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Local Group
Mario L Mateo
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Encyclopedia of Astronomy & Astrophysics
P. Murdin
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Local Group
Local Group
Not long after EDWIN HUBBLE established that galaxies are
‘island universes’ similar to our home galaxy, the MILKY
WAY, he realized that a few of these external galaxies are
considerably closer to us than any others. In 1936 he
first coined the term ‘Local Group’ in his famous book
The Realm of the Nebulae to identify our nearest galactic
neighbors. More than 60 yr later, the galaxies of the
Local Group remain particularly important to astronomers
because their proximity allows us to obtain our most
detailed views of the properties of normal galaxies beyond
our own. These nearby systems also provide our clearest
views of how galaxies interact with one another in the
relatively small volume of space of the Local Group.
The brightest members of the Local Group are so close
to us that on a clear, dark night away from city lights it is
possible to see them with the unaided eye: in the southern
hemisphere the LARGE MAGELLANIC CLOUD (LMC) and SMALL
MAGELLANIC CLOUD (SMC) shine brightly, while in the north
the ANDROMEDA and TRIANGULUM GALAXY can be seen as faint
smudges of light in the sky. These two galaxies are, in fact,
the most distant objects visible with the naked eye. From
both hemispheres, the gossamer glow of the Milky Way
reveals the presence of billions of stars spread throughout
the thin disk of our home galaxy. These five galaxies
constitute the most luminous and massive members of the
Local Group.
Although the existence, if not the true nature, of
the five naked-eye Local Group galaxies and the Milky
Way has been known to humans since antiquity, the first
member identified telescopically was Messier 32 (more
typically referred to as M32) by G-J Le Gentil in 1749.
Since then, astronomers have steadily identified additional
members of the Local Group. By the time Hubble first
introduced the concept of the Local Group in 1936, he was
able to list 11 galaxies that he considered to be members of
the group. At present (1999), 43 galaxies can be catalogued
as probable members of the Local Group; these systems are
listed in table 1 along with the dates of discovery for each.
Remarkably, more Local Group members have been found
in the past 30 yr than in all previous human history. Also,
the era of discovery is almost certainly not over as future
surveys uncover more members or as new nearby galaxies
are found serendipitously.
Why are the galaxies of the Local Group so difficult to
identify? The principal reason is that, apart from our Milky
Way Galaxy and the large Andromeda and Triangulum
galaxies (known also as M31 and M33, respectively), the
known members of the Local Group are DWARF GALAXIES. By
definition, these systems have low intrinsic luminosities.
They usually also exhibit very low surface brightness
(see LOW SURFACE BRIGHTNESSES GALAXIES). This property is a
measure of how spread out the galaxy’s light is on the sky.
In the case of nearly every dwarf galaxy member of the
Local Group, the surface brightnesses are lower than that
E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S
of the night sky, lending them a ghostly appearance and
making them very difficult to detect, even at close range1 .
Much of the recent success in finding new Local
Group members is due to the availability of the many largescale photographic surveys of the sky carried out since the
seminal Palomar Sky Survey of the 1950s. Soon after these
surveys were begun, visual searches of the photographic
plates identified new nearby galaxies. Starting in the
1970s, automated measurements and analyses of the plates
from these surveys helped uncover nearly all of the most
recently discovered Local Group members. However,
even the most complete optical survey cannot find all of
the galaxies in the sky. For example, searches for galaxies
near the bright band of the Milky Way itself are severely
hindered by the high stellar density in this part of the
sky and by the clouds of gas and dust within the plane
of our Galaxy. This INTERSTELLAR MATTER effectively blocks
all optical light from distant objects, making it impossible
to find galaxies lurking in the background. Ongoing and
planned surveys in the infrared and radio wavelengths can
penetrate the haze of the Milky Way by detecting radiation
that is unaffected by dust obscuration. These searches are
almost certain to reveal several new Local Group members
in coming years.
Another complication in producing a complete census
of the Local Group is the uncertainty involved with
defining the group’s boundary. The best way to establish
this is to determine which local galaxies are gravitationally
bound to one another. Since M31 and the Milky Way
dominate the mass of all probable Local Group members,
this process requires a good estimate of the masses of
these two giant galaxies (see below). In addition, we
need accurate information on the distances and motions
of individual candidate Local Group galaxies to determine
whether they are physically bound to the M31–Milky Way
system. Table 1 lists the 43 galaxies that appear to be likely
members of the Local Group based on this approach.
Although well defined, this method of identifying and
counting members of the Local Group is highly uncertain.
