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Posted: **Tue Sep 12, 2023 4:05 am**

Galaxy Cluster Abell 370 and Beyond

Explanation: Some 4 billion light-years away, massive galaxy cluster Abell 370 is captured in this sharp Hubble Space Telescope snapshot. The cluster of galaxies only appears to be dominated by two giant elliptical galaxies and infested with faint arcs. In reality, the fainter, scattered bluish arcs, along with the dramatic dragon arc below and left of center, are images of galaxies that lie far beyond Abell 370. About twice as distant, their otherwise undetected light is magnified and distorted by the cluster's enormous gravitational mass, overwhelmingly dominated by unseen dark matter. Providing a tantalizing glimpse of galaxies in the early universe, the effect is known as gravitational lensing. A consequence of warped spacetime, lensing was predicted by Einstein almost a century ago. Far beyond the spiky foreground Milky Way star at lower right, Abell 370 is seen toward the constellation Cetus, the Sea Monster. It was the last of six galaxy clusters imaged in the Frontier Fields project.

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Posted: **Tue Sep 12, 2023 5:36 am**

: Galaxy Cluster Abell 370 and Beyond. Image Credit: NASA, ESA, Jennifer Lotz and the HFF Team (STScI)

APOD Robot wrote:

The cluster of galaxies only appears to be dominated by two giant elliptical galaxies and infested with faint arcs.

I'd say that Abell 370 is dominated by two giant elliptical galaxies and adorned by two amazing arcs, one big fat arc of a lensed galaxy, and one arc of at least 12 elliptical galaxies forming a semicircle over the two behemoths of the cluster!

Ann

Posted: **Tue Sep 12, 2023 8:18 am**

I can see four stars with diffraction spikes, so are ALL the other objects in that photo beyond the Milky Way?

If so, well, Gosh!
But another question, why do distant object not raise spikes?
Thanks,
John

Posted: **Tue Sep 12, 2023 8:35 am**

The universe is definitely bigger than we can ever imagine!

This is an interesting addition of the (inferred) Dark Matter from ESA, shown as an overlay in blue. Note that the orientation is slightly rotated.

I wonder if this disprove MOND?

Posted: **Tue Sep 12, 2023 9:53 am**

JIm,
Since we cannot detect Dark Matter in the Solar System let alone 4 billion light years away, that blue cloud must be theoretical , not observation. It's derived from conventional physics theory, and 'photoshopped' in.

I read recently that the latest attempts to push definition and detection to try and find Dark Matter approach the "Neutrino Fog" (I think that was the expression), where quantum uncertainty clouds any attempt at demonstrating DM. Perhaps they will find it, but if they don't we need to look elsewhere, and currently MOND is the best guess!
On the same trail, this week's New Scientist has a main article on The Standard Model of the basic structure of matter, that it is as rock-solid as any theory, but is inadequate in many ways. New paradigms are needed!
John

Posted: **Tue Sep 12, 2023 11:34 am**

JohnD wrote: ↑Tue Sep 12, 2023 8:18 am But another question, why do distant object not raise spikes?

It's because stars are point sources whereas distant galaxies have a measurable angular size, with their light spread out over a larger area.

(In reality any light entering the telescope will diffract but most of the light ends up in a point in the centre, which is why we don't notice diffraction spikes on extended objects or on really dim stars. Some very bright galaxy cores do also show diffraction spikes, especially in JWST images.)

Posted: **Tue Sep 12, 2023 12:45 pm**

Knight of Clear Skies wrote: ↑Tue Sep 12, 2023 11:34 am
JohnD wrote: ↑Tue Sep 12, 2023 8:18 am But another question, why do distant object not raise spikes?
It's because stars are point sources whereas distant galaxies have a measurable angular size, with their light spread out over a larger area.

(In reality any light entering the telescope will diffract but most of the light ends up in a point in the centre, which is why we don't notice diffraction spikes on extended objects or on really dim stars. Some very bright galaxy cores do also show diffraction spikes, especially in JWST images.)

And, of course, most nearby point sources don't produce visible spikes, either.

