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Extragalac)c Astronomy
Lecture 10: DARK MATTER
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DARK MATTER -‐ HISTORY
• 1933: Zwicky studied the dynamics
of the Coma cluster. Virial
theorem implies that the mass
is 4X larger than the sum
of the individual galaxy masses.
• 1937: Smith studied the dynamics
of the Virgo cluster. Same
conclusion
• 1972: Freeman analyzed the HI
rota)on curve of NGC 300. He
found that there is as much
dark maXer than visible maXer
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DARK MATTER -‐ history
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DARK MATTER
There is no reason to suppose
that every types of maXer in
the Universe emit detectable photons:
1. No reason why the different
processes of SF would not
have produced a large number of
stars with M* < 0.08 Msun
2. If it was not of the
spin transi)on of H at 21
cm, we would not know about
10% of the visible mass in
spirals
3. Dust in galaxies was discovered
because the size of the grains
happen to be of the same
order as the visible light
(light was not only absorbed
but reddened)
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DARK MATTER
Ø Mass is not correlated with
light
95% light M* > Msun
Ø Solar neighborhood
95% mass M* < Msun
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DARK MATTER: defini)on
We call DARK MATTER any form
of maXer that doesn’t emit any
detectable photon at any wavelength
(γ-‐rays, X-‐rays, UV, visible, IR,
radio, …) of the electromagne)c
spectrum but of which the
existence is deduced by its
gravita)onal effects.
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DARK MATTER
Ø White dwarfs: while a large
number may have cooled so that
they are now invisible, they
cannot cons)tute the dark maXer
since we can deduce their
presence by:
1. The study of the density of
white dwarfs wrt the MS stars
in the solar neighborhood
2. With the aid of the theories
of stellar evolu)on 3. With the
aid of the SF history in
our neighborhood
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DARK MATTER in Spirals • In the
inner regions, visible maXer (gas
& stars) can explain the
rota)onal veloci)es
• At the edge of the stellar
disk, visible maXer and dark
maXer contribute almost equally to
the veloci)es
• In the outer regions, the mass
is totally dominated by dark
maXer
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DARK MATTER in dwarfs
• Dark maXer halo dominates at
all radii
• There is even more luminous
mass in gas than in stars
• Dark maXer contributes to 90%
of the mass
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DARK MATTER in clusters
• NGC 2300 (X-‐rays)
• X-‐rays: hot gas
• Hot gas should dissipate
• Confine in the center by dark
maXer
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DARK MATTER Type of object Size Ratio
(M/L) % of dark matter
Solar neighborhood 100 pc 3-5 33%
Spirals 30-50 kpc 10-20 50-90%
Binary systems 50-100 kpc 20-30 90%
Groups 0.5-1.5 