For example, apart from a few of the nearest galaxies, we
cannot measure the PROPER MOTION—the angular movement
across the sky—of external galaxies. A galaxy that may
be moving towards or away from us at a moderate speed
may be moving very rapidly across our line of sight. Thus,
some of the galaxies in the table that we believe are bound
to the Local Group may actually only be ‘passing through
the neighborhood’. A second problem is that distances
to local galaxies are notoriously difficult to determine
reliably. Methods that work for the Magellanic Clouds
may not be applicable to other nearby galaxies, and vice
1
Unlike the apparent brightness of a galaxy, the surface
brightness of an extended object does not change as a function
of distance—at least for distances up to a few hundred million
light-years. This makes low-surface-brightness galaxies difficult
to detect anywhere. Consequently, a very large number of lowsurface-brightness galaxies may still remain hidden throughout
the universe, enough possibly to fundamentally change our views
of the distribution and numbers of galaxies in the universe.
Copyright © Nature Publishing Group 2001
Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998
and Institute of Physics Publishing 2001
Dirac House, Temple Back, Bristol, BS1 6BE, UK
1
Local Group
E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S
Table 1. Galaxies of the Local Group.
Galaxy
Other name
M31
Milky Way
M33
LMC
SMC
WLM
M32
NGC 205
NGC 3109
IC 10
NGC 185
NGC 147
NGC 6822
IC 5152
IC 1613
Sextans A
Sextans B
Sagittarius
Fornax
Pegasus
EGB 0427+63
SagDIG
And VII
UKS2323-326
Leo I
And I
GR 8
Leo A
And II
Sculptor
Antlia
And VI
LGS 3
And III
And V
Phoenix
DDO 210
Tucana
Leo II
Sextans
Carina
Ursa Minor
Draco
NGC 224
NGC 598
NGC 292
DDO 221
NGC 221
M110
DDO 236
UGC 192
UGC 396
DDO 3
DDO 209
DDO 8
DDO 75
DDO 70
DDO 216
UGCA 92
UKS1927-177
Cassiopeia
UGCA 438
DDO 74
DDO 155
DDO 69
Peg dSph
Pisces
Aquarius
DDO 93
DDO 199
DDO 208
Year of
discovery
RA
(2000)
Declination
(2000)
Type
Subgroup
Distance
(Mly)
VT
Luminosity
(106 L )
Mass
(106 M )
–
–
–
–
–
1923
1749
1864
1864
1895
1864
1864
1864
1895
1906
1942
1955
1994
1938
1958
1984
1977
1998
1978
1955
1972
1956
1942
1972
1938
1985
1998
1978
1972
1998
1976
1959
1985
1950
1990
1977
1955
1955
00h 42.7m
17h 45.7m
01h 33.9m
05h 23.6m
00h 52.7m
00h 02.0m
00h 42.7m
00h 40.4m
10h 03.1m
00h 20.4m
00h 39.0m
00h 33.2m
19h 44.9m
22h 02.7m
01h 04.9m
10h 11.1m
10h 00.0m
18h 55.1m
02h 40.0m
23h 28.6m
04h 32.0m
19h 30.0m
23h 26.5m
23h 26.5m
10h 08.5m
00h 45.7m
12h 58.7m
09h 59.4m
01h 16.5m
01h 00.2m
10h 04.1m
23h 51.7m
01h 03.9m
00h 35.3m
01h 10.3m
01h 51.1m
20h 46.8m
22h 41.8m
11h 13.5m
10h 13.1m
06h 41.6m
15h 09.2m
17h 20.3m
+41◦ 16
−29◦ 01
+30◦ 40
−69◦ 45
−72◦ 50
−15◦ 28
+40◦ 52
+41◦ 41
−26◦ 10
+59◦ 18
+48◦ 20
+48◦ 31
−14◦ 48
−51◦ 18
+02◦ 08
−04◦ 43
+05◦ 20
−30◦ 29
−34◦ 27
+14◦ 45
+63◦ 36
−17◦ 41
+50◦ 42
−32◦ 23
+12◦ 19
+38◦ 00
+14◦ 13
+30◦ 45
+33◦ 26
−33◦ 43
−27◦ 20
+24◦ 36
+21◦ 53
+36◦ 31
+47◦ 38
−44◦ 27
−12◦ 51
−64◦ 25
+22◦ 09
−01◦ 37
−50◦ 58
+67◦ 13
+57◦ 55
SbI–II
Sbc
ScII–III
IrrIII–IV
IrrIV–V
IrrIV–V
E2
E5p/dSph–N
IrrIV–V
dIrr
dSph/dE3p
dSph/dE5
IrrIV–V
dIrr
IrrV
dIrr
dIrr
dSph-N
dSph
dIrr/dSph
dIrr
dIrr
dSph
dIrr
dSph
dSph
dIrr
dIrr
dSph
dSph
dIrr/dSph
dSph
dIrr/dSph
dSph
dSph
dIrr/dSph
dIrr/dSph
dSph
dSph
dSph
dSph
dSph
dSph
M31
MW
M31
MW
MW
LGC
M31
M31
N3109
