APOD: Rubin's Galaxy (2021 Sep 18)

Comments and questions about the APOD on the main view screen.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by alter-ego » Sun Sep 19, 2021 7:46 pm

johnnydeep wrote: Sun Sep 19, 2021 4:19 pm Let me add one more highly questionable "source" for a 800+ Kly diameter. From http://www.poyntsource.com/Richard/ugc_2885.htm
Type = Sc Spiral Galaxy in Perseus
RA = 3h 53.5m 2.5s DEC = +35° 35' 18'' Distance ~ 96 Mpc Diameter ~ 250 kpc
m = +13.5 Apparent size = 5.5'
Mass = 2 x 1012 M¤ (Milky Way = 5 x 1011 M¤ ) Redshift = 5,800 km/sec, not unusual

The spiral galaxy UGC 2885 is the largest known spiral galaxy being some 250,000 parsecs in size (815,000 light years). It is some 10 times larger than the Milky Way ! The diagram below shows the comparison of UGC 2885's size vs. some other well known galaxies.

This makes UGS 2885 appear comically large, and almost certainly incorrect since it makes the visible disk itself appear to be over 800 Kly in diameter! PS - that seemingly second galactic nucleus is in fact the very brightest foreground star in the APOD!

It's unclear where they got the source for the size, but one size is listed in this rather old - pre Hubble - reference from 1993.

"Canzian, B. Allen, R.J., Tilanus, R.P.J., 1993, Spiral Structure of the Giant Galaxy UGC 2885: Hά Kinematics, Astrophysical Journal, 406, p. 457."

Which I found at https://ui.adsabs.harvard.edu/abs/1993A ... C/abstract, or https://www.researchgate.net/publicatio ... kinematics, in which it states a Holmberg radius:
The velocity field of the exceptionally large Sc galaxy UGC 2885 (Holmberg radius 84 h^-1^ kpc, where h = H_0_/100 km s^-1^ Mpc^-1^) has been mapped in Hα emission with the TAURUS I imaging Fabry-Perot spectrometer using the 2.5 m telescope at La Palma. The rotation curve extracted from the velocity field agrees with published data. Ripples in the velocity field around the minor axis indicate radial flows across the spiral arms. The radial flow speeds in the plane of the disk show 50-70 km s^-1^ peak-to-peak variation, suggesting that a strong density wave is present in the underlying stellar disk. Such high speeds may alternatively be a natural consequence of the open arm spiral pattern. In addition, strong density waves may naturally occur in large spiral galaxies or in spiral galaxies as massive as UGC 2885. (Its mass is over 10^12^ h^-1^ M_sun_ within the radius to which spiral arms reach, 52 h^-1^ kpc.) A strong density wave may also be necessary for the effective maintenance of the orderly, two-armed spiral pattern that is visible in the outer disk of UGC 2885, where the gas has made only about a dozen revolutions in a Hubble time.
So, the Holmberg diameter would be 168 kpc, or 547 kly. Now, what's a Holmberg radius? From http://astro.vaporia.com/start/holmbergradius.html
The Holmberg radius of a galaxy (RH or RHO) is a measure of its radius (along the semi-major axis) based upon surface brightness. It is specifically the radius to the region of the galaxy's surface where the surface brightness has an apparent magnitude of 26.5 per square arcsecond through a B filter. In other words, that size area of that amount of surface brightness would provide light equivalent to a star of such magnitude. Since surface brightness generally does not decline with distance, the radius generally indicates the same portion of the galaxy, no matter how distant.
• Your first link is a good example of how easy it is to get wrong / misleading information.
• A 10% difference between our visible image size calcs is certainly within reasonable error:
   1) I chose 4.5' for the max angular size, and 2) I used the average SIMBAD distance =75Mpc ± ~10%
• The Holmberg radius does explain the 800kly diameter. It appears you chose Ho = 100 km∙[sec∙Mpc]-1? For this work, the average Hubble constant was close to 68 km∙[sec∙Mpc]-1, yielding a Holmberg diameter = 805 kly.

