the local group

The Local Group• Approximately 41 members• All within 5 million light years of the Milky Way• Clustered in two groups around the two dominant

spiral galaxies (The Milky Way and Andromeda)• Average apparent Magnitude = 11• Average Absolute Magnitude = -13• Average Diameter = 5 kpc

The following images represent only the most visible members of the local group.

The Large Magellanic Cloud

The Small Magellanic Cloud

The Large Magellanic Cloud

The Small Magellanic Cloud

NGC 6822NGC 6822

Characteristic Properties of the Local Group of Galaxies

Just as we used the nearby stars to characterize the distribution of stars in the Milky Way

galaxy, we will use the character of the nearest galaxies (i.e. The Local Group) to characterize

the distribution of galaxies in the larger Universe.

As you “click” through the following figures please pay attention to the (hopefully) useful notes and highlights that appear.

L o c a l G r o u p G a l a x ie s7 %

5 %

3 7 %

1 7 %

2 4 %

1 0 %

S p i r a l

d E l l i p t i c a l

d S p h

I r r e g u la r

d I r r

d I r r / d S p h

76% of Local Group galaxies are dwarf galaxies.

T h e L o c a l G r o u p

0 %

2 5 %

5 0 %

7 5 %

1 0 0 %

161 11 6A p p a r e n t M a g n i t u d e

Cu

mu

lati

ve F

racti

on

90%

66% 90% are invisible to the naked eye being dimmer than 6th magnitude

66% are 100 times dimmer than the limit of visibility.

T h e L o c a l G r o u p

0

2

4

6

8

1 0

1 2

0 2 0 0 4 0 0 6 0 0 8 0 0 1 , 0 0 0 1 , 2 0 0 1 , 4 0 0 1 , 6 0 0 1 , 8 0 0 2 , 0 0 0

D is t a n c e D is t r ib u t io n , k p c

Local Group galaxies appear to be clustered around the two largest members of the group: The Milky Way and the Andromeda (M31) galaxies which are separated by about 670 kpc (2 Mlyrs). The entire Local Group has a diameter of about 5 Mlyrs.

The Milky Way Group

Andromeda Group

T h e L o c a l G r o u p

- 5 0 0

0

5 0 0

1 0 0 0

1 5 0 0

- 1 5 0 0 - 1 0 0 0 - 5 0 0 0 5 0 0 1 0 0 0

G a l a c t ic X , k p c

Gala

ctic Y

, kpc

Andromeda Group

The Milky Way Group

Each of the small red dots represents a Local Group galaxy as see from above the plane of the Milky Way

T h e L o c a l G r o u p

0 %

2 5 %

5 0 %

7 5 %

1 0 0 %

- 2 5- 2 0- 1 5- 1 0- 5

A b s o l u t e M a g n i t u d e

Cu

mu

lati

ve F

racti

on

Absolute Magnitude of the Milky Way is about -20

About 80% of Local Group galaxies are 5 magnitudes less luminous or, equivalently, 100

times dimmer than the Milky Way.

About 33% of Local Group galaxies are 10 magnitudes less luminous or 10,000

times dimmer thatn the Milky Way.

T h e L o c a l G r o u p

0 %

2 5 %

5 0 %

7 5 %

1 0 0 %

0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0

D ia m e t e r , k p c

Cum

ula

tive F

raction

80% are less than 5 kpc in diameter (smaller than the bulge of the Milky Way).

T h e L o c a l G r o u p

- 2 0 0

- 1 0 0

0

1 0 0

2 0 0

3 0 0

4 0 0

0 . 0 5 . 0 1 0 . 0 1 5 . 0 2 0 . 0 2 5 . 0 3 0 . 0 3 5 . 0

D i a m e t e r , k p c

Rad

ial

Velo

cit

y,

km

/sec

Positive Radial Velocities represent Motion Away from the Milky Way

Negative Radial Velocities represent Motion Toward the Milky Way

The Andromeda Galaxy (M31) is “falling” toward the Milky Way at 59 km/s.

Summary of the Local Group Galaxies• The Local Group consists of about 41 galaxies.

• 76% of Local Group galaxies are dwarf galaxies.

• 90% are invisible to the naked eye and 66% are 100 times dimmer than the limit of visibility.

• Local Group galaxies appear to be clustered around the two largest members of the group: The Milky Way and the Andromeda (M31) galaxies which are separated by about 670 kpc (2 Mlyrs). The entire Local Group has a diameter of about 5 Mlyrs.

Summary of the Local Group Galaxies• 80% of Local Group Galaxies are less than 0.01 LMW

and 33% are less than 0.0001 LMW

• 80% are less than 5 kpc in diameter (smaller than the bulge of the Milky Way).

• The Andromeda Galaxy is approaching the Milky Way and will impact our galaxy in less than 11 Byrs.