NIKA2 observations of 3 low-mass galaxy clusters at $z\sim 1$: pressure profile and $Y_{\rm SZ}-M$ relation

Galaxy clusters are important astrophysical objects that provide insights into the formation and evolution of cosmic structures. Understanding the properties of their intracluster medium (ICM) is crucial for both cosmology and astrophysics. The thermal pressure profile of the ICM provides valuable information about the matter distribution within galaxy clusters. It reflects how the gas is compressed within the cluster’s gravitational potential well. Additionally, the $Y_{\rm SZ}-M$ scaling relation is an important tool for measuring cluster masses, as it relates to the Sunyaev-Zel’dovich (Sunyaev1972, SZ, see) flux ($Y_{\rm SZ}$, which directly measures the thermal energy of the ICM) to the total cluster mass, $M$ (see Mroczkowski2019, for a review).

While the thermal pressure profile and the $Y_{\rm SZ}-M$ scaling relation have been extensively studied up to intermediate redshifts (Arnaud2010; PlanckV2013, e.g.,), there is a lack of detailed observations in the low-mass, high-redshift regime. In this proceeding, we bridge this observational gap by presenting detailed SZ observations of three low-mass ($M_{500}\sim 2\times 10^{14}$ M_⊙) galaxy clusters at $z\sim 1$ selected from the XXL X-ray survey. These clusters, namely XLSSC 072, XLSSC 100, and XLSSC 102 were imaged using the NIKA2 millimeter camera on the IRAM 30-meter telescope at 150 GHz and 260 GHz (Adam2018a).

By combining the SZ observations with X-ray and optical data, we investigate the dynamic state of the clusters and derive their thermal pressure profiles. We also extract the SZ fluxes and compare them with expectations based on the standard evolution of the ICM properties calibrated on nearby massive systems. This analysis allows us to explore any deviations from the expected evolution in the ICM properties for low-mass clusters at high redshifts, where astrophysical processes are expected to have a more significant impact (Fakhouri2010; Pop2022).

This proceeding is organized as follows: Section 2 presents the cluster selection and the observations, Section 3 discusses the data analysis used to address the cluster dynamical state, extract the pressure profile and the $Y_{\rm SZ}-M$ relation, in Section 4, we summarize our main findings and conclusions. See adam2023xxl for the full results. See also (Ricci2020) for the detailed analysis of the first cluster XLSSC 102.

We selected our target clusters from the XXL survey (Pierre2016). The XXL survey’s selection function enabled the identification of clusters at low mass and high redshift (Pacaud2016). We specifically focused on the securely detected XXL clusters (C1) from the northern region (XXL-N), observable from the IRAM 30m telescope. Optical detections using galaxy overdensity were used for independent confirmation, with robust spectroscopic redshift estimates (Adami2018). We selected three clusters, XLSSC 072, XLSSC 100, and XLSSC 102, at a redshift $z\sim 1$ and $M_{500}\sim 1-2\times 10^{14}$ M_⊙, for NIKA2 observations.

The observations were conducted between January 2018 and February 2020. XLSSC 072, XLSSC 100, and XLSSC 102 were observed for 10, 10, and 6.6 hours, respectively. The data reduction followed the procedure described in Adam2015 and Adam2016. The data analysis included estimating the astrophysical signal filtering induced by the data reduction and deriving the noise statistical properties through power spectrum analysis and Monte Carlo simulations. Overall, the observational details and data reduction process for all three clusters were consistent with the characteristics of the NIKA2 instrument (Perotto2020).

After smoothing the NIKA2 data to an effective resolution of 27 arcsec, XLSSC 072, XLSSC 100, and XLSSC 102, are detected with a peak signal-to-noise ratio (S/N) of -9.7, -9.2, and -6.9, respectively. Radio and submillimeter point sources were removed from the SZ images prior to further analysis.

Our study aimed to investigate the SZ structure and properties of the ICM in three low-mass galaxy clusters at a redshift $z\sim 1$. By utilizing the NIKA2 camera, we obtained valuable resolved SZ data for these clusters, representing some of the first detailed observations of such low-mass systems at high redshifts.

Our analysis revealed evidence of ongoing merging activity in all three clusters, as indicated by their disturbed morphologies and deviations from compact spherically symmetric distributions. The pressure profiles of these clusters, rescaled according to standard evolution in mass and redshift, were found to be in agreement with those of local dynamically disturbed systems. Despite their perturbed ICM, low masses, and high redshifts, we did not observe any significant deviations in the $Y_{\rm SZ}-M$ scaling relation for our target clusters. While the sample size is not sufficient to draw statistical conclusions, these findings provide an initial indication that the scaling relations are stable down to masses of $M_{500}\sim 2\times 10^{14}$ M_⊙ and redshifts of $z\sim 1$. This may indicate that the cluster formation physics is already in place down to these masses and up to these redshifts

It is important to note that the comparison of the pressure profiles and scaling relation to those of local samples is limited by uncertainties in the mass estimates. This highlights the challenges in accurately determining and robustly estimating the mass in this relatively unexplored regime.