The Holmberg radius at least defines a standard dimension. However, I'd expect it could be problematic in regions of the sky where scattered light from nearby nebula or the MW IFN would bias the result to larger sizes, e.g. the average IFN brightness is near the Holmberg criteria (e.g. M81 & M82). Maybe this bias would not be a problem for most galaxies.
   
I've not seen reference to the Holmberg radius before, possibly because common galaxy images don't go that faint without overexposure of the main body. The M51 example would be a great baseline comparison to an equivalently imaged UGC 2885.
 
DEEP IMAGING OF M51: A NEW VIEW OF THE WHIRLPOOL’S EXTENDED TIDAL DEBRIS (2018)
M51 - Extended Size.jpg
 
So, I'd be happier if the APOD description had simply noted the 800-kly dimension as the Holmberg radius (also linked).
However, I'd guess defining the Holmberg radius for the MW would be a bigger challenge and have a large(r) uncertainty.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Fred the Cat » Sun Sep 19, 2021 8:12 pm

Explored has been dark matter’s effect on time. If time dilation occurs for the stars closer to a galaxy’s halo, shouldn’t the distance traveled per unit time be altered? We study the effects on humans and, by using more sensitive clocks, dark matter but how can we measure rotation of distant objects if time or size is different than we perceive from Earth? :?

[Time -(Physics) a quantity measuring duration, usually with reference to a periodic process such as the rotation of the earth or the vibration of electromagnetic radiation emitted from certain atoms. In classical mechanics, time is absolute in the sense that the time of an event is independent of the observer. According to the theory of relativity it depends on the observer's frame of reference. Time is considered as a fourth coordinate required, along with three spatial coordinates, to specify an event]
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by NGC3314 » Sun Sep 19, 2021 9:00 pm

johnnydeep wrote: Sat Sep 18, 2021 7:34 pm The longer the photographic exposure, the more the dimmer star stuff starts appearing at the outer edges of any galaxy, thereby increasing its apparent size. I think we need a definition of diameter something like: the minimal extent within which 95% (say) of the mass is concentrated.
A recently popular measure of galaxy sizes (standard in most uses of the Sloan Digital Sky Survey catalogs, for example) has become the Petrosian radius, the radius where the surface brightness has dropped to some fraction (usually 20%) of its average value inside that radius. (Almost accidentally, this has the advantage of being remarkably insensitive to galaxy shape even if one adds the intensity in circular rather than elliptical apertures). In principle, this is insensitive to the detection threshold (in practice, one can be fooled if much of the galaxy light is too faint compared to the noise level).

Using some red-light data taken with the 1-meter remotely-operated SARA telescope in Arizona, I did this measurement before there were Sloan or Legacy Survey data in this region. UGC 2885 has a Petrosian radius 97 arcseconds or 38 kpc. For comparison, NGC 6872 (the Condor), with its tidally stretched arms, has Petrosian radius 185 arcsec or 32.5 kpc.

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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Chris Peterson » Sun Sep 19, 2021 9:10 pm

Fred the Cat wrote: Sun Sep 19, 2021 8:12 pm Explored has been dark matter’s effect on time.
Neither of those papers appear to address what you suggest they do.
If time dilation occurs for the stars closer to a galaxy’s halo, shouldn’t the distance traveled per unit time be altered? We study the effects on humans and, by using more sensitive clocks, dark matter but how can we measure rotation of distant objects if time or size is different than we perceive from Earth? :?
The gravitational fields created by galaxies, with both normal matter and dark matter, are vastly too small to result in significant (or even measurable) time dilation effects. We don't see gravitational time dilation outside the realm of black holes or very near the surface of condensed matter bodies like neutron stars.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by alter-ego » Mon Sep 20, 2021 3:46 am

