  Hot gaseous haloes surrounding galaxies and extending well beyond the
distribution of stars are a ubiquitous prediction of galaxy formation
scenarios. The haloes are believed to consist of gravitationally trapped gas
with a temperature of millions of Kelvin. The existence of such hot haloes
around massive elliptical galaxies has been established through their X-ray
emission. While gas out-flowing from starburst spiral galaxies has been
detected, searches for hot haloes around normal, quiescent spiral galaxies have
so far failed, casting doubts on the fundamental physics in galaxy formation
models. Here we present the first detection of a hot, large-scale gaseous halo
surrounding a normal, quiescent spiral galaxy, NGC 5746, alleviating a
long-standing problem for galaxy formation models. In contrast to starburst
galaxies, where the X-ray halo can be powered by the supernova energy, there is
no such power source in NGC 5746. The only compelling explanation is that we
are here witnessing a galaxy forming from gradually in-flowing hot and dilute
halo gas.
