SALT spectroscopic observations of galaxy clusters detected by ACT and a Type II quasar hosted by a brightest cluster galaxy

TL;DR

This study uses SALT spectroscopic follow-up to analyze galaxy clusters detected by ACT via the SZ effect, revealing their masses, redshifts, and a rare Type II quasar in a brightest cluster galaxy, indicating active quasar feedback.

Contribution

First spectroscopic confirmation of ACT-detected clusters in the southern hemisphere and identification of a rare Type II quasar in a massive cluster.

Findings

Clusters have masses of (5--20) x 10^14 solar masses.

The cluster ACT-CL J0320.4+0032 hosts a Type II quasar.

Clusters span redshifts 0.3 to 0.55.

Abstract

We present Southern African Large Telescope (SALT) follow-up observations of seven massive clusters detected by the Atacama Cosmology Telescope (ACT) on the celestial equator using the Sunyaev-Zel'dovich (SZ) effect. We conducted multi-object spectroscopic observations with the Robert Stobie Spectrograph in order to measure galaxy redshifts in each cluster field, determine the cluster line-of-sight velocity dispersions, and infer the cluster dynamical masses. We find that the clusters, which span the redshift range 0.3 < z < 0.55, range in mass from (5 -- 20) x 10$^{14}$ solar masses (M200c). Their masses, given their SZ signals, are similar to those of southern hemisphere ACT clusters previously observed using Gemini and the VLT. We note that the brightest cluster galaxy in one of the systems studied, ACT-CL J0320.4+0032 at z = 0.38, hosts a Type II quasar. Only a handful of such systems are currently known, and therefore ACT-CL J0320.4+0032 may be a rare example of a very massive halo in which quasar-mode feedback is actively taking place.