Starship Asterisk*

by **Chris Peterson** » Fri Oct 07, 2022 2:43 pm

johnnydeep wrote: ↑Fri Oct 07, 2022 2:36 pm
Chris Peterson wrote: ↑Thu Oct 06, 2022 9:30 pm
johnnydeep wrote: ↑Thu Oct 06, 2022 9:02 pm And to quote the venerable, yet sometimes wrong Wikipedia article about the meaning of any given measured red shift, there are THREE main contributors to it:

(I take relativistic Doppler redshift to be simply the fully accurate way to measure the effect that relative velocity has on redshift.)

As an interesting example, all three causes could be significantly contributing to the measured redshift of a neutron star (deep gravity well effect) in a fast orbit around a black hole (relativistic Doppler shift effect) at a true distance of 5 Gly (meaningful cosmological expansion effect)!
I didn't include gravitational redshift because It plays almost no role in images like this, or in contributing significantly to the measured redshift of objects like galaxies. It's really only a factor when looking at light emerging from a deep gravitational well.

(Processes like scattering don't change the frequency of electromagnetic radiation, they just change the ratios between different frequencies. So the apparent color might change (as we see with the reddened light coming through dusty regions... or sunsets). But a source of some wavelength will still have the same wavelength after that optical process.)
Thanks for that extra clarification. So a true "redshift" will shift all frequencies of light toward the red end of the spectrum (always equally?), whereas scattering, absorption, etc., will only preferentially lessen or remove energy from certain frequencies? E.g., dust reddens by effectively removing blue wavelengths from our line of sight.

Exactly.

by **johnnydeep** » Fri Oct 07, 2022 2:36 pm

Chris Peterson wrote: ↑Thu Oct 06, 2022 9:30 pm
johnnydeep wrote: ↑Thu Oct 06, 2022 9:02 pm And to quote the venerable, yet sometimes wrong Wikipedia article about the meaning of any given measured red shift, there are THREE main contributors to it:

https://en.wikipedia.org/wiki/Redshift wrote: Redshift

In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a negative redshift, or blueshift. The terms derive from the colours red and blue which form the extremes of the visible light spectrum.

In astronomy and cosmology, the three main causes of electromagnetic redshift are:

• The radiation travels between objects which are moving apart ("relativistic" redshift, an example of the relativistic Doppler effect)
• The radiation travels towards an object in a weaker gravitational potential, i.e. towards an object in less strongly curved (flatter) spacetime (gravitational redshift)
• The radiation travels through expanding space (cosmological redshift). The observation that all sufficiently distant light sources show redshift corresponding to their distance from Earth is known as Hubble's law.

Relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. Gravitational waves, which also travel at the speed of light, are subject to the same redshift phenomena.

Examples of strong redshifting are a gamma ray perceived as an X-ray, or initially visible light perceived as radio waves. Subtler redshifts are seen in the spectroscopic observations of astronomical objects, and are used in terrestrial technologies such as Doppler radar and radar guns.

Other physical processes exist that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; however, the resulting changes are distinguishable from (astronomical) redshift and are not generally referred to as such (see section on physical optics and radiative transfer).

The value of a redshift is often denoted by the letter z, corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by the wavelength ratio 1 + z (which is >1 for redshifts, <1 for blueshifts).
(I take relativistic Doppler redshift to be simply the fully accurate way to measure the effect that relative velocity has on redshift.)

As an interesting example, all three causes could be significantly contributing to the measured redshift of a neutron star (deep gravity well effect) in a fast orbit around a black hole (relativistic Doppler shift effect) at a true distance of 5 Gly (meaningful cosmological expansion effect)!
I didn't include gravitational redshift because It plays almost no role in images like this, or in contributing significantly to the measured redshift of objects like galaxies. It's really only a factor when looking at light emerging from a deep gravitational well.

(Processes like scattering don't change the frequency of electromagnetic radiation, they just change the ratios between different frequencies. So the apparent color might change (as we see with the reddened light coming through dusty regions... or sunsets). But a source of some wavelength will still have the same wavelength after that optical process.)

