Gas around black holes and interstellar medium distribution are key factors in understanding the growth of supermassive black holes and the evolution of their host galaxies. However, as a crucial parameter, gas density is hard to determine reliably, because the general method is not applicable to all quasars.
![](https://scx1.b-cdn.net/csz/news/tmb/2021/quasar.jpg)
Source:
https://phys.org/news/2021-03-sharp-broad-absorption-line-variations.html