Starship Asterisk*

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.

• UGC 2885 is Abby Normal

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

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 10^1.3 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

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.

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

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 10^1.3 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.

• Log(M) ~ 12.2
  Log(R) ~ 1.53

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

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:

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.

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.

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.

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]

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]

Fred the Cat wrote: ↑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? :?

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.

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.

Click to view full size image

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.

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.

Click to view full size image

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.

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.

alter-ego wrote: ↑Sun Sep 19, 2021 4:12 am

Chris Peterson wrote: ↑Sat Sep 18, 2021 8:00 pm

johnnydeep wrote: ↑Sat Sep 18, 2021 7:34 pm

True. 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.

It's analogous to measuring beam diameters in optics and radio technology. Beams don't typically have sharp edges. And there are well defined criteria for assigning diameters, like 1/e² or full width at half maximum (FWHM), the latter being very common for measuring stellar image diameters.

I think an image or link should be referenced for to back up that 800-kly size. For example, maybe it has to do with a measured globular cluster distribution. I don't like the fact the stated size is ~2.5x larger than the size of the presented Hubble image. At least state the visible size in the APOD image (~320,000ly), then state the extended size and what measurements it's based on, with the appropriate links. Even lacking a formal diameter definition, I'd be satisfied with any image, or data plot that could accurately show a very faint fuzz-ball surrounding the galaxy 2.5x the displayed visible size. Also, in order to claim a relative size, the same data must exist in some form for the MW.
Yeah, I'm not happy with the APOD description here. To me, it's misleading considering the presented image.

johnnydeep wrote: ↑Sat Sep 18, 2021 3:51 pm

Ann wrote: ↑Sat Sep 18, 2021 11:02 am

Tom Fleming wrote: ↑Sat Sep 18, 2021 10:32 am To quote today Sep 18th ...it has around 1 trillion stars. That's about 10 times as many stars as the Milky Way. So a population of 100 billion for our MW. Modern convention puts the actual number around 3-400 billion as I'm given to understand. Is this just a rounding off for a bit of dramatic presentation?
Thanks

The way I understand it - and I haven't tried to look it up, mind you - is that the Milky Way has a huge halo that really pushes the number of stars belonging to our galaxy to whole new levels.

I think a figure of 100 billion stars is way too low for our galaxy.

Ann

Also, the Wikipedia article for UGC 2885 cites its size as "only" 463000 ly, not 800000 ly, and that it could have grown this large, not via mergers, but the slow and steady accumulation of intergalactic gas. From https://en.wikipedia.org/wiki/UGC_2885:

UGC 2885 is classified as a field galaxy—a class of galaxies found in remote, under-dense and “vacant” sections of space, far from other major galaxies. NASA has reported that the theorized main source for disk growth for UGC 2885 came from the accretion of intergalactic hydrogen gas, rather than through the repeated process of galactic collision, as most galaxies are thought to grow.

The lack of interaction is evident from the near-perfect structure of the spiral arms and disk, lack of tidal tails, and modest rate of star formation—approximately 0.5 solar masses/year.

Additionally, despite being originally classified as an unbarred spiral galaxy, new Hubble images clearly show the presence of a small bar cutting across the ring structure of the core. This is peculiar, as most bars are thought to form through minor gravitational perturbations brought on by satellite and neighboring galaxies, which is something this galaxy lacks. This galaxy highlights that bars are able to form in spiral galaxies without the influence of another galaxy—this indicates that other forces, such as interactions between stars, gas and dust, as well as the gravitational influence of dark matter, might play a role in their development.

Chris Peterson wrote: ↑Sat Sep 18, 2021 8:00 pm

johnnydeep wrote: ↑Sat Sep 18, 2021 7:34 pm

Chris Peterson wrote: ↑Sat Sep 18, 2021 7:24 pm

Between uncertainties in distance and the lack of any well defined edge, I'm skeptical of the claimed diameters of any galaxies. I'm not sure it's even that meaningful of a concept in many cases.

True. 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.

It's analogous to measuring beam diameters in optics and radio technology. Beams don't typically have sharp edges. And there are well defined criteria for assigning diameters, like 1/e² or full width at half maximum (FWHM), the latter being very common for measuring stellar image diameters.

