Astronomy Answers: Universe Family Tree: Galaxy Cluster

# Astronomy AnswersUniverse Family Tree: Galaxy Cluster

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 Type :  Galaxy Cluster Cloud ───  Galaxy Cluster ───  Galaxy Size :  23 Mpc ───  8 Mpc ───  50 kpc Earth :  Canes Venatici Cloud ───  Local Group ───  the Galaxy

A galaxy cluster is a set of galaxies that appear to be associated with each other. Figuring out which galaxies belong to which galaxy cluster usually suffers from the same problems as figuring out which galaxy clusters belong to which supercluster.

Galaxy clusters are commonly divided into rich and poor clusters. Rich clusters have many member galaxies and the galaxies are usually concentrated toward the middle of the cluster, with one or more very bright and large galaxies in the very center. Poor clusters have few members and usually show no particular concentration toward the center.

## 1. Local Group

Our own Galaxy is part of the Local Group, which is a poor galaxy cluster: it has only two bright members (our own galaxy and the Andromeda Nebula - M31) and no obvious center. It is well separated from all other galaxy clusters, though: there are no galaxies known at distances between 1.3 and 2.4 Mpc from us. The members of the Local Group that I know about are listed in the following table. "Ident" is a common identification, often taken from the constellations they appear in, "Type" is the type of galaxy, "B" is the apparent total B magnitude, "diam" the diameter, "DM" the distance measure, "Dist" the distance from us, and "Source" the source of this information. Numbers between parentheses refer to the notes below the table. Numbers between curly braces {} refer to the bibliography. APOD means an Astronomy Picture of the Day of this object is available.

Table 1: Local Group of Galaxies

Ident Type B Diam DM Dist Source
kpc kpc
Galaxy Sc 30 {3} 1994
NGC 147 E5 10.6 1 660 {3} 1994
NGC 185 E5 10.3 1 660 {3} 1994
NGC 205 E5 8.9 2 640 {3} 1994 APOD
M 32 (1) E0 9.1 1 660 {3} 1994 APOD
M 31 (1) Sb 4.4 50 24.77±0.11 900 {7} 1997 APOD
M 33 (1) Sc 6.3 8 730 {3} 1994 APOD
NGC 6822 IB(s)m 9.3 2 470 {3} 1994
IC 10 S 11.7 1260 {3} 1994
IC 1613 Im 10.1 1 740 {3} 1994
SMC (1) Irr 2.8 3 50 {3} 1994 APOD
Sculptor dE 9.2 1 65 {3} 1994
Fornax dE 9.1 2 170 {3} 1994
LMC (1) Irr? 0.6 7 18.70±0.10 55 {7} 1997 APOD
Leo A Irr 13.1 1100 {3} 1994
Sextans B Im 12.0 {1} 1975
Sextans C dE 140 {3} 1994
Leo I dE 11.3 0.7 230 {3} 1994 APOD
Sextans A Irr 11.7 1000 {3} 1994
Leo II dE 12.9 0.7 230 {3} 1994
Ursa Maior dE 120 {3} 1994
Ursa Minor dE 67 {3} 1994
Serpens dE {1} 1975
Draco dE 1 67 {3} 1994
Capricornus dE {1} 1975
Pegasus dE 170 {1} 1975
WLM Irr 870 {3} 1994
LGS-3 (Psc) dIrr-dSph 0.7 23.9±0.3 600 {5} 1996
Sagittarius dE 4 24 {6} 1995
Cassiopeia I Irr 14.6 (2) 790 {4} 1996
Dwingeloo 1 SBb-c 14.0 (2)
Dwingeloo 2
Maffei 1
MB 1
MB 2
MB 3
Cam B

Notes:

1. M 31 = Andromeda Nebula, M 33 = Triangulum Nebula, SMC = Small Magellanic Cloud (Nubeculum Minor), LMC = Large Magellanic Cloud (Nubeculum Maior), M 32 = NGC 221.
2. V magnitude.

## 2. Nearby Galaxy Clusters

Properties of nearby galaxy clusters (centered on our Galaxy, M 31, M 81, NGC 5128, NGC 5236, and M 101) are listed in the following table, which was taken from {8}. Only those galaxies were included that appear bound by gravity to one of the big galaxies.