Posted: **Tue Sep 12, 2023 12:46 pm**

There it is again, the very familiar idea of gravity (be it lensing or just gravity in general) as "warped spacetime", which describes what we see, but does it tell us what is actually going on? I remember Feynman saying somewhere that general relativity does not tell us what the "machinery" of gravity is. And I am itching to understand this! How does mass communicate its effect to its surroundings?

When gravity distorts light, we actually mean that it is not light per se that is affected by gravity but its path (if that is a correct way to put it). Same with physical bodies: planets don't have forces that "pull" on their moons, they just distort their path with their mass. Fair enough. What I am itching to understand is: WHAT exactly is being distorted? Please don't say "spacetime", this is going around in circles! Please don't say "this is how gravity works", this is saying: gravity works by virtue of the workings of gravity!

If spacetime is not some bendy elastic fabric that gets bent and curved, if this is just a naive explanatory image, then I ask again: WHAT is bending? Because light's path indeed gets bent, so what is that path "made of" in order to bend?
I'm desperate to understand gravity...

Posted: **Tue Sep 12, 2023 12:52 pm**

JohnD wrote: ↑Tue Sep 12, 2023 8:18 am I can see four stars with diffraction spikes, so are ALL the other objects in that photo beyond the Milky Way?

If so, well, Gosh!
But another question, why do distant object not raise spikes?
Thanks,
John

The galaxy on the bottom/middle of the frame edge seems to have faint spikes unless… it’s superposed by a star that just gives that impression.

Posted: **Tue Sep 12, 2023 1:11 pm**

The larger APOD image (click on the image a couple of times) is very busy. Lots of light sources with diffraction spikes, galaxies everywhere, lensed (stretched) sources abound.

Posted: **Tue Sep 12, 2023 1:25 pm**

Chris Alex wrote: ↑Tue Sep 12, 2023 12:46 pm There it is again, the very familiar idea of gravity (be it lensing or just gravity in general) as "warped spacetime", which describes what we see, but does it tell us what is actually going on? I remember Feynman saying somewhere that general relativity does not tell us what the "machinery" of gravity is. And I am itching to understand this! How does mass communicate its effect to its surroundings?

When gravity distorts light, we actually mean that it is not light per se that is affected by gravity but its path (if that is a correct way to put it). Same with physical bodies: planets don't have forces that "pull" on their moons, they just distort their path with their mass. Fair enough. What I am itching to understand is: WHAT exactly is being distorted? Please don't say "spacetime", this is going around in circles! Please don't say "this is how gravity works", this is saying: gravity works by virtue of the workings of gravity!

If spacetime is not some bendy elastic fabric that gets bent and curved, if this is just a naive explanatory image, then I ask again: WHAT is bending? Because light's path indeed gets bent, so what is that path "made of" in order to bend?
I'm desperate to understand gravity...

Spacetime is being distorted, which alters the path light takes. Because that is how gravity works. There is nothing circular here. Gravity is easy to understand if you can understand general relativity.

Posted: **Tue Sep 12, 2023 1:26 pm**

Tszabeau wrote: ↑Tue Sep 12, 2023 12:52 pm
JohnD wrote: ↑Tue Sep 12, 2023 8:18 am I can see four stars with diffraction spikes, so are ALL the other objects in that photo beyond the Milky Way?

If so, well, Gosh!
But another question, why do distant object not raise spikes?
Thanks,
John
The galaxy on the bottom/middle of the frame edge seems to have faint spikes unless… it’s superposed by a star that just gives that impression.

As noted elsewhere, galaxies produce spikes just like stars do. They're just all spread out so they are usually not visible against the background noise level.

Posted: **Tue Sep 12, 2023 1:51 pm**

Chris Peterson wrote: ↑Tue Sep 12, 2023 1:25 pm
Spacetime is being distorted, which alters the path light takes. Because that is how gravity works. There is nothing circular here. Gravity is easy to understand if you can understand general relativity.

