The Extended Northern ROSAT Galaxy Cluster Survey (NORAS II). I. Survey Construction and First Results

Abstract

As the largest, clearly defined building blocks of our Universe, galaxy clusters are interesting astrophysical laboratories and important probes for cosmology. X-ray surveys for galaxy clusters provide one of the best ways to characterise the population of galaxy clusters. We provide a description of the construction of the NORAS II galaxy cluster survey based on X-ray data from the northern part of the ROSAT All-Sky Survey. NORAS II extends the NORAS survey down to a flux limit of 1.8 × 10 −12 erg s −1 cm −2 (0.1 -2.4 keV) increasing the sample size by about a factor of two. The NORAS II cluster survey now reaches the same quality and depth of its counterpart, the Southern REFLEX II survey, allowing us to combine the two complementary surveys. The paper provides information on the determination of the cluster X-ray parameters, the identification process of the X-ray sources, the statistics of the survey, and the construction of the survey selection function, which we provide in numerical format. Currently NORAS II contains 860 clusters with a median redshift of z = 0.102. We provide a number of statistical functions including the logN-logS and the X-ray luminosity function and compare these to the results from the complementary REFLEX II survey. Using the NORAS II sample to constrain the cosmological parameters, σ 8 and Ω m , yields results perfectly consistent with those of REFLEX II. Overall, the results show that the two hemisphere samples, NORAS II and REFLEX II, can be combined without problems to an all-sky sample, just excluding the Zone-of-Avoidance.

Observational evidence for a local underdensity in the Universe and its effect on the measurement of the Hubble constant

“…We already found good observational support for this concept with our galaxy cluster surveys (Böhringer et al , 2004(Böhringer et al , 2017a. We showed that the density fluctuation power spectrum of galaxy clusters is an amplified version of the power spectrum of galaxies and of the inferred power spectrum of the underlying dark matter distribution, where the bias is dependent on the lower cluster mass limit exactly as predicted from theory (Balaguera-Antolinez et al 2010.…”

“…The CLASSIX galaxy cluster survey and its extension is based on the X-ray detection of galaxy clusters in the ROSAT All-Sky Survey (RASS, Trümper 1993, Voges et al 1999. The source detection for the survey, the construction of the survey and the survey selection function as well as tests of the completeness of the survey are described in Böhringer et al (2013Böhringer et al ( , 2017a. In summary, the nominal unabsorbed flux limit for the galaxy cluster detection in the RASS is 1.8 × 10 −12 erg s −1 cm −2 in the 0.1 -2.4 keV energy band.…”

Constraints from thermal Sunyaev-Zel’dovich cluster counts and power spectrum combined with CMB

“…Recent studies, using the LSS probes, also seem to show a disagreement with the best cosmology of the CMB. This includes studies based on cluster samples (Vikhlinin et al 2009;Hasselfield et al 2013;Benson et al 2013;Böhringer & Chon 2016;Böhringer et al 2017;Schellenberger & Reiprich 2017), on the linear growth rate data (Moresco & Marulli 2017, and references therein), or on cosmic shear (Hildebrandt et al 2017;van Uitert et al 2018;Joudaki et al 2017). Despite intrinsic limitations to each of these probes (e.g.…”

The HectoMAP Cluster Survey. II. X-Ray Clusters

“…Large galaxy cluster catalogs are also based on Xray identification. The ROSAT All-Sky Survey (RASS) data have been especially important (Ebeling et al 1998;Böhringer et al 2000Böhringer et al , 2004Ebeling et al 2010;Böhringer et al 2013Böhringer et al , 2017. The Sunyaev-Zel'dovich (SZ) effect (Sunyaev & Zeldovich 1972) is a more recent but equally powerful tool for identifying clusters (Vanderlinde et al 2010;Marriage et al 2011;Planck Collaboration et al 2015Bleem et al 2015).…”