Periodic Reporting for period 1 - CLEVeR (The CLuster and group Environment as Viewed by eROSITA)
Reporting period: 2022-06-01 to 2024-11-30
Galaxy groups play a crucial role in modern cosmology by serving as key tests for the predictions of large-scale structure and galaxy formation and evolution models. Their hot gas content, which determines their X-ray properties, is significantly influenced by the feedback from the supermassive black hole hosted by the central galaxy—a fundamental aspect of contemporary cosmological models. Unlike galaxy clusters, the predictions for the baryonic content of galaxy groups vary greatly depending on the implementation of feedback mechanisms. Thus, galaxy groups are an ideal laboratory for providing critical constraints that anchor these simulations.
Currently, there are no comprehensive observational constraints to guide theoretical models and differentiate between various scenarios. CLEVeR (The CLuster and group Environment and Viewed by eROSITA) aims to fill this critical gap in our understanding by characterizing the baryonic content of galaxy groups, from their hot gas on megaparsec scales to the properties of their galaxy populations on kiloparsec scales.
The primary limitation in this area has been the lack of X-ray observations of galaxy groups, largely due to the low sensitivities of previous surveys. CLEVeR aims to overcome this by leveraging the unprecedented statistics and sensitivity of the eROSITA All Sky Survey, which is particularly effective at detecting galaxy groups. Additionally, we will complement eROSITA data with upcoming 4MOST spectroscopic surveys of galaxy groups in the optical range, as well as with ancillary datasets that capture specific physical processes related to feedback and the central galaxy, such as MaNGA, MUSE, and LOFAR.
With this comprehensive dataset, we will be able to study and constrain all relevant components on all relevant scales, enabling us to refine our understanding of the physics of galaxy groups, their interactions with black hole feedback, and, consequently, the large-scale structure simulations of the Universe.
Currently, there are no comprehensive observational constraints to guide theoretical models and differentiate between various scenarios. CLEVeR (The CLuster and group Environment and Viewed by eROSITA) aims to fill this critical gap in our understanding by characterizing the baryonic content of galaxy groups, from their hot gas on megaparsec scales to the properties of their galaxy populations on kiloparsec scales.
The primary limitation in this area has been the lack of X-ray observations of galaxy groups, largely due to the low sensitivities of previous surveys. CLEVeR aims to overcome this by leveraging the unprecedented statistics and sensitivity of the eROSITA All Sky Survey, which is particularly effective at detecting galaxy groups. Additionally, we will complement eROSITA data with upcoming 4MOST spectroscopic surveys of galaxy groups in the optical range, as well as with ancillary datasets that capture specific physical processes related to feedback and the central galaxy, such as MaNGA, MUSE, and LOFAR.
With this comprehensive dataset, we will be able to study and constrain all relevant components on all relevant scales, enabling us to refine our understanding of the physics of galaxy groups, their interactions with black hole feedback, and, consequently, the large-scale structure simulations of the Universe.
For the first time, CLEVeR has demonstrated that most groups identified through galaxy clustering properties in spectroscopic surveys remain undetected by eROSITA's X-ray selection, as well as in other surveys of similar depth. The stacked X-ray surface brightness profile of these groups, which lack an X-ray counterpart in surveys like eRASS (eROSITA All Sky Survey), suggests they do contain hot gas, albeit with different spatial distributions compared to X-ray-detected groups. Specifically, they exhibit a much lower central gas concentration. This finding indicates that relying solely on X-ray surveys, which often probe only shallow depths over large areas, is insufficient for capturing the full range of density, entropy, and temperature profiles in low-mass halos, leading to biased conclusions.
The population of galaxy groups undetected by eROSITA represents the majority of the underlying dark matter halo distribution. Their lower central gas concentration may be attributed to supermassive black hole feedback expelling gas outward. This observation underscores potential biases in X-ray-based group selection and highlights the importance of using alternative techniques for studying gas distribution and black hole feedback, particularly at the group mass scale.