M31
M31
M31
LGC
LGC
M31
N3109
N3109
MW
MW
LGC
M31
LGC
M31
LGC
MW
M31
GR8
MW
M31
MW
N3109
M31
M31
M31
M31
MW
LGC
LGC
MW
MW
MW
MW
MW
2.5
0.03
2.7
0.16
0.19
3.0
2.6
2.6
4.1
2.7
2.0
2.3
1.6
5.2
2.3
4.7
4.4
0.08
0.45
3.1
4.2
3.4
2.5
4.3
0.81
2.6
4.9
2.2
1.7
0.26
4.1
2.7
2.6
2.5
2.6
1.4
2.6
2.9
0.66
0.28
0.33
0.21
0.27
3.4
–
5.9
0.4
2.0
10.4
8.1
8.1
9.9
11.6
9.1
9.4
9.1
11.2
9.6
11.3
11.4
4.0
7.6
12.0
13.9
13.5
15.2
13.8
10.1
12.8
14.4
12.8
12.7
8.5
14.8
14.1
14.3
14.2
15.0
13.2
14.7
15.2
12.0
10.3
10.9
10.3
10.9
25 000
8 300
3 000
2 100
580
500
380
370
160
160
130
130
94
70
64
56
41
18
16
12
9.1
6.9
5.7
5.3
4.8
4.7
3.4
3.0
2.4
2.2
1.7
1.4
1.3
1.1
1.0
0.9
0.8
0.6
0.6
0.5
0.4
0.3
0.3
700 000
350 000
30 000
20 000
1 000
150
2 120
740
6 550
1 580
130
110
1 640
400
795
395
885
–
68
58
–
9.6
–
–
22
–
7.6
11
–
6.4
12
–
13
–
–
33
5.4
–
9.7
19
13
23
22
versa. It is quite common that galaxies once believed to be
Local Group members are later removed from the list of
members as we refine our estimates of their distances and
motions. Indeed, two of the galaxies Hubble first proposed
to be Local Group members are now known to be located
much further away. Thus we can not only expect additions
to the table of Local Group members as new galaxies are
discovered but also subtractions from the list as we learn
that some of the galaxies are not members after all.
Global properties of the galaxies of the Local
Group
The individual galaxies of the Local Group span a large
range of basic properties. The luminosities of Local Group
galaxies range from a minimum of about 250 000 times
the luminosity of the SUN, to a maximum of more than
20 billion times the luminosity of the Sun, a range of a
factor of 75 000. It is also clear that the galaxies of the Local
Group span a very large range in size even though their
extreme dimensions are impossible to determine precisely.
The smallest systems in the Local Group are approximately
1000 ly in diameter, while the luminous parts of the giant
galaxies M31 and the Milky Way span over 100 000 ly from
end to end. This range is comparable with the size range
exhibited by mammals, from the tiny bumblebee bat to
the blue whale. Figure 1 illustrates the relative sizes of the
visible portions of most of the Local Group galaxies.
Virtually every major galaxy type is represented in
the Local Group. M31, the Milky Way and M33 are all
examples of SPIRAL GALAXIES, but each represents a slightly
different subclass of this family of galaxies. M31, for
example, exhibits a prominent central bulge and welldefined spiral arms throughout the thin disk of the galaxy;
it is classified as an Sb galaxy (see GALAXIES: CLASSIFICATION).
M33 has a very weak, possibly non-existent central bulge
and very poorly defined spiral arms; it is an Sc galaxy.
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Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998
and Institute of Physics Publishing 2001
Dirac House, Temple Back, Bristol, BS1 6BE, UK
2
Local Group
E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S
Figure 1. A family portrait of many of the members of the Local Group. The galaxies are shown to scale and to roughly the correct
range of relative brightness. The Milky Way is shown as it may appear to a viewer located outside the galaxy. This montage was
produced by Bruno Binggeli of the University of Basel.
