Supernova remnants (SNRs) and stellar winds drive strong shock waves into the ISM and create bubbles and superbubbles filled with hot thin plasma (10⁶–10⁷ K), which emits soft X-ray lines and continuum emission. The ROSAT all-sky survey has revealed large structures (10 or more) in the X-ray sky, e.g., the nearby SNR Monogem Ring or the diffuse Galactic ridge emission. These structures are too large to be studied as a whole in pointed observations with today’s X-ray missions XMM-Newton or Chandra. Only the all-sky survey data from eROSITA will allow us to study the temperature, abundances, density, ionization, etc., of these large, nearby objects on 1 pc scales and to improve our understanding of the hot phase of the ISM and the nature of the observed diffuse X-ray emission. In addition, the study of X-ray shadows of dark clouds over the sky will allow us to obtain a better understanding of the local hot ISM.

In the first funding period of eRO-STEP (2021-2024), we have studied the emission of the diffuse X-ray background observed with eROSITA including the emission from the Local Hot Bubble, the Galactic X-ray ridge, and the very nearby SNRs. As the diffuse X-ray emission observed with eROSITA is now better understood, during the second funding period (2024-2027), we are extending our studies to that of the diffuse emission from hot interstellar plasma in Galactic superbubbles and in the entire ISM in the Magellanic Clouds. This will allow us to obtain a better understanding of the energy sources and budget, chemical abundances, properties of the plasma, 3-D structure, and the interaction between the hot and the colder phases in the ISM.