The SRG/eROSITA all-sky survey: X-ray scaling relations of galaxy groups and clusters in the western Galactic hemisphere

TL;DR

This work leverages the largest ICM/IGrM-detected sample from the eROSITA eRASS1 survey (3061 clusters and groups across 0.05<z<1.07 and M500 from 1.1×10^13 to 1.6×10^15 M⊙) to constrain X-ray scaling relations among L_X, T, M_gas, and Y_X, explicitly modeling selection and redshift evolution. Using a forward, Bayesian framework with a detailed selection function derived from end-to-end simulations, the authors fit four relations (L_X–T, L_X–M_gas, L_X–Y_X, M_gas–T) with normalization A, slope B, evolution C, and intrinsic scatter σ_Y|X, reporting slopes generally steeper than self-similar and redshift evolution consistent with self-similar expectations. The results align with previous observational and simulation studies that include non-gravitational physics, and demonstrate the power of large, well-characterized samples to tightly constrain cluster physics and cosmology. The findings imply that non-gravitational processes significantly shape ICM/IGrM properties, while the evolution of these relations remains close to the self-similar prediction, underscoring the potential of upcoming deeper eROSITA data to extend these constraints to lower masses and higher redshifts, improving cosmological applications.

Abstract

The soft X-ray telescope on board the Spectrum-Roentgen-Gamma (SRG) mission, eROSITA (extended ROentgen Survey with an Imaging Telescope Array), has produced the largest sample to date of galaxy groups and clusters detected via their intracluster/intragroup medium (ICM/IGrM) emission. Scaling relations between the intrinsic properties of these systems provide valuable insight into their formation and evolution. In this work, we investigate the scaling relations between key physical properties, such as soft band X-ray luminosity, temperature, gas mass, and the low-scatter mass proxy $Y_{\rm X}$, for the galaxy groups and clusters detected in the first eROSITA All-Sky Survey (eRASS1). Our analysis fully accounts for selection effects and the redshift evolution of the observable distributions. We construct a high-purity sample of $3061$ galaxy groups and clusters spanning the redshift range $0.05<z<1.07$ and mass range of $1.1\times10^{13}<M_{500}/$M$_{\odot}<1.6\times10^{15}$. This represents the largest sample to date used for scaling relation analysis. The selection function, derived from state-of-the-art simulations of the eROSITA sky, is rigorously incorporated into our modeling. We report best-fit parameters - normalization, slope, redshift evolution, and intrinsic scatter - for a set of scaling relations: $L_{\mathrm{X}}-T$, $L_{\mathrm{X}}-M_{\rm gas}$, $L_{\mathrm{X}}-Y_{\rm X}$, as well as the $M_{\rm gas}-T$ relation. Our best-fit models indicate that the slopes of the scaling relations deviate significantly from self-similar expectations, while the redshift evolution remains consistent with the self-similar model. The fits exhibit small statistical uncertainties, likely owing to the large sample size. Our results are in good agreement with previous observational studies that account for selection effects, as well as with simulations that incorporate non-gravitational physics.

Figures (18)

• Figure 1: Redshift ( left panel) and mass distribution ( right panel) of the $3061$ eRASS1 clusters and groups used in this work. The median redshift and mass of the sample are shown by the vertical dashed black lines.
• Figure 2: Hexbin plot of the soft band X-ray luminosity ($0.5-2.0$ keV energy band) and redshift distribution of the selected $3061$ eRASS1 clusters. Solid black curves show the PDF of the hypothetical $L_{\rm X}-z$ distribution given by the selection function (see Section \ref{['subsect:sel_func']} for details).
• Figure 3: Soft band ($0.5-2.0$ keV energy band) X-ray luminosity--Temperature, $L_{\rm X}-T$, scaling relation of the selected galaxy group and cluster eRASS1 sample. Data points are color-coded according to their redshift. Arrows indicate temperature lower limits for objects in the sample with poorly constrained temperatures. The black line shows the best-fit scaling relation model. The black shaded area indicates the $1\sigma$ uncertainty of the mean of the log-normal model, and the dashed black lines indicate the best-fit standard deviation ($\sigma_{L_{\rm X}|T}$) around the mean.
• Figure 4: Comparison of the slope $B$ for the $L_{\rm X}-T$ scaling relation with literature results from Bahar2022, Migkas2020, Giles2016, Lovisari2015, Kettula2015, Eckmiller2011, and Pratt2009. All the shown errors are at the $1\sigma$ level. The vertical shaded areas represent our best-fit measurements at the $1-2\sigma$ confidence level. The vertical dashed black line display the self-similar expected value.
• Figure 5: Same as Fig. \ref{['fig:Lx_T_best_fit_model']}, but for soft band ($0.5-2.0$ keV energy band) X-ray luminosity--Gas mass, $L_{\rm X}-M_{\rm gas}$, scaling relation.