The Local Group

The Local Group of Galaxies

This is “our” group of galaxies.

It was first recognized by Hubble, in the time of the first distance

determinations and redshift measurements.

Messier objects:

The Andromeda Galaxy

M31

and its satellites

M32

and

M110,

as well as the Triangulum galaxy

M33.

Other members (over 30 in all) include our

Milky Way Galaxy, the Large and the Small Magellanic

Cloud (LMC and

SMC), which have been

known before the invention of the

telescope, as well as several smaller galaxies which were discovered more

recently. These galaxies are spread in a volume of nearly 10 million light

years diameter, centered somewhere between the Milky Way and M31.

Membership is not certain for all these galaxies, and there are possible other

candidate members.

Of the Local Group member galaxies, the Milky Way and M31 are by for the most

massive, and therefore dominant members. Each of these two giant spirals has

accumulated a system of satellite galaxies, where

  • the system of the Milky Way contains many (nearby) dwarf galaxies,

    spread all over the sky, namely Sag DEG,

    LMC, SMC, and the dwarf galaxies in Ursa Minor, Draco, Carina,

    Sextans (dwarf), Sculptor, Fornax, Leo I and Leo II,

  • the system of the Andromeda galaxy is seen from outside, and thus

    grouped around its main galaxy M31 in Andromeda, containing bright

    nearby M32 and M110 as well as fainter and more far-out NGCs 147 and 185,

    the very faint systems And I, And II, And III and possibly And IV

    (but for this one see remark below).

The third-largest galaxy, the Triangulum spiral M33, may or may not be

an outlying gravitationally bound companion of M31, but has itself probably

the dwarf LGS 3 as a satellite.

The other members cannot be assigned to one of the main subgroups, and float

quite alone in the gravitational field of the giant group members.

The substructures of the group are probably not stable. Observations and

calculations suggest that the group is

highly dynamic and has changed significantly

in the past: The galaxies around the large elliptical Maffei 1 have

probably been once part of our galaxy group.

As this shows, the Local Group is not isolated, but in gravitional

interaction, and member exchange, with the nearest surrounding groups,

notably:

  • the Maffei 1 group, which

    besides the giant elliptical galaxy Maffei 1 also contains smaller

    Maffei 2, and is associated with nearby IC 342.

    Highly obscurred by dark dust near the Milky Way’s equatorial plane

  • the Sculptor Group or South Polar Group

    (with members situated around the South Galactic pole), dominated by

    NGC 253

  • the M81 group
  • the M83 group

In the future, interaction between the member galaxies and with the cosmic

neighborhood will continue to change the Local Group. Some astronomers speculate

that the two large spirals, our Milky Way and the Andromeda Galaxy, may perhaps

collide and merge in some distant future, to form a giant elliptical.

In addition, there is evidence that our nearest big cluster of galaxies, the

Virgo Cluster, will probably stop our cosmological

recession away from it, accelerate the Local Group toward itself so that it will

finally fall and merge into this huge cluster of galaxies, see our

Virgo Cluster & Local Group page.

A table of the currently known Local Group member galaxies follows. While the

positions are known very acurately, the distances are only very vaguely known

for some members, and the sources even disagree for the most prominent members

such as M31 and M33. If interested, look at our compilation of

distances from various sources.

Please note that this table is currently under review, as new data (Hipparcos

distances, discovery of new members) keep our knowledge in flow.

Local Group Member Galaxies
Galaxy RA Dec Type m_v dim RV Dist
WLM 00:02.0 -15:28 IB(s) IV-V 10.9 12 x 4 – 42 4200
IC 10 00:20.4 +59:18 KBm? 10.3 7.3 x 6.4 – 83 4200:
NGC 147 00:33.2 +48:31 dE5 pec 9.5 15.0 x 9.4 + 89 2400:
And III 00:35.4 +36:31 dE2 13.5p 2900:
NGC 185 00:39.0 +84:20 dE3 pec 9.2 14.5 x 12.5 + 39 2500:
M110 00:41.3 +41:41 E5 pec 8.5 19.5 x 12.5 – 1 2900
And IV 00:42.5 +40:34 Irr ? 2900:
M 32 00:42.7 +40:52 E2 (cE2) 8.1 11.0 x 7.3 + 35 2900
M 31 00:42.7 +41:16 SA(s)b I-II 3.4 185.0 x 75.0 – 59 2900
And I 00:45.7 +38:00 dE3 pec ? 13.2 2900:
SMC 00:51.7 -73:14 SB(s)m pec 2.3 280 x 160 – 30 210
Scl dw 01:00.0 -33:42 dE3 pec 10.5p +162 300:
LGS 3 01:03.8 +21:53 Irr 15.4p 2 3000:
IC 1613 01:05.1 +02:08 IAB(s)m V 9.2 20.0 x 18.5 -125 2900:
And V 01:10.3 +47:38 d 2900:
And II 01:16.4 +33:27 dE0 13: 2900:
M 33 01:33.9 +30:39 SA(s)cd II-III 5.7 67.0 x 41.5 + 3 3000
Phe dw 01:49.0* -44:42* Irr 1600:
For dw 02:39.9 -34:32 dE2 8. 530:
And VII 03:27.8 +50:35 dSph 2900:
UGCA 86 03:59.9 +67:08 Irr ? S0 ? +262
UGCA 92 04:27.4 +63:30 Irr ? S0 ? + 66 3000:
LMC 05:19.7 -68:57 SB(s)m 0.1 650 x 550 179
Car dw 06:14.6 -50:58 dE3 300:
Leo A 09:59.4 +30:45 IBm V 7000:
Sex B 10:00.0 +05:20 Ir+ IV-V 4000:
NGC 3109 10:03.1 -26:09 Ir+ IV-V 10. p 16.0 x 2.9 +131 4100:
Ant dw 10:04.1 -27:20 dE3 14.8 4100:
Leo I 10:05.5 +12:19 dE3 9.8 880:
Sex A 10:11.1 -04:43 Ir+ V 4000:
Sex dw 10:13.2 -01:37 dE3 300:
Leo II 11:13.5 +22:10 dE0 pec 800:
GR 8 12:58.7 +14:13 Im V 14.5 1.2 x 1.1 +165 5000:
UMi dw 15:08.8 +67:12 dE4 240:
Dra dw 17:20.1 +57:55 dE0 pec 280
Milky Way 17:45.6 -28:56 SAB(s)bc I-II ? 0 28
SagDEG 18:55 -30:30 dE7 80
SagDIG 19:30.1 -17:42 IB(s)m V 15: 2000:
NGC 6822 19:44.9 -14:49 IB(s)m IV-V 9 + 66 1700:
Aqr dw 20:46.8 -12:51 Im V 2000:
IC 5152 22:06.1 -51:17 IAB(s)m IV 10.6 4.9 x 3.0 + 30 3000:
Tuc dw 22:41.7 -64:25 dE5 3000:
Peg dw 23:28.6 +14:45 Im V 6000:
And VI 23:51.7 +24:36 dSph 2900:

