Star-formation quenching of cluster galaxies as traced by metallicity and presence of active galactic nuclei, and galactic conformity
Authors: Maier C. , Haines C. P. , Ziegler B. L. 
Published: November 2021

Aims. We strive to explore the differences in the properties and quenching processes of satellite galaxies in a sample of massive clusters with passive and star-forming (SF) brightest cluster galaxies (BCGs). One aim is to investigate galactic conformity effects, manifested in a correlation between the fraction of satellite galaxies that halted star formation and the state of star formation in the central galaxy.

Methods. We explored 18 clusters from the Local Cluster Substructure Survey at 0.15 < z < 0.26, using spectra from the Arizona Cluster Redshift Survey Hectospec survey of about 1800 cluster members at R < R200 in a mass-complete sample. Nine clusters have a SF BCG and nine have a passive BCG, which enable the exploration of galactic conformity effects. We measured the fluxes of emission lines of cluster members, allowing us to derive O/H gas metallicities and to identify active galactic nuclei (AGN). We compared our cluster galaxy sample with a control field sample of about 1300 galaxies with similar masses and at similar redshifts observed with Hectospec as part of the same survey. We used the location of SF galaxies, recently quenched galaxies (RQGs) and AGN in the projected velocity versus the position phase-space (phase-space diagram) to identify objects in the inner regions of the clusters and to compare their fractions in clusters with SF and passive BCGs.

Results. The metallicities of log(M/M⊙)≥10 SF cluster galaxies with R < R200 were found to be enhanced with respect to the mass-metallicity relation obtained for our sample of coeval field SF galaxies. This metallicity enhancement among SF cluster galaxies is limited to lower-mass satellites (10 < log(M/M⊙) < 10.7) of the nine clusters with a passive BCG, with no metallicity enhancement seen for SF galaxies in clusters with active BCGs. Many of the SF galaxies with enhanced metallicities are found in the core regions of the phase-space diagram expected for virialized populations. We find a higher fraction of log(M/M⊙)≥10.7 SF galaxies at R < R500 in clusters with active BCGs as compared to clusters with passive BCGs, which stands as a signal of galactic conformity. In contrast, much higher fractions at R < R500 of AGN and, particularly of RQGs, are found in clusters with passive BCGs in comparison to clusters with active BCGs.

Conclusions. We deduce that strangulation is initiated in clusters with passive BCGs when SF satellite galaxies pass R200, by stopping the pristine gas inflow that would otherwise dilute the interstellar medium and would keep their metallicities at the level of values similar to those of field galaxies at similar redshifts. These satellite galaxies continue to form stars by consuming the available gas in the disk. For galaxies with massses above log(M/M⊙)∼10.7 that manage to survive and remain SF when traveling to R < R500 of clusters with passive BCGs, we assume that they suffer a rapid quenching of star formation, likely due to AGN triggered by the increasing ram pressure stripping toward the cluster center, which can compress the gas and fuel AGN. These AGN can rapidly quench and maintain quenched satellite galaxies. On the other hand, we found that surviving SF massive satellite galaxies around active BCGs are less affected by environment when they enter R < R500, since we observe R < R500 SF galaxies with masses up to M ∼ 1011 M⊙ and with metallicities typical of coeval field galaxies. This observed galactic conformity implies that active BCGs must maintain their activity over timescales of at least ∼1 Gyr.