  We present our XMM-Newton observation of the fastest rotating spiral galaxy
UGC 12591. We detect hot gas halo emission out to 110 kpc from the galaxy
center, and constrain the halo gas mass to be smaller than 3.5e11 solar masses.
We also measure the temperature of the hot gas as T=0.64\pm0.03 keV. Combining
our X-ray constraints and the near-infrared and radio measurements in the
literature, we find a baryon mass fraction of 0.03-0.04 in UGC 12591,
suggesting a missing baryon mass of 75% compared with the cosmological mean
value. Combined with another recent measurement in NGC 1961, the result
strongly argues that the majority of missing baryons in spiral galaxies does
not reside in their hot halos. We also find that UGC 12591 lies significantly
below the baryonic Tully-Fisher relationship. Finally, we find that the baryon
fractions of massive spiral galaxies are similar to those of galaxy groups with
similar masses, indicating that the baryon loss is ultimately controlled by the
gravitational potential well. The cooling radius of this gas halo is small,
similar to NGC 1961, which argues that the majority of stellar mass of this
galaxy is not assembled as a result of cooling of this gas halo.
