based on Maier, Ziegler, Haines & Smith, 2019a, A&A, 621, A131

Slow-then-rapid star formation quenching in z~0.2 cluster galaxies

& on Maier, Hayashi, Ziegler & Kodama,

2019b, A&A, 626, A14

Cluster induced quenching of galaxies

at z~1.5 traced by enhanced metallicities

Slow-then-rapid quenching and strangulation of cluster galaxies at z~0.2 and z~1.5 as traced by enhanced metallicities

Christian Maier

Slow quenching: strangulation/starvation/suffocation

In humans: death by strangulation is a slow process

- victim uses up oxygen in the lungs but keeps producing carbon dioxide, which remains trapped in the body

- high levels of carbon dioxide in the blood of a corpse suggest suffocation

In galaxies: strangulation is a slow star formation quen-ching process (Peng et al. 2015, Maier et al. 2016, 2019)

- the supply of pristine gas onto the galaxy disk is halted, but star formation can continue, using the gas available in the disk until it is completely used up

- instead of building carbon dioxide (cf. humans), the strangled galaxies accumulate metals produced by massive stars (no pristine gas dillution)

- higher metallicities: evidence for galactic “suffocation”

Lilly et al. (2013):

Bathtub model

Cluster induced Slow-then-rapid star-formation quenching

Wetzel et al. (2013):

Delayed-then-rapid quenching scenario: SFR unaffected after first infall for a few Gyr: controversial, because hard to understand and simulate galaxies flying at thousands km/s through ICM (intra-cluster medium) of clusters remaining uninfluenced by environment for the delay time of several Gyr

January 2019: Maier et al. (2019):

Slow-then rapid quenching in z~0.2 cluster galaxies as traced by enhanced metallicities and by quenching timescales from observed fraction of star-forming galaxies as a function of clustercentric radius

March 2019: Roberts et al. (2019):

Slow-then rapid quenching in local cluster galaxies as traced by observed quenched fraction of galaxies as a function of ICM density profiles (derived from Chandra X-ray data)

Slow-then-rapid quenching (stopping of star formation) at z~0.2

Timescales:

1-2 Gyr for slow-quenching using part of available gas reservoir,

~0.5-1Gyr for rapid quenching when ICM density higher, and galaxies travel to denser inner regions of cluster

Galaxies fly towards the center of the cluster:

first, after passing Rvirial (R200), slow

quenching (supply of pristine gas onto galaxy disk halted),

then, above an ICM threshold, rapid quenching (quenched fraction increases faster) with cold gas affected by ram-pressure, the pressure exerted on a galaxy flying through ICM

Roberts et al (2019, ApJ, 873, 42) & Maier et al. (2019, A&A, 621, A131)

Cluster induced Slow-then-rapid star-formation quenching

Lilly et al. (2013):

Bathtub model

One massive cluster XMM2215 at z~1.5, M200~6*1014Msun, with KMOS spectro-

scopy of Hα and [NII] covering about one virial radius R200 (Maier et al. 2019b)

Higher Redshift z~1.5: Do we see slow quenching (strangulation) of cluster galaxies as traced by higher

metallicities ?

HST H-band image and spectroscopic cluster members XMM2215 R<0.5R200

Phase-space diagram for cluster XMM2215 at z~1.5

Maier et al. (2019b)

- Phase-space diagram (left panel) characterizes accretion state of cluster galaxies relatively free from effects due to the 2D projected positions with respect to the cluster center (right panel)

- Dashed large triangle: virialized region derived by Rhee et al (2017) using cosmological hydrodynamic simulations of clusters

Cluster-centric radius vs. line-of-sight velocity relative to cluster redshift

XMM2215 z~1.5 galaxies inside half R200 have higher metallicities by ~0.1dex (and higher [NII]/Hα ratios by ~0.2dex) than KMOS3D field galaxies at z~1.5, and slightly lower SFRs than field galaxies, but are still star-forming → slow quenching (strangulation)

Mass-metallicity relation (MZR) & enhanced [NII]/Hα ratios

Maier et al. (2019b)

Cluster-centric radius vs. line-of-sight velocity relative to cluster redshift

Lower Redshift: Phase-space diagram of 7 z~0.2 clusters

Star-forming galaxies, non star-forming, with gas-phase metallicities O/H measured

Mlow: 10≤log(M/Msun)<10.5, Mhigh: 10.5≤log(M/Msun)≤11

SF galaxies at R<R200: higher line-of-sight velocities: infalling population

- fSF and MZR in three (density) regions: R<R200, R200<R<2R200, 2R200<R<3R200.

