AS1001:Extra-Galactic Astronomy
Lecture 3: Galaxy Fundamentals
Simon Driver Theatre B
[email protected]
http://www-star.st-and.ac.uk/~spd3
Galaxy Fundamentals
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How many stars are in a galaxy ?
How did galaxies form ?
How many galaxies are there ?
How far apart are they ?
How are they clustered ?
What is the mass of a typical galaxy ?
What is the mass density of the Universe ?
Extra-galactic Distances
In Extragalactic astronomy we work in
6
1 Mpc = 10 pcs
m = M + 5 log10 ( d ) ! 5
d in pc
6
m = M + 5 log10 ( d " 10 ) ! 5
d in Mpc
6
m = M + 5 log10 ( d ) + 5 log10 (10 ) ! 5
m = M + 5 log10 ( d ) + 25
NB: M still defined to be at 10pc
How many stars in a Galaxy ?
e.g., Andromeda which is at 0.9Mpc
and has an apparent magnitude mv=3.5
1) M = m ! 5 log10 (d ) ! 25 = !21.3mags
2) Assume M*=5.48 (i.e., Solar)
3)
f GAL = n* f*
M GAL
f GAL
n* f*
# M * = #2.5 log10 (
) = #2.5 log10 (
)
f*
f*
n* = 10#0.4 ( M GAL # M * ) " 5 ! 1010
4)
n*=50 billion stars
How did Galaxies Form ?
TWO COMPETING SCENARIOES
• Hierarchical Merging
• Initial Collapse
How did Galaxies Form?
• Hierarchical merging
• For
• Initial Collapse
• For
– Mergers seen
– Ellipticals are old
– Ellipticals in high density
– Ellipticals seen at high z
environments
– Spirals/Irrs rotating
– Irrs isolated
– Irregulars forming today
• Against
– Ellipticals seen at early epochs • Against
– Irregulars forming today
– Mergers seen
PROBABLY SOME OF EACH OCCURRING
The Antennae Galaxy: mid-merger
Cluster Formation Simulation
John Dubinski: www.cita.utoronto.ca/~dubinski
Cluster Formation Simulation
John Dubinski: www.cita.utoronto.ca/~dubinski
How many galaxies are there ?
STEP1: Take all sky photos
STEP2: Count galaxies brighter than some magnitude
STEP3: Assume most galaxies are like the MW*
STEP4: Calculate depth and volume of sky
STEP5: Calculate the SPACE-DENSITY of galaxies
[* This is a bit of a fudge but works because the most easily detected
galaxies are like the Milky Way, i.e., big bright spirals. This does not
mean they’re the most numerous just the most visible !]
The Space Density of Galaxies
• For example the MW has MB = -20 and there are ~10,000 MWlike galaxies known, brighter than 14th mag over the whole sky.
How many galaxies are there per Mpc cubed ?
m = M + 5 log10 ( d ) + 25
0.2[ m ! M ! 25 ]
0.2[14 !( !20 ) ! 25 ]
d
=
10
=
10
Use magnitude equation
to get the distance =>
d = 63Mpc
4
4
V = "d 3 = " (63)3
3
3
Use geometry to get
the volume =>
V = 106 Mpc 3
n = number density
N 104
n=
= 6 = 10!2 gals / Mpc 3
V 10
• i.e., There is ~1 MW-like galaxy every 100Mpc cubed
How far apart are they ?
• The mean separation of galaxies is therefore
1/3
~(100) = 4.6 Mpc
100Mpc
4.6 Mpc
• In reality though we know that galaxies are
strongly clustered
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Large Scale Structure in the Universe
The Stickman: 1980
p
M
0
15
Geller & Huchra
c
Large Scale Structure in the Universe
The Great Wall: 1988
Large Scale Structure in the Universe
Mass-to-Light Ratios
• Let us assume that the amount of light a
galaxy emits relates to its mass
• i.e., there exists a mass-to-light ratio
[we will explore the validity of this later]
• Typically this is expressed in solar units:
Mass-to-light ratio
X=1 For our sun
X~10 For a galaxy
M
M!
=X
L
L!
Solar Mass
Solar Luminosity
The Mass of M31
Given that the average mass-to-light ratio is about
10 what mass does this imply for M31 which has
MV = -20.5 mags ?
M"
M
L
M = L = 10
L = 10M "
L
L"
L"
#0.4(M V #M V " )
M = 10M "10
30
M = 10 $ 2 $10 $10
41
#0.4(#20.5# 4.6)
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M = 2.2 " 10 kg = 1.1 " 10 M !
!
The Density of the Universe
• By multiplying together the space density of
galaxies and the mass of the typical galaxy
we can get an approximate value for the
density of the visible Universe:
$ = nM = 1.1 # 109 M " / Mpc 3 ~ 10!28 kg / m 3
• This is the matter that we can see.
However…
• More accurate observations incorporating
dynamics of galaxies and clusters, yields the
total density (includes luminous and dark
matter):
# ~ 2 ! 4 " 10!27 kg / m 3
• Most of matter in Universe we cannot see!
• Mass of hydrogen atom: mH=1.7 10-27 kg
• If the Universe was smoothly spread out
there would be a couple of hydrogen atoms
per cubic metre. But the air we breathe
contains about 1025 atoms per cubic metre.