Although it is clear that the Milky Way is a spiral galaxy—
the thin structure of the Milky Way and the existence of a
concentration of stars and GLOBULAR CLUSTERS towards the
constellation Sagittarius all confirm this classification—the
specific spiral subtype is extremely difficult to determine
reliably from our unfavorable vantage point inside the
Galaxy. Various indirect indicators suggest that our
Galaxy can be classified as an Sbc galaxy, intermediate
between the properties described for M31 and M33. Some
recent studies at radio, infrared and optical wavelengths
also suggest that our Galaxy contains an elongated central
bar composed of old, metal-rich stars. If true, then the
Milky Way is an example of a BARRED SPIRAL GALAXY, and its
specific subtype is SBbc, where the upper-case ‘B’ denotes
a barred system. All three galaxies are typically considered
‘giant’ spirals, despite the fact that M33 is only about
10% as luminous or massive as M31. By comparison, our
Galaxy is about half as luminous and massive as M31.
All the remaining galaxies of the Local Group are
dwarf systems of various types. The galaxies that exhibit
the most variation in appearance and in their global
properties are the DWARF IRREGULAR GALAXIES. The most
massive example of this type of galaxy in the Local Group
is the LMC, one of the satellites of the Milky Way and
the second closest external galaxy to the Sun. The LMC
is a massive dwarf, and its global properties place it
near the ill-defined boundary separating dwarf irregular
galaxies from small spirals. Careful studies reveal features
in the LMC normally found in spiral galaxies, such as
a central bar-like concentration of older stars, and some
evidence of indistinct spiral arms. The LMC’s companion,
the SMC, is a true irregular with little sign of large-scale
structure. The remaining irregular galaxies of the Local
Group are identified in the table. All of these systems are
substantially smaller and less luminous than the LMC.
The principal reason that dIrr galaxies appear to
have such chaotic structures is that they typically form
stars in small clumps called associations embedded within
the galaxies. Because young stars are typically very
luminous, they are often the most prominent stars seen
in optical images of these types of dwarf galaxies. If the
star-forming regions are irregularly distributed—and they
usually are—they give a strong impression that the entire
galaxy has a highly distorted, chaotic overall structure.
Figure 2 illustrates the appearance of a dwarf irregular
galaxy Sextans A, both from a ground-based image and
from an image taken with the HUBBLE SPACE TELESCOPE. The
luminous young blue and red stars are distributed nonuniformly, lending this galaxy its highly disorganized
appearance.
This apparent lack of large-scale organization in dIrr
galaxies is somewhat of an illusion produced by the
luminous young stars that generally make up a minority of
the total stellar population of such galaxies. Observations
in the red and infrared are most sensitive to the light
output of the dominant population of old and middleaged stars in these galaxies. When astronomers try to
determine the distribution of these oldest stars, even the
Copyright © Nature Publishing Group 2001
Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998
and Institute of Physics Publishing 2001
Dirac House, Temple Back, Bristol, BS1 6BE, UK
3
Local Group
E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S
Figure 2. (Left panel) A ground-based color image of the Local Group irregular galaxy Sextans A. The entire central region of the
galaxy is included. Notice how the bright clumps in the body of the galaxy are dominated by blue–white stars. Such stars are very hot
and young objects indicating that these clumps are regions of relatively recent star formation in the galaxy. This image was taken by
Diedre Hunter of Lowell Observatory. This figure is reproduced as Color Plate 9. (Right panel) An image of the central region of
Sextans A obtained with the Hubble Space Telescope. In this image the individual stars in the clump located to the left of the galaxy’s
center in the left-hand panel are now easily apparent. Note too how red stars are more common away away from the bright clump of
blue stars; these redder stars trace star formation in the galaxy long ago and show clearly that star formation has gone on over an
extended period in Sextans A. This complex spatial and temporal star formation history is common for most of the dwarfs of the Local
Group, especially the irregular galaxies. The HST photograph was provided by Robbie Dohm-Palmer of the University of Minnesota
and the University of Michigan. This figure is reproduced as Color Plate 10.
most irregular dwarfs exhibit a much smoother, more
symmetric appearance. An analogy is the surface of a pot
of slowly boiling soup: although bubbles erupt at different
locations at different times, the overall distribution of the
soup is relatively uniform. The active star-forming regions
of irregulars correspond to the bubbles: very prominent
when active, but short lived and all interspersed in a more
uniform medium.
With the striking exception of the Magellanic Clouds,
the dIrr galaxies of the Local Group tend to be found far
from the two large galaxies (M31 and the Milky Way) of the
Local Group. A few of the least luminous dwarf irregulars
are also among the most metal-poor galaxies known.
Because stars produce heavy elements which they then
eject back into the interstellar medium at the ends of their
lives, the low abundances of such elements in the smaller
dwarfs suggest that these galaxies are relatively pristine,
unevolved systems. These galaxies may be forming stars
in large quantities for the first time in their lives. As such,
these galaxies are invaluable ‘living fossils’ that can tell us
of the properties of gas and stars in the early universe.