Table 2: Nearby Galaxy Clusters

Group Galaxy M31 M 81 N5128 N5236 M101
Dist/Mpc 0.00 0.77 3.58 3.5 4.2 6.58
log(M1/Msun) 11.64 11.89 11.58 11.81 11.47 11.66
Rk/kpc 141 85 86 269 91 244
t 0.089 0.032 0.047 0.19 0.058 0.20
N 13 11 15 10 5 7
N* 3 6 4 7 2 7
M1/Mtot 0.92 0.89 0.88 0.74 0.98 0.86
g 15.4 26.3 18.2 19.5 6.6 19.5

The rows contain: Dist/Mpc: the average distance of the group to us, in Mpc; log(M1/Msun): the decimal logarithm of the mass of the principal (i.e., most massive) member of the group in solar masses; Rk/kpc: the average distance (projected unto the sky) of the companions to the principal member, in kpc; t: the characteristic crossing time of the group relative to the age of the universe (assuming a Hubble constant of 75 km/s Mpc); N: the number of companion members (including probable members) that are gravitationally bound to the principal member; N*: the number of companion members with a mass exceeding one thousand million solar masses (log M/Msun > 9), which should be detectable out to the distance of M 101; M1/Mtot: the fraction of the total mass of the group that is in the principal member; g: a number indicating the typical gravitational bond between the companion members and the principal member. The bigger the value for g, the more closely the galaxies are bound. (The g in this table is equal to 10-log g from the original publication).

## 3. Nearby Rich Galaxy Clusters

A survey of rich galaxy clusters down to a visual magnitude of about +18 (corresponding to an average redshift of about z = 0.14 or a distance of about 550 Mpc) yielded 4076 rich cluster (each having at least 30 member galaxies with magnitudes between 0 and 2 magnitudes above that of the third brightest galaxy) {2}. The following table lists those that have a redshift published in the survey that is less than z = 0.02 (80 Mpc), and some that have a proper name. For about one fifth of the rich galaxy clusters listed in the survey a redshift was included. "ACO" stands for the identifying number of the galaxy cluster in the survey, "Dist" for the distance (derived from the redshift value in the survey, in Mpc, and "m10A" for the magnitude of the tenth brightest galaxy in the cluster, corrected for galactic extinction.

Table 3: Nearby Rich Galaxy Clusters

ACO Name Dist m10A
194 74 13.9
262 67 13.3
347 78 13.3
426 Perseus 76 12.5 <a href="//antwrp.gsfc.nasa.gov/apod/ap970426.html">APOD
569 82 13.8
1060 Hydra I 47 12.7
1656 Coma 97 13.5
2065 Corona Borealis 300 15.6
2151 Hercules 154 13.8
3526 46 13.2
3537 69 14.3
3565 46 14.0
3574 58 13.4
3627 60 13.5

## 4. References

1. "Nearby Groups of Galaxies", G. de Vaucouleurs, Chapter 14 in "Galaxies and the Universe", Editors: Allan Sandage, Mary Sandage, and Jerome Kristian, Volume 9 of "Stars and Stellar Systems", General Editor: Gerard P. Kuiper; University of Chicago Press (Midway Reprint), ISBN 0-226-45970-5 (1975).
2. "A Catalogue of Rich Clusters of Galaxies." G.O. Abell, H.G. Corwin Jr., R.P. Olowin. Astrophys. J. Suppl. Ser. 70, 1 (1989) [ADS 1989ApJS...70....1A]
3. "The Cambridge Atlas of Astronomy", 3rd edition, General editors: Jean Adouze, Guy Israël, Cambridge University Press (1994)
4. "The irregular galaxy Cassiopeia 1 -- a new member of the Local group", N. Tikhonov, Astronomische Nachrichten, vol. 317, no. 3, p. 175 (1996) [ADS 1996AN....317..175T]
5. "A colour-magnitude diagram of the Pisces dwarf galaxy", Tikhonov, N.; Makarova, L., Astronomische Nachrichten vol. 317, no. 3, p. 179 (1996) [ADS 1996AN....317..179T]
6. "Sagittarius: the nearest dwarf galaxy", Ibata, Rodrigo A.; Gilmore, Gerard; Irwin, Michael J., Mon. Not. Royal Astron. Soc., vol. 277, p 781 (1995) [ADS 1995MNRAS.277..781I]
7. "The Cepheid PL Zero-Point from Hipparcos Trigonometrical Parallaxes", M. W. Feast, R. M. Catchpole, Mon. Not. R. Astron. Soc., vol. 286, p L1 (1997) [ADS 1997MNRAS.286L...1F]
8. "The Local Group in comparison with other nearby groups of galaxies", I. Karachentsev, Astron. Astrophys. 305, 33-41 (1996) [ADS 1996A&A...305...33K].

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Last updated: 2020-07-18