The composition of the interstellar medium in the Galaxy as seen through X-rays

The term, "Milky Way" usually refers to the hundreds billion stars of our Galaxy, which emit most of its visible light. This name derives from its appearance as a weak, "milky" band which arches across the night sky. It is made of individual stars that cannot be distinguished by the naked eye as it is also the case for the other galaxies. However, the environment around the stars is not empty, it hosts a very tenuous medium which is called "interstellar medium" (ISM). The ISM contains matter in the form of gas and dust. Thermonuclear fusion in stellar interiors enrich the ISM with heavy elements like oxygen and iron. Eventually dust and molecules are produced. Part of the interstellar matter is then used to give birth to new stars whose chemical structure and metallicity differ from the previous generations. The ISM is therefore an active component of the Galaxy which exchanges matter with stars and affects many of their properties, and highly influences the Galactic evolution. This work provides a successful and new method to determine metal abundances and chemistry properties of the ISM through its absorption lines in the X-ray spectra of background sources like X-ray binaries and Active Galactic Nuclei. This technique reveals ionic species and molecular compounds that are difficult to detect with other methods and provides a self-consistent way to determine the abundance gradients of the ISM in the Galaxy and its connection with the stellar evolution. Our estimates of metallicity gradients and predictions of abundances in the Solar neighborhood agree with those estimated at long wavelengths with other methods.