APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

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APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by APOD Robot » Wed Mar 06, 2024 5:05 am

Image M102: Edge-on Disk Galaxy

Explanation: What kind of celestial object is this? A relatively normal galaxy -- but seen from its edge. Many disk galaxies are actually just as thin as NGC 5866, the Spindle galaxy, pictured here, but are not seen edge-on from our vantage point. A perhaps more familiar galaxy seen edge-on is our own Milky Way galaxy. Also cataloged as M102, the Spindle galaxy has numerous and complex dust lanes appearing dark and red, while many of the bright stars in the disk give it a more blue underlying hue. The blue disk of young stars can be seen in this Hubble image extending past the dust in the extremely thin galactic plane. There is evidence that the Spindle galaxy has cannibalized smaller galaxies over the past billion years or so, including multiple streams of faint stars, dark dust that extends away from the main galactic plane, and a surrounding group of galaxies (not shown). In general, many disk galaxies become thin because the gas that forms them collides with itself as it rotates about the gravitational center. The Spindle galaxy lies about 50 million light years distant toward the constellation of the Dragon (Draco).

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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by AVAO » Wed Mar 06, 2024 6:37 am

APOD Robot wrote: Wed Mar 06, 2024 5:05 am Image M102: Edge-on Disk Galaxy

Explanation: What kind of celestial object is this? A relatively normal galaxy -- but seen from its edge. Many disk galaxies are actually just as thin as NGC 5866, the Spindle galaxy, pictured here, but are not seen edge-on from our vantage point. A perhaps more familiar galaxy seen edge-on is our own Milky Way galaxy. Also cataloged as M102, the Spindle galaxy has numerous and complex dust lanes appearing dark and red, while many of the bright stars in the disk give it a more blue underlying hue. The blue disk of young stars can be seen in this Hubble image extending past the dust in the extremely thin galactic plane. There is evidence that the Spindle galaxy has cannibalized smaller galaxies over the past billion years or so, including multiple streams of faint stars, dark dust that extends away from the main galactic plane, and a surrounding group of galaxies (not shown). In general, many disk galaxies become thin because the gas that forms them collides with itself as it rotates about the gravitational center. The Spindle galaxy lies about 50 million light years distant toward the constellation of the Dragon (Draco).

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Image Credit: NASA, ESA, Hubble; Processing: Ehsan Ebahimian
ZOOM IN: https://esahubble.org/images/opo0624a/zoomable/
Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Rauf » Wed Mar 06, 2024 6:53 am

Just a small correction, the name of image processor is apparently misspelled. I think it is Ebrahimian.

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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Ann » Wed Mar 06, 2024 7:48 am

Yes, today's APOD showcases an interesting-looking galaxy!


Let's analyse it. The dust disk is thick and dark, with little blue or white flecks of either star clusters or star formation. Light brown dust bunnies rise from the disk, remnants of violent occurrences in the past, most likely supernovas. Behind the dust disk is a diffuse, bright, yellow-white light: That would be the light from the central yellow bulge.

And then there is that blue, blue light, shooting out like a light saber from the end of the dust disk at right. The blue laser beam seems to shoot out from the dust lane handle. What is it?

Let's compare NGC 5866 (M102) with another very edge-on spiral galaxy, NGC 891:



We recognize the dark dust lanes, the dust bunnies rising from it, and the yellowish galactic bulge behind it. But the blue laser beam isn't there! Also note that the dust lane in NGC 891 is much longer than the dust lane in NGC 5866. It extends to the edge of the visible disk, which is clearly not the case for NGC 5866.


NGC 5866 reminds me of M64, the Black-Eye galaxy, which has a very small dust disk indeed:


If you look at an 1.5 MB version of the closeup of the center of M64, here, you can see more clearly how blue stars stream away from the dust. You can also see that while you can discern individual stars or clusters in the "blue stream", the stream is mostly smooth. It appears to contain large numbers of smallish blue stars like Sirius and Vega.


Take a look at small galaxy NGC 5364:

NGC 5394 Gemini Observatory annotated.png
The bright white crescent "above" the nucleus of NGC 5394
is a remnant of past star formation. It contains huge numbers of
smallish blue star like Sirius and Vega.


