Observational strategy

The X-GAP project is the brain child of a series of review papers on the physical properties of galaxy groups (see here). The writing of these review papers lead us to realize that our knowledge of the galaxy group population is currently hampered by two main problems:

• Existing group samples are most likely not representative of the group population as a whole. Indeed, samples based on compact source detection in X-rays are highly biased toward centrally peaked objects. On the other hand, optical group catalogues with few members are prone to projection effects and unvirialized structures.
• Previous programs targeting galaxy groups in X-rays (e.g., CLoGS) have focused on the properties of the brightest, most nearby groups. The spatial extent of these systems largely exceeds the field of view of current X-ray telescopes, such that the properties of the IGrM beyond the cores are still largely unknown.

Our sample selection method is designed to overcome the difficulties highlighted above. Our selection based on extended X-ray emission gets rid of the bias toward highly peaked systems, whereas the cross-correlation between optical groups and X-ray emission greatly reduces the impact of projection effects. On top of that, our criterion on the spatial extent of the groups ensures that R₅₀₀ will be contained within the XMM-Newton field of view.

We searched the XMM-Newton archive for systems matching our selection criteria and performed a preliminary analysis of 4 systems that were already observed by XMM-Newton. In the image below we show an RGB image of the A1213 system (z=0.047, M₅₀₀=7e13 M_⊙), with R=SDSS r-band, G=SDSS g-band, and B=XMM-Newton 0.5-2 keV map. We can see that the X-ray emission is very diffuse, with a low central surface brightness and a bimodality associated with a massive galaxy in the North. This analysis demonstrates that our selection method is able to detect systems without a bright central X-ray peak.

We analyzed the existing data for these four systems and extracted surface-brightness and temperature profiles, which we then deprojected to extracted 3D profiles of gas density, temperature, entropy, and pressure. We also fitted with a Navarro-Frenk-White model to the data under the assumption that the gas is in hydrostatic equilibrium and we estimated the total gravitating mass profile of the groups.

We can see that the selected groups appear to have low densities in their central regions in comparison with massive clusters (indicated by the X-COP sample average, see Ghirardini et al. 2019). The selected systems also exhibit flat entropy profiles with a large entropy excess all the way out to R₅₀₀. We then looked at the hydrostatic mass and gas fraction profiles of these systems, and found that they all show a very low gas fraction in their inner regions, quickly rising outwards. We can also see that our observational strategy allows us to measure the gas fraction at R₅₀₀ without requiring any extrapolation.

The X-GAP team will provide similar analyzes for the full sample. We are committed to providing high-level data products for all the systems to the community in an easy-to-use and intuitive format.