In astronomy, “metals” refers to all the elements of the periodic table other than hydrogen and helium. This is because the two lightest elements were formed in the early moments of the Big Bang and make up the vast majority of the atomic matter in the cosmos. Other elements such as carbon, oxygen, iron, and gold were formed in astrophysical processes such as stellar mergers, supernovae, and neutron star collisions. Therefore, the distribution of metals in the Milky Way tells us a lot about the history of our galaxy.
Unfortunately, measuring the metallicity of the Milky Way is challenging because metals can absorb light. So when Swapnaneel Dey, a student at the University of Arizona, wanted to measure galactic metallicity, he used a method known as Radio Recombination Lines (RRL). This technique provides a good measure of the temperature of free electrons in interstellar hydrogen clouds. Since metals are heavier than hydrogen, their presence tends to cool the electrons, so there is a relationship between the temperature of the electrons and the amount of metals in a cloud.
Swapnaneel used data from the Herschel space telescope and the Very Large Array (VLA) to measure the metallicity of hydrogen clouds in the Milky Way, particularly the ratio of oxygen to hydrogen. What he found was that the variation in this proportion was quite large. So large that he would seem to disagree with previous studies on galactic metallicity. Swapnaneel believes this wide variation is also due to a calibration problem between the Herschel and VLA data. Standard Herschel calibrations assume that extended objects are fairly uniform, but interstellar hydrogen clouds can vary quite a bit.
This means that the relationship between metallicity and electron temperature is more subtle than we thought. Swapnaneel hopes that future studies can better understand these subtleties.
