Redshifts of nearby galaxies

[Vanadium 50]

We have had a number of threads lately on the redshifts of Local Group galaxies. Replies have focused on how these are not cosmological in origin. This is of course true,

But there is something more basic. These redshifts are measured from Earth, not the center of the Milky Way. The Earth is in motion with respect to the galaxy as a whole and Galaxy X can be moving towards Earth but away from the galaxy as a whole or vice versa,

I write this so one can link to it next time it comes up,

 

[PeterDonis]

These redshifts are measured from Earth, not the center of the Milky Way.

Even the center of the Milky Way is not, AFAIK, comoving (i.e., a hypothetical observer at the center of the Milky Way and at rest relative to that center would not see the CMB as isotropic). To match against cosmological models, what we really want is the redshift relative to a comoving observer at our location. CMB temperatures are customarily corrected for this (to remove the dipole anisotropy due to the Earth's motion relative to comoving worldlines in our vicinity). Are redshift measurements similarly corrected? I don't see why they couldn't be.

 

[Vanadium 50]

Yumpin' Yiminy I came right out and said these are non-cosmological, What is the point of quibbling that they are non-cosmological? I know it - because I just said it,

The point I am making here is that these measurements are not what they are purported to be, Galaxy X can be moving towards Earth but away from the galaxy as a whole or vice versa.

 

[PeterDonis]

The point I am making here is that these measurements are not what they are purported to be

If the measurements are just given directly, without any corrections, then yes, of course they're not what one should be comparing to theoretical models.

My question is whether the measurements are in fact just given directly, without any corrections, in the literature, or whether corrections are applied and the redshifts that are given in the literature are not just the direct measurements but corrected values.

 

[Vanadium 50]

Yes, the measurements are given directly because that's where the telescopes are.

If you want some other number, you can convert. (But what should that other number be? MW-centered? Local Group-centered? Virgo Superclutter centered? CMBR centered?) If you like, what is reported is what the redshift is, and not what it would be if some other measurement were done.

 

[PeterDonis]

the measurements are given directly because that's where the telescopes are.

The fact that the telescopes are at a particular location in spacetime with a particular state of motion does not mean scientists are forced to only report the raw data taken from the telescopes. If doing so is in fact common practice, that's fine, but I'm wondering if there is something more to show that than just "that's where the telescopes are". Presumably the scientists reporting redshifts are aware of the state of motion of the telescopes relative to other objects of interest.

what should that other number be?

If one is comparing measured redshifts with cosmological models, it seems to me that, as I said before, the obvious number would be the redshift that would be measured by a comoving observer at the same point in spacetime as the actual observation event. Similarly, if one were trying to assess, say, the depth of the gravity well of the local group, or the Virgo supercluster, one would want numbers centered on that gravity well.

Even if "raw" redshifts are reported by experimentalists doing telescopic observations, I would expect scientists doing statistical analysis to compare with cosmological or other models to apply corrections.

 

[Bandersnatch]

Yes, the measurements are given directly because that's where the telescopes are.

If you want some other number, you can convert. (But what should that other number be? MW-centered? Local Group-centered? Virgo Superclutter centered? CMBR centered?) If you like, what is reported is what the redshift is, and not what it would be if some other measurement were done.

Have you read Hubble's seminal paper? It's entire thrust concerns removing the motion of the solar system from observables, and then reporting the result. Those velocities in the famous graph are w/r to the Galaxy, after correction.

 

[Vanadium 50]

You are correct. I should have said sun-centered and not earth-centered. This is so January and June data can be understood. It's also 3 parts in 23.

The Hubble paper was important, but not exactly right. The galaxies were largely too close. The distance ladder was also a we bit creaky.

The earth's...pardon, the sun's galactic motion works out to a bias in measurement of ~10 MLy, Anything inside that is trouble, anything well outside is OK.

 

[Ibix]

Just to add some numbers here, WMAP says (section 29.3.2) that the local group does around 620km/s wrt the CMB rest frame and the Earth does around 370km/s. Note the two orders of magnitude tighter bounds on the latter figure (~100m/s vs ~15km/s). I don't know what reporting conventions are (I note that the WMAP CMB images usually have the dipole subtracted out), but there is quite a difference between a Milky Way-centered velocity and a Sun-centered velocity when talking about nearby objects.

However, that peculiar-velocity-induced Doppler corresponds to adding or subtracting about 0.002 to the $z$ value, so it's a marginal note to anything with $z$ anywhere near 0.1 or larger. I presume a similar analysis is the basis of V50's ~10Mly rule of thumb in #8.

 

[Vanadium 50]

That's one problem. Here's the other.

If a galaxy is far away, its own internal motion is largely irrelevant. If it's close, what do you do? Pick the center. For M33 this is easy. M31 is not as nice, but it's easy to get close.

But what do you do for irregulars? Where is the "center" of the SMC?

 

[Jaime Rudas]

But what do you do for irregulars?

You could take spectrograms of different areas of the galaxy as Vesto Slipher did more than a century ago to discover the rotation of spiral galaxies.

 

[Vanadium 50]

Yes, and now you have these spectra. Which one represents the One True Velocity of the Irregular?

 

[Jaime Rudas]

Yes, and now you have these spectra. Which one represents the One True Velocity of the Irregular?

But what use is there in knowing the One True Velocity (whatever that means)?

 

[Jaime Rudas]

The Hubble paper was important, but not exactly right. The galaxies were largely too close. The distance ladder was also a we bit creaky.

Yes, but the redshifts (which is what the original post was about) were accurate enough despite being measured just over 100 years ago.

 

[Vanadium 50]

When you plot x vs y, it helps to measure both well.