Thanks for your answer! Yes I get it, spacetime is being distorted. What I don't get is what spacetime is! Metrically I get it, concretely no. It surely is not a "thing" like, say, dark matter, so if we are not to reify it, what's gravity distorting in the reality of things? That is my question. "Gravity distorts spacetime" - can you replace the word "spacetime" with anything else?

Posted: **Tue Sep 12, 2023 1:56 pm**

Chris Alex wrote: ↑Tue Sep 12, 2023 1:51 pm
Chris Peterson wrote: ↑Tue Sep 12, 2023 1:25 pm
Spacetime is being distorted, which alters the path light takes. Because that is how gravity works. There is nothing circular here. Gravity is easy to understand if you can understand general relativity.
Thanks for your answer! Yes I get it, spacetime is being distorted. What I don't get is what spacetime is! Metrically I get it, concretely no. It surely is not a "thing" like, say, dark matter, so if we are not to reify it, what's gravity distorting in the reality of things? That is my question. "Gravity distorts spacetime" - can you replace the word "spacetime" with anything else?

Why replace it? You are falling here to what I call the "Billiard Ball Fallacy". The expectation that the operation of the Universe must somehow be intuitive, that every element must have some equivalent to what we see around us every day.

Spacetime is a property of the Universe, and one that we can accurately describe with mathematics. That is what it is. What else does it need to be? How is this different from other properties, like magnetic or electric fields?

Posted: **Tue Sep 12, 2023 2:27 pm**

A picture such as this makes it even more foreign to me that "there are more possible moves in a game of Go than there are particles, including zero mass particles, in the observable Universe". For that matter, I believe Chess also holds that distinction. I know it all merely comes down to math, but to relegate it this way is beguiling.
I wonder how much more of the observable Universe has to be discovered before the statement is no longer certain.

Posted: **Tue Sep 12, 2023 2:55 pm**

Chris Peterson wrote: ↑Tue Sep 12, 2023 1:25 pm
Why replace it? You are falling here to what I call the "Billiard Ball Fallacy". The expectation that the operation of the Universe must somehow be intuitive, that every element must have some equivalent to what we see around us every day.

Spacetime is a property of the Universe, and one that we can accurately describe with mathematics. That is what it is. What else does it need to be? How is this different from other properties, like magnetic or electric fields?

I'm all for spacetime! I only meant replacing the word in that sentence alone, to help the 6 year old in me understand a little more.

You ask: how is it different from other fields like the electric and magnetic? This might help clarify my clumsy question. With those fields, we can say a few things about what's going on, e.g. with electrons and photons etc. But can we say what's going on when a planet pulls on a moon? How does gravity connect the two bodies, by which medium?

Posted: **Tue Sep 12, 2023 3:13 pm**

Chris Alex wrote: ↑Tue Sep 12, 2023 2:55 pm
Chris Peterson wrote: ↑Tue Sep 12, 2023 1:25 pm
Why replace it? You are falling here to what I call the "Billiard Ball Fallacy". The expectation that the operation of the Universe must somehow be intuitive, that every element must have some equivalent to what we see around us every day.

Spacetime is a property of the Universe, and one that we can accurately describe with mathematics. That is what it is. What else does it need to be? How is this different from other properties, like magnetic or electric fields?

I'm all for spacetime! I only meant replacing the word in that sentence alone, to help the 6 year old in me understand a little more.

You ask: how is it different from other fields like the electric and magnetic? This might help clarify my clumsy question. With those fields, we can say a few things about what's going on, e.g. with electrons and photons etc. But can we say what's going on when a planet pulls on a moon? How does gravity connect the two bodies, by which medium?

No, you can't say what's "really" going on with electric and magnetic fields. You're just pushing the problem back one level. In the end, the only way we have to understand them at the lowest level is with mathematical descriptions involving "fields" created by "interactions" between obscure "particles". It really is no different.

Gravity connects two bodies by virtue of the shape of spacetime. The math is perfectly clear.

Posted: **Tue Sep 12, 2023 4:24 pm**

Galaxy cluster 370 and Beyond! We are getting close to seeing
the Big Bang Happen!