One particularly interesting Local Group irregular
galaxy is IC 10. This galaxy is forming stars at an
unsustainably rapid rate. If it were to continue it would
soon exhaust its raw materials for making stars (gas and
dust) in only a few million years. The implication is that
unless it started out with an astoundingly large reservoir
of gas from which to form stars, IC 10 has probably been
caught during a particularly active—but short-lived—
phase in its star-formation history. Such galaxies that
appear to form stars at unsustainably high rates are known
as STARBURST GALAXIES. IC 10 is probably the closest example
of a true starburst galaxy, although one active region of star
formation in the LMC—the 30 Doradus Region—exhibits
similar characteristics on a subgalactic scale.
The remaining dwarfs of the Local Group are
ellipsoidal systems. These galaxies are characterized by
a roughly circular or elliptical outline on the sky, and by a
smooth, centrally concentrated distribution of light. One
of these, M32, is considered to be an example of a true
DWARF ELLIPTICAL GALAXY. As such, it represents the lowluminosity end of the very large family of elliptical galaxies
which includes some of the largest, most luminous and
most massive individual galaxies known. M32 is also
noteworthy because it appears to harbor a massive BLACK
HOLE in its extremely bright nucleus, it may be a unique
local example of a galaxy with no ancient stars and, as
a companion of M31, it shows distortions that indicate a
strong gravitational interaction with its massive parent.
The remaining ellipsoidal Local Group galaxies are
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and Institute of Physics Publishing 2001
Dirac House, Temple Back, Bristol, BS1 6BE, UK
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Local Group
known as DWARF SPHEROIDAL GALAXIES (or dSph galaxies).
About half of all galaxies in the Local Group are of
this type and they are apparently similarly common
in other groups and clusters.
Consequently, these
dim, unassuming galaxies probably represent the most
common type of galaxy in the entire universe. Within
the Local Group, dSph galaxies are typically found in
the company of a larger parent galaxy. For example,
of the 13 close companions of the Milky Way, nine are
dSph satellites. Virtually all the remaining dSph galaxies
of the Local Group are found near M31. The most
luminous dSph galaxy, NGC205, is a highly distorted
companion of M31 (figure 3). The two lowest-luminosity
galaxies known are dSph companions of the Milky Way:
the Draco and Ursa Minor systems. These galaxies
emit less light than some individual globular clusters—
massive, compact star clusters typically found within
extended halos surrounding elliptical, spiral and larger
dwarf galaxies (see HALO, GALACTIC). Nevertheless, the large
dimensions and large masses (most in the form of matter
that we cannot see directly) of galaxies such as Draco
and Ursa Minor distinguish them from the more compact
clusters.
Five of the dwarf galaxies of the Local Group are
difficult to classify as ellipsoidal or irregular systems
because they exhibit some of the properties of both. These
‘transition’ galaxies may represent the late stages of starformation episodes in dwarf irregulars fortuitously caught
during the period when the youngest stars begin to fade
from prominence. This picture is consistent with the starformation histories we measure for transition systems and
with the growing evidence that dSph and dIrr galaxies
have undergone complicated star-formation histories over
the entire lifetime of the universe. A few hundred million
years after periods of active star formation, a dSph galaxy
that may initially have looked like a dIrr system could
become a transition galaxy. This intimate relation between
dIrr and dSph galaxies in which ‘transition’ systems act
as a ‘missing’ evolutionary link remains controversial.
Nonetheless, there is little doubt that some dSph galaxies
looked a lot like low-luminosity dIrr systems at some
point(s) in the past.
Galaxies are not the only inhabitants of intergalactic
space within the Local Group. There is growing evidence
of the existence of isolated clouds of gas, usually in the
form of neutral hydrogen, distributed throughout the
group. This material may represent gas expelled from
other galaxies or may correspond to primordial matter
that has yet to collapse into small stellar systems such as
globular clusters or dwarf galaxies. Some remote globular
star clusters could plausibly be ‘free-floating’ members
of the Local Group that were ejected from their parent
galaxies during past interactions of individual galaxies
within the group.
