The quenching of galaxies in clusters from z=1.5 to z=0.1 as traced by their gas-phase metallicities

Bianca-iulia Ciocan 1 , Christian Maier 1 , Bodo Ziegler 1

  • 1 University Of Vienna, Department Of Astrophysics, Vienna, Austria

Abstract

Gas-phase metallicities offer insight into the chemical evolution of galaxies, as they reflect the recycling of gas through star formation, galactic inflows and outflows. Environmental effects such as star-formation quenching mechanisms play an important role in shaping the evolution of galaxies, and intermediate redshift clusters of galaxies are expected to be the sites where such environmental processes can be clearly observed with present-day telescopes.
We used gas-phase metallicities for galaxies in clusters from z=1.5 to z=0.1 as a tracer for star formation quenching. We interpret the observed enhanced gas-phase abundances of cluster galaxies to be caused by the suppressed gas inflow, which would otherwise dilute the ISM with primordial elements, and we found indications for a slow-then-rapid quenching scenario.
Our sample comprises Hectospec/MMT spectroscopy for seven LoCuSS clusters at 0.15 < z < 0.26 and VLT spectroscopy for two CLASH Frontier Fields clusters, RX J2248-443 at z = 0.348 and MACS J0416.1-2403 at z = 0.3972 (Maier et al. 2019a, Ciocan et al. 2020, Maier et al. 2016). We also explore a high-z cluster, XMMXCSJ2215.9-1738, at z~1.5 based on KMOS spectroscopy (Maier et al. 2019b).
The fluxes of strong emission lines ([OII] λ3727, Hβ, [OIII] λ5007, Hα and [NII] λ6584) enabled the derivation of (O/H) gas metallicities, star formation rates, and contamination from active galactic nuclei. We compared our samples of cluster galaxies to a population of field galaxies at similar redshifts. We used the location of galaxies in projected phase-space to distinguish between cluster and field galaxies. Both populations follow the star-forming sequence in the diagnostic diagrams, which allow the ionising sources in a galaxy to be disentangled, with only a low number of galaxies classified as Seyfert II. Both field and cluster galaxies follow the "main sequence" in the mass-specific SFR relation, with no substantial difference observed between the two populations. The gas-phase metallicities of cluster galaxies with intermediate and low masses are shifted to higher values than those of their counterparts residing in lower density environments i.e. at large cluster-centric radii or in the field. Comparisons to the expected (O/H)s from the Fundamental Metallicity relation by Lilly et al. (2013) show that the cluster members located at small cluster-centric radii deviate strongly from the model predictions, while galaxies residing at larger cluster-centric radii or in the field are in accordance with the model predictions. We interpret these elevated (O/H)s of intermediate and low mass cluster galaxies as the result of ram pressure stripping of the gas in the galaxies’ halos and shutting of the inflow of primordial gas along cosmic filaments by the cluster.
Comparing our observational results with simulations of Bahé and Schaye et al., we concluded that ram-pressure stripping can produce a slow-then-rapid quenching phase. Slow quenching starts when the galaxy passes R200, as the ram pressure of the ICM exceeds the restoring pressure of the hot diffuse gas confined in the galaxies halo, leading to its removal. During this phase, cluster galaxies are still star-forming, as their cold gas disk is left unperturbed, but they show elevated metallicities tracing the ongoing quenching. This phase lasts for 1-2 Gyr, and during this time, the galaxies travel to denser inner regions of the cluster. The “rapid" quenching phase is then initiated, which leads to a complete quenching of star formation owing to the increasing ram pressure towards the cluster centre that can also strip the cold gas disk.

CLASH-VLT: Strangulation in z~0.35 RXJ 2248-4431 cluster galaxies

Galaxies fly towards the center of the cluster:

  • after passing Rvirial (R200) ➡ slow quenching (supply of pristine gas onto galaxy disk halted)
  • above an ICM threshold ➡ rapid quenching (cold gas affected by ram-pressure)

Data:

  • CLASH-VLT VIMOS spectra &  WFI photometry (UBVRIz) for  field (0.3<z<0.4) + RXJ2248-4431 cluster (0.33<z<0.36) 
  • For metallicity study: matched local comparison sub-sample of SDSS emission line galaxies (0.04 < z < 0.08)
Results:

Phase space analysis: clustercentric radius vs. line-of-sight velocity for the sample of CLASH galaxies with 0.3 < z < 0.4.

  • Based on the phase-space analysis (Carlberg et al. 1997), we identified the cluster and field galaxies, as well as the cluster specific parameters: R200 = 2.57 ± 0.05 Mpc; M200 = 1.77 ± 0.03 × 1015M⊙ 
  • based on the BPT (Baldwin et al. 1981) diagnostic diagram, we exclude all AGNs from the sample
  • The SFRs are derived from extinction-corrected Hα luminosities

sSFR–M relation for the R2248 cluster members (filled circles) and for the comparison sample field galaxies (open circles) with available measurements of the Hα emission line.  The red solid line stands for the the main sequence of SF galaxies for a redshift of z ∼ 0.4, as derived by Peng et al. (2010). In this study, the “mass complete” sample is represented by the intermediate bin (9.2 < log(M/M⊙) < 10.2). In this mass bin, the median sSFR values of both cluster and field galaxies are comparable given the errors. 

