Is there a radial gradient of redshifts within individual galaxies?

[hkyriazi]

TL;DR Summary

Is there a small, radial gradient of stellar redshifts in individual galaxies, with bluer light coming from the center, and redder light from stars in the galaxy periphery?

The summary says it all. Such small gradients, if they exist, would be visible in the Milky Way and local galaxies in our cluster. I'm not familiar enough with the raw data--and haven't tried to search the astronomical literature--to know whether any such small effect has been reported. (If such effects were large, I'm sure I'd have already heard about them, as they'd be discussed actively. Tiny effects, however, might be ascribed to gravitational red/blueshifts, star temperature differences, or other unknown causes, and not widely known.)

 

[Bandersnatch]

One can measure galactic rotation by comparing relative blue vs redshift of light from the approaching vs receding limb. It's routinely done.
I'm not sure why you think there should be any blueshift from the centre vs redshift from the limbs.

 

[hkyriazi]

Thanks. I should've remembered or thought of that!

Do you happen to know whether the spectrometers measuring such rotational shifts have the resolution to isolate the two sides of galaxies that happen to be edge on with respect to us? In other words, can we actually identify one half as redshifted and the other as blueshifted, or does the spectrometer take in a smear of light from the whole galaxy, and we calculate the rotation rate based on the spread of the wavelengths (and simply assuming the redshifted portion is receding, etc.)? Perhaps they do both, depending upon the galaxy's "visual size" (subtended angle).

Alternatively, are there any radial gradients of red/blueshift seen in galaxies whose plane is perpendicular to our line of sight? I suspect not, or you'd have mentioned it.

As to why I suspect there may be such shifts, I'm pursuing a hypothesis within a larger theory, related to Milgrom's MOND hypothesis, that matter varies in "strength," and inertia, depending upon its proximity to other matter. Regions of space dense with matter would have stronger matter, and matter in the periphery would be weaker and with less inertia (more greatly affected by gravity, and having lower frequencies of light emitted).

 

[phyzguy]

Do you happen to know whether the spectrometers measuring such rotational shifts have the resolution to isolate the two sides of galaxies that happen to be edge on with respect to us? In other words, can we actually identify one half as redshifted and the other as blueshifted, or does the spectrometer take in a smear of light from the whole galaxy, and we calculate the rotation rate based on the spread of the wavelengths (and simply assuming the redshifted portion is receding, etc.)? Perhaps they do both, depending upon the galaxy's "visual size" (subtended angle).

If the galaxy is near enough, and hence large enough in angular size, the spectrograph can definitely resolve the varying redshift across the galaxy. Here's an example from the Hubble STIS.

 

[Bandersnatch]

Here's another example (UGC 2936):

As a rule of thumb, if a galaxy is resolvable as a line , you can place a spectrograph across the image.

 

[hkyriazi]

Lots of info in this single plot! Thanks!

 

[mfb]

Apart from radial velocities (which dominate) there is a small blueshift redshift from the center as the light is emitted in a deeper gravity well and redshifts while leaving it.

 

[Bandersnatch]

there is a small blueshift from the center

Did you mean to say redshift?

 

[hkyriazi]

Apart from radial velocities (which dominate) there is a small blueshift from the center as the light is emitted in a deeper gravity well and redshifts while leaving it.

Now I'm getting confused. Do you mean tangential velocities, not radial? Ah...never mind. You mean the radial velocity of the entire galaxy, not the rotationally-induced shifts.
I would think the deeper gravity well in the center would lead to a greater gravitational redshift, and not at all blue.

 

[hkyriazi]

Here's another example (UGC 2936):
View attachment 247734
As a rule of thumb, if a galaxy is resolvable as a line , you can place a spectrograph across the image.

Anyone have any idea what accounts for the breadth of the spectral lines at the galaxy center? Is it just a smear due to over-exposure, or simply to the finite size of the slit used to filter the light, along with the smear of tangential velocities that passed through the slit filter?

 

[mfb]

Now I'm getting confused. Do you mean tangential velocities, not radial? Ah...never mind. You mean the radial velocity of the entire galaxy, not the rotationally-induced shifts.

Radial as seen from us (distance to us), not radial in the galaxy.

I meant redshift where I wrote blueshift, my mistake.

Anyone have any idea what accounts for the breadth of the spectral lines at the galaxy center? Is it just a smear due to over-exposure, or simply to the finite size of the slit used to filter the light, along with the smear of tangential velocities that passed through the slit filter?

Radial velocities increase near the central black hole and all these stars might be too close to resolve that region.