Nearby Non-Member (?) Galaxies
Galaxy RA Dec Type m_v dim RV Dist
NGC 404 01:09.4 +35:43 E0 10.1 4.4 x 4.1 +178 8000:
Argo dw 07:04.5* -58:27* Irr 12000:
2318-42 23:18.1* -42:00* Irr
UKS2323-326 23:23.8* -32:40* Irr 4500:

Key:

RA, Dec:

Right Ascension and Declination for epoch 2000.0

(* indicates epoch 1950.0 coordinates)

Type:

Classification type from Tom Polakis’ article, or elsewhere

m_v

Apparent visual brightness in Magnitudes

dim

Angular dimension in arc minutes

RV:

Radial velocity wrt galactic center in km/sec

Dist:

Distance in 1000 light years (kly)

More data of and remarks on Local Group Galaxies
Galaxy Other name R Diam Mass
WLM (*) 8 ?
IC 10
NGC 147 DDO 3
And III
NGC 185
M 110 NGC 205 17 10000
And IV (*)
M 32 NGC 221 8 3000
M 31 NGC 224 195 400000
And I
SMC NGC 292 25 6000
Scl dw E351-G30
LGS 3 Psc dw (*)
IC 1613 DDO 8
And V
And II
M 33 NGC 598 60 25000
Phe dw (I)
For dw E356-G04
And VII Cas dSph
UGCA 86 (?)
UGCA 92 EGB0427+63 (I)
LMC 30 20000
Car dw E206-G220
Leo A Leo III (?)
Sex B DDO 70 (I)
NGC 3109 (I)
Ant dw (I) 5
Leo I Regulus G.
Sex A DDO 75
Sex dw
Leo II Leo B
GR 8 DDO 155 (?)
UMi dw DDO 199
Dra dw DDO 208
Milky Way 100 750000
SagDEG Sgr I dw (*)
SagDIG Sgr dw (*)
NGC 6822 Barnard’s
Aqr dw DDO 210 (?)
IC 5152 (?)
Tuc dw
Peg dw DDO 216
And VI Peg dSph

Key:

R:

Remarks: (*), (?) and (I); see below

Diam:

Diameter in 1000 light years (kly)

Mass:

Mass in million solar masses

Remarks (*):

  • “WLM” is “Wolf-Lundmark-Melotte”
  • Andromeda IV is probably a large open cluster in M31,

    not an independent galaxy

    (ApJ

    105:933-7, 3/1993)

  • “LGS” stands for “Local Group (suspected)” (LGS 3)
  • “SagDEG” is the “Sagittarius Dwarf Elliptical Galaxy”

    (discovered 1994, containing M54)

  • “SagDIG” is the “Sagittarius Dwarf Irregular Galaxy”

Galaxies marked with “(?)” may be non-members as they are not in the

list of Irwin et.al. 1997 (this list has 35 Local Group members),

in particular:

  • UGCA 86 may be a member of the

    Maffei 1 group

  • Leo A, GR8, the Aquarius Dwarf and IC 5152 are listed as nearby

    non-member galaxies in Irwin’s list.

Galaxies marked with “(I)” are newly taken into the list from Irwin’s

list (besides the newly discovered Antlia dwarf, these are the Phoenix

dwarf, UGCA 92, Sextans B, and NGC 3109.

Below we list some nearby but probably non-member field galaxies (which

are also not members of one of the neighboring groups listed above), in

the same format as above:

As our Milky Way obscures parts of the sky, there is still a steady flow

of new discoveries of galaxies, in low galactic latitudes (i.e., near the

equatorial plane of our galaxy, where the obscuring dust is most dense).

Also, some of the galaxies are of extreme low surface brightness, and it

was only recently possible to detect them. Therefore, it must be expected

that more Local Group members exist, obscurred by dust, or extremely

faint, and are still waiting for their detection somewhen in the future.

Jeff Bondono has

compiled a comprehensive list

of Local Group member and member candidate galaxies.

Links

You are invited to contribute more links which are of interest for this page;

please email the maintainer.

References

  • Tom Polakis has published an interesting article for observers

    of local group galaxies, Observing the Local Group, in the

    Deep Sky magazine (#36), which was reprinted in

    David Eicher‘s book

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