One prominent class of normal galaxy is not found
within the Local Group: giant elliptical galaxies. This is
not entirely surprising; since large ellipticals are relatively
rare in the local universe, we would have been ‘lucky’
E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S
Figure 3. An image of NGC 205, a dwarf spheroidal companion
of M31. The bright galaxy to the lower right of the figure is M31
itself. Note how close to the larger galaxy NGC 205 appears to
be and how the outer extent of NGC 205 appears distorted from
the smooth elliptical shape apparent near the galaxy core. This
distortion is due to the strong tides that are induced in NGC 205
during its close passage to its much larger parent, M31. M32 and
the Sagittarius dwarf also show evidence of tidal disruption by
M31 and the Milky Way, respectively. The Magellanic Clouds
show clear evidence of tidal distortion due to mutual
interactions over the past few billion years. This image was
obtained by Paul Harding, Heather Morrison and Anne Fry of
Case Western Reserve University.
to find one closeby. Moreover, giant ellipticals tend to
be found in regions with a high density of galaxies, a
manifestation of the so-called GALAXY MORPHOLOGY–DENSITY
RELATION. Because the Local Group is a loose, low-density
collection of galaxies, it would have been unusual—
although not impossible—for it to contain one or more big
elliptical galaxies. The nearest giant elliptical galaxy to us
is probably MAFFEI 1. The slightly more distant Centaurus A
(NGC 5128) is somewhat easier to study because it is not
located so close to the Milky Way in the sky and suffers
far less obscuration by interstellar dust in the plane of our
Galaxy.
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Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998
and Institute of Physics Publishing 2001
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5
Local Group
The motions of galaxies within the Local Group
The motions of galaxies in the Local Group appear
to violate HUBBLE’S LAW that the universe is uniformly
expanding. The reason for this has to do with the
definition of the Local Group as the collection of nearby
galaxies that are gravitationally bound to one another.
Just as the planets of the solar system do not appear
to be expanding away from us because they are bound
to the Sun, the mutual attraction of galaxies within the
Local Group has overcome the universal expansion in our
immediate neighborhood. The most striking example of
this is M31, which is currently approaching our galaxy at
nearly 50 km s−1 : if on a true collision course, the two
galaxies will meet in about 8–10 billion years. More likely,
M31 and the Milky Way make up a binary system in which
the two galaxies orbit their common center of gravity.
Inevitably, tidal effects will cause the two galaxies to merge
into one giant system at some time in the distant future.
Many of the other galaxies of the Local Group also exhibit
motions towards us, signifying that they too are either in
orbit about the Milky Way or currently moving along orbits
about M31 or other Local Group galaxies that cause them
to move towards us at the present time. In all of these cases,
the orbital motions are larger than the universal expansion
velocities we would expect to measure for such nearby
galaxies based on Hubble’s law.
The mutual attraction of M31 and the Milky Way can
be used to estimate the mass of the Local Group. Much
as a ball thrown in the air first rises, stops and then falls,
M31 and the Milky Way are now falling towards each other
after their initial movement apart after the BIG BANG. If one
measures the relative velocities and locations of the two
galaxies and one estimates how long it has been since
they were together—essentially the age of the universe
estimated from the Hubble constant or from the ages of
the oldest stars—it is possible to estimate the combined
mass of M31 and the Milky Way. This sort of analysis was
first carried out by F D Kahn and L Woltjer in 1959. Modern
applications of this technique reveal that the combined
mass of M31 and the Milky Way is in the range between 500
billion and 2 trillion times the mass of the Sun. Because
virtually all the matter of the Local Group is located in
these two giant spiral galaxies, this is also our best estimate
of the total mass of the group.
As massive as the Local Group is, the total light output
of all of the galaxies in the group M31 and the Milky Way
is equivalent to ‘only’ 30 billion times the luminosity of
the Sun. This is unusual because for normal stars the ratio
of their mass (in units of the mass of Sun, or 2 × 1030 kg)
and their luminosity (also in units of the luminosity of the
Sun, or 4 × 1026 W) is around 1. For the Local Group the
ratio is much higher, approximately 50. This suggests that
the group is dominated by ‘DARK MATTER’ which contributes
to the local gravitational field but remains invisible at any
wavelength of ELECTROMAGNETIC RADIATION. The large massto-light ratio of the Local Group implies that only 2–5%
of the total matter in the group is visible to us through
emitted or reflected radiation.
E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S
We can also measure the masses of Local Group
galaxies individually by determining the range of
velocities of individual stars and gas clouds that orbit
within individual systems. So long as Newton’s law
of gravity remains valid over the physical scales of the
galaxies, astronomers find that Local Group galaxies
typically exhibit mass-to-light ratios in the range 3–100
(see table 1). Thus, the dark matter within the Local
Group is not spread out uniformly throughout the group,
but is instead concentrated about the individual galaxies
we detect optically. This suggests that possibility that the
Local Group may contain some nearby examples of ‘dark’
galaxies consisting of only dark matter with no luminous
material. No such systems have been detected yet. The
dwarf galaxies of the Local Group do offer one important
clue about the nature of dark matter. Certain types of dark
matter that have been postulated in the past—in particular
NEUTRINOS—cannot account for the surprisingly high mass
densities required to account for the total masses of these
small galaxies.