Thanks for that extra clarification. So a true "redshift" will shift all frequencies of light toward the red end of the spectrum (always equally?), whereas scattering, absorption, etc., will only preferentially lessen or remove energy from certain frequencies? E.g., dust reddens by effectively removing blue wavelengths from our line of sight.

by **Chris Peterson** » Thu Oct 06, 2022 9:30 pm

johnnydeep wrote: ↑Thu Oct 06, 2022 9:02 pm And to quote the venerable, yet sometimes wrong Wikipedia article about the meaning of any given measured red shift, there are THREE main contributors to it:

https://en.wikipedia.org/wiki/Redshift wrote: Redshift

In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a negative redshift, or blueshift. The terms derive from the colours red and blue which form the extremes of the visible light spectrum.

In astronomy and cosmology, the three main causes of electromagnetic redshift are:

• The radiation travels between objects which are moving apart ("relativistic" redshift, an example of the relativistic Doppler effect)
• The radiation travels towards an object in a weaker gravitational potential, i.e. towards an object in less strongly curved (flatter) spacetime (gravitational redshift)
• The radiation travels through expanding space (cosmological redshift). The observation that all sufficiently distant light sources show redshift corresponding to their distance from Earth is known as Hubble's law.

Relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. Gravitational waves, which also travel at the speed of light, are subject to the same redshift phenomena.

Examples of strong redshifting are a gamma ray perceived as an X-ray, or initially visible light perceived as radio waves. Subtler redshifts are seen in the spectroscopic observations of astronomical objects, and are used in terrestrial technologies such as Doppler radar and radar guns.

Other physical processes exist that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; however, the resulting changes are distinguishable from (astronomical) redshift and are not generally referred to as such (see section on physical optics and radiative transfer).

The value of a redshift is often denoted by the letter z, corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by the wavelength ratio 1 + z (which is >1 for redshifts, <1 for blueshifts).
(I take relativistic Doppler redshift to be simply the fully accurate way to measure the effect that relative velocity has on redshift.)

As an interesting example, all three causes could be significantly contributing to the measured redshift of a neutron star (deep gravity well effect) in a fast orbit around a black hole (relativistic Doppler shift effect) at a true distance of 5 Gly (meaningful cosmological expansion effect)!

I didn't include gravitational redshift because It plays almost no role in images like this, or in contributing significantly to the measured redshift of objects like galaxies. It's really only a factor when looking at light emerging from a deep gravitational well.

(Processes like scattering don't change the frequency of electromagnetic radiation, they just change the ratios between different frequencies. So the apparent color might change (as we see with the reddened light coming through dusty regions... or sunsets). But a source of some wavelength will still have the same wavelength after that optical process.)

by **johnnydeep** » Thu Oct 06, 2022 9:02 pm

And to quote the venerable, yet sometimes wrong Wikipedia article about the meaning of any given measured red shift, there are THREE main contributors to it:

https://en.wikipedia.org/wiki/Redshift wrote: Redshift

In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in frequency and energy, is known as a negative redshift, or blueshift. The terms derive from the colours red and blue which form the extremes of the visible light spectrum.

In astronomy and cosmology, the three main causes of electromagnetic redshift are:

• The radiation travels between objects which are moving apart ("relativistic" redshift, an example of the relativistic Doppler effect)
• The radiation travels towards an object in a weaker gravitational potential, i.e. towards an object in less strongly curved (flatter) spacetime (gravitational redshift)
• The radiation travels through expanding space (cosmological redshift). The observation that all sufficiently distant light sources show redshift corresponding to their distance from Earth is known as Hubble's law.

Relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. Gravitational waves, which also travel at the speed of light, are subject to the same redshift phenomena.

Examples of strong redshifting are a gamma ray perceived as an X-ray, or initially visible light perceived as radio waves. Subtler redshifts are seen in the spectroscopic observations of astronomical objects, and are used in terrestrial technologies such as Doppler radar and radar guns.

Other physical processes exist that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; however, the resulting changes are distinguishable from (astronomical) redshift and are not generally referred to as such (see section on physical optics and radiative transfer).

The value of a redshift is often denoted by the letter z, corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by the wavelength ratio 1 + z (which is >1 for redshifts, <1 for blueshifts).

(I take relativistic Doppler redshift to be simply the fully accurate way to measure the effect that relative velocity has on redshift.)

