What assumptions are we making when we infer the mass of a galaxy cluster from observations of the spectra of its constituent galaxies?

The assumptions  we make when we infer the mass of a galaxy cluster from observations of the spectra of its constituent galaxies is thermal hydrostatic equilibrium

Explanation:

In galaxy cluster observation, thermal X-ray emission from the hot intracluster gas measures directly the gas density and temperature distributions within the cluster. This hot gas is confined by and expected to trace the underlying three-dimensional gravitational potential. Under the assumption of thermal hydrostatic equilibrium, these gas properties may be used to infer the cluster total mass distribution.

Hence, the assumptions  we make when we infer the mass of a galaxy cluster from observations of the spectra of its constituent galaxies is thermal hydrostatic.

the atomic mass is the bottom number on the element.

-steel jelly

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)