Our home galaxy consists not only of stars and planets, but also of large amounts of interstellar gases that are distributed between the celestial bodies. Until now, this interstellar medium was considered to be relatively well mixed and homogeneous. But now new observational data show that the gas in the Milky Way is far more lumpy and inhomogeneously distributed than previously assumed. Above all, the content of heavier elements, which are released into the galactic environment by supernovae, among other things, is up to ten times lower in many places than in the vicinity of our solar system. This calls common models into question.
Although the Milky Way is our galactic home, it always causes surprises. Because astronomers have by no means revealed all of their secrets. One reason for this is the location of our solar system: Because we are part of the rotating disk in which most of the stars, gases and dust rotate around the galactic center, we do not have an unrestricted view. Many areas of the galaxy are covered by dust clouds or the galactic center, and it is difficult to map large-scale structures from within. For this reason, among other things, some assumptions about the evolution and composition of galaxies are based on theoretical models and conclusions from observations of similar galaxies in our cosmic neighborhood.
According to these models, the interstellar medium of our galaxy is primarily composed of three components: the hydrogen gas flowing in from intergalactic space, the gas between the stars that is enriched with heavier elements by supernovae and the dust that arises from the condensation of the elements present in this gas . “Galaxies are rejuvenated by the ‘virgin’ gas coming from outside, because this is the raw material for star formation,” explains first author Annalisa De Cia from the University of Geneva. It is only through nuclear fusion in the stars that heavier elements are created, which in astronomy are all referred to as metals, regardless of their chemical behavior.
Starlight reveals heavy elements
So far, models have assumed that all three components of the interstellar medium mix with one another due to the rotation and currents within the Milky Way, so that a largely homogeneous distribution is created overall. Specifically, some researchers recently determined that, for example, a local tenfold enrichment with metals by a supernova is distributed over a volume of 0.1 cubic kiloparsecs within around 280 million years, as De Cia and her colleagues report. According to this, there may be local, temporary deviations due to fresh gas inflows and supernovae, but in the long term the proportion of gas in the Milky Way should be composed relatively similarly everywhere.
De Cia and her colleagues have now checked whether this is actually the case with the help of spectral measurements in ultraviolet light. To do this, they sighted 25 different stars with the Hubble Space Telescope and the Very Large Telescope (VLT) of the European Southern Observatory in Chile. “When we observe a star, the metals that are in the gas between the star and us absorb a small portion of the light at specific frequencies,” explains co-author Patrick Petitjean from the Sorbonne University in Paris. “This not only allows us to detect their presence, but also to determine which element it is and how common it is.” Because the dust, which is also present in the interstellar medium, can falsify the measurements, the astronomers used the relative proportions of the various Metals in order to correct the values so that its influence is taken into account.
Inhomogeneous and less metallic
The analyzes reveal that the distribution of elements in the interstellar medium is far less homogeneous than the theoretical models predict. “The most notable result is that the largest fluctuations in our lines of sight differ by more than an order of magnitude,” report De Cia and her team. “Two thirds of our samples also show sub-solar metallicities.” According to this, the proportion of heavy elements – at least in the measured sections of the interstellar medium – does not correspond to that in the solar environment, but averages 55.7 percent of the solar value, as astronomers do have determined. In addition, they found clear local differences, in which some areas were rather metal-rich, while others only had a metallicity of 17 percent of the solar value.
According to De Cia and her team, this indicates that the gas flowing in from the outside in particular mixes less quickly and thoroughly with the interstellar medium than the theory suggests. As a result, pockets of original, low-metal gas could not only be preserved in the Milky Way longer than previously thought, they could even occur very frequently, as the researchers explain. “This discovery plays a key role in the development of theoretical models for the formation and evolution of galaxies,” says co-author Jens-Kristian Krogager from the University of Geneva. “From now on we have to increase the resolution in our simulations in order to be able to capture these local changes in the metallicity at different locations in the Milky Way.”
Source: Annalisa De Cia (University of Geneva) et al., Nature, doi: 10.1038 / s41586-021-03780-0