Distances to Galactic Clusters
I expect accepted distances of distant galaxies are wrong; some sizes are probably wrong too.
A table with distances is below. The trivia can be interesting.
If distances to galaxies were calculated by standard candles, the distances should be 'close enough.' However those standards have limitations.
These are limitations for using Cepheids:
Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble constant can be established.
Chief among the uncertainties tied to the classical and type II Cepheid distance scale are: the nature of the period-luminosity relation in various passbands, the effects of photometric contamination (blending) and a changing (typically unknown) extinction law on Cepheid distances. All these topics are actively debated in the literature.
The accuracy of the distance measurements to Cepheid variables and other bodies within 7,500 lightyears is vastly improved by combining images from Hubble taken six months apart when the Earth and Hubble are on opposite sides of the Sun.
From a 2007 document from Cornell University in arxiv:
Extragalactic Cepheids are the basic rungs of the cosmic distance scale. They are excellent standard candles, although their luminosities and corresponding distance estimates can be affected by the particular properties of the host galaxy. Therefore, the accurate analysis of the Cepheid population in other galaxies, and notably in the Andromeda Galaxy (M31), is crucial to obtaining reliable distance determinations. We obtained accurate photometry of 416 Cepheids in M31 over a five year campaign within a survey aimed at the detection of eclipsing binaries. The resulting Cepheid sample is the most complete in M31 and has almost the same period distribution as the David Dunlap Observatory sample in the Milky Way. The large number of epochs (~250 per filter) has permitted the characterization of the pulsation modes of 356 Cepheids, with 281 of them pulsating in the fundamental mode and 75 in the first overtone. The period-luminosity relationship of the fundamental mode Cepheids has been studied and a new approach has been used to estimate the effect of blending. We find that the blending contribution is as important as the metallicity correction when computing Cepheid distance determinations to M31 (~0.1 mag). Since large amplitude Cepheids are less affected by blending, we have used those with an amplitude >0.8 mag to derive a distance to M31.
I see this process as: Hubble's Law is a given, using a red shift to get a distance and only the Hubble Constant for the inverse expansion gets periodic adjustments. Cepheids can provide that tuning within their luminosity/distance constraints.
When no Cepheid or supernova standard candle is available, only a red shift could be used to calculate a distance.
When Hubble's Law is used, so the distance comes from the NH red shift, then this technique is inaccurate as the density of intergalactic neutral hydrogen changes among clusters.
Both Magellanic Clouds are surrounded by NH causing high red shifts though visibly very close. M81 and M82 have a NH bridge. Some quasars are known to be in a NH cloud.
Using the NH absorption line for the foundation of cosmology is clumsy. ' [NH] makes up 75% of the universe's mass.
An analysis of the velocities and distances to the IC 342/Maffei Group as measured by M. J. Valtonen and collaborators suggested that IC 342 and Maffei 1 were moving faster than what could be accounted for in the expansion of the universe.
Maffei 2 in the Maffei group has a distance of 9.8 Mly with blue shift of -17 km/s.
IC342 in the Maffei group has a distance of 10.7 Mly with red shift of +31 km/s. The combination must be a problem. Blue shifts are typically a calcium absorption line near the Local Group, not NH.
A distance [to M31] of 11 million light years was determined in 1993 using the Hubble observations of 32 Cepheid variables in the galaxy. The distance was later corrected using data from ESA's Hipparcos satellite.
This [Hipparcos data] allows surveys at different wavelengths to be directly correlated with the Hipparcos stars, and ensures that the catalogue proper motions are, as far as possible, kinematically non-rotating. The determination of the relevant three solid-body rotation angles, and the three time-dependent rotation rates, was conducted and completed in advance of the catalogue publication. This resulted in an accurate but indirect link to an inertial, extragalactic, reference frame.
I was not aware of this extragalactic reference frame for calculating distances, or how often it is used.
The Virgo cluster:
As with many other rich galaxy clusters, Virgo's intracluster medium is filled with a hot, rarefied plasma at temperatures of 30 million kelvins that emits X-Rays. Within the intracluster medium (ICM) are found a large number of intergalactic stars (up to 10% of the stars in the cluster), including some planetary nebulae.
Astronomers should not assume intergalactic space is empty space.
I cannot identify exactly how a distance is calculated for any particular object.
This is how very distant galactic structures are measured:
Because these structures [Fornax Wall, Sculptor Wall, Great Wall], are so large, it is convenient to estimate their size by measuring their redshift.
None of these measurements are accurate using that method.
Whenever a distance is stated, its calculation is never identified.
With much confidence in the expanding universe I suspect the red shift is used more often rather than the more difficult luminosity calculation, especially given the candles have a limited range.
Hubble's Law is applied universally though it should not. Its distance calculations ignore the limitations for its use. I am suspicious of all distances mentioned here. IC1101 (below) is an obvious anomaly. Perhaps its distance is just wrong.
A few galaxies for reference:
LMC 165 Kly
SMC 195 Kly
M31 2.5 Mly
M33 2.7 Mly
NGC 3184 40 Mly
M87 53 Mly
Markarian's Chain about 55 Mly - 8 galaxies 'lie along a smoothly curved line' (includes M84, M86, others)
NGC 6872 212 Mly
IC1101 1 Bly - "possibly one of the largest and most luminous galaxies in the universe"
For galaxy groups that are not too distant, like the M81 group, the red shift for individual galaxies are sometimes available but their distances are not always shown.
A few galaxy groups for reference:
Local Group 0 Mly
Maffei Group 10 Mly
M81 Group 12 Mly
M101 Group 21 Mly
Norma Cluster 22 Mly - aka Abell 3627
M51 Group 23 Mly
M66 Group - Leo Triplet 35 Mly
M96 Group - Leo I Group 37 Mly
NGC 5866 Group 50 Mly
Virgo Cluster 54 Mly - known for its Virgo-centric Flow - Local Group higher red shifts imply Local Group is moving faster than this more distant group
Fornax Cluster 62 Mly
Leo II galaxy groups (11 of them) 70-90 Mly
Eridanus Cluster 75 Mly
Abell 262 galaxy cluster 214 Mly
Abell 426 galaxy cluster 240 Mly
A1367 or Leo or West Coma cluster 295 Mly
A1656 East Coma cluster 295 Mly
CfA2 Great Wall 300 Mly at nearest - consists of Coma, Hercules, Leo superclusters, and Coma Filament
Abell 2151 500 Mly - aka Hercules cluster
Abell 133 galaxy cluster 763 Mly
Bootes supercluster (11 clusters) 815-1025 Mly
Abell 2029 galaxy cluster 1 Bly - has IC1101
Abell 222 galaxy cluster 2.4 Bly
Abell 223 galaxy cluster 2.4 Bly - a filament connects 222 to 223
Abell 383 galaxy cluster 2.5 Bly
Abell 370 galaxy cluster 4 Bly
The last clusters have extreme red shifts. I suspect quasars are in these distant groups; I assume only one spectrum is sampled for a collection of objects to determine a velocity for the group. Different velocities of objects in a group would be awkward especially when a quasar is seen next to a low red shift object. If the highest red shift is used for a group that is an invalid conclusion.
For determining motion of nearby galaxies correctly we could use emission lines of individual stars to avoid NH absorption lines due to the intervening space. That requires proximity for resolution to individual stars.
I do not see an alternative for measuring distant galaxies or clusters. They are always in the intergalactic medium.
I wonder where those galaxies really are.
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