Chris Peterson wrote: Sun Sep 19, 2021 9:10 pm
Fred the Cat wrote: Sun Sep 19, 2021 8:12 pm Explored has been dark matter’s effect on time.
Neither of those papers appear to address what you suggest they do.
If time dilation occurs for the stars closer to a galaxy’s halo, shouldn’t the distance traveled per unit time be altered? We study the effects on humans and, by using more sensitive clocks, dark matter but how can we measure rotation of distant objects if time or size is different than we perceive from Earth? :?
The gravitational fields created by galaxies, with both normal matter and dark matter, are vastly too small to result in significant (or even measurable) time dilation effects. We don't see gravitational time dilation outside the realm of black holes or very near the surface of condensed matter bodies like neutron stars.
Not sure if I'm connecting with your point, but time dilations have been measured for both local and galactic cluster environments:
Certainly gravitational time dilation has been observed on and around Earth with extremely accurate clocks:
https://en.wikipedia.org/wiki/Time_dilation] wrote:Experimental testing
• In 1959, Robert Pound and Glen A. Rebka measured the very slight gravitational redshift in the frequency of light emitted at a lower height,   where Earth's gravitational field is relatively more intense. The results were within 10% of the predictions of general relativity.
   In 1964, Pound and J. L. Snider measured a result within 1% of the value predicted by gravitational time dilation.[34] (See Pound–Rebka    experiment)
• In 2010, gravitational time dilation was measured at the Earth's surface with a height difference of only one meter, using optical atomic clocks.[22]
More directed towards astronomy and measuring wavelength shift (doppler & gravitational):
https://en.wikipedia.org/wiki/Gravitational_redshift wrote:Astronomical observations
...
In 2020, a team of scientists published the most accurate measurement of the solar gravitational redshift so far, made by analyzing Fe spectral lines in sunlight reflected by the moon; their measurement of a mean global 638 ± 6 m/s lineshift is in agreement with the theoretical value of 633.1 m/s.[13],[14] Measuring the solar redshift is complicated by the Doppler shift caused by the motion of the sun's surface, which is of similar magnitude as the gravitational effect.[14]
 
In 2011 the group of Radek Wojtak of the Niels Bohr Institute at the University of Copenhagen collected data from 8000 galaxy clusters and found that the light coming from the cluster centers tended to be red-shifted compared to the cluster edges, confirming the energy loss due to gravity.[15]
If my calculations are correct:
• The solar gravitational redshift measurement yields a time dilation ≈ 10 ppb, and
   for a nominal 500nm Fe line, the measured wavelength shift ≈ 5fm (they started with over 300 lines, and ended up with 97 in the final set)
• Granted, sampling 8000 galaxy clusters to see a tendency for greater redshift toward the cluster center implies small redshift changes, but the    measurement does not include black holes or neutron stars.

I have to admit, these latter redshift measurements seem very challenging and I haven't researched the details enough have more confidence in the claims.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Chris Peterson » Mon Sep 20, 2021 4:11 am

alter-ego wrote: Mon Sep 20, 2021 3:46 am
Chris Peterson wrote: Sun Sep 19, 2021 9:10 pm
Fred the Cat wrote: Sun Sep 19, 2021 8:12 pm Explored has been dark matter’s effect on time.
Neither of those papers appear to address what you suggest they do.
If time dilation occurs for the stars closer to a galaxy’s halo, shouldn’t the distance traveled per unit time be altered? We study the effects on humans and, by using more sensitive clocks, dark matter but how can we measure rotation of distant objects if time or size is different than we perceive from Earth? :?
The gravitational fields created by galaxies, with both normal matter and dark matter, are vastly too small to result in significant (or even measurable) time dilation effects. We don't see gravitational time dilation outside the realm of black holes or very near the surface of condensed matter bodies like neutron stars.
Not sure if I'm connecting with your point, but time dilations have been measured for both local and galactic cluster environments:
Certainly gravitational time dilation has been observed on and around Earth with extremely accurate clocks:
https://en.wikipedia.org/wiki/Time_dilation] wrote:Experimental testing
• In 1959, Robert Pound and Glen A. Rebka measured the very slight gravitational redshift in the frequency of light emitted at a lower height,   where Earth's gravitational field is relatively more intense. The results were within 10% of the predictions of general relativity.
   In 1964, Pound and J. L. Snider measured a result within 1% of the value predicted by gravitational time dilation.[34] (See Pound–Rebka    experiment)
• In 2010, gravitational time dilation was measured at the Earth's surface with a height difference of only one meter, using optical atomic clocks.[22]
More directed towards astronomy and measuring wavelength shift (doppler & gravitational):
https://en.wikipedia.org/wiki/Gravitational_redshift wrote:Astronomical observations
...
In 2020, a team of scientists published the most accurate measurement of the solar gravitational redshift so far, made by analyzing Fe spectral lines in sunlight reflected by the moon; their measurement of a mean global 638 ± 6 m/s lineshift is in agreement with the theoretical value of 633.1 m/s.[13],[14] Measuring the solar redshift is complicated by the Doppler shift caused by the motion of the sun's surface, which is of similar magnitude as the gravitational effect.[14]
 