As an interesting example, all three causes could be significantly contributing to the measured redshift of a neutron star (deep gravity well effect) in a fast orbit around a black hole (relativistic Doppler shift effect) at a true distance of 5 Gly (meaningful cosmological expansion effect)!

by **Chris Peterson** » Thu Oct 06, 2022 8:46 pm

johnnydeep wrote: ↑Thu Oct 06, 2022 8:41 pm
Ann wrote: ↑Thu Oct 06, 2022 7:53 pm
De58te wrote: ↑Thu Oct 06, 2022 7:12 pm We are told of the galaxy's yellowish core, dark dust clouds, bright blue star clusters, and red star forming regions are easy to spot.But what are the little green spots? You can see them best in the expanded large image. There is a green spot about a third way up the tail, and another green spot in the companion galaxy. But I find most curious of all is that if we assume that the whale's head is to the left, that at the lower back side of the head just below that bright red and white area which I assume could be the whale's eye you can see a long green rectangle that might be appear like a ladder is placed up next to the side. Of maybe a broken necklace still hanging in place. Could it be a photography glitch, or something really there that is green?
No large green "ladders" exist in galaxies. That's a photographic artifact.

Ann
So, are ALL the green spots likely to be photographic artifacts? For instance, all these:

green spots in ngc4361.JPG

Yes. The only thing that can create green extended zones would be emission nebulas, and there is no reason to think that's what we're seeing here.

BTW, I take this as a warning sign that the processing created a good deal of fine structure that isn't real. We see it with the green, because those areas can't be natural. But odds are that similar artifacts exist as well in the red and blue channels, but are pretty much impossible to see.

by **johnnydeep** » Thu Oct 06, 2022 8:41 pm

Ann wrote: ↑Thu Oct 06, 2022 7:53 pm
De58te wrote: ↑Thu Oct 06, 2022 7:12 pm We are told of the galaxy's yellowish core, dark dust clouds, bright blue star clusters, and red star forming regions are easy to spot.But what are the little green spots? You can see them best in the expanded large image. There is a green spot about a third way up the tail, and another green spot in the companion galaxy. But I find most curious of all is that if we assume that the whale's head is to the left, that at the lower back side of the head just below that bright red and white area which I assume could be the whale's eye you can see a long green rectangle that might be appear like a ladder is placed up next to the side. Of maybe a broken necklace still hanging in place. Could it be a photography glitch, or something really there that is green?
No large green "ladders" exist in galaxies. That's a photographic artifact.

Ann

So, are ALL the green spots likely to be photographic artifacts? For instance, all these:

by **Ann** » Thu Oct 06, 2022 7:53 pm

De58te wrote: ↑Thu Oct 06, 2022 7:12 pm We are told of the galaxy's yellowish core, dark dust clouds, bright blue star clusters, and red star forming regions are easy to spot.But what are the little green spots? You can see them best in the expanded large image. There is a green spot about a third way up the tail, and another green spot in the companion galaxy. But I find most curious of all is that if we assume that the whale's head is to the left, that at the lower back side of the head just below that bright red and white area which I assume could be the whale's eye you can see a long green rectangle that might be appear like a ladder is placed up next to the side. Of maybe a broken necklace still hanging in place. Could it be a photography glitch, or something really there that is green?

No large green "ladders" exist in galaxies. That's a photographic artifact.

Ann

by **De58te** » Thu Oct 06, 2022 7:12 pm

We are told of the galaxy's yellowish core, dark dust clouds, bright blue star clusters, and red star forming regions are easy to spot.But what are the little green spots? You can see them best in the expanded large image. There is a green spot about a third way up the tail, and another green spot in the companion galaxy. But I find most curious of all is that if we assume that the whale's head is to the left, that at the lower back side of the head just below that bright red and white area which I assume could be the whale's eye you can see a long green rectangle that might be appear like a ladder is placed up next to the side. Of maybe a broken necklace still hanging in place. Could it be a photography glitch, or something really there that is green?

by **Chris Peterson** » Thu Oct 06, 2022 6:47 pm

Ann wrote: ↑Thu Oct 06, 2022 6:31 pm
Chris Peterson wrote: ↑Thu Oct 06, 2022 5:57 pm
Ann wrote: ↑Thu Oct 06, 2022 5:43 pm
Okay. According to Wikipedia, M100 has a redshift of 1571 ± 1 km/s - yes, I know that there are different ways to measure redshifts...
To be clear, that is not redshift. That is distance, which is much less precise than redshift. There are not different ways to measure redshift. Redshift is a primary measurement, made directly, and the only error on it will be whatever instrumental error exists. Distance in astronomy is almost always a secondary or derived metric, not something that can be measured directly.
I'm probably mixing up redshift with radial velocity, then.

