Why is it more challenging to measure dark matter in elliptical galaxies compared to spiral galaxies?

Measuring dark matter in elliptical galaxies is more challenging primarily because they tend to lack atomic hydrogen gas, which is crucial for determining the rotational speeds of stars and gas within galaxies. In spiral galaxies, the presence of significant amounts of atomic hydrogen allows astronomers to use techniques such as the 21 cm line emission to gauge velocities, which can then be used to infer the presence and distribution of dark matter.

In contrast, elliptical galaxies generally have much less atomic hydrogen and often consist of older stars, meaning there are fewer gas clouds to observe. Without this atomic hydrogen, it becomes more difficult to apply the same methods used in spiral galaxies, making it challenging to accurately determine the mass distribution and the amount of dark matter present.

While other choices touch on aspects related to elliptical galaxies, such as their distance or concentration of dark matter, the lack of atomic hydrogen fundamentally limits the methods available for measuring their rotation and consequently their dark matter content.