  • Gas phase abundances were derived employing the O3N2 calibration of Kewley et al. 2013

Mass–metallicity relation (MZR) for R2248 cluster members (filled circles) and the comparison sample of field galaxies (open circles) using the O3N2 metallicity calibration of Kewley et al. (2013).  In the mass complete bin (9.2 < log(M/M⊙) < 10.2), the median (O/H) for the cluster galaxies is higher than the median (O/H) for the field galaxies by 0.065 dex, with a 1.8σ significance. 

  • The observed Z(M,SFR) was compared to the theoretical predictions from Lilly et al. 2013 

Difference between the measured (O/H)s for cluster galaxies (filled circles) and the comparison sample of field galaxies (open circles) and the expected (O/H)s from the formulations of Lilly et al. (2013) for a primordial infall metallicity Z0 relative to the yield y, Z0/y = 0. The (O/H)s of the cluster galaxies (big filled circle) deviate more strongly from the FMR model predictions (by ∼0.12 dex) than the ones for the field galaxies (big open circle), which are in quite good agreement with the model predictions. 

(O/H) ~ SFR/inflow * yield

➡ If inflow decreases or stops by almost constant SFR ➡ O/H increases

Conclusion ➡ Strangulation

Ciocan et al. 2020, A&A, 633, A139

CLASH-VLT: Strangulation in z = 0.39 MACSJ0416.1-2403 cluster galaxies

Data: 

  • CLASH-VLT VIMOS spectra & Subaru photometry (BRz) for  field+MASJ 0416 cluster

Phase-space diagram based on all MAC0416 spectroscopic members (black open circles); cluster members  from the "mass complete" bin (9.2 < log(M/M⊙) < 10.2)  with measured metallicities are indicated by large (blue and red) symbols. High metallicity objects are depicted as red symbols and galaxies with lower metallicities as blue symbols. The dashed vertical line indicates R200 ∼ 1.8 Mpc of the cluster; the dashed magenta line indicates the caustic determined by Balestra et al. 2016. The accreted objects (within the caustic) have predominantly higher metallicities than the infalling systems and the field counterparts (beyond the caustic). 

Conclusion:   

 ➡Strangulation

Maier et al. 2016, A&A, 590, A108

Strangulation of z ∼ 1.5 XMMXCS J2215.9-173 cluster galaxies

Data:

  •  XMMXCS J2215.9-1738 cluster and field galaxies at z ∼ 1.5 with KMOS spectroscopy 

MZR for cluster and field galaxies at z ∼ 1.5, using the N2 method of Pettini & Pagel (2004) to derive oxygen abundances. The SDSS local MZR relation is shown by the red solid line. Red triangles are XMM2215 cluster galaxies from the SF virialized sample, blue star symbols are XMM2215 infalling galaxies, and blue filled circles depict KMOS3D field galaxies at 1.42 < z < 1.52 from Wuyts et al. (2016), while the blue filled squares show the stacked values for the KMOS3D field sample. The object shown as a filled red circle is the active BCG of XMM2215.  We observe an enhancement of the O/Hs of about 0.1 dex for XMM2215 cluster galaxies (red dotted curve) compared to infalling and field galaxies (blue dotted curve).

Conclusion:

➡ Strangulation

Maier et al. 2019, A&A, 626, A14

LoCuSS: Strangulation in z~0.2 cluster galaxies

Data:

  • LoCuSS cluster and field galaxies with 0.15 < z < 0.26 from the mass complete sample of the ACReS Hectospec survey 

MZR and dependence on environment cluster and  galaxies. Magenta curves and symbols depict the MZR of the matched SDSS sample. The median O/Hs in the mass-complete  bins are shown as big filled circles; the field median values (depicted as cyan symbols) are shown in all panels. The median O/H value of low M galaxies in the more central part of the clusters (R < R200 ) is higher than the median O/H field value, by about 0.06 dex, with a 2.4σ significance.

Conclusions:

➡ Strangulation

➡ quenching timescales  recovered by comparison to Millennium cosmological simulation: ∆t = 1−2 Gyr  

Maier et al.  2019, A&A, 621, A131

Conclusions

log(M/M☉)>9.2 cluster galaxies show more  enhanced metallicities compared to their counterparts from lower density environemts ➡ strangulation.

➡Strangulation is a mechanism that causes the removal of the diffuse hot gas reservoir confined in the galaxy halo, initiating the slow quenching phase.

➡The gas disc is left unperturbed and while the galaxies continue to form stars using their cold gas reservoir, they enhance their metallicities, as no fresh pristine gas accretion dilutes their ISM. 

➡Meanwhile, as galaxies travel to denser inner regions of the cluster, their star formation is likely completely quenched by ram-pressure stripping of their cold gas disc in the rapid quenching phase.