My software Guide informs me that the galaxy RV of M87 (which I assumes means its radial velocity) from optical observation is 1271 ± 75.537 km/s. Its radial velocity relative to the Local Group is 1153.384 km/s, its radial velocity relative to the GSR is 1218.634 km/s, its radial velocity corrected for the Virgo infall is 1323.549 km/s, and its radial velocity relative to the 3K background is 1595.702 km/s.

This has nothing to do with redshift, then?

Ann

And in general, radial velocity is not much used except in a relative sense, such as one side of a body compared with another to derive rotation information. Cosmological redshift does not represent a radial velocity, but is a consequence of the expansion of space.

by **Ann** » Thu Oct 06, 2022 6:31 pm

Chris Peterson wrote: ↑Thu Oct 06, 2022 5:57 pm
Ann wrote: ↑Thu Oct 06, 2022 5:43 pm
Okay. According to Wikipedia, M100 has a redshift of 1571 ± 1 km/s - yes, I know that there are different ways to measure redshifts...
To be clear, that is not redshift. That is distance, which is much less precise than redshift. There are not different ways to measure redshift. Redshift is a primary measurement, made directly, and the only error on it will be whatever instrumental error exists. Distance in astronomy is almost always a secondary or derived metric, not something that can be measured directly.

I'm probably mixing up redshift with radial velocity, then.

My software Guide informs me that the galaxy RV of M87 (which I assumes means its radial velocity) from optical observation is 1271 ± 75.537 km/s. Its radial velocity relative to the Local Group is 1153.384 km/s, its radial velocity relative to the GSR is 1218.634 km/s, its radial velocity corrected for the Virgo infall is 1323.549 km/s, and its radial velocity relative to the 3K background is 1595.702 km/s.

This has nothing to do with redshift, then?

Ann

by **Chris Peterson** » Thu Oct 06, 2022 5:57 pm

Ann wrote: ↑Thu Oct 06, 2022 5:43 pm
Okay. According to Wikipedia, M100 has a redshift of 1571 ± 1 km/s - yes, I know that there are different ways to measure redshifts...

To be clear, that is not redshift. That is distance, which is much less precise than redshift. There are not different ways to measure redshift. Redshift is a primary measurement, made directly, and the only error on it will be whatever instrumental error exists. Distance in astronomy is almost always a secondary or derived metric, not something that can be measured directly.

by **Ann** » Thu Oct 06, 2022 5:43 pm

rstevenson wrote: ↑Thu Oct 06, 2022 4:00 pm Okay Ann and Chris, now I’m confused (not for the first time.) I took the 30 Mly figure from the Wikipedia article on NGC 4631. (The APOD description gives 25 Mly.) Then I found the SIMBAD distance in Mpc for NGC 4627 and converted it to Mly and got the difference of 6.4 Mly compared to the 30 Mly figure.

Just now I decided to go back to SIMBAD and get the distance to NGC 4631 in Mpc also for a direct comparison, and I find it’s almost identical (with a good range of error) to the distance to NGC 4627. So now I wonder where the WIKIpedia estimate of 30 Mly come from? Usually Wikipedia science articles are carefully tended by experts.

Anyway, you’re right Ann, as I should have assumed.

Rob

From SIMBAD
NGC 4627
distance Q unit| err- err+ | method | reference |
-----------------------------------------------------------------------
| 7.30 Mpc | |redshift|2018MNRAS.479.4136K|
| 9.29 Mpc | | |2016AJ....152...50T|
| 9.29 Mpc | -0.14 +0.14 | |2013AJ....146...86T|
| 9.38 Mpc | |SBF |2008ApJ...676..184T|
| 9.0 Mpc | | |2007ApJS..173..185G|

NGC 4631
distance Q unit| err- err+ | method | reference |
-----------------------------------------------------------------------
| 7.35 Mpc | |redshift|2018MNRAS.479.4136K|
| 7.36 Mpc | -0.20 +0.20 |T-RGB |2020ApJ...905..104K|
| 7.35 Mpc | |T-RDB |2017AJ....153....6K|
| 7.35 Mpc | | |2016AJ....152...50T|
| 7.35 Mpc | -0.10 +0.10 | |2013AJ....146...86T|
| 7.1 Mpc | |redshift|2011MNRAS.413..813C|
| 5.7 Mpc | -1.0 +0.9 |T-F |2008ApJ...676..184T|
| 9.0 Mpc | | |2007ApJS..173..185G|
| 7.35 Mpc | |redshift|2007ApJ...655..790C|
| 6.9 Mpc | | |2004ApJ...602..231C|

Thanks for the praise, Rob. I take a keen interest in galaxies and hot blue stars and try to be knowledgeable about those things for my own sake, but I know almost zilch when it comes to asteroids, lunations, technical astronomical hardware (and software) and math. Among other things.

