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Some galaxies have a blue shift

All or nearly all galaxies with a blue shift are near Earth.
One online reference lists 35 objects in our local group and among them are 16 with blue shift. Among these 16 are the two large spirals, M33 and M31, and a few of its satellite galaxies.

Maffei Galaxy Group is one of the closest galaxy groups to our Local Group. Unfortunately the group is quite obscured by the Milky Way core in the line of sight.  Paolo Maffei discovered several members in 1967 via their infrared emissions.

This group is notable with one large spiral galaxy Maffei 2, one medium spiral galaxy NGC1560, one irregular galaxy Camelopardalis A, and an isolated pair of dwarf galaxies NGC 1569 and UGCA 92; these five have a rare blue shift.

I was recently surprised to discover details about this group:

Maffei 1 is close to Maffei 2 in celestial positions with a difference in RA of only 0h5m and in dec of 0d3'  but they are moving in opposite directions (red vs blue) and of course have different distances.
Maffei 1 is a giant elliptical (perhaps the closest of this type); if not obscured it is 3/4 the visible size of a full moon.(I didn't know that!)
Maffei 2 is an intermediate spiral, also obscured, and is a rare blue shift object.

Andromeda M31 Galaxy Spectrum

When calculating the velocity of M31 relative to Earth NASA uses the shift in two absorption lines for calcium. These lines are blue shifted for M31.
Most galaxies in the universe have a red shifted hydrogen absorption line (a future topic). I noted earlier that quasars have a red shifted hydrogen emission line.

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 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 in that element.

The light from M31 is the summation of the electromagnetic radiation covering a broad spectrum from roughly 1 trillion 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.
2) the calcium is closer to Earth than M31 so the light from all the stars passes through it.

The 2nd choice seems much more likely than the first. The fact nearly all blue shift objects are usually close to Earth also implies this conclusion. Blue shifts are observed only in our local group and the nearby Maffei galaxy group.

I cannot find online whether all the blue shift objects are using the same calcium absorption lines as M31. I assume that is true. M33 distance is similar to that for M31 in our local group; they also have similar blue shifts; but while at the same RA their declination is different by about 11 degrees. I find no detailed spectrum data for M33.

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.

Attached are the relative velocities of many individual objects in the M31 galaxy in Andromeda to monitor its rotation.

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, as expected.
However I see an apparent uniformity in the distribution around the chart's zero-zero. 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.

Because there is an apparent uniformity around zero velocity then my conclusion is M31 is not moving in the direction of the Milky Way. This data provides nothing about lateral motion.
I cannot find the actual individual velocities for this plot so a better analysis to identify an overall motion  is not possible.

Based on just this coarse visual presentation one could conclude the relative velocity of M31 toward the Milky Way is probably small.

Certainly this conclusion can be debated the plot shows no significant tendency toward red or blue.

This should not be a complete surprise if the two large spiral galaxies in the local group are actually moving together at this large scale. There is no real reason why they cannot. Both the Milky Way and M31 have their own troop of satellite galaxies and perhaps M31 and its troop might be moving roughly together.


I cannot find online this type of red shift / blue shift data for the rotation of objects in the M33 galaxy or any other.
The only M33 rotation curve I found online had only positive numbers so maybe they were absolute values or some other manipulation had been done.

The accepted M31 relative velocity value (via the calcium line) is -301 km/s, which is faster than any of these stars showing a limit around +/- 100 km/s

Perhaps there is another method to determine the relative velocity of an entire galaxy but this use of individual stars and their emission lines should work, if only for an estimate.

Blue shifts are probably handled incorrectly when trying to assign a velocity.

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