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.

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

The Local Group of Galaxies

Links

References