Anyway. You shouldn't blindly trust Wikipedia. Not everyone who writes or edits those Wikipedia articles, let alone those who write or edit those Wikipedia stubs, are very knowledgeable. Very recently I looked up a Wikipedia entry about a galaxy - sorry, I can't remember which one - and Wikipedia gave me two quite different distances to this one object. I know I have come across at least one other blatant mistake in a Wikipedia text on astronomy - and I even talked about that Wikipedia mistake here, and Chris promised he would contact Wikipedia about it.

As Chris said, however, galaxies in clusters move around among themselves this way and that, and when we measure the redshifts of nearby galaxies (such as NGC 4631/NGC 4627) they are indeed going to move relative to one another, which can indeed mean that appear to have very different redshifts, even though they are at basically at the same distance from us. If one galaxy is moving away from us in its orbit around another galaxy, and the other galaxy is moving toward us in its corresponding orbit around the center of gravity of the two-galaxy system, then the two galaxies can indeed appear to be separated by millions of light-years, even if the true distance between them is smaller than the size of the larger galaxy's disk.

Consider galaxies M90 and M100, two of the largest galaxies of the Virgo Cluster:

: M90. Image: Judy Schmidt, who used Hubble data to create the picture.

: M100. Image: ESO.

Okay. According to Wikipedia, M100 has a redshift of 1571 ± 1 km/s - yes, I know that there are different ways to measure redshifts, but I can't be bothered to try to wrap my mind around that mess - and the corresponding distance to M100 is 55 million light-years. The distance to Virgo Cluster member M100 seems reasonable, because again according to Wikipedia, the distance to the center of the Virgo Cluster itself is 53.8 ± 0.3 Mly.

All right. But what about the distance to Virgo Cluster member M90? What is its redshift? Well, surprise. M90 has no redshift. This galaxy is blueshifted, so it is indeed approaching us.

For now. The combined gravity of the Virgo Cluster will pull it back in and make it recede again along with the other galaxies. And if indeed it were to escape the clutches of Virgo, it still wouldn't make it to the Local Group before the expansion of the Universe would pull it back.

But since M90 has no redshift, we can't use redshift to estimate its distance form us. However, its distance is probably not too different from the distance to M100, since we have very good reasons to believe that M90 is indeed a member of the Virgo Cluster.

I like this simulation of stellar motions inside a star cluster:

Click to play embedded YouTube video.

Stars and galaxies are different, of course. But stars in a star cluster and galaxies in a galaxy cluster are similar in that they move around relative to one another.

Consider the Virgo Cluster:

: The Virgo Cluster. Photo: Kees Scherer.

The galaxies in the Virgo Cluster look so static. Look how Markarian's Chain runs from the top to the lower left, with the big elliptical galaxies, roundish M84 and elongated swollen M86 at top, and the "Eyes" galaxies, bystander's signature, to the lower left of them.

Haven't these galaxies always looked like that? Will they not stay like that, in their current form and position, forever?

They won't, of course. Look at this dancer. When this picture was taken, she was airborne. But that was a fleeting moment in time, just like the position of galaxies are as they move in the vast fabric of spacetime.

Ann

by **rstevenson** » Thu Oct 06, 2022 5:22 pm

Thanks Chris. Good explanation.

Rob

by **Chris Peterson** » Thu Oct 06, 2022 4:57 pm

rstevenson wrote: ↑Thu Oct 06, 2022 4:00 pm Okay Ann and Chris, now I’m confused (not for the first time.) I took the 30 Mly figure from the Wikipedia article on NGC 4631. (The APOD description gives 25 Mly.) Then I found the SIMBAD distance in Mpc for NGC 4627 and converted it to Mly and got the difference of 6.4 Mly compared to the 30 Mly figure.

Just now I decided to go back to SIMBAD and get the distance to NGC 4631 in Mpc also for a direct comparison, and I find it’s almost identical (with a good range of error) to the distance to NGC 4627. So now I wonder where the WIKIpedia estimate of 30 Mly come from? Usually Wikipedia science articles are carefully tended by experts.

