First-hand information for everyone
A team using the South Pole Telescope’s SPT-3G camera reports a new catalog of galaxy clusters found through the thermal Sunyaev–Zel’dovich (SZ) effect. The catalog contains 8,892 candidate objects detected above a significance threshold (ξ=4). Using optical and infrared data the team confirmed 7,190 of those candidates as galaxy clusters. For the higher-significance subset (ξ≥5) there are 4,480 candidates with an expected purity above 99 percent; the full ξ≥4 sample has an expected purity above 82 percent.
The observations come from five years of arcminute-resolution maps at three millimeter-wave bands. The maps reach white-noise depths of about 3.2, 2.5, and 8.9 microkelvin-arcmin at 95, 150, and 220 GHz, respectively. The thermal SZ effect is the small change in the cosmic microwave background caused when hot gas in a galaxy cluster scatters CMB photons. Because the SZ signal does not fade with distance the method is useful for finding clusters across a wide range of redshifts.
The confirmed sample spans a broad range of masses and distances. Reported cluster masses (M500c) go from roughly 7.9×10^13 to 1.6×10^15 solar masses (scaled to h70), with a median mass of 1.65×10^14 solar masses. The redshift range is 0.037 ≲ z ≲ 2, with a median redshift zmed = 0.73. In this sample 1,780 clusters lie at z>1 and 271 at z>1.5. The SPT-3G sample is deeper than previous millimeter-wave SZ samples, with per-cluster detection signal-to-noise about 2–4 times higher and a confirmed cluster density of about 4.5 clusters per square degree.
The team cross-matched the SPT-3G catalog with eRASS1, the first eROSITA All-Sky Survey release, finding 1,279 matches to eRASS1 clusters and 1,319 matches to eRASS1 point sources. The SPT and eROSITA mass estimates are reported to be in relatively good agreement. The many matches to eROSITA point sources could help studies of X-ray active galactic nuclei inside clusters and of how eROSITA selects high-redshift clusters. The catalog also flags a number of clusters as candidate strong gravitational lenses of distant optical or dusty star-forming galaxies.
The authors ran internal validation tests and compared the sample to external catalogs. These checks reveal a clear increase in correlated dusty emission with redshift. In particular, the 220 GHz temperature increment associated with clusters is about 17 times larger at z≈1.5 than at z≈0.25. They find only weak evidence for correlated synchrotron (radio) emission. The paper notes that previous analyses indicate the SPT-3G cluster selection is robust at this level of dust contamination, but the changing contamination with redshift is an uncertainty to account for.
This catalog is important because it extends SZ-selected cluster samples to higher depth and higher redshift than many previous surveys. That makes it a rich resource for studying cluster astrophysics, the population of high-redshift clusters, and rare objects such as strong lenses. The authors emphasize that further joint analyses and detailed work on the astrophysical and cosmological implications of the full SPT-3G sample will be the subject of upcoming papers.