Observing the X-ray-bright gas in the halo of the Milky Way, ESA's XMM-Newton has gathered new data which favour a process involving fountains of hot gas in our Galaxy.

Such a scenario, with the gas flowing from the galactic disc into the halo where it then condenses into cooler clouds and subsequently falls back to the disc, confirms the importance of supernova explosions in forging the evolution of the interstellar medium and of the entire Galaxy.

The interstellar medium (ISM) in the Milky Way is a complex, dynamical system consisting of gas in different phases, spanning a wide range of densities and temperatures. The interplay among the various phases in the ISM, namely the hot, warm and cold gas, determines the entire history of star formation in our Galaxy, by shaping the birthplaces of stars.

The most massive stars, in particular, have a deep influence on the ISM, as they release copious amounts of energy both during their lives and with their eventual dramatic demises in the form of supernova explosions. Understanding the structure and dynamics of the ISM is a key element in figuring out the processes of star formation in the Galaxy, and of the evolution of spiral galaxies in general.

One phase of the ISM, the hot gas, has very low densities (below 0.01 cm-3) and temperatures as high as a few million Kelvin, hence it is hot enough to emit X-rays.

The existence of the hot component of the ISM was first proposed in the 1970s, not long after the new spectral window of X-ray astronomy opened; since then, it has become clear that the hot phase represents an important component of the ISM, as it most directly traces the injection of energy into the ISM from stars and supernovae.

Supernova explosions heating the ISM can drive hot gas out of the disc in so-called galactic fountains, forming a halo of hot gas around the Milky Way.

Such a halo was first detected by the ROSAT X-ray telescope in the early 1990s, and similar halos have also been detected around other spiral galaxies. In the galactic fountain scenario, as the gas rises above and below the disc, reaching heights of a few kiloparsecs, it emits radiation and thus becomes cooler.

This cooled gas starts to condense into clouds which then fall back into the disc, in a fashion that resembles a fountain: this creates a global circulation of gas in the Galaxy which dynamically connects the disc and the halo.

Radio observations of hydrogen gas in our Galaxy show structures that are thought to be superbubbles bursting out of the disc, giving rise to galactic fountains. However, we cannot see the hot gas rising into the halo in these structures, because the X-rays from this hot gas are absorbed by intervening material in the disc.

"Although we cannot directly observe the hot gas rising out of the disc, it has long been suspected that galactic fountains are responsible for the hot gas observed in the Milky Way's halo," explains David Henley from the University of Georgia, in the US, who led a study that provides new evidence supporting a galactic fountain mechanism in our Galaxy.

"We have taken spectra of the X-ray-emitting hot gas in the galactic halo and compared them to detailed predictions coming from different models. The galactic fountain scenario turned out to be the one that best describes our data," he adds.

The study relies on a series of spectroscopic observations performed with ESA's X-ray observatory, XMM-Newton, targeting the emission from gas in the galactic halo, which is dominated by highly ionised oxygen atoms in the XMM-Newton energy band.

Henley and collaborators compared the data to predictions from three different models that have been put forward to explain the origin of the hot gas in the halo: in one case, the hot gas is accreted from extragalactic material; another model accounts for the heating of the halo gas in terms of individual supernova explosions taking place in the halo itself; finally, a third model relies on supernovae powering the turbulent dynamics of the ISM and producing, among other features, galactic fountains.

"The high-quality spectra collected by XMM-Newton were essential in discriminating between the various models, pointing towards the significant contribution of supernova-driven galactic fountains to the X-ray emission of the galactic halo," comments Norbert Schartel, XMM-Newton Project Scientist.

This result shows that galactic fountains are a major player in the mixing and distribution of gas in the ISM, thus confirming previous clues about the crucial role of supernovae in the global evolution of the Milky Way.

"There are still some open issues but we feel a step closer to answering the question of the origin of the hot halo gas. Further observations, expanding the extent of the current survey, as well as more detailed simulations on the theoretical front will surely shed new light on this issue," Henley concludes.