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

Comments and questions about the APOD on the main view screen.
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johnnydeep
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Re: APOD: Galaxy Cluster Abell 370 and Beyond (2023 Sep 12)

Post by johnnydeep » Wed Sep 13, 2023 2:59 pm

Ann wrote: 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


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

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

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. :ssmile:

Ann
Thanks. This paper from 2004 discusses a bunch of other "local groups" and clusters near the Local Group - https://arxiv.org/pdf/astro-ph/0410065.pdf. It also includes tables with the major members.
The Local Group and other neighboring galaxy groups
I. D. Karachentsev
Special Astrophysical Observatory of Russian Academy of Sciences, N.Arkhyz, KChR, 369167, Russia
ikar@luna.sao.ru

ABSTRACT
Over the last few years, rapid progress has been made in distance measurements for nearbygalaxies based on the magnitude of the tip of red giant branch stars. Current CCD surveys withHST and large ground- based telescopes bring∼10%-accurate distances for roughly a hundredgalaxies within 5 Mpc. The new data on distances to galaxies situated in (and around) thenearest groups: the Local Group, M81 group, CenA/M83 group, IC342/Maffei group, Sculptorfilament, and Canes Venatici cloud allowed us to determine their total mass from the radius ofthe zero- velocity surface,R0, which separates a group as bound against the homogeneous cosmicexpansion. The values ofR0 for the virialized groups turn out to be close each other, in the rangeof 0.9 – 1.3 Mpc. As a result, the total masses of the groups are close to each other, too, yieldingtotal mass-to-blue luminosity ratios of 10 – 40M⊙/L⊙. The new total mass estimates are 3 –5 times lower than old virial mass estimates of these groups. Because about half of galaxies inthe Local Volume belong to such loose groups, the revision of the amount of dark matter (DM)leads to a low local density of matter, Ωm≃0.04, which is comparable with the global baryonicfraction Ωb, but much lower than the global density of matter, Ωm = 0.27.  
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Christian G.
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Re: APOD: Galaxy Cluster Abell 370 and Beyond (2023 Sep 12)

Post by Christian G. » Wed Nov 01, 2023 4:01 pm

Chris Peterson wrote: 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.
Sorry for returning to this (and you have been more than generous with your answers as it is!) but one clarification regarding your "it really is no different" - are you implying that gravity too is at the lowest level a field with obscure particles interacting, in other words are you of the view that gravity must be quantized? Or of the view that GR and the curvature of spacetime is a complete theory of gravity which needs no quantum mechanics?

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

Post by Chris Peterson » Wed Nov 01, 2023 4:31 pm

Chris Alex wrote: Wed Nov 01, 2023 4:01 pm
Chris Peterson wrote: Tue Sep 12, 2023 3:13 pm
Chris Alex wrote: Tue Sep 12, 2023 2:55 pm


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.
Sorry for returning to this (and you have been more than generous with your answers as it is!) but one clarification regarding your "it really is no different" - are you implying that gravity too is at the lowest level a field with obscure particles interacting, in other words are you of the view that gravity must be quantized? Or of the view that GR and the curvature of spacetime is a complete theory of gravity which needs no quantum mechanics?
I reserve judgment. The inability to tie gravity to the other forces remains arguably the largest unsolved problem of fundamental physics. I find arguments both ways to depend more on philosophy than science. I simply don't think there's enough evidence at this point to form any sort of "belief".
Chris

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

Post by JohnD » Thu Nov 02, 2023 11:37 am

The main article in this weeks New Scientist is "In Search of Quantum Gravity". See: https://www.sciencedirect.com/science/a ... 7923020122 if you don't have a subscription to NS. The author is a journalist, not a scientist!
She describes the work of Kathryn Zurek, who proposes "gravitons popping in and out of existence", causing fluctuations in space-time. I suppose that this is analogous to virtual pairs in conventional quantum theory, vacuum bubbles, but assumes that there are not only gravitons, but antigravitons too! Since we cannot, as yet, detect the first, surely the second is theory too far?
JOhn