The distribution of galaxies within the Local
Group
Figure 4 shows a stereoscopic picture of the Local Group.
It is quite clear from this three-dimensional image of the
Local Group that the group’s volume is not uniformly
filled with galaxies. Instead, most of the galaxies appear to
congregate into three clumps; the remaining galaxies are
spread out into a larger ‘cloud’ of galaxies that occupies a
large fraction of the total volume of the Local Group. The
smaller ‘subgroups’ correspond to M31 and its satellites,
the Milky Way and its companions, and the ‘NGC3109
subgroup’ comprising five small galaxies that are well
separated from M31 and the Milky Way. The galaxies
of the more extended cloud—the so-called ‘Local Group
Cloud’—are not obviously associated with any single
larger galaxy. Table 1 lists the subgroup with which each
Local Group galaxy is most likely associated.
In general, each subgroup corresponds to a set of
gravitationally bound galaxies. The 13 satellites of the
Milky Way, for example, are all likely to be in orbit about
our Galaxy while M31 maintains a similar stable of small
companion galaxies. This situation is analogous to the
moons that orbit individual planets of the Solar System:
although Jupiter and Saturn orbit the Sun, both planets
possess their own large families of bound satellites. In the
case of the Local Group, the subgroups are weakly bound
together, and there is evidence that some satellites may
be occasionally ‘swapped’ from one subgroup to another.
For example, some models of the dynamical evolution of
the Local Group suggest that the Magellanic Clouds and
perhaps the Leo I dSph galaxy may have first formed near
M31, but are now satellites of our Galaxy.
Some Local Group galaxies show unmistakable signs
of strong mutual gravitational interactions. As noted
above, the Milky Way and M31 have overcome the initial
expansion and are now falling in towards one another.
The close companions of M31—NGC 205 and M32—both
Copyright © Nature Publishing Group 2001
Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998
and Institute of Physics Publishing 2001
Dirac House, Temple Back, Bristol, BS1 6BE, UK
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Local Group
E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S
Figure 4. (Top panel) A stereo map of the Local Group showing its approximate three-dimensional structure. The view is from a point
located about 7 million ly directly in front of the Sun’s current motion about the center of the Milky Way. The large cross near the
center of each panel denotes the position of the Milky Way, while the large off-center cross is M31. The smaller cross represents M33.
The open squares show the locations of the dwarf ellipticals and dwarf spheroidal galaxies; the solid squares denote the dwarf
irregulars; and the small ×s are the transition galaxies. The MW, M31 and NGC 3109 groups are easily visible; the latter is on the ‘far’
side of the figure. The large diffuse ‘bubble’ of galaxies extending to the lower right and towards the viewer is the Local Group Cloud.
(Lower panel) A guide to the identities of individual galaxies in the stereo image. The systems are denoted with abbreviations that
should—in conjunction with the names listed in table 1—uniquely define each galaxy. The galaxies closest to M31 and the Milky Way
are not identified in order to minimize confusion in those crowded regions of the figure.
show global distortions that may be due to strong tidal
effects induced by their close passage to their parent galaxy
(figure 3 shows the distortions induced in NGC 205 by
its close passage by M31). The Magellanic Clouds reveal
considerable evidence that they too have interacted in
the past. For example, the MAGELLANIC STREAM is a long
arc of neutral gas that was probably ejected from one or
both of the Clouds as they passed close to one another
and passed the Milky Way. Because the SMC is the least
massive component of the interacting pair, it has suffered
the most. One clear sign of this is the fact that the SMC is
elongated significantly along our line of sight, resembling
a cigar whose long axis is roughly pointed towards us.
This elongation reflects the stretching of the galaxy as it is
literally pulled apart by its abusive neighbor. Detailed
models of the dynamical evolution of the Magellanic
Clouds indicate that they will both fall into the Milky Way
in the next few billion years. Although the Clouds will be
Copyright © Nature Publishing Group 2001
Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998
and Institute of Physics Publishing 2001
Dirac House, Temple Back, Bristol, BS1 6BE, UK
7
Local Group
completely disrupted in the process, even the much-larger
Milky Way will be affected as its disk is puffed up by the
energy injected by the infalling dwarfs.