There's a reason that most of the literature restricts the information to redshift, without converting to distance. That is not a high accuracy conversion given that it relies on the Hubble Constant, which is quite uncertain itself.

To reiterate what I explained earlier, nearby objects like this have a very small cosmological redshift, and the smaller that value, the greater the uncertainty. That's because what we're measuring is the shift of a known emission line from its true wavelength to an observed wavelength. Two things can create that shift. The first is cosmological redshift, which is always "red" because of the expansion of spacetime. The second is Doppler shift, caused by the motion of the body, and that can shift a line towards either red or blue, depending on the direction of the motion relative to us. For very distant objects, Doppler shift is small compared with cosmological redshift, and can largely be ignored. But for nearby objects like these, with intrinsically small cosmological redshift, Doppler shift introduces a significant uncertainty. So distance calculations based on redshift are not terribly accurate. We can't use redshift in this case to determine the relative distance between the two bodies, only the approximate distance of the pair from us. (In many cases we can look at galaxies in clusters, in which case a more accurate redshift can be obtained because all of the galaxies should have the same cosmological redshift, but as they are moving in lots of different directions, the Doppler shift should average out to near zero.)

by **rstevenson** » Thu Oct 06, 2022 4:00 pm

Okay Ann and Chris, now I’m confused (not for the first time.) I took the 30 Mly figure from the Wikipedia article on NGC 4631. (The APOD description gives 25 Mly.) Then I found the SIMBAD distance in Mpc for NGC 4627 and converted it to Mly and got the difference of 6.4 Mly compared to the 30 Mly figure.

Just now I decided to go back to SIMBAD and get the distance to NGC 4631 in Mpc also for a direct comparison, and I find it’s almost identical (with a good range of error) to the distance to NGC 4627. So now I wonder where the WIKIpedia estimate of 30 Mly come from? Usually Wikipedia science articles are carefully tended by experts.

Anyway, you’re right Ann, as I should have assumed.

Rob

From SIMBAD
NGC 4627
distance Q unit| err- err+ | method | reference |
-----------------------------------------------------------------------
| 7.30 Mpc | |redshift|2018MNRAS.479.4136K|
| 9.29 Mpc | | |2016AJ....152...50T|
| 9.29 Mpc | -0.14 +0.14 | |2013AJ....146...86T|
| 9.38 Mpc | |SBF |2008ApJ...676..184T|
| 9.0 Mpc | | |2007ApJS..173..185G|

NGC 4631
distance Q unit| err- err+ | method | reference |
-----------------------------------------------------------------------
| 7.35 Mpc | |redshift|2018MNRAS.479.4136K|
| 7.36 Mpc | -0.20 +0.20 |T-RGB |2020ApJ...905..104K|
| 7.35 Mpc | |T-RDB |2017AJ....153....6K|
| 7.35 Mpc | | |2016AJ....152...50T|
| 7.35 Mpc | -0.10 +0.10 | |2013AJ....146...86T|
| 7.1 Mpc | |redshift|2011MNRAS.413..813C|
| 5.7 Mpc | -1.0 +0.9 |T-F |2008ApJ...676..184T|
| 9.0 Mpc | | |2007ApJS..173..185G|
| 7.35 Mpc | |redshift|2007ApJ...655..790C|
| 6.9 Mpc | | |2004ApJ...602..231C|

by **Ann** » Thu Oct 06, 2022 1:46 pm

rstevenson wrote: ↑Thu Oct 06, 2022 11:47 am Hi Ann,

You say, “ To me it looks as if NGC 4627 is about to dive right through the disk of NGC 4631. If that were to happen, it would have a tremendous effect on NGC 4631.” Yes it would, but…

I was curious about how close that companion galaxy is to the whale of a galaxy. Their respective Wikipedia entries aren’t immediately helpful, with the Whale listing a redshift as well as a distance of 30 Mly, but the companion listing just a redshift. But SIMBAD gave me the megaparsec distance so I used an online converter to turn that into 23.6 Mly, so roughly 6.4 Mly closer to us than the Whale. I don’t think it’s going to dive in to swim with the Whale any time soon, though it may have passed close enough in the distant past to cause some of those faint streamers.

(Errors and omissions excepted.)

