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Mock HUBS observations of hot gas with IllustrisTNG


The lack of adequate X-ray observing capability is seriously impeding the progress in understanding the hot phase of circumgalactic medium (CGM), which is predicted to extend to the virial radius of a galaxy or beyond, and thus in acquiring key boundary conditions for studying galaxy evolution. To this end, the Hot Universe Baryon Surveyor (HUBS) is proposed. HUBS is designed to probe hot CGM by detecting its emission or absorption lines with a non-dispersive X-ray spectrometer of high resolution and high throughput. The spectrometer consists of a 60 × 60 array of microcalorimeters, with each detector providing an energy resolution of 2 eV, and is placed in the focal plane of an X-ray telescope of 1 field-of-view. With such a design, the spectrometer is highly optimized for detecting X-ray-emitting hot gas in the CGM of local galaxies, as well as in intra-group medium (IGrM), intra-cluster medium (ICM), or intergalactic medium (IGM). To assess the scientific potential of HUBS, in this work, we created mock observations of galaxies, groups, and clusters at different redshifts with the IllustrisTNG simulation. Focusing exclusively on emission studies, we took into account the effects of light cone, Galactic foreground emission, and background AGN contribution in the mock observations. From the observations, we made mock X-ray images and spectra, analyzed them to derive the properties of the emitting gas in each case, and compared the results with the input parameters from the simulation. The results show that HUBS is well suited for studying hot CGM at low redshifts. The redshift range is significantly extended for measuring IGrM and ICM. The sensitivity limits are also presented for detecting extended emission of low surface brightness.

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Data Availability

The IllustrisTNG halo catalogues and the simulation snapshots [47] are publicly available at The rest of the data underlying the article will be available from the corresponding author on reasonable request.











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We thank Dan McCammon for providing the XQC filter data and for useful discussions, and Zhansan Wang for providing preliminary data on the HUBS optics. We would also like to thank John ZuHone for useful suggestion on using some of the software packages, and Taotao Fang for advice on making mock observations. This work made use of several Python packages for astronomy, including pyXSIM [40, 41], photutils [42], astropy [43, 44]. The figures in this paper were made using the python matplotlib [45] and seaborn [46] package.


This work was supported in part by the Ministry of Science and Technology of China through its National Key R&D Program, Grant 2018YFA0404502, and by the National Natural Science Foundation of China through Grant 11821303.

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Zhang, YN., Li, C., Xu, D. et al. Mock HUBS observations of hot gas with IllustrisTNG. Exp Astron (2022).

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  • X-ray astronomy
  • X-ray spectroscopy
  • X-ray mission
  • Circumgalactic medium
  • Intra-group medium
  • Intra-cluster medium