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# Shifts in Emission Lines and Absorption Lines for M31

Attached are the relative velocities of many individual objects in the M31 galaxy in Andromeda. Emission lines were used.

One side of M31 is red shifted so it is assumed to be receding.
The other side is blue shifted so it is assumed to be approaching.
The combination indicates the disk is rotating.
I see an apparent uniformity in the distribution of red/blue. Initially that might be expected.
However I assumed each object would reflect its own motion as well as the galaxy it is moving with, as one piece in the package of a trillion.
Therefore I assumed there would be slightly more red shift and less blue shift if the M31 were receding. If M31 were approaching I would expect to see the opposite.

I do not know whether the data were manipulated, like all were offset by a certain value of km/s.

Because there is an apparent uniformity around zero velocity then my impression is they are offset.
If the plot is of raw data one might conclude  M31 is moving in the direction of the Milky Way at a slow velocity. I can find no details for the plot.

M31 Absorption Lines

When calculating the velocity of M31 relative to Earth NASA uses the blue shift in two absorption lines for calcium.

Is that calcium absorption line  really telling us the motion of the entire galaxy? I doubt it.

An absorption line results from a particular element or compound absorbing that specific wavelength because an electron can move to a higher energy state. This absorption can occur anywhere between the source and observer.

By comparison an emission line occurs when an electron drops to a lower energy state and that energy is released in the specific wavelength for that state change.

When we view the light from M31 it is the summation of the electromagnetic radiation covering a broad spectrum from all 1 trillion (or so) stars in our view of that galaxy. I expect in that mix any variations in emission lines and absorption lines probably get washed out.

Somewhere between the stars in M31 and us on Earth there are 'loose' calcium atoms absorbing their two wavelengths.

There are two possibilities:

1) it is possible that all the stars in M31 (or others with blue shift) have the calcium in or near their atmosphere and the summation never finds a star with no calcium, to fill in that wavelength.
This seems unlikely.

2) the calcium is closer to Earth than M31 so the light from all the stars passes through it.

Galaxies with blue shifts are observed only in our local group and the nearby Maffei galaxy group.
That observation  implies the choice (2) is correct.

I cannot find online whether all the blue shift objects are using the same calcium absorption lines as M31. M33 distance is similar to that for M31 in our local group; both also have similar blue shifts; but while at the same RA their declination is different by about 11 degrees. I found a plot of the M33 rotation curve but all velocities are unsigned so I assume they are an absolute value.

Regardless, any absorption lines are not necessarily absorbed by elements only in the star's atmosphere.
If we are using the motion of Calcium atoms in the space within our local group to calculate the motions of objects behind the calcium then this is clearly not justified.

When NASA uses Calcium absorption lines to calculate the relative velocity for M31 the result is -301 km/s (blue shift or approaching)
That value is not seen here when checking individual stars.

This value leads to silly stories of M31 crashing into the Milky Way in the distant future.

This resulted in a comment:

'
First you better check out what happens to all that theory if *redshift does not measure distance*
'

my response:

the red shift associated with distance is caused by many hydrogen atoms(at low density) in intergalactic space absorbing  that wavelength across that great distance.  A red or blue shift in an emission line is the motion of that atom at its current location and unrelated to distance.