Rob

6.4 Mly means, unless I'm very much mistaken, 6.4 million light-years. Note that the distance between the Milky Way and Andromeda is approximately 2.5 million light-years. Therefore, if the distance between NGC 4631 and NGC 4627 is indeed 6.4 Mly, then these two galaxies would be more than twice as far from one another as the Milky Way and Andromeda.

That's impossible. Not only are NGC 4631 and NGC 4627 very close together in the sky, but in my opinion, you can actually see how NGC 4627 is being stretched in the direction of the disk of NGC 4631, almost certainly as a consequence of gravitational forces (or of the warping of spacetime near NGC 4631). The chances that NGC 4627 would be a foreground or background galaxy, completely unrelated to NGC 4631, are in my opinion negligible.

I'd argue that the spheroidal nature of NGC 4627 boosts the already overwhelming chances that NGC 4627 is gravitationally bound to NGC 4631, and that it has become spheroidal by losing its gas (and thus its starforming ability) during its orbits around NGC 4631.

Also, check out this image again, the one I posted in my first post here:

: This sequence shows NGC 4631 (and NGC 4627) in optical light (at left in black and white), an X-ray and ultraviolet composite in the middle, and an X-ray and H-alpha image at right. Credit: Left panel: NOAO/AURA/NSF Middle panel: X-ray: NASA/CXC/UMass/D.Wang et al., UV: NASA/GSFC/UIT Right panel: X-ray: NASA/UMass/D.Wang et al., Optical: NASA/HST/D.Wang et al.

Note in the middle image that the hot halo of X-ray-emitting gas (mapped as blue) is asymmetrical and "stretched" in the direction of NGC 4627. I'd say that that is because the hot gas is surrounding the center of gravity of the NGC 4631/NGC 4627 system.

So, in short: The distance between NGC 4631 and NGC 4627 is nowhere near 6.4 Mly. The distance between them may be similar to the distance between the Milky Way and the Large Magellanic Cloud, which is ~180,000 ly.

To me it looks as if it might be even smaller.

Ann

by **orin stepanek** » Thu Oct 06, 2022 1:19 pm

Beautiful galaxy! Similar size to M.W.

Nice annotation of and around M.W.

by **Chris Peterson** » Thu Oct 06, 2022 1:00 pm

rstevenson wrote: ↑Thu Oct 06, 2022 11:47 am Hi Ann,

You say, “ To me it looks as if NGC 4627 is about to dive right through the disk of NGC 4631. If that were to happen, it would have a tremendous effect on NGC 4631.” Yes it would, but…

I was curious about how close that companion galaxy is to the whale of a galaxy. Their respective Wikipedia entries aren’t immediately helpful, with the Whale listing a redshift as well as a distance of 30 Mly, but the companion listing just a redshift. But SIMBAD gave me the megaparsec distance so I used an online converter to turn that into 23.6 Mly, so roughly 6.4 Mly closer to us than the Whale. I don’t think it’s going to dive in to swim with the Whale any time soon, though it may have passed close enough in the distant past to cause some of those faint streamers.

(Errors and omissions excepted.)

Rob

At this close distance cosmological redshift and Doppler redshift are pretty hard to separate. That is, you can't use cosmological redshift to reliably decide how far apart these two object are, because they could reasonably have Doppler red/blue shifts on the same order as the total difference in redshift between them.

by **rstevenson** » Thu Oct 06, 2022 11:47 am

Hi Ann,

You say, “ To me it looks as if NGC 4627 is about to dive right through the disk of NGC 4631. If that were to happen, it would have a tremendous effect on NGC 4631.” Yes it would, but…

I was curious about how close that companion galaxy is to the whale of a galaxy. Their respective Wikipedia entries aren’t immediately helpful, with the Whale listing a redshift as well as a distance of 30 Mly, but the companion listing just a redshift. But SIMBAD gave me the megaparsec distance so I used an online converter to turn that into 23.6 Mly, so roughly 6.4 Mly closer to us than the Whale. I don’t think it’s going to dive in to swim with the Whale any time soon, though it may have passed close enough in the distant past to cause some of those faint streamers.

(Errors and omissions excepted.)

Rob

by **Ann** » Thu Oct 06, 2022 6:12 am

: NGC 4631, the Whale Galaxy, and companion NGC 4627. Image: Michael Sherick.