Perhaps the most spectacular example of an interaction within the Local Group involves the Sagittarius dSph
galaxy. Discovered only in 1994, the SAGITTARIUS DWARF
GALAXY is now known to be the closest galaxy to the Milky
Way. Sadly for Sagittarius, this is too close, and the dwarf
is being severely torn apart by the gravitational tidal forces
exerted on it by our much more massive Galaxy. A clear
indication that this process has already begun is the fact
that Sagittarius has been stretched along a long arc that
currently is known to extend nearly all of the way around
the sky. Like the Magellanic Clouds, Sagittarius does not
have long to live as a separate galaxy before it too is completely disrupted and merges into the main body of the
Milky Way.
Cosmological implications of the Local Group
Although most studies of the origin of the universe focus
on radiation coming from extremely distant galaxies, the
Local Group also plays a critical role in cosmological
studies. For example, with the advent of large groundbased and space-based telescopes and the use of highsensitivity electronic detectors, it has become possible
to measure the complete fossil record of star formation
in many of the galaxies of the Local Group. These
investigations provide a critical point of comparison with
studies aimed at understanding how galaxies formed in
the first place by studying objects detectable at the edge of
the visible universe. So far, observations of Local Group
systems reveal that normal galaxies have formed stars in
unexpectedly complex and varied ways. Some nearby
dwarf spheroidal galaxies are composed almost entirely
of ancient stars formed soon after the big bang, but most
of these galaxies formed large numbers of stars over long
periods of time extending as recently as the past few
hundred million years. The dwarf irregulars continue to
form stars today, in some cases perhaps for the first time.
Astronomers are just now beginning to try to reconcile
these detailed observations of Local Group galaxies with
studies of larger numbers of extremely remote galaxies.
The Local Group also offers insights into the question
of whether small galaxies have merged over time to
form larger systems such as M31, the Milky Way or the
Magellanic Clouds. Some of the mergers can be studied
today such as the disruption of the Sagittarius dwarf and
the interactions of the Magellanic Clouds and our Galaxy
described above. However, most mergers that led to the
formation of larger galaxies must have occurred long ago
when the universe was still quite young; we are only now
beginning to understand how to disentangle evidence of
these past encounters within our own Galaxy. The merger
histories of Local Group galaxies will eventually shed new
light on the conditions of the early universe and on the
nature of the dark matter that helped drive the formation
of galaxies in the first place.
E N C Y C LO P E D IA O F A S T R O N O M Y AN D A S T R O P H Y S I C S
On an even larger scale, the Local Group itself
seems to interact significantly with other nearby groups of
galaxies. One of these, the Sculptor Group, appears to be
strongly elongated along a line projecting back to the Local
Group. Because our group is considerably more massive,
it is probable that the gravitational force of the Local
Group has significantly distorted the Sculptor Group. One
result of this interaction is the lack of a clear boundary
between the Sculptor Group and our own. Some galaxies
traditionally associated with Sculptor are occasionally
catalogued with the Local Group and vice versa. Unlike
the Local Group, Sculptor lacks any giant galaxies, but
does contain a number of lower-luminosity spirals and
many dwarf galaxies. On roughly the opposite side of
the sky the M81–Maffei Group represents another nearby
concentration of galaxies that may have a significant
effect on the evolution and internal motions of the Local
Group. Unlike the Sculptor Group, the M81–Maffei Group
contains some giant galaxies, including the large spiral
M81 and the elliptical galaxy Maffei 1. As with the
Sculptor Group, some of the galaxies assigned to the M81–
Maffei Group have been considered at times to be Local
Group members and vice versa. On top of the interactions
with neighboring groups, the Local Group is also ‘falling’
into the nearby VIRGO CLUSTER of galaxies. The attempt
to understand the details and implications of these local
interactions and large scale motions remains a very active
field of modern research.
Bibliography
Hodge P 200 An Atlas of Local Group Galaxies (Dordrecht:
Kluwer) (an atlas of photographs of all Local Group
galaxies)
Mateo M 1998 Dwarf galaxies of the Local Group Ann.
Rev. Astron. Astrophys. 36 435–506 (a recent technical
review of the Local Group)
Sandage A and Bedke J 1994 The Carnegie Atlas of Galaxies
(Washington, DC: Carnegie Institution) (stunning
photographs of many galaxies including most of the
brighter members of the Local Group)
van den Bergh S 2000 The Galaxies of the Local Group
(Cambridge: Cambridge University Press) (a galaxyby-galaxy description of the Local Group; many
historical comments included)
All of these contain many additional references.
Copyright © Nature Publishing Group 2001
Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998
and Institute of Physics Publishing 2001
Dirac House, Temple Back, Bristol, BS1 6BE, UK
Mario L Mateo
8