Lunch talk on May. 20th, 2019
Detection of galaxy clusters in X-ray wavelength
Speaker: Weiwei Xu (PKU/NAOC)
Venue: Room 2317, SWIFAR Building
Time: 12:30 PM, Monday, 20th May, 2019
Some indications for tension have long been identified between cosmological constraints obtained from galaxy clusters and primary cosmic microwave background (CMB) measurements. Typically, assuming the matter density and fluctuations, as parameterized with Omega_m and sigma_8, estimated from CMB measurements, many more clusters are expected than those actually observed. This has been reinforced recently by the Planck collaboration. One possible explanation could be that certain types of galaxy groups or clusters were missed in previous samples, resulting in a higher incompleteness than estimated. In this talk, I will present the project of detecting a hypothetical class of very extended, low-surface-brightness galaxy groups or clusters have been missed in previous X-ray cluster surveys based on the ROSAT All-Sky Survey (RASS). We applied a dedicated source-detection algorithm sensitive to more unusual group or cluster surface-brightness distributions. The cross-correlation of the candidates with optical/infrared surveys is also used for cluster identification and redshift estimation. We obtain ~1000 galaxy cluster candidates in almost all-sky region. Besides many known clusters, we also detect a number of new candidates, which are not included in any previous cluster catalogs detected in X-ray band or from Sunyaev-Zel’dovich effect. By the research about a pilot sample in detail, we find some of the new-detected galaxy groups are with low redshift, low mass and flat surface-brightness profile that lie above the nominal flux limits of previous RASS-based cluster catalogs. In this project, we demonstrate that some galaxy groups were indeed missed in previous RASS surveys, possibly due to the flat surface-brightness distributions of this potential new population.