Chris Peterson wrote: ↑Sat Sep 18, 2021 8:40 pm

Ann wrote: ↑Sat Sep 18, 2021 8:31 pm

Red light image of galaxy NGC 4565. Uploaded by Wen-Ping Chen at Researchgate.

---

Take a look at the above image of galaxy NGC 4565 in red light, 6660 Å (666 nm).

It sure looks as if the bright optical disk of this galaxy has an edge, albeit a diffuse one.

Ann

Which is what a criterion like I referenced above would allow a number to be assigned to. But in fact, some or most galaxies do have a large halo of gravitationally bound stars around them, with a much lower density than the more visible galaxy. So how does that get measured and quantified? It's kind of like our own Solar System. At its extreme, it may extend two light years- meaning that there are probably gravitationally bound objects orbiting the Sun at that distance. Where do we draw the line, though, in defining its size?

johnnydeep wrote: ↑Sat Sep 18, 2021 7:04 pm

orin stepanek wrote: ↑Sat Sep 18, 2021 12:58 pm RubinsGalaxy_hst1024.jpg

Does extended Galaxy refer to it's large size?
...

It seems so. From the link https://ui.adsabs.harvard.edu/abs/2017h ... H/abstract

UGC 2885 was discoverd to be the most extended disk galaxy [250 kpc diameter] by Vera Rubin in the 1980's. We ask for HST observations of UGC 2885 as it is close enough to resolve the GC population with HST but it is a substantially more extended disk than any studied before.

BTW, this 2017 reference at least agrees with the 800 kly diameter figure in the APOD text (250 kpc * 3.26 = 815 kly). The Wikipedia article quotes a diameter of only 463 kly based on a reference to several sources but I can't find that value actually confirmed in any of them. Personally, I like the larger value since it increases the pure awesomeness of Rubin's Galaxy!

Ann wrote: ↑Sat Sep 18, 2021 8:31 pm

Red light image of galaxy NGC 4565. Uploaded by Wen-Ping Chen at Researchgate.

---

Take a look at the above image of galaxy NGC 4565 in red light, 6660 Å (666 nm).

It sure looks as if the bright optical disk of this galaxy has an edge, albeit a diffuse one.

Ann

johnnydeep wrote: ↑Sat Sep 18, 2021 7:34 pm

Chris Peterson wrote: ↑Sat Sep 18, 2021 7:24 pm

johnnydeep wrote: ↑Sat Sep 18, 2021 7:04 pm

It seems so. From the link https://ui.adsabs.harvard.edu/abs/2017h ... H/abstract



BTW, this 2017 reference at least agrees with the 800 kly diameter figure in the APOD text (250 kpc * 3.26 = 815 kly). The Wikipedia article quotes a diameter of only 463 kly based on a reference to several sources but I can't find that value actually confirmed in any of them. Personally, I like the larger value since it increases the pure awesomeness of Rubin's Galaxy!

Between uncertainties in distance and the lack of any well defined edge, I'm skeptical of the claimed diameters of any galaxies. I'm not sure it's even that meaningful of a concept in many cases.

True. 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.

Chris Peterson wrote: ↑Sat Sep 18, 2021 7:24 pm

johnnydeep wrote: ↑Sat Sep 18, 2021 7:04 pm

orin stepanek wrote: ↑Sat Sep 18, 2021 12:58 pm RubinsGalaxy_hst1024.jpg

Does extended Galaxy refer to it's large size?
...

It seems so. From the link https://ui.adsabs.harvard.edu/abs/2017h ... H/abstract

UGC 2885 was discoverd to be the most extended disk galaxy [250 kpc diameter] by Vera Rubin in the 1980's. We ask for HST observations of UGC 2885 as it is close enough to resolve the GC population with HST but it is a substantially more extended disk than any studied before.

BTW, this 2017 reference at least agrees with the 800 kly diameter figure in the APOD text (250 kpc * 3.26 = 815 kly). The Wikipedia article quotes a diameter of only 463 kly based on a reference to several sources but I can't find that value actually confirmed in any of them. Personally, I like the larger value since it increases the pure awesomeness of Rubin's Galaxy!

Between uncertainties in distance and the lack of any well defined edge, I'm skeptical of the claimed diameters of any galaxies. I'm not sure it's even that meaningful of a concept in many cases.