Identification of low redshift groups and clusters of galaxies in the X-CLASS survey and the X-ray luminosity-temperature relation

Abstract

Properties of the hot intracluster and intragroup medium are mostly set by the underlying gravitational potential well, although complex astrophysical processes at play during their buildup may leave a significant imprint. Observational constraints on the degree and scales of such non-gravitational processes require well-selected samples of objects and deep observations of their gas content. We aim to study the scaling relation between two global properties of the hot gas, namely its soft-band X-ray luminosity ($L_X$) and its temperature ($T$), by studying a sample of low-mass systems associated with precise redshifts, simultaneously accounting for sample selection biases and associated measurement uncertainties. This work takes as input a large catalogue of X-ray-selected galaxy clusters (X-CLASS). We perform a thorough revision of the redshifts of sources using deep photometric data from the Legacy Surveys and our own tailored spectroscopic follow-up of 52 low-redshift systems. We devise a spectroscopically complete sample of 155 low-redshift ($0.07<z<0.2$) systems, and we measure properties of their X-ray emitting gas, with median $\overline{T}=1.7$ keV and median $\overline{L_X}=10^{43}$ erg s$^{-1}$. We infer the relation between $L_X$ and $T$ in a Bayesian framework. Our sample of groups and clusters with median total mass $\sim 6 \times 10^{13}M_\odot$ reveals a relation $L_X-T$ steeper than predicted by the self-similar model, with a slope $B=3.2 \pm 0.1$. This result fits well within recent studies that together indicate a trend of increasing slope with decreasing median halo mass. This work supports a scenario of a stronger decrease in luminosity with decreasing mass in the group regime than for massive galaxy clusters. This effect is possibly due to strong and sustained feedback expelling gas efficiently from their relatively shallower potential wells.

Figures (9)

• Figure 1: Performances of the photometric redshift estimates. Top: photometric redshifts vs. spectroscopic redshifts for confirmed X-CLASS clusters. Bottom: Bias (red) and NMAD scatter (blue) of the photometric redshift estimates as a function of spectroscopic redshifts for confirmed X-CLASS clusters. From left to right: (i) bias-corrected photXclus redshift, (ii) redMaPPer redshift, and (iii) best photometric redshift estimate. The quantities $\sigma_{\rm NMAD}$, bias (defined as the median of $\Delta z = z_{\rm phot} - z_{\rm spec}$), and the outlier fraction are defined in the main text.
• Figure 2: Spectra of two galaxies (G2 and G3) in xclass1365 as observed with MISTRAL. The redshift measurements are based on the main absorption lines detected and identified using the template spectrum of an early-type galaxy (in orange, redshifted to $z=0.1554$ for G2 and $z=0.1593$ for G3 as measured with the cross-correlation tool fxcor). The intensities are in arbitrary units, and neither spectrum is flux calibrated. The shaded areas correspond to atmospheric absorption lines except around 5700 Å, where there is a defect in the MISTRAL optics. Both spectra have been graded with a confidence level of 3 for the redshift measurement.
• Figure 3: Three-color image of xclass1365 extracted from the LS DR9. The yellow circles represent galaxies observed with MISTRAL, along with their redshifts. The blue circle indicates the source was also measured in the SDSS (with concordant redshift). North is up and east is left, and the field of view is 1.87 wide. The white contours correspond to the X-ray contours smoothed and filtered with a wavelet algorithm.
• Figure 4: Left: Redshift distribution of the X-CLASS-LR sample composed of 155 groups and clusters of galaxies. The dashed line indicates the median redshift. Right: Estimated mass distribution of the X-CLASS-LR sample. The dashed lines indicate the median redshift and the median mass (logarithmic scale).
• Figure 5: X-ray luminosity ($L_X$) versus redshift for the X-CLASS-LR sample (red circles). X-CLASS-LR sources with measurement uncertainties greater than 10% are plotted in purple. Red dashed lines represent the redshift limits of the sample. Additional samples with measurement uncertainties below 10% are shown: the MCXC-II survey in black 2024Sadibekova, the Cosmology Sample of the eROSITA DR1 in cyan 2024Bulbul, the XXL DR2 survey in green 2018bAdami, and the eROSITA Final Equatorial-Depth Survey (eFEDS) sample in blue 2022Liu.