Today's APOD is a fine image. But because I find this galaxy pair, NGC 4631 and its companion NGC 4627, so interesting, I will post more images to try to show you what kind of beasts the Whale and its Pup really are. Let's start by looking at these two galaxies in ultraviolet light!

: NGC 4631 and NGC 4627 in ultraviolet light. Photo: NASA/GALEX.

: NGC 4627. in ultraviolet light. Photo: NASA/GALEX.

In ultraviolet images, blue means far ultraviolet light and very hot stars. Yellow means near ultraviolet light and not very hot stars. So as you can see, NGC 4631 contains a lot of very hot stars, but NGC 4627 lacks them.

NGC 4631 emits not only ultraviolet light but also X-rays:

: This sequence shows NGC 4631 (and NGC 4627) in optical light (at left in black and white), an X-ray and ultraviolet composite in the middle, and an X-ray and H-alpha image at right. Credit: Left panel: NOAO/AURA/NSF Middle panel: X-ray: NASA/CXC/UMass/D.Wang et al., UV: NASA/GSFC/UIT Right panel: X-ray: NASA/UMass/D.Wang et al., Optical: NASA/HST/D.Wang et al.

Note that we can hardly see NGC 4627 at all in the middle panel. In the right-hand image it is off panel, but I think it would have been invisible anyway. The blue X-ray halo surrounding NGC 4631 is made of million-degrees hot gas, which has been expelled from the galaxy due to large numbers of supernovas and strong stellar winds from massive young stars.

We can take a look at NGC 4631 and NGC 4631 in the SDSS palette, too:

: NGC 4631 and NGC 4627 in the SDSS palette. Image: Donald Pelletier and SDSS.

In the SDSS g-r-i palette, red emission nebulas look green. Note greenish and cyan splotches in the disk of NGC 4631. Note that NGC 4631 is both yellower, greener and bluer than NGC 4627, which is muted and fairly uniform in color, although perhaps slightly bluer in the middle.

So NGC 4631 has a massive yellow center and brilliantly starforming arms full of bright blue stars and red emission nebulas. NGC 4627 is a spheroidal galaxy where not much is going on.

To me it looks as if NGC 4627 is about to dive right through the disk of NGC 4631. If that were to happen, it would have a tremendous effect on NGC 4631. A bulls-eye hit might transform NGC 4631 into another Cartwheel Galaxy!

NGC 4627 is most certainly affecting NGC 4631 very strongly already, as we can see from the larger galaxy's whale-like shape. NGC 4627 is also almost certainly stirring up the gas in NGC 4631 and fanning its ongoing star formation.

But NGC 4627 is not the only galaxy affecting the shape and amount of star formation in NGC 4631. Another neighbor is almost certainly a part of this galactic and gravitational tug-of-war:

NGC 4656 (left) and NGC 4631/NGC 4627 (right). Photo: Jean-Baptiste Auroux

NGC 4656 is a weird galaxy. It is called the Hockey Stick Galaxy

, and it is almost all blue, as if it contained no yellow center at all - well it does, but it is very puny - and it has very few and very faint emission nebulas, except in the "blade" (which may or may not be another small galaxy that has collided with NGC 4656).

It is as if NGC 4656 started out as a little wisp of a galaxy surrounded by a huge halo of gas, which suddenly caught fire like wildfire and set of star formation absolutely everywhere in NGC 4656. And then the fire was spent, the nebulas died down, and all that was left was a tremendous population of early B-type stars.

Surely the gravitational influence of NGC 4631 must have had something to do with the pretty incredible star formation history of NGC 4656? Unless it is the "blade" that is responsible for everything?

Ann

by **APOD Robot** » Thu Oct 06, 2022 4:06 am

NGC 4631: The Whale Galaxy

Explanation: NGC 4631 is a big beautiful spiral galaxy. Seen edge-on, it lies only 25 million light-years away in the well-trained northern constellation Canes Venatici. The galaxy's slightly distorted wedge shape suggests to some a cosmic herring and to others its popular moniker, The Whale Galaxy. Either way, it is similar in size to our own Milky Way. In this sharp color image, the galaxy's yellowish core, dark dust clouds, bright blue star clusters, and red star forming regions are easy to spot. A companion galaxy, the small elliptical NGC 4627 is just above the Whale Galaxy. Faint star streams seen in deep images are the remnants of small companion galaxies disrupted by repeated encounters with the Whale in the distant past. The Whale Galaxy is also known to have spouted a halo of hot gas glowing in X-rays.

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