Posted: **Tue Sep 12, 2023 5:05 pm**

There’s a lot of information in the “Frontier Fields” site mentioned. The picture subtends approximately 1 square arcminute; a grain of sand held at arm’s length. How many square arcminutes make up the whole sky globe? I don’t know how to figure this out.

Posted: **Tue Sep 12, 2023 5:14 pm**

Roy wrote: ↑Tue Sep 12, 2023 5:05 pm There’s a lot of information in the “Frontier Fields” site mentioned. The picture subtends approximately 1 square arcminute; a grain of sand held at arm’s length. How many square arcminutes make up the whole sky globe? I don’t know how to figure this out.

Around 149 million.

Posted: **Tue Sep 12, 2023 9:15 pm**

Are the galaxies in the Abell cluster more closely packed that in the local group that includes the Milky Way? It certainly seems so from this Hubble image, even excluding the warped arcs of the background galaxies that are not part of Abell. I base this on the fact that the M-W and Andromeda are roughly 25 of their diameters apart, and the average distance seen here in Abell seems to be much less than that. But perhaps the M-W and Andromeda are not representative of other members of the whatever cluster they are part of? Is the local group considered a full-fledged cluster?

Posted: **Tue Sep 12, 2023 11:36 pm**

Roy wrote: ↑Tue Sep 12, 2023 5:05 pm There’s a lot of information in the “Frontier Fields” site mentioned. The picture subtends approximately 1 square arcminute; a grain of sand held at arm’s length. How many square arcminutes make up the whole sky globe? I don’t know how to figure this out.

Roy, you would need to figure out the radius of the celestial sphere in arcminutes, and then plug this value into the equation for the surface area of the sphere (4piR^2), which will yield your answer in square arcminutes. The radius of this sphere can be determined by first remembering that the circumference of a circle is 2piR, so that radius R = circumference/2pi. The circumference in arcminutes is 360 degrees * 60 arcminutes per degree, or 21600 arcminutes, meaning that the radius R is 21600/2pi, or 3438 arcminutes. The surface area of the sphere in square arcminutes (i.e., what you want to calculate) is therefore 4piR^2 = 4pi*(3438^2) = 148532540 square arcminutes (approximately 149 million).

Steve Cannistra

www.starrywonders.com

Posted: **Wed Sep 13, 2023 3:58 am**

johnnydeep wrote: ↑Tue Sep 12, 2023 9:15 pm Are the galaxies in the Abel cluster more closely packed that in the local group that includes the Milky Way? It certainly seems so from this Hubble image, even excluding the warped arcs of the background galaxies that are not part of Abel. I base this on the fact that the M-W and Andromeda are roughly 25 of their diameters apart, and the average distance seen here in Abel seems to be much less than that. But perhaps the M-W and Andromeda are not representative of other members of the whatever cluster they are part of? Is the local group considered a full-fledged cluster?

Are the galaxies in the Abel cluster more closely packed that in the Local Group that includes the Milky Way? Yes, absolutely!

The Local Group of Galaxies is not called a group for nothing. It's a group, not a cluster, so it is much smaller than a cluster. The density of the Local Group is absolutely nothing compared with Abell 370:

The Local Group. Illustration: Pablo Carlos Budassi

: Galaxy Cluster Abell 370 and Beyond Image Credit: NASA, ESA, Jennifer Lotz and the HFF Team (STScI)

My impression is that Abell 370 is very closely packed even for a galaxy cluster. Compare it with the Virgo Cluster:

: The Virgo Cluster. Image Credit & Copyright: Fernando Pena

Ann

Posted: **Wed Sep 13, 2023 12:58 pm**

Ann wrote: ↑Wed Sep 13, 2023 3:58 am
johnnydeep wrote: ↑Tue Sep 12, 2023 9:15 pm Are the galaxies in the Abel cluster more closely packed that in the local group that includes the Milky Way? It certainly seems so from this Hubble image, even excluding the warped arcs of the background galaxies that are not part of Abel. I base this on the fact that the M-W and Andromeda are roughly 25 of their diameters apart, and the average distance seen here in Abel seems to be much less than that. But perhaps the M-W and Andromeda are not representative of other members of the whatever cluster they are part of? Is the local group considered a full-fledged cluster?