In 2011 the group of Radek Wojtak of the Niels Bohr Institute at the University of Copenhagen collected data from 8000 galaxy clusters and found that the light coming from the cluster centers tended to be red-shifted compared to the cluster edges, confirming the energy loss due to gravity.[15]
If my calculations are correct:
• The solar gravitational redshift measurement yields a time dilation ≈ 10 ppb, and
   for a nominal 500nm Fe line, the measured wavelength shift ≈ 5fm (they started with over 300 lines, and ended up with 97 in the final set)
• Granted, sampling 8000 galaxy clusters to see a tendency for greater redshift toward the cluster center implies small redshift changes, but the    measurement does not include black holes or neutron stars.

I have to admit, these latter redshift measurements seem very challenging and I haven't researched the details enough have more confidence in the claims.
Okay, I'll accept measurable (with some difficulty). But the point remains that gravitational time dilation has no significant impact on any of the usual measurements we make of galaxies, as apparently suggested in the post I was responding to.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by johnnydeep » Mon Sep 20, 2021 4:10 pm

Ok, back to the size of Rubin's galaxy. I watched a talk by astronomer Benne Holwerda, University of Louisville, on the Hubble Space Telescope channel here -

Click to play embedded YouTube video.

After 10 minutes of background on Vera Rubin there's an hour long talk followed by some questions and answers. In it, Benne discusses Rubin's galaxy and this Hubble pic specifically, and the various things now know about it. The talk was done several months after the last time this same pic was in an APOD (yes, it's a repeat), which was Jan 25, 2020.

I was surprised to learn that the pic I posted previously with the seemingly wildly off base galaxy sizes actually appeared in a paper by Rubin herself! See about 26:05 into the video. The picture is apparently from the second paper by Rubin in 1980. Here's what's shown in the video:

rubins galaxy size comparison.JPG

Beene doesn't question any of the sizes shown, nor specifically where the huge size of Rubin's galaxy is coming from. Later, starting at 55:45, there's a discussion of a graph of galaxy mass vs radius that seems to indicate an "effective radius" for Rubin's galaxy of about 101.3 kpc (=20 kpc), but that's much smaller than other numbers given:

rubins galaxy radius.JPG

So, what's this about? Am I misinterpreting the graph? I'm tempted to email Beene and ask him to weigh in here and to explain where the huge size of Rubin's galaxy is coming from.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Fred the Cat » Mon Sep 20, 2021 4:30 pm

Chris Peterson wrote: Mon Sep 20, 2021 4:11 am
alter-ego wrote: Mon Sep 20, 2021 3:46 am
Chris Peterson wrote: Sun Sep 19, 2021 9:10 pm
Neither of those papers appear to address what you suggest they do.