So, yes. The blue laser beam of NGC 5866 is actually the remnant of past star formation. It's more than that: The dust lane where the stars were born has probably receded, and the stars have been left behind. They may just have "streamed out of it", but my bet is that the dust lane has receded and the stars have been left behind.

The reason why the small blue stars like Sirius and Vega are left behind to form a light saber beam of blue light in NGC 5866 is of course that they are a lot more more long-lived than the blue behemoths that once kept them company. Also, they are much, much more numerous than the heavyweight OB stars ever were. But their light is certainly not as "electric blue" as the APOD seems to suggest.


And how about this gorgeous spiral galaxy that contains no dust lanes whatsoever except a small dust ring around the center, where we also find star formation?

NGC 4314 G Fritz Benedict Andrew Howell Inger Jorgensen David Chapell Jeffery Kenney Beverly J Smith and NASA.png
Can you see the tiny blue ring around the center?
Credit: G. Fritz Benedict et al.

NGC 4314 must have contained a lot of dust in its youth, because if it didn't, I just can't see how it could have formed such a gorgeous spiral shape. Now the dust is all gone, except in a small ring. Don't ask me how it happened, although the Youtube video below just maybe will give you a clue!

Click to play embedded YouTube video.

And finally, there is the question of the flatness of many galactic disks.

APOD Robot wrote:

In general, many disk galaxies become thin because the gas that forms them collides with itself as it rotates about the gravitational center.

Like pizza dough, you mean?

Click to play embedded YouTube video.

Ann
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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Roy » Wed Mar 06, 2024 2:20 pm

We are assured that large quantities of dark matter surround all these galaxies, but can be detected only gravitationally. But DM’s gravity must be pushing gravity, not pulling gravity. Ellipticals are getting squashed out flat; spiral arms are squirting out; these are pushed into spirals or ultimately rings as we saw in the polar ring galaxy posting; cores are pushed so hard they squirt out jets top and bottom and even collapse into black holes. Got to be pushing gravity from the dark matter, denser around some galaxies than others.

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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Chris Peterson » Wed Mar 06, 2024 2:44 pm

Roy wrote: Wed Mar 06, 2024 2:20 pm We are assured that large quantities of dark matter surround all these galaxies, but can be detected only gravitationally. But DM’s gravity must be pushing gravity, not pulling gravity. Ellipticals are getting squashed out flat; spiral arms are squirting out; these are pushed into spirals or ultimately rings as we saw in the polar ring galaxy posting; cores are pushed so hard they squirt out jets top and bottom and even collapse into black holes. Got to be pushing gravity from the dark matter, denser around some galaxies than others.
There is no such thing as "pushing gravity". The visible effects of DM's gravity is exactly as we'd expect for baryonic matter. Ellipticals are not flattening, spirals aren't "squirting out". When mass collapses gravitationally it commonly speeds up and flattens (we see this at all scales in the Universe... or in the local pizza parlor). When spiral galaxies are disrupted by interacting with other galaxies, they commonly lose their flatness and their spirals and evolve into elliptical galaxies. Galactic cores don't collapse into black holes, they contain black holes. And when matter falls into a black hole, it is gravity that provides the energy to create the magnetic fields that expel jets. Those jets aren't being "pushed" by gravity!
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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by ThoughtfulStarGazer » Wed Mar 06, 2024 3:05 pm

This is a great image.

But the text is slightly misleading in terms of discussing the "thinness" of the edge on view. Like the Pencil Galaxy, with its edge on view, one used to looking at angular spiral galaxies has no ide how "thick" they are, yet their broadness is often discussed, e.g., 100K light years across, etc.

It would be useful to know how deep the galactic planes of these edge on views are. 500 LY, 1,000 LY?