Are the galaxies in the Abel cluster more closely packed that in the Local Group that includes the Milky Way? Yes, absolutely!

The Local Group of Galaxies is not called a group for nothing. It's a group, not a cluster, so it is much smaller than a cluster. The density of the Local Group is absolutely nothing compared with Abell 370:

The Local Group Pablo Carlos Budassi.png
The Local Group. Illustration: Pablo Carlos Budassi
Galaxy Cluster Abell 370 and Beyond Image Credit: NASA, ESA, Jennifer Lotz and the HFF Team (STScI)

My impression is that Abell 370 is very closely packed even for a galaxy cluster. Compare it with the Virgo Cluster:

The Virgo Cluster. Image Credit & Copyright: Fernando Pena
Ann

Thanks for that impressively detailed diagram of the Local Group. So the Local Group seems to contain only two large galaxies - the MW and Andromeda - one much smaller galaxy - M33 (aka Triangulum) - and 70+ dwarf galaxies, with the MW and Andromeda dominating their respective "lobes" (of the dumbbell shape that Wikipedia describes the LG as looking like). Is the Local Group part of any larger Galaxy Cluster, or is there no grouping between the LG and the much larger Virgo Super Cluster that it apparently is a part of?

Posted: **Wed Sep 13, 2023 1:22 pm**

johnnydeep wrote: ↑Wed Sep 13, 2023 12:58 pm
Ann wrote: ↑Wed Sep 13, 2023 3:58 am
johnnydeep wrote: ↑Tue Sep 12, 2023 9:15 pm Are the galaxies in the Abel cluster more closely packed that in the local group that includes the Milky Way? It certainly seems so from this Hubble image, even excluding the warped arcs of the background galaxies that are not part of Abel. I base this on the fact that the M-W and Andromeda are roughly 25 of their diameters apart, and the average distance seen here in Abel seems to be much less than that. But perhaps the M-W and Andromeda are not representative of other members of the whatever cluster they are part of? Is the local group considered a full-fledged cluster?

Are the galaxies in the Abel cluster more closely packed that in the Local Group that includes the Milky Way? Yes, absolutely!

The Local Group of Galaxies is not called a group for nothing. It's a group, not a cluster, so it is much smaller than a cluster. The density of the Local Group is absolutely nothing compared with Abell 370:

The Local Group Pablo Carlos Budassi.png
The Local Group. Illustration: Pablo Carlos Budassi
Galaxy Cluster Abell 370 and Beyond Image Credit: NASA, ESA, Jennifer Lotz and the HFF Team (STScI)

My impression is that Abell 370 is very closely packed even for a galaxy cluster. Compare it with the Virgo Cluster:

The Virgo Cluster. Image Credit & Copyright: Fernando Pena
Ann
Thanks for that impressively detailed diagram of the Local Group. So the Local Group seems to contain only two large galaxies - the MW and Andromeda - one much smaller galaxy - M33 (aka Triangulum) - and 70+ dwarf galaxies, with the MW and Andromeda dominating their respective "lobes" (of the dumbbell shape that Wikipedia describes the LG as looking like). Is the Local Group part of any larger Galaxy Cluster, or is there no grouping between the LG and the much larger Virgo Super Cluster that it apparently is a part of?

I think the Local Group is considered to be an outlier of the Virgo Cluster. Or maybe that should be the Virgo Supercluster.

One of the absolutely closest groups to us is the M81 Group. I don't know if it considered to be associated with the Virgo Supercluster (but yes, it is). Another very nearby galaxy is NGC 5128, or Centaurus A. Not sure if NGC 5128 is considered to be a member of a group, or if it is an isolated galaxy (I think not, though). IC 342 is also very close, and it belongs to the IC 342/Maffei group.

I guess anything that is really close to us is considered to be a part of the Virgo Supercluster!

I'm too lazy to google. But I guess I did google just a little a bit anyway.

Ann

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APOD: Galaxy Cluster Abell 370 and Beyond (2023 Sep 12)

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