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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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:

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:

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

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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