\(\def\|{&}\DeclareMathOperator{\D}{\bigtriangleup\!} \DeclareMathOperator{\d}{\text{d}\!}\)
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.
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:
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).
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 N | ame D | ist | m10A | |
---|---|---|---|---|
194 | 74 | 13.9 | ||
262 | 67 | 13.3 | ||
347 | 78 | 13.3 | ||
426 P | erseus | 76 | 12.5 < | a href="//antwrp.gsfc.nasa.gov/apod/ap970426.html">APOD |
569 | 82 | 13.8 | ||
1060 H | ydra I | 47 | 12.7 | |
1656 C | oma | 97 | 13.5 | |
2065 C | orona Borealis 3 | 00 | 15.6 | |
2151 H | ercules 1 | 54 | 13.8 | |
3526 | 46 | 13.2 | ||
3537 | 69 | 14.3 | ||
3565 | 46 | 14.0 | ||
3574 | 58 | 13.4 | ||
3627 | 60 | 13.5 |
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Last updated: 2021-07-19