a large fraction of the gas into stars in a few hundred Myr. The same inflow feeds the black hole and triggers an AGN. Then the gas is gone — burned in the starburst, expelled or heated by the AGN — and the violent relaxation has scrambled the ordered disks into a pressure-supported spheroid. The remnant is a gas-poor elliptical on the red sequence. This is the classic merger pathway from blue spiral to red elliptical, and it neatly couples quenching to morphological transformation.

Worked example: ram-pressure stripping in the Virgo cluster

Let's put numbers on environmental quenching. Will a Milky-Way-like spiral falling into the Virgo cluster have its cold gas stripped? The Gunn-Gott criterion says stripping occurs when the ram pressure of the intracluster medium exceeds the gravitational restoring force per unit area on the cold disk gas:

ρ_ICM · v²   >   2π · G · Σ_star · Σ_gas

ram pressure          gravitational restoring force / area

Take representative Virgo values. The ICM density at intermediate cluster radius is ρ_ICM ≈ 10⁻²⁷ g/cm³ (about 10⁻⁴ particles/cm³). An infalling galaxy moves near the cluster velocity dispersion, v ≈ 1000 km/s = 10⁸ cm/s. So:

P_ram = ρ_ICM · v²
      = (10⁻²⁷ g/cm³) · (10⁸ cm/s)²
      = 10⁻¹¹ dyne/cm²

Now the restoring force per unit area in the outer disk, where the stellar surface density is Σ_star ≈ 10 M☉/pc² and the gas surface density Σ_gas ≈ 5 M☉/pc² (converted to CGS, 1 M☉/pc² ≈ 2 × 10⁻⁴ g/cm²):

F_grav/A = 2π · G · Σ_star · Σ_gas
         ≈ 2π · (6.7×10⁻⁸) · (2×10⁻³) · (1×10⁻³)
         ≈ 8 × 10⁻¹³ dyne/cm²

The ram pressure (10⁻¹¹) beats the restoring force (~8 × 10⁻¹³) by more than a factor of ten in the outer disk. The galaxy's loosely bound outer gas is stripped on its first cluster passage; only the tightly bound central gas survives. With no extended reservoir left (strangulation has already removed the hot halo), the galaxy exhausts its central gas and slides into the green valley within a few hundred Myr to ~1 Gyr — fast enough to keep the valley underpopulated, exactly as observed.

Mass quenching vs. environmental quenching: the two-axis picture

One of the most important results in this field is that the two dominant channels are separable and roughly independent. Peng et al. (2010), analyzing SDSS and zCOSMOS, showed that the quiescent fraction can be written as a product of a mass term and an environment term — quenching from mass and quenching from environment act like two independent multiplicative probabilities. In practice:

• Central galaxies (the dominant galaxy of a halo) quench mainly by mass — i.e. by AGN feedback once they grow massive enough. Environment barely matters for them.
• Satellite galaxies quench by both: they carry the mass-quenching probability of any galaxy of their mass, plus an additional environmental probability that grows with host halo mass and with time since infall.
• Morphological quenching is a proposed fourth, gentler regime: in galaxies with a large stellar bulge or spheroid, the deep, slowly varying gravitational potential stabilizes the gas disk against fragmentation, suppressing star formation even with gas present (Martig et al. 2009). It is more a modulation than a kill switch.

Why the gas runs out: a depletion-time argument

It is worth seeing why removing the supply of gas, not just the existing gas, is what makes quenching permanent. A star-forming galaxy converts cold gas to stars on a depletion timescale:

t_dep = M_gas / SFR  ≈  1 – 2 Gyr   (typical for a main-sequence spiral)

This is short — only ~10% of the age of the universe. A galaxy in steady state must therefore be continuously refueled: cold gas accreting from the cosmic web and from cooling of the hot halo replaces what is turned into stars. This is the "gas-regulator" or "bathtub" model: stars form at a rate set by the balance between inflow, the existing reservoir, and outflow. Star formation is sustained only as long as inflow keeps the tub from draining.

Now cut off the inflow — by strangulation removing the hot halo, by AGN radio-mode keeping that halo too hot to cool, or by stripping removing the reservoir outright. The tub drains in one depletion time. Even without removing a single gram of the existing cold gas, the galaxy quenches in 1–2 Gyr simply because nothing replaces what it burns. This is why "preventive" feedback (stopping fuel from arriving) is at least as important as "ejective" feedback (blowing gas out): it explains both the speed of the green-valley crossing and why massive galaxies in hot halos stay dead.