IYKWIM

Roy

Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Roy » Wed Mar 06, 2024 7:21 pm

Chris Peterson wrote: Wed Mar 06, 2024 2:44 pm
Roy wrote: Wed Mar 06, 2024 2:20 pm We are assured that large quantities of dark matter surround all these galaxies, but can be detected only gravitationally. But DM’s gravity must be pushing gravity, not pulling gravity. Ellipticals are getting squashed out flat; spiral arms are squirting out; these are pushed into spirals or ultimately rings as we saw in the polar ring galaxy posting; cores are pushed so hard they squirt out jets top and bottom and even collapse into black holes. Got to be pushing gravity from the dark matter, denser around some galaxies than others.
There is no such thing as "pushing gravity". The visible effects of DM's gravity is exactly as we'd expect for baryonic matter. Ellipticals are not flattening, spirals aren't "squirting out". When mass collapses gravitationally it commonly speeds up and flattens (we see this at all scales in the Universe... or in the local pizza parlor). When spiral galaxies are disrupted by interacting with other galaxies, they commonly lose their flatness and their spirals and evolve into elliptical galaxies. Galactic cores don't collapse into black holes, they contain black holes. And when matter falls into a black hole, it is gravity that provides the energy to create the magnetic fields that expel jets. Those jets aren't being "pushed" by gravity!
Pizza dough is elastic, and starts out as a glob which the maker flattens, then throws it up rotating, so the centrifugal force slowly pulls it out into a disc. He supplies the flattening force and rotational impulse. Early on, galaxies were thought to evolve from ellipticals to spirals to polar galaxies. Are you claiming spirals become ellipticals, or only due to collisions?
Normal gravitation wil provide the elasticity. Of the spacial dough, and a certain amount of the flattening. But how much of the dark matter is involved falling in? How can we know it is not pressing in? If only we could see it!

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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Chris Peterson » Wed Mar 06, 2024 7:30 pm

Roy wrote: Wed Mar 06, 2024 7:21 pm
Chris Peterson wrote: Wed Mar 06, 2024 2:44 pm
Roy wrote: Wed Mar 06, 2024 2:20 pm We are assured that large quantities of dark matter surround all these galaxies, but can be detected only gravitationally. But DM’s gravity must be pushing gravity, not pulling gravity. Ellipticals are getting squashed out flat; spiral arms are squirting out; these are pushed into spirals or ultimately rings as we saw in the polar ring galaxy posting; cores are pushed so hard they squirt out jets top and bottom and even collapse into black holes. Got to be pushing gravity from the dark matter, denser around some galaxies than others.
There is no such thing as "pushing gravity". The visible effects of DM's gravity is exactly as we'd expect for baryonic matter. Ellipticals are not flattening, spirals aren't "squirting out". When mass collapses gravitationally it commonly speeds up and flattens (we see this at all scales in the Universe... or in the local pizza parlor). When spiral galaxies are disrupted by interacting with other galaxies, they commonly lose their flatness and their spirals and evolve into elliptical galaxies. Galactic cores don't collapse into black holes, they contain black holes. And when matter falls into a black hole, it is gravity that provides the energy to create the magnetic fields that expel jets. Those jets aren't being "pushed" by gravity!
Pizza dough is elastic, and starts out as a glob which the maker flattens, then throws it up rotating, so the centrifugal force slowly pulls it out into a disc. He supplies the flattening force and rotational impulse. Early on, galaxies were thought to evolve from ellipticals to spirals to polar galaxies. Are you claiming spirals become ellipticals, or only due to collisions?
Normal gravitation wil provide the elasticity. Of the spacial dough, and a certain amount of the flattening. But how much of the dark matter is involved falling in? How can we know it is not pressing in? If only we could see it!
The presumption is that when galaxies form, they are mostly gas and dark matter, which at it collapses speeds up (due to conservation of angular momentum) and is dense enough for the material to interact with itself, and is therefore behaving as a fluid. As such, it flattens (just like any accretion disk). Arms usually form in flat disks because of gravitational resonances. As these galaxies age, there is less and less internal friction, so they are only maintained as disks by ordinary Newtonian mechanics. If they are perturbed significantly, there is little to keep them flat, and they become irregular galaxies and eventually ellipticals. With enough time, they will become spherical.

We theorize the existence of dark matter because, gravitationally, it behaves exactly like baryonic matter. In terms of how it alters the paths of the matter we do see, in terms of how it bends spacetime and deflects light, and mathematically in how it supports our dominant cosmological theories.
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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by johnnydeep » Wed Mar 06, 2024 8:40 pm

ThoughtfulStarGazer wrote: Wed Mar 06, 2024 3:05 pm This is a great image.

