Astronomers using the NASA/ESA Hubble Space Telescope have measured the metallicity of the neutral interstellar medium towards 25 bright stars in our Milky Way Galaxy.
The interstellar medium — the space between the stars — comprises gases at different temperatures and densities and dust particles.
The neutral interstellar medium is dominated by neutral hydrogen and has ionization fractions of up to 8%.
The concentration of chemical elements heavier than helium — the so-called metallicity — spans orders of magnitudes in Milky Way stars, because they formed at different times.
However, the gas in the vicinity of the Sun is assumed to be well mixed and to have a level of chemical enrichment similar to the Sun’s atmosphere, called the solar metallicity.
“Until now, theoretical models considered that the neutral interstellar medium was homogeneously mixed and reached the solar composition everywhere in our Galaxy, with a slight increase in metallicity in the center, where the stars are more numerous,” said Dr. Patrick Petitjean, an astronomer at the Institut d’Astrophysique de Paris at Sorbonne University.
“We wanted to observe this in detail using Hubble’s ultraviolet spectrograph.”
Using Hubble’s Space Telescope Imaging Spectrograph (STIS), Dr. Petitjean and colleagues obtained and analyzed near-ultraviolet spectra of 25 bright O- and B- type stars in the Milky Way.
They found large variations in metallicity over a factor of ten (with an average of 55% solar metallicity), including many regions of low metallicity, down to about 17% solar metallicity and possibly below.
They suggest that pristine gas falling onto the Milky Way’s disk in the form of rapidly moving clouds can cause the observed chemical inhomogeneities on scales of tens of light-years.
They think that this low-metallicity gas does not efficiently mix into the interstellar medium.
“This discovery plays a key role in the design of theoretical models on the formation and evolution of galaxies,” said Dr. Jens-Kristian Krogager, an astronomer in the Department of Astronomy at the University of Geneva.
“From now on, we will have to refine the simulations by increasing the resolution, so that we can include these changes in metallicity at different locations in the Milky Way.”
Originally Published By SciNews