In conjunction with our observational analysis, we also produced mock observations that accurately reflect real observational datasets. Specifically, we generated mock galaxy catalogs and eROSITA mock observations using a 30x30 deg² light cone in the local Universe (z < 0.2) and a 25 deg² light cone at 0.6 < z < 1.1 based on the Magneticum simulation. This simulation is noted for its precise reproduction of optical and X-ray observables for galaxy groups and clusters. Mock eROSITA observations were created for the eRASS4 and eRASS8 depths, incorporating all instrumental effects via the SIXTE simulator. This pioneering experiment, which integrated galaxy spatial distribution with X-ray emission data, allowed us to test selection effects and better understand the physical processes modeled in the Magneticum simulation.
The mock eROSITA observations were crucial for evaluating the eRASS cluster and group selection functions against optical selection methods. We assessed the detection algorithm and selection function for eRASS groups and clusters (eSASS), revealing eROSITA's tendency to detect groups with high central gas concentration and low central entropy. Using a GAMA-like mock galaxy catalog, we evaluated different optical selection algorithms, confirming the robustness of optical group selection, which yields clean samples with minimal contamination. Comparisons between eROSITA and optical selection functions revealed that, as with observations, many optically selected groups remain undetected in the mock eROSITA data. These groups tend to show lower gas concentrations and higher entropy, possibly due to the effects of AGN feedback.
The population of galaxy groups undetected by eROSITA represents the majority of the underlying dark matter halo distribution. Their lower central gas concentration may be attributed to supermassive black hole feedback expelling gas outward. This observation underscores potential biases in X-ray-based group selection and highlights the importance of using alternative techniques for studying gas distribution and black hole feedback, particularly at the group mass scale.
In conjunction with our observational analysis, we also produced mock observations that accurately reflect real observational datasets. Specifically, we generated mock galaxy catalogs and eROSITA mock observations using a 30x30 deg² light cone in the local Universe (z < 0.2) and a 25 deg² light cone at 0.6 < z < 1.1 based on the Magneticum simulation. This simulation is noted for its precise reproduction of optical and X-ray observables for galaxy groups and clusters. Mock eROSITA observations were created for the eRASS4 and eRASS8 depths, incorporating all instrumental effects via the SIXTE simulator. This pioneering experiment, which integrated galaxy spatial distribution with X-ray emission data, allowed us to test selection effects and better understand the physical processes modeled in the Magneticum simulation.
The mock eROSITA observations were crucial for evaluating the eRASS cluster and group selection functions against optical selection methods. We assessed the detection algorithm and selection function for eRASS groups and clusters (eSASS), revealing eROSITA's tendency to detect groups with high central gas concentration and low central entropy. Using a GAMA-like mock galaxy catalog, we evaluated different optical selection algorithms, confirming the robustness of optical group selection, which yields clean samples with minimal contamination. Comparisons between eROSITA and optical selection functions revealed that, as with observations, many optically selected groups remain undetected in the mock eROSITA data. These groups tend to show lower gas concentrations and higher entropy, possibly due to the effects of AGN feedback.
CLEVeR was designed to capitalize on eROSITA's unparalleled statistical capabilities in detecting galaxy groups, owing to the instrument's high sensitivity in the soft X-ray band. Initially, we did not anticipate that a significant portion of the group population would remain unobserved at the eRASS depth. Demonstrating the existence of this undetected population was a major achievement. Ignoring this population, which can only be detected through other selection techniques such as optical methods and which constitutes the majority of the underlying dark matter structures, would lead to biased conclusions.
CLEVeR's methodology, involving rigorous testing of each analysis step with mock observations, allows us to provide clear, robust, and unbiased constraints on the average thermodynamical properties of hot gas in galaxy groups. This approach also enables us to examine the impact of gravitational processes, such as AGN feedback, on these properties, thereby providing the most robust and enduring observational constraints for future large-scale structure and galaxy evolution models.
CLEVeR's methodology, involving rigorous testing of each analysis step with mock observations, allows us to provide clear, robust, and unbiased constraints on the average thermodynamical properties of hot gas in galaxy groups. This approach also enables us to examine the impact of gravitational processes, such as AGN feedback, on these properties, thereby providing the most robust and enduring observational constraints for future large-scale structure and galaxy evolution models.