But the text is slightly misleading in terms of discussing the "thinness" of the edge on view. Like the Pencil Galaxy, with its edge on view, one used to looking at angular spiral galaxies has no ide how "thick" they are, yet their broadness is often discussed, e.g., 100K light years across, etc.

It would be useful to know how deep the galactic planes of these edge on views are. 500 LY, 1,000 LY?

IYKWIM
It is commonly stated the the Milky Way is 90,000 light years in diameter but only 1000 light years thick. How exactly those numbers are determined matters, and that's where the "isophotal diameter" (and I suppose the "isophotal thickness"?) come into play.
https://en.wikipedia.org/wiki/Milky_Way#:~:text=of%20many%20galaxies.-,The%20Milky%20Way,-is%20a%20barred wrote: Milky Way
The Milky Way is a barred spiral galaxy with a D25 isophotal diameter estimated at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,600 light-years),[10] but only about 1,000 light-years thick at the spiral arms (more at the bulge). Recent simulations suggest that a dark matter area, also containing some visible stars, may extend up to a diameter of almost 2 million light-years (613 kpc).
https://en.wikipedia.org/wiki/Galaxy#Isophotal_diameter wrote: Isophotal diameter
The isophotal diameter is introduced as a conventional way of measuring a galaxy's size based on its apparent surface brightness.[126] Isophotes are curves in a diagram - such as a picture of a galaxy - that adjoins points of equal brightnesses, and are useful in defining the extent of the galaxy. The apparent brightness flux of a galaxy is measured in units of magnitudes per square arcsecond (mag/arcsec2; sometimes expressed as mag arcsec−2), which defines the brightness depth of the isophote. To illustrate how this unit works, a typical galaxy has a brightness flux of 18 mag/arcsec2 at its central region. This brightness is equivalent to the light of an 18th magnitude hypothetical point object (like a star) being spread out evenly in a one square arcsecond area of the sky.[127] For the purposes of objectivity, the spectrum of light being used is sometimes also given in figures. As an example, the Milky Way has an average surface brightness of 22.1 B-mag/arcsec−2,[128][129][130] where B-mag refers to the brightness at the B-band (445 nm wavelength of light, in the blue part of the visible spectrum).
Admittedly, I'm not very clear on this definition, but I believe it amounts to saying that "most" of the Milky Way's magnitude is within so many arcseconds of the brightest central point. So, for example, I could say that 50% or 90% of the apparent magnitude (brightness) is within 4 arcseconds of the center. But how the '25' in "25 isophotal diameters" relates to the particular percentage being used - and what that is! - I don't know. I happily await a more learned reply to school me!
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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Chris Peterson » Wed Mar 06, 2024 11:44 pm

johnnydeep wrote: Wed Mar 06, 2024 8:40 pm
ThoughtfulStarGazer wrote: Wed Mar 06, 2024 3:05 pm This is a great image.

But the text is slightly misleading in terms of discussing the "thinness" of the edge on view. Like the Pencil Galaxy, with its edge on view, one used to looking at angular spiral galaxies has no ide how "thick" they are, yet their broadness is often discussed, e.g., 100K light years across, etc.

It would be useful to know how deep the galactic planes of these edge on views are. 500 LY, 1,000 LY?

