Astronomers are certain any absorption or emission lines in a galaxy spectrum indicate the galaxy motion.
This is a fundamental mistake in cosmology. A galaxy is not a single light source in motion. A galaxy has millions of stars. Some galaxies have more dust clouds than others. A galaxy spectrum depends on the observer's line of sight. The observed spectrum from Earth is certainly different when the galaxy is face-on or edge-on, or in between.
Every absorption or emission line in a galaxy spectrum is from atoms in the line of sight or in the intergalactic medium(IGM). If either line shifts toward blue or red, this shift occurs in the IGM. A line shift in the IGM indicates nothing about the distant galaxy.
Initially there was a rough correlation between a hydrogen absorption line and distance because the IGM accumulated a redshift in the absorption line as the distance of light travel in the IGM increased. This initial correlation resulted in Hubble's law as an initial distance metric.
In 1936 Hubble put our Local Group on an island separate from the Hubble Flow to excuse Local Group spectrum anomalies like blue shifts for those galaxies in the direction of M31.
As distances to the galaxies being measured increased, more anomalies arose like the Virgo Centric Flow. VCF refers to the observation of nearer galaxies appear faster than distant galaxies, in the direction of the constellation Virgo. This is impossible for the claimed uniform expansion of the universe centered on Earth where velocity must consistently increase with visible distance.
Despite that history, astronomers remain convinced these lines indicate a galaxy velocity.
This mistaken certainty has consequences. Here are just 5.
1) Public sources like Wikipedia will publish a red shift value as z or a velocity in km/s.
These sources never provide the spectrum to justify this claimed shift. The certainty results in no attempt to provide supporting data.
This lack of data prevents any public accountability for any claimed velocities.
I find this negligence appalling. When working in industry, I had to prove all claims of precision with laser interferometers to get customer satisfaction. The printouts were provided both for reference and to assist future machine maintenance.
Astronomers never prove their claims.
This practice of no spectrum publication has persisted for 100 years.
Perhaps not by coincidence, but by consistent negligence, LIGO never has evidence for their GW detection claims and the scientific community never demands proof from LIGO.
2) With no galaxy spectrum data, it is impossible to learn about the IGM.
Each galaxy spectrum is affected by the IGM in its line of sight so the spectrum reveals what is in the IGM to the extent its contents can affect light. Loose protons and electrons might not be detectable, but atoms like calcium will be. Calcium atoms are in IGM toward M31. This is known only because NASA publicly published the M31 absorption lines for a teacher's page with a student's exercise.
3) Astronomers incorrectly have certainty the absorption lines are in the galaxy and so they fail to comprehend the absorption lines are from the IGM.
4) Cosmology seeks to understand the universe and yet the pervasive IGM is simply ignored because absorption lines coming from IGM are wrongly assigned to the galaxy.
One would think the IGM is important considering its volume but its contents are currently mostly unknown by negligence in providing public data.
Unfortunately, the bottom line here is: astronomers and cosmologists honestly don't understand a galaxy spectrum and its absorption lines.
5) The mistake with high redshifts resulting from the IGM has compounded into more mistakes:
a) there is no justification for a claimed universe expansion.
b) the claimed undetectable entity called dark energy, and
c) a big bang theory which proposes an explosion to trigger the expansion while having absolutely no evidence for its entire scenario.
When accountability is not enforced from the beginning, mistakes proliferate.