Thanks 94JZA80 for your help. About my question, I agree with you that the answer is yes, but if we exclude the fact that a part of the orbit of the gas is traveling away from us and another part of the orbit of the gas is traveling towards us, I mean, let pretend we don't have this specific information. If we only know that the the supermassive black hole that form the active core of M87 is a strong source of radiation made from at many wavelengths and emits a jet of energetic plasma extending at least 5000 light years and that astronomers have measured the velocity of gas which surrounds and orbits the central black hole of galaxy M87 using the emission lines of various atoms, how the orbital motion of gas about the central black hole in M87 affect its spectrum when observed from the Earth? How therefore could we see an oscillation of the emission/absorption lines in the gas' spectrum between lower frequencies (longer wavelengths) and higher frequencies (shorter wavelengths)? Might have something to do with the fact that the gas are not still but are rotating around the black hole? Is there another observations we could make or consider in order to answer yes to the question? Thanks!