Study of X-ray emission from nearby normal galaxies using eROSITA All-Sky Survey data

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X-ray observations of galaxies allow us to study the population of compact objects formed at the end of the stellar life and the diffuse interstellar gas, which has been ionised and heated mainly by stellar winds and supernovae. As most of the stars are formed in multiple systems, compact objects often have a companion star, from which the compact object accretes matter. Accreting neutron stars and black holes are the brightest stellar X-ray sources in normal galaxies and are called X-ray binaries (XRBs). Since binary evolution and formation of compact objects depend on the underlying stellar population, XRBs are ideal tracers of stellar population and star formation histories in galaxies. Strong stellar winds of massive stars and supernovae cause strong shock waves in the interstellar medium (ISM) and the expelled stellar material, which will form the diffuse hot ionised phase in the ISM. Since the cooling of the hot low-density plasma is very inefficient, it will persist for a long time, and diffuse X-rays can be observed over the entire galaxy. X-ray observations in the last decades have allowed us to constrain the emission components in normal galaxies. In nearby galaxies, the source populations and the hot gas have been studied in detail, while studies of the integrated emission from further distant galaxies have shown that there is a relation between the integrated X-ray luminosity L_X and the star-formation rate (SFR), also separately for the XRBs and the diffuse hot gas. Moreover, the L_X/SFR relations also depend on the metallicity, and hence also on the age or on the distance of galaxies. Therefore, X-ray study of galaxies at higher distances will help us to improve our understanding of the evolution of galaxies in the Universe. The study of the diffuse hot gas has so far only been possible in the closest galaxies due to the relativity low sensitivity of the telescopes at lower X-ray energies. Thanks to eROSITA's high sensitivity in the lower-energy band and its all-sky coverage, we now have the ideal X-ray telescope to study the diffuse X-ray emission from a complete sample of normal galaxies. Based on our studies of the diffuse X-ray emission in very nearby galaxies (D < 10 Mpc), we will study the X-ray emission of the diffuse hot gas in galaxies using eROSITA All-Sky Survey data. First, we will study the emission of the hot diffuse gas in galaxies up to D ~ 20 Mpc, in which we can resolve the emission in the galaxies. We will analyse the spectrum using emission models based on the results of our XMM-Newton or Chandra study of the very nearby galaxies. In the second step, we will use the models to study the stacked spectra of further distant galaxies in different intervals (up to ~200 Mpc) and the relation between the X-ray luminosity and SFR, distance, or metallicity. In the last step, we will also perform simulations for future X-ray missions to estimate the expected sample size and depth of similar studies.