Imagine a city map where streets are replaced by car speeds and buildings by the metal content of stars. Such a map can tell you where the center is and where the outskirts are, even without knowing exact distances. This is precisely the approach astronomers took with our Galaxy, using longitude–velocity (ℓ–v) diagrams.
They gathered oxygen data from ionized hydrogen regions (hydrogen, glowing under the influence of hot stars) across the entire disk of the Milky Way. Then they compared these data with computer models of galaxies from the FIRE-2 project. It turned out that these diagrams reveal a radial metallicity gradient: the farther from the center, the fewer heavy elements. It's like having more expensive shops downtown and fewer on the outskirts.
Moreover, the models exhibited 'anomalous gas motions'—the tail of the velocity distribution there is three times larger than in our Galaxy. This means that gas in the models moves too chaotically compared to reality. Such a discrepancy is important for understanding how stars are born and die, enriching the interstellar medium with metals.
So, longitude–velocity diagrams are a powerful but still underused tool. They allow us to study the chemical history of the Galaxy without relying on imprecise distance measurements. Perhaps they will help piece together the full picture of how the Milky Way became what it is.
Oxygen is the most abundant heavy element in the Universe, yet in ionized hydrogen regions its abundance is millions of times lower than that of hydrogen.
In Liu Cixin's novel 'The Three-Body Problem', aliens study the structure of galaxies to understand their history — an approach without direct distance measurements that echoes the idea of ℓ–v diagrams.