Surprisingly dim coronae around Milky Way-like galaxies

Galaxies at least as massive as the Milky Way have virial temperatures exceeding a million degrees and can thus sustain massive, extended hot halos that regulate galaxy growth. The existence of such halos is supported by analytic theory, numerical simulations, and X-ray and SZ-effect measurements around nearby galaxies, including stacked X-ray emission around galaxies observed by eROSITA. These results predict X-ray coronae within 10 kpc of galaxy disks with 0.5-2 keV luminosities of a few x 10³⁹ erg/s and surface brightness exceeding 10³⁵ erg/s/kpc². Here we show that several edge-on Milky Way-sized spiral galaxies with very deep XMM-Newton exposures have significantly fainter coronae only visible within a few kpc of the disk. We discuss the tension with eROSITA stacks and the Milky Way itself, the connection to SZ measurements, and the prospects for X-ray observations at higher energy resolution in absorption with Arcus-Probe or in emission with the Line Emission Mapper Probe.