The gravitational fields created by galaxies, with both normal matter and dark matter, are vastly too small to result in significant (or even measurable) time dilation effects. We don't see gravitational time dilation outside the realm of black holes or very near the surface of condensed matter bodies like neutron stars.
Not sure if I'm connecting with your point, but time dilations have been measured for both local and galactic cluster environments:
Certainly gravitational time dilation has been observed on and around Earth with extremely accurate clocks:
https://en.wikipedia.org/wiki/Time_dilation] wrote:Experimental testing
• In 1959, Robert Pound and Glen A. Rebka measured the very slight gravitational redshift in the frequency of light emitted at a lower height, where Earth's gravitational field is relatively more intense. The results were within 10% of the predictions of general relativity.
In 1964, Pound and J. L. Snider measured a result within 1% of the value predicted by gravitational time dilation.[34] (See Pound–Rebka experiment)
• In 2010, gravitational time dilation was measured at the Earth's surface with a height difference of only one meter, using optical atomic clocks.[22]
More directed towards astronomy and measuring wavelength shift (doppler & gravitational):
https://en.wikipedia.org/wiki/Gravitational_redshift wrote:Astronomical observations
...
In 2020, a team of scientists published the most accurate measurement of the solar gravitational redshift so far, made by analyzing Fe spectral lines in sunlight reflected by the moon; their measurement of a mean global 638 ± 6 m/s lineshift is in agreement with the theoretical value of 633.1 m/s.[13],[14] Measuring the solar redshift is complicated by the Doppler shift caused by the motion of the sun's surface, which is of similar magnitude as the gravitational effect.[14]

In 2011 the group of Radek Wojtak of the Niels Bohr Institute at the University of Copenhagen collected data from 8000 galaxy clusters and found that the light coming from the cluster centers tended to be red-shifted compared to the cluster edges, confirming the energy loss due to gravity.[15]
If my calculations are correct:
• The solar gravitational redshift measurement yields a time dilation ≈ 10 ppb, and
for a nominal 500nm Fe line, the measured wavelength shift ≈ 5fm (they started with over 300 lines, and ended up with 97 in the final set)
• Granted, sampling 8000 galaxy clusters to see a tendency for greater redshift toward the cluster center implies small redshift changes, but the measurement does not include black holes or neutron stars.

I have to admit, these latter redshift measurements seem very challenging and I haven't researched the details enough have more confidence in the claims.
Okay, I'll accept measurable (with some difficulty). But the point remains that gravitational time dilation has no significant impact on any of the usual measurements we make of galaxies, as apparently suggested in the post I was responding to.
It was a simple thought experiment. We know the stars, for some reason, orbit galaxies at speeds explained by presence of dark matter in their halos. Presume their speeds were equal but time was stretched. We would see them moving faster than expected. In our time reference a star moving 10 miles per second near the center. In the galactic halo star time reference 1 second is 10 seconds. It might appear to us the star is moving 100 miles.

As to size in our reference, if dark matter pervades the universe and affects time, distances might appear to us greater than we measure.

Again, just a scat-brain thought experiment if dark matter affects time rather than gravity. :roll:
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by Chris Peterson » Mon Sep 20, 2021 4:56 pm

Fred the Cat wrote: Mon Sep 20, 2021 4:30 pm
Chris Peterson wrote: Mon Sep 20, 2021 4:11 am Okay, I'll accept measurable (with some difficulty). But the point remains that gravitational time dilation has no significant impact on any of the usual measurements we make of galaxies, as apparently suggested in the post I was responding to.
It was a simple thought experiment. We know the stars, for some reason, orbit galaxies at speeds explained by presence of dark matter in their halos. Presume their speeds were equal but time was stretched. We would see them moving faster than expected. In our time reference a star moving 10 miles per second near the center. In the galactic halo star time reference 1 second is 10 seconds. It might appear to us the star is moving 100 miles.

As to size in our reference, if dark matter pervades the universe and affects time, distances might appear to us greater than we measure.

Again, just a scat-brain thought experiment if dark matter affects time rather than gravity. :roll:
Except there is no theoretical basis for believing that dark matter affects time, and no observational evidence that it does so, other than the well known relationship between gravity and time... which depends on mass, and we know the mass of dark matter quite accurately- and it isn't enough to have any effect much above the noise floor of our measurement abilities.

(And the orbits of stars in galaxies is only one piece of evidence for dark matter. Affecting time wouldn't explain others, such as the structure of the CMB or the nature of gravitational lenses.)
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by neufer » Mon Sep 20, 2021 7:31 pm

Chris Peterson wrote: Mon Sep 20, 2021 4:56 pm
(And the orbits of stars in galaxies is only one piece of evidence for dark matter.

