The similarity of observed X-ray coronae associated with L* disc and
  elliptical galaxies
  The existence of hot, X-ray luminous gaseous coronae surrounding present day
L* galaxies is a generic prediction of galaxy formation theory in the cold dark
matter cosmogony. While extended X-ray emission has been known to exist around
elliptical galaxies for a long time, diffuse extra-planar emission has only
recently been detected around disc galaxies. We compile samples of elliptical
and disc galaxies that have Chandra and XMM-Newton measurements, and compare
the scaling of the coronal X-ray luminosity (L_X) with both the K-band
luminosity (L_K) and the coronal X-ray temperature (T_X). The X-ray flux
measurements are corrected for non-thermal point source contamination by
spatial excision and spectral subtraction for resolved and unresolved sources
respectively. We find that the properties of the extended X-ray emission from
galaxies of different morphological types are similar: for both elliptical and
disc galaxies, the L_X - L_K and L_X - T_X relations have similar slope,
normalisation and scatter. The observed universality of coronal X-ray
properties suggests that the bulk of this emission originates from gas that has
been accreted, shock-heated and compressed during the assembly of the galaxy
and that outflows triggered by stellar processes make only a minor contribution
to the X-ray emission. This reservoir of cooling gas is a potential source of
morphological transformation; it provides a fresh supply of material for discs
to grow around galaxies of all morphological types.
