Sorry to contradict the wikipedia entry, which is supposed to be the source of all wisdom, but the
actual situation regarding the status and extent of the Sculptor group is quite different from what is described in your post. Wikis were supposed to be self-correcting, as they supposedly embody “the knowledge of all humankind”, but what we get in practice is some entries by experts and other entries that misinterpret the facts.
(The comments you quote from wikipedia are probably a misinterpretation of some poorly worded sections in the two belowmentioned papers by Igor Karachentsev)
The Sculptor Group of galaxies does not exist. The overdensity of nearby galaxies in the direction of Sculptor is, in reality, a freely expanding cloud or association of galaxies:
A internally consistent and fairly accurate distance scale for the nearby groups of galaxies and for their members has been measured by a loose group of astronomers whose informal leader is Igor Karachentsev. A lot of the papers by this group aim to characterize the entire contents of what has become known as "The Local Volume", that is, to make a three-dimensional "map" of
everything that can be found within a distance of 10 megaparsecs (= 32.6 million light years).
The goal of these astronomers is to estimate the distances, masses, luminosities, sizes, etc., of
all of the objects that exist within this volume, down to very low levels of galaxy Luminosity and galaxy Surface Brightness. The premise of this work, apart from the “wow” factor of creating the first-ever accurate map of the local universe, is that we can (perhaps) discover everything we need to know about galaxy formation and about the history of the galaxies, and about matter, by looking very closely at the Local Volume of galaxies!
A review of the properties of the nearest groups of galaxies can be found in:
I.D. Karachentsev, (2005), Astronomical Journal, Vol. 129, p.178.
A review of the distribution of the galaxies within the Sculptor Cloud can be found in:
(2003) , Astronomy & Astrophysics, Volume 404, page 93
Karachentsev (and his many associates) found that the overdensity of galaxies formerly known as the Sculptor Group of Galaxies is
not actually gravitationally bound;
when we consider the distances between these galaxies and when we consider the total mass of all of these galaxies (luminous matter, plus “dark matter”), and we do the appropriate calculations, it can be shown that the "group" is
much too spatially extended to hold together under the influence of its own gravity.
Instead, what we observe in this part of the sky is actually "The Sculptor Cloud" (or Sculptor Filament) of galaxies, a
freely expanding association of galaxies that, while it does constitute an
overdensity of galaxies relative to the distribution of field galaxies,
does not hold together and therefore actually participates in the expansion of the universe. (a group of galaxies, in the strict and formal definition that is adopted by professional astronomers, holds together, and does not expand in this way)
The freely expanding Sculptor Cloud is an intrinsically elongated (prolate) structure that stretches from the margins of the Local Group out to a distance of at least 16 million light years, and its longer dimension is extended along our line-of-sight towards the constellation Sculptor (the longer axis of the cloud is approximately in our line-of-sight).
Some astronomers are of the opinion that the Sculptor Cloud , and our own Local Group of galaxies, and one or two other galaxy groups, are all part of a single elongated supergalactic structure in the shape of a 10 Megaparsec long filament.
The distended and low-density Sculptor Cloud is well shown in the following diagram (NGC 253 is getting closer to the far end, from the perspective of an observer in our own Galaxy):
But....within the Sculptor Cloud there do exist several rather small REAL groups of galaxies, in that their constituent galaxies
are probably gravitationally bound.
Two examples of small galaxy groups found within the Sculptor Cloud of galaxies:
- There is a compact Very Poor group composed of NGC 300 and NGC 55 and a few low luminosity galaxies.
- There is a concentration of galaxies, significantly further away, around NGC 253 and NGC 247.
The Low Surface Brightness & Very Extended spiral NGC 45 is getting near to the far end of the Sculptor Cloud....an approximate distance for this galaxy, as currently quoted in the scientific literature, is 16 million light years. (This estimate of the NGC 45 distance appears to originate in a very old distance measurement that was based on inaccurate methods that were in use; decades ago.)
Incidentally, recent measurements of the distances to NGC 300 and NGC 55 using the "gold standard" method that utilizes Cepheid Variables, have shown that the figure of 7 million light years is a bit greater than the actual distance of these two galaxies.
NGC 300 and NGC 55 are both at a distance of about 6.1 to 6.3 million light years, according to the latest measurement using Cepheid variables. At any rate, the true distance of these galaxies is closer to 6 million than 7 million.
My personal feeling, from consideration of the relative degree of resolution of NGC 55 and
NGC 300, has always been that NGC 55
seems to be significantly closer than NGC 300!
Cheers,
the Mad astronomer
