Unveiling the Sources of X-ray Luminosity in DESI Galaxy Groups: Insights from the SRG/eROSITA All-Sky Survey

TL;DR

This work analyzes the sources of X-ray luminosity in DESI galaxy groups by leveraging the SRG/eROSITA all-sky survey (eRASS1) and a rich multi-wavelength dataset. Using optical DESI group catalogs, spectroscopic information, and X-ray/radio counterparts, the authors stack X-ray images to separate contributions from central AGN (modeled via the PSF) and extended hot gas across halo masses $M_h$ and redshifts $z_g$. They find a clear hierarchy in total X-ray luminosity with group type, and show that central AGN dominate in low-mass halos while extended gas becomes more significant at $M_h \\gtrsim 10^{13.5} h^{-1} M_\odot$, though the coupling between AGN activity and extended gas remains inconclusive due to potential positional offsets and selection effects. The study demonstrates the power of combining all-sky X-ray data with large optical spectroscopic samples to quantify X-ray components in galaxy groups and to inform models of AGN feedback and group assembly.

Abstract

We use the first eROSITA all-sky survey (eRASS1) to investigate the contributions of AGN and extended gas to the total X-ray luminosity ($L_X$) of galaxy groups with different halo masses ($M_h$) at different redshifts. The presence of AGN in their central galaxies is identified using multi-wavelength catalogs, including the X-ray counterparts, the ASKAP radio catalog, and the DESI spectroscopic measurements. We apply the stacking method to obtain sufficient statistics for the X-ray surface brightness profile and the $L_X$ for groups with different central AGN properties. We find that the X-ray groups exhibit the highest $L_X$, followed by groups with QSO, radio, BPT-AGN, and non-AGN centrals. Moreover, the $L_X$ of the $M_h \lesssim 10^{13}h^{-1}M_\odot$ groups is dominated by the central AGN, while the X-ray emission from extended gas tends to be more prominent in the $M_h \gtrsim 10^{13}h^{-1}M_\odot$ groups. In groups where the AGN play a major role in X-ray emission, the contribution from extended gas is minor, resulting in significant uncertainties concerning the extended X-ray emission. When the subset containing the X-ray detected counterparts is excluded, the extended gas component becomes easier to obtain. A correlation has been identified between the X-ray luminosity of the central AGN and extended gas. However, once we account for the positional offset, their correlation becomes less prominent. Currently, the results are not conclusive enough to confirm whether there is a connection between the AGN feedback and extended gas. However, they provide a new perspective on the feedback processes in the history of group assembly.

Figures (14)

• Figure 1: The sky coverages depicted in the DESI (enclosed by bold teal lines), ASKAP Hale..2021, and eRASS1 (orange filled region), with the exposure times in the eRASS1 regions represented by the color bar. The regions enclosed by solid pink lines are the eRASS1 Counterpart Catalog Salvato..2025Kluge..2024. The thin grey contour lines represent the galactic hydrogen column density along the line of sight to each point, based on HEALPIX resampling of the Leiden/Argentine/Bonn Survey of Galactic HI Kalberla..2005. The filled neon contours display the spectroscopic completeness for galaxies in each region. The bold dashed grey line represents the region with $\delta < 30^{\circ}$, which corresponds to the area of data used in this study.
• Figure 2: The distributions of $z_g$ (left) and $M_h$ (right) for the spectroscopic (upper) and full (lower) samples, respectively, categorized by their central characteristics: X-ray groups (yellow dotted and shaded), radio centrals (red dashed), QSO centrals (green solid), BPT-AGN centrals (blue dashdot), and non-AGN centrals or others (grey solid and shaded). Note that the full sample does not include systems with QSO or BPT-AGN centrals.
• Figure 3: Venn diagrams illustrating the overlap among four AGN classifications: X-ray (yellow filled), Radio (red hatched), QSO (green hatched), and BPT-AGN (blue filled) for galaxy groups with BGGs having spectroscopic measurement. The left panel corresponds to groups with redshift $0 < z_g < 0.5$, while the right panel shows groups in the range $0.5 < z_g < 1.0$.
• Figure 4: Stacked rest-frame $0.5-2.0$ keV band images of DESI groups with different central properties at different $M_h$ and $z_g$ bins. The dashed circles represent the regions within a radius of $R_{180}$. Only the data bins with at least 100 groups are shown here.The bottom row of each column indicates the radial scale range corresponding to the respective $M_h$ and $z_g$ bins. The number of subsamples for each category within a given bin is shown in the top-right corner of each panel.
• Figure 5: Left two columns: Mean X-ray surface brightness profiles of $z_g < 0.5$ DESI groups with different central properties at different $M_h$ bins. The leftmost corresponds to different catagories, whereas the rightmost of the two left columns shows the subsamples after removing the X-ray detected ones. The solid lines represent the convolved PSF normalized by their individual count rates, while the dashed lines represent the convolved PSF normalized without weighting. Only the data bins with at least 100 groups are shown here. Right two columns: The same as the left two columns, but for the groups with $z_g > 0.5$.