Affecting time wouldn't explain others, such as the structure of the CMB or the nature of gravitational lenses.)
https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence wrote:
Dark Matter Observational evidence:
  • 1. Galaxy rotation curves
    2. Velocity dispersions
    3. Galaxy clusters
    4. Gravitational lensing
    5. Cosmic microwave background
    6. Structure formation
    7. Bullet Cluster
    8. Type Ia supernova distance measurements
    9. Sky surveys and baryon acoustic oscillations
    10. Redshift-space distortions
    11. Lyman-alpha forest
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by neufer » Mon Sep 20, 2021 10:07 pm

johnnydeep wrote: Mon Sep 20, 2021 4:10 pm
Click to play embedded YouTube video.
I watched a talk by astronomer Benne Holwerda, University of Louisville, on the Hubble Space Telescope channel here :arrow:

After 10 minutes of background on Vera Rubin there's an hour long talk followed by some questions and answers. In it, Benne discusses Rubin's galaxy and this Hubble pic specifically, and the various things now know about it. The talk was done several months after the last time this same pic was in an APOD (yes, it's a repeat), which was Jan 25, 2020.

I was surprised to learn that the pic I posted previously with the seemingly wildly off base galaxy sizes actually appeared in a paper by Rubin herself! See about 26:05 into the video. The picture is apparently from the second paper by Rubin in 1980.

Beene doesn't question any of the sizes shown, nor specifically where the huge size of Rubin's galaxy is coming from. Later, starting at 55:45, there's a discussion of a graph of galaxy mass vs radius that seems to indicate an "effective radius" for Rubin's galaxy of about 101.3 kpc (=20 kpc), but that's much smaller than other numbers given.

So, what's this about? Am I misinterpreting the graph? I'm tempted to email Beene and ask him to weigh in here and to explain where the huge size of Rubin's galaxy is coming from.
The Andromeda Galaxy (Messier 31) fits the upper curves:
  • Log(M) ~ 12.2
    Log(R) ~ 1.53
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by alter-ego » Tue Sep 21, 2021 4:09 am

Chris Peterson wrote: Mon Sep 20, 2021 4:11 am
alter-ego wrote: Mon Sep 20, 2021 3:46 am
Chris Peterson wrote: Sun Sep 19, 2021 9:10 pm
Neither of those papers appear to address what you suggest they do.



The gravitational fields created by galaxies, with both normal matter and dark matter, are vastly too small to result in significant (or even measurable) time dilation effects. We don't see gravitational time dilation outside the realm of black holes or very near the surface of condensed matter bodies like neutron stars.
Not sure if I'm connecting with your point, but time dilations have been measured for both local and galactic cluster environments:
Certainly gravitational time dilation has been observed on and around Earth with extremely accurate clocks:
https://en.wikipedia.org/wiki/Time_dilation] wrote:Experimental testing
• In 1959, Robert Pound and Glen A. Rebka measured the very slight gravitational redshift in the frequency of light emitted at a lower height,   where Earth's gravitational field is relatively more intense. The results were within 10% of the predictions of general relativity.
   In 1964, Pound and J. L. Snider measured a result within 1% of the value predicted by gravitational time dilation.[34] (See Pound–Rebka    experiment)
• In 2010, gravitational time dilation was measured at the Earth's surface with a height difference of only one meter, using optical atomic clocks.[22]
More directed towards astronomy and measuring wavelength shift (doppler & gravitational):
https://en.wikipedia.org/wiki/Gravitational_redshift wrote:Astronomical observations
...
In 2020, a team of scientists published the most accurate measurement of the solar gravitational redshift so far, made by analyzing Fe spectral lines in sunlight reflected by the moon; their measurement of a mean global 638 ± 6 m/s lineshift is in agreement with the theoretical value of 633.1 m/s.[13],[14] Measuring the solar redshift is complicated by the Doppler shift caused by the motion of the sun's surface, which is of similar magnitude as the gravitational effect.[14]
 