Maier et al. (2019a)

Median O/H for log(M/Msun)<10.5 galaxies at R<R200 is higher than field and R>R200 median values; also higher Mlow ΔO/H values at R<R200 cf. field

The enhanced metallicities of cluster galaxies inside R200

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Maier et al. (2019a)

Assumption: quenching is initiated when galaxies pass within R200 for the

first time (no dillution of ISM because supply of gas Φ halted

resulting in enhanced metallicities at R<R200):

Zeq=SFR/Φ*y

instantaneous complete quench at R200 or after a time delay

Quenching timescales from comparison with observed fSF

Wetzel et al. (2013):Delayed-then-rapid quenching

Lilly et al. (2013):Bathtub model

Zeq=SFR/Φ*y

pristine gas inflow Φ, y: yield

Observed fraction of star-forming galaxies fSF vs. predictions from the Millennium simulation (curves) assuming quenching is initiated when galaxies pass within R200 for the first time:

instantaneous complete quenchat R200 or after time delay (fSF higher)

Quenching timescales from comparison with observed fSF

Maier et al. (2019a):

Slow-then-rapid quenching

Quenching timescales of 1-2Gyr deduced, shorter than gas consumption timescales of 2-3Gyr

→rapid quenching of star-formation after 1-2Gyr when galaxies travel to inner regions of the cluster, before slow-quenching consumes all gas in 2-3Gyr

→ candidate: ram-pressure stripping

Gas removal: ram pressure Pram > restoring pressure (Gunn & Gott 1972)

Pram at R200 too low to strip cold gas, but sufficient to strip hot gas: slow quenching

Ram-pressure stripping --> slow-then-rapid quenching

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Bahe et al. (2013), hydrodynamic simulations

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Gas removal: ram pressure Pram > restoring pressure (Gunn & Gott 1972)Pram at R200 too low to strip cold gas, but sufficient to strip hot gas: slow quenching

Pram at R<<R200 higher, enough to strip also cold gas: rapid quenching

Ram-pressure stripping --> slow-then-rapid quenching

Removal of hot halo gas at R200, then at R<<R200 (at higher density of ICM) of cold gas by ram-pressure stripping produces a slow-then-rapid quenching

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Bahe et al. (2013), hydrodynamic simulations

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- Inflow of gas stops when cluster galaxies pass R200, likely due to the removal of the hot halo by ram pressure stripping effects: metallicities increase because no dillution of ISM with pristine gas inflow --> slow quenching (strangulation)- During slow quenching phase of 1-2Gyr galaxies are still star-forming- After 1-2Gyr galaxies have travelled to inner regions of the cluster where ram pressure > gravitational restoring pressure of cold gas → rapid quenching by removal of cold gas

Summary: Slow-then-rapid quenching scenario

Maier et al. (2019a) for z~0.2 clusters & Roberts et al. (2019) for local clusters

Maier, Ziegler, Haines & Smith, 2019a, A&A, 621, A131- Enhanced metallicities of z~0.2 cluster galaxies inside R200 indicate slow quenching initiated by removing hot halo gas when galaxies pass R200

- Quenching timescales derived from comparison of observed fractions of SF galaxies as a function of clustercentric radius with simulations indicate a slow-then-rapid quenching scenario for cluster galaxies at z~0.2 with ram-pressure stripping a likely candidate for the main actor in this scenario

Maier, Hayashi, Ziegler & Kodama, 2019b, A&A, 626, A14 - KMOS H-band observations of [NII] and Hα for XMM2215 cluster galaxies at z~1.5 reveal higher metallicities of galaxies inside half R200: evidence for slow quenching (strangulation)

Conclusions