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Luminous X-ray gas coronae in the dark matter halos of massive spiralgalaxies are a fundamental prediction of structure formation models, yet only afew such coronae have been detected so far. In this paper, we study the hotX-ray coronae beyond the optical disks of two normal massive spirals, NGC1961and NGC6753. Based on XMM-Newton X-ray observations, hot gaseous emission isdetected to ~60 kpc - well beyond their optical radii. The hot gas has abest-fit temperature of kT~0.6 keV and an abundance of ~0.1 Solar, and exhibitsa fairly uniform distribution, suggesting that the quasi-static gas resides inhydrostatic equilibrium in the potential well of the galaxies. The bolometricluminosity of the gas in the (0.05-0.15)r_200 region (r_200 is the virialradius) is ~6e40 erg/s for both galaxies. The baryon mass fractions of NGC1961and NGC6753 are f_b~0.1, which fall short of the cosmic baryon fraction. Thehot coronae around NGC1961 and NGC6753 offer an excellent basis to probestructure formation simulations. To this end, the observations are confrontedwith the moving mesh code Arepo and the smoothed particle hydrodynamics codeGadget. Although neither model gives a perfect description, the observedluminosities, gas masses, and abundances favor the Arepo code. Moreover, theshape and the normalization of the observed density profiles are betterreproduced by Arepo within ~0.5r_200. However, neither model incorporatesefficient feedback from supermassive black holes or supernovae, which couldalter the simulated properties of the X-ray coronae. With the further advanceof numerical models, the present observations will be essential in constrainingthe feedback effects in structure formation simulations.

Hot X-ray coronae around massive spiral galaxies: a unique probe of structure formation models