Quenching mechanisms compared

Mechanism	Acts on	Gas reservoir attacked	Timescale	Dominates when	Result
AGN radiative (quasar) mode	Centrals, post-merger	Cold ISM — ejected by winds	~10²–10³ Myr	High accretion, M★ > 10¹⁰·⁵ M☉	Fast ejective quench
AGN kinetic (radio) mode	Massive centrals	Hot halo — kept from cooling	Gyr, maintained	Low accretion, massive halos	Permanent (preventive)
Strangulation / starvation	Satellites	Hot halo — supply cut off	~4 Gyr	M★ < 10¹¹ M☉ in groups/clusters	Slow quench
Ram-pressure stripping	Satellites	Cold disk gas — blown off	~100s Myr	Dense ICM, high velocity	Fast quench, jellyfish tails
Major merger	Pairs (1:1–1:4)	Cold gas — burned + ejected	~100s Myr	Gas-rich encounters	Red elliptical remnant
Morphological quenching	Bulge-dominated	None removed — stabilized	Gradual	Large spheroid present	Suppressed, not killed
Harassment (cumulative tides)	Cluster dwarfs	Disk disrupted, gas loosened	Several Gyr	Many fast fly-bys in clusters	Transforms to dwarf spheroidal

Observational status and frontier

• The bimodality is rock-solid. Strateva et al. (2001) and Baldry et al. (2004) established the blue/red split from SDSS; it persists across redshift out to z ~ 1 and beyond, and combining ultraviolet (clean SFR tracer) with infrared (dust correction) sharpens rather than fills the green valley.
• Mass and environment quenching are separable. Peng et al. (2010) decomposed the quiescent fraction into independent mass and environment terms — a key empirical anchor for every modern model.
• JWST found quenching far too early. Spectroscopically confirmed massive quiescent galaxies now appear at z ~ 4–5, within the first ~1.5 Gyr after the Big Bang. They are too red and too dead too soon for older models, demanding very efficient early (almost certainly AGN-driven) quenching.
• Jellyfish galaxies are the smoking gun for stripping. Surveys like GASP have caught dozens of cluster spirals mid-strip, with ionized-gas tails confirming ram pressure in action.
• Simulations reproduce the red sequence only with AGN feedback. Cosmological simulations (Illustris-TNG, EAGLE, SIMBA) all require strong AGN feedback to prevent massive galaxies from overcooling into blue monsters; without it, the red sequence simply does not form. This is indirect but powerful confirmation that AGN feedback is the mass-quenching engine.
• The cosmic context. The cosmic star formation rate density peaked at cosmic noon (z ≈ 2) and has fallen roughly tenfold to today (Madau & Dickinson 2014). That decline is the cumulative history of galaxies quenching across cosmic time.

Common pitfalls and misconceptions

• "Quenched means no gas at all." Not necessarily. Many quiescent galaxies still contain gas — but it is hot, or too stable (morphological quenching), or not cooling because the halo is kept hot. Quenching is about the inability to form stars, not the strict absence of all gas.
• "The green valley is a true gap." It is a deficit, not a gap. Galaxies live there; they just don't linger. Treating it as empty mis-states the physics — the point is the short crossing time, not zero occupancy.
• "Red equals old equals dead — always." Edge-on, dusty star-forming disks can look red in optical colors. This is why a UV+IR or spectroscopic sSFR, not optical color alone, is the trustworthy quenching diagnostic.
• "Mergers always quench." Major gas-rich mergers do, eventually. But minor mergers can add gas and rejuvenate a red galaxy, briefly turning it blue again — the green valley has traffic in both directions.
• "One mechanism explains everything." There is no single cause. Mass quenching (AGN) governs massive centrals; environmental quenching (strangulation + stripping) governs satellites; mergers couple quenching to morphology. Which dominates depends on mass, environment, and epoch.
• "Quenching is irreversible." Mostly, for massive galaxies in hot halos — but rejuvenation is real for intermediate-mass galaxies that capture fresh cold gas, and a measurable fraction of red-sequence galaxies show recent star formation.