In 2011 the group of Radek Wojtak of the Niels Bohr Institute at the University of Copenhagen collected data from 8000 galaxy clusters and found that the light coming from the cluster centers tended to be red-shifted compared to the cluster edges, confirming the energy loss due to gravity.[15]
If my calculations are correct:
• The solar gravitational redshift measurement yields a time dilation ≈ 10 ppb, and
   for a nominal 500nm Fe line, the measured wavelength shift ≈ 5fm (they started with over 300 lines, and ended up with 97 in the final set)
• Granted, sampling 8000 galaxy clusters to see a tendency for greater redshift toward the cluster center implies small redshift changes, but the    measurement does not include black holes or neutron stars.

I have to admit, these latter redshift measurements seem very challenging and I haven't researched the details enough have more confidence in the claims.
Okay, I'll accept measurable (with some difficulty). But the point remains that gravitational time dilation has no significant impact on any of the usual measurements we make of galaxies, as apparently suggested in the post I was responding to.
Yup, I agree with that. :)
Oh, my calcs need correcting. I've been bothered by just how small the shifts are. Turns out I got stuck on using the wrong escape velocity for the Sun.
The time dilation = 2 ppm, and the wavelength shift = 1 pm. Hey, ~1000x larger is not to be ignored.
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by johnnydeep » Tue Sep 21, 2021 12:41 pm

neufer wrote: Mon Sep 20, 2021 10:07 pm
johnnydeep wrote: Mon Sep 20, 2021 4:10 pm
Click to play embedded YouTube video.
I watched a talk by astronomer Benne Holwerda, University of Louisville, on the Hubble Space Telescope channel here :arrow:

After 10 minutes of background on Vera Rubin there's an hour long talk followed by some questions and answers. In it, Benne discusses Rubin's galaxy and this Hubble pic specifically, and the various things now know about it. The talk was done several months after the last time this same pic was in an APOD (yes, it's a repeat), which was Jan 25, 2020.

I was surprised to learn that the pic I posted previously with the seemingly wildly off base galaxy sizes actually appeared in a paper by Rubin herself! See about 26:05 into the video. The picture is apparently from the second paper by Rubin in 1980.

Beene doesn't question any of the sizes shown, nor specifically where the huge size of Rubin's galaxy is coming from. Later, starting at 55:45, there's a discussion of a graph of galaxy mass vs radius that seems to indicate an "effective radius" for Rubin's galaxy of about 101.3 kpc (=20 kpc), but that's much smaller than other numbers given.

So, what's this about? Am I misinterpreting the graph? I'm tempted to email Beene and ask him to weigh in here and to explain where the huge size of Rubin's galaxy is coming from.
The Andromeda Galaxy (Messier 31) fits the upper curves:
  • Log(M) ~ 12.2
    Log(R) ~ 1.53
Apparently. But then UGC 2885 is still shown on the chart (with a Y cross) having an effective radius less than that of Andromeda. I have to ask: WTH? Unless "effective radius" means something special I'm not aware of.
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neufer
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Re: APOD: Rubin's Galaxy (2021 Sep 18)

Post by neufer » Tue Sep 21, 2021 1:45 pm

johnnydeep wrote: Tue Sep 21, 2021 12:41 pm
neufer wrote: Mon Sep 20, 2021 10:07 pm
johnnydeep wrote: Mon Sep 20, 2021 4:10 pm
I'm tempted to email Beene and ask him to weigh in here
and to explain where the huge size of Rubin's galaxy is coming from.
The Andromeda Galaxy (Messier 31) fits the upper curves:
  • Log(M) ~ 12.2 : Log(R) ~ 1.53
Apparently. But then UGC 2885 is still shown on the chart (with a Y cross) having an effective radius less than that of Andromeda. I have to ask: WTH? Unless "effective radius" means something special I'm not aware of.
Click to play embedded YouTube video.
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.
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  • UGC 2885 is Abby Normal :arrow:
Art Neuendorffer