IYKWIM
It is commonly stated the the Milky Way is 90,000 light years in diameter but only 1000 light years thick. How exactly those numbers are determined matters, and that's where the "isophotal diameter" (and I suppose the "isophotal thickness"?) come into play.
https://en.wikipedia.org/wiki/Milky_Way#:~:text=of%20many%20galaxies.-,The%20Milky%20Way,-is%20a%20barred wrote: Milky Way
The Milky Way is a barred spiral galaxy with a D25 isophotal diameter estimated at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,600 light-years),[10] but only about 1,000 light-years thick at the spiral arms (more at the bulge). Recent simulations suggest that a dark matter area, also containing some visible stars, may extend up to a diameter of almost 2 million light-years (613 kpc).
https://en.wikipedia.org/wiki/Galaxy#Isophotal_diameter wrote: Isophotal diameter
The isophotal diameter is introduced as a conventional way of measuring a galaxy's size based on its apparent surface brightness.[126] Isophotes are curves in a diagram - such as a picture of a galaxy - that adjoins points of equal brightnesses, and are useful in defining the extent of the galaxy. The apparent brightness flux of a galaxy is measured in units of magnitudes per square arcsecond (mag/arcsec2; sometimes expressed as mag arcsec−2), which defines the brightness depth of the isophote. To illustrate how this unit works, a typical galaxy has a brightness flux of 18 mag/arcsec2 at its central region. This brightness is equivalent to the light of an 18th magnitude hypothetical point object (like a star) being spread out evenly in a one square arcsecond area of the sky.[127] For the purposes of objectivity, the spectrum of light being used is sometimes also given in figures. As an example, the Milky Way has an average surface brightness of 22.1 B-mag/arcsec−2,[128][129][130] where B-mag refers to the brightness at the B-band (445 nm wavelength of light, in the blue part of the visible spectrum).
Admittedly, I'm not very clear on this definition, but I believe it amounts to saying that "most" of the Milky Way's magnitude is within so many arcseconds of the brightest central point. So, for example, I could say that 50% or 90% of the apparent magnitude (brightness) is within 4 arcseconds of the center. But how the '25' in "25 isophotal diameters" relates to the particular percentage being used - and what that is! - I don't know. I happily await a more learned reply to school me!
The "25" just references the brightness (25 mag per square arcsec). So this metric defines the edge of a galaxy based on where the brightness drops below a certain level. It's very simple. That same article discusses lots of other approaches, including the radius inside which some percentage (like 50%) of the total brightness lies. They are all somewhat arbitrary, as a galaxy has a very fuzzy edge, and when dark matter is included, is usually much larger than the visible part.

(The value of 25 mag/sq arcsec seems to have been decided on because that's near the noise floor set by the atmosphere. From space, and at other wavelengths we can see much deeper. Which means that a galaxy might look quite a bit bigger.)
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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Ann » Thu Mar 07, 2024 5:09 am

I can't help thinking of the rings of Saturn when we discuss the diameter versus the "thickness" of spiral galaxies. Obviously galaxies are "proportionally thicker" for their diameter than the rings of Saturn. The thickness of the central dust lane versus the thickness of the stellar disk is another parameter of this discussion.

Of course, spiral galaxies are so much more complex than the rings of the sixth planet of our solar system. And so so so much larger!


Okay - on second thought, I guess I should have compared the rings of Saturn with the dust disk of M104, or what do you think?


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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by johnnydeep » Thu Mar 07, 2024 6:43 pm

Ann wrote: Thu Mar 07, 2024 5:09 am I can't help thinking of the rings of Saturn when we discuss the diameter versus the "thickness" of spiral galaxies. Obviously galaxies are "proportionally thicker" for their diameter than the rings of Saturn. The thickness of the central dust lane versus the thickness of the stellar disk is another parameter of this discussion.

Of course, spiral galaxies are so much more complex than the rings of the sixth planet of our solar system. And so so so much larger!


Okay - on second thought, I guess I should have compared the rings of Saturn with the dust disk of M104, or what do you think?


Ann
So how is the thickness of of Saturn' rings arrived? Is there an "isomassal" measurement? I.e., "80% of the ring's mass lies within some distance of the midline"?
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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by johnnydeep » Thu Mar 07, 2024 6:47 pm

Chris Peterson wrote: Wed Mar 06, 2024 11:44 pm
johnnydeep wrote: Wed Mar 06, 2024 8:40 pm
ThoughtfulStarGazer wrote: Wed Mar 06, 2024 3:05 pm This is a great image.

But the text is slightly misleading in terms of discussing the "thinness" of the edge on view. Like the Pencil Galaxy, with its edge on view, one used to looking at angular spiral galaxies has no ide how "thick" they are, yet their broadness is often discussed, e.g., 100K light years across, etc.

It would be useful to know how deep the galactic planes of these edge on views are. 500 LY, 1,000 LY?

IYKWIM
It is commonly stated the the Milky Way is 90,000 light years in diameter but only 1000 light years thick. How exactly those numbers are determined matters, and that's where the "isophotal diameter" (and I suppose the "isophotal thickness"?) come into play.
https://en.wikipedia.org/wiki/Milky_Way#:~:text=of%20many%20galaxies.-,The%20Milky%20Way,-is%20a%20barred wrote: Milky Way
The Milky Way is a barred spiral galaxy with a D25 isophotal diameter estimated at 26.8 ± 1.1 kiloparsecs (87,400 ± 3,600 light-years),[10] but only about 1,000 light-years thick at the spiral arms (more at the bulge). Recent simulations suggest that a dark matter area, also containing some visible stars, may extend up to a diameter of almost 2 million light-years (613 kpc).
https://en.wikipedia.org/wiki/Galaxy#Isophotal_diameter wrote: Isophotal diameter
The isophotal diameter is introduced as a conventional way of measuring a galaxy's size based on its apparent surface brightness.[126] Isophotes are curves in a diagram - such as a picture of a galaxy - that adjoins points of equal brightnesses, and are useful in defining the extent of the galaxy. The apparent brightness flux of a galaxy is measured in units of magnitudes per square arcsecond (mag/arcsec2; sometimes expressed as mag arcsec−2), which defines the brightness depth of the isophote. To illustrate how this unit works, a typical galaxy has a brightness flux of 18 mag/arcsec2 at its central region. This brightness is equivalent to the light of an 18th magnitude hypothetical point object (like a star) being spread out evenly in a one square arcsecond area of the sky.[127] For the purposes of objectivity, the spectrum of light being used is sometimes also given in figures. As an example, the Milky Way has an average surface brightness of 22.1 B-mag/arcsec−2,[128][129][130] where B-mag refers to the brightness at the B-band (445 nm wavelength of light, in the blue part of the visible spectrum).
Admittedly, I'm not very clear on this definition, but I believe it amounts to saying that "most" of the Milky Way's magnitude is within so many arcseconds of the brightest central point. So, for example, I could say that 50% or 90% of the apparent magnitude (brightness) is within 4 arcseconds of the center. But how the '25' in "25 isophotal diameters" relates to the particular percentage being used - and what that is! - I don't know. I happily await a more learned reply to school me!
The "25" just references the brightness (25 mag per square arcsec). So this metric defines the edge of a galaxy based on where the brightness drops below a certain level. It's very simple. That same article discusses lots of other approaches, including the radius inside which some percentage (like 50%) of the total brightness lies. They are all somewhat arbitrary, as a galaxy has a very fuzzy edge, and when dark matter is included, is usually much larger than the visible part.

(The value of 25 mag/sq arcsec seems to have been decided on because that's near the noise floor set by the atmosphere. From space, and at other wavelengths we can see much deeper. Which means that a galaxy might look quite a bit bigger.)
Thanks. I don't know why I stopped reading at the part I quoted. The next paragraph would have cleared up my confusion!
Roderick Oliver Redman in 1936 suggested that the diameters of galaxies (then referred to as "elliptical nebulae") should be defined at the 25.0 mag/arcsec2 isophote at the B-band, which is expected to cover much of the galaxy's light profile.[132] This isophote then became known simply as D25 (short for "diameter 25"), and corresponds to at least 10% of the normal brightness of the night sky, which is very near the limitations of blue filters at that time. This method was particularly used during the creation of the Uppsala General Catalogue using blue filters from the Palomar Observatory Sky Survey in 1972.
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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Chris Peterson » Thu Mar 07, 2024 6:48 pm

johnnydeep wrote: Thu Mar 07, 2024 6:43 pm
Ann wrote: Thu Mar 07, 2024 5:09 am I can't help thinking of the rings of Saturn when we discuss the diameter versus the "thickness" of spiral galaxies. Obviously galaxies are "proportionally thicker" for their diameter than the rings of Saturn. The thickness of the central dust lane versus the thickness of the stellar disk is another parameter of this discussion.

Of course, spiral galaxies are so much more complex than the rings of the sixth planet of our solar system. And so so so much larger!


Okay - on second thought, I guess I should have compared the rings of Saturn with the dust disk of M104, or what do you think?


Ann
So how is the thickness of of Saturn' rings arrived? Is there an "isomassal" measurement? I.e., "80% of the ring's mass lies within some distance of the midline"?
They don't have a "fuzzy" top and bottom. It's a very sharp transition between ring and no ring.
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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by johnnydeep » Thu Mar 07, 2024 7:58 pm

Chris Peterson wrote: Thu Mar 07, 2024 6:48 pm
johnnydeep wrote: Thu Mar 07, 2024 6:43 pm
Ann wrote: Thu Mar 07, 2024 5:09 am ...

Okay - on second thought, I guess I should have compared the rings of Saturn with the dust disk of M104, or what do you think?
...

Ann
So how is the thickness of of Saturn' rings arrived? Is there an "isomassal" measurement? I.e., "80% of the ring's mass lies within some distance of the midline"?
They don't have a "fuzzy" top and bottom. It's a very sharp transition between ring and no ring.
How sharp? And what's the definition of "sharp"?
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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Chris Peterson » Thu Mar 07, 2024 8:08 pm

johnnydeep wrote: Thu Mar 07, 2024 7:58 pm
Chris Peterson wrote: Thu Mar 07, 2024 6:48 pm
johnnydeep wrote: Thu Mar 07, 2024 6:43 pm

So how is the thickness of of Saturn' rings arrived? Is there an "isomassal" measurement? I.e., "80% of the ring's mass lies within some distance of the midline"?
They don't have a "fuzzy" top and bottom. It's a very sharp transition between ring and no ring.
How sharp? And what's the definition of "sharp"?
There's really no density gradient across the ring system. You've got icy bodies around you, and you move a few meters and you don't.
Chris

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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by johnnydeep » Thu Mar 07, 2024 8:15 pm

Chris Peterson wrote: Thu Mar 07, 2024 8:08 pm
johnnydeep wrote: Thu Mar 07, 2024 7:58 pm
Chris Peterson wrote: Thu Mar 07, 2024 6:48 pm

They don't have a "fuzzy" top and bottom. It's a very sharp transition between ring and no ring.
How sharp? And what's the definition of "sharp"?
There's really no density gradient across the ring system. You've got icy bodies around you, and you move a few meters and you don't.
So the size of those icy bodies doesn't decrease gradually with the distance from the center plane?
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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by Chris Peterson » Thu Mar 07, 2024 8:20 pm

johnnydeep wrote: Thu Mar 07, 2024 8:15 pm
Chris Peterson wrote: Thu Mar 07, 2024 8:08 pm
johnnydeep wrote: Thu Mar 07, 2024 7:58 pm

How sharp? And what's the definition of "sharp"?
There's really no density gradient across the ring system. You've got icy bodies around you, and you move a few meters and you don't.
So the size of those icy bodies doesn't decrease gradually with the distance from the center plane?
Not so far as anybody can tell at this point. The exact thickness isn't even known, given how extremely thin it is.

If you look at the mass distribution of a galaxy, there's no sharp edge in comparison to the size. That is, the density decreases over some sizeable percentage of the total size of the galaxy. The transition zone of the rings is very small compared to the ring thickness.

The dynamics are very different, of course. A body orbiting Saturn must orbit around a single point (the center of the planet) and if it's part of the ring system, with extremely low inclination. Anything else would have crashed into something and been absorbed or ejected long ago. A galaxy has no such constraint, so many stars are orbiting at a significant inclination (nearly spherical at the core, of course).
Chris

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Re: APOD: M102: Edge-on Disk Galaxy (2024 Mar 06)

Post by johnnydeep » Thu Mar 07, 2024 10:28 pm

Chris Peterson wrote: Thu Mar 07, 2024 8:20 pm
johnnydeep wrote: Thu Mar 07, 2024 8:15 pm
Chris Peterson wrote: Thu Mar 07, 2024 8:08 pm

There's really no density gradient across the ring system. You've got icy bodies around you, and you move a few meters and you don't.
So the size of those icy bodies doesn't decrease gradually with the distance from the center plane?
Not so far as anybody can tell at this point. The exact thickness isn't even known, given how extremely thin it is.

If you look at the mass distribution of a galaxy, there's no sharp edge in comparison to the size. That is, the density decreases over some sizeable percentage of the total size of the galaxy. The transition zone of the rings is very small compared to the ring thickness.

The dynamics are very different, of course. A body orbiting Saturn must orbit around a single point (the center of the planet) and if it's part of the ring system, with extremely low inclination. Anything else would have crashed into something and been absorbed or ejected long ago. A galaxy has no such constraint, so many stars are orbiting at a significant inclination (nearly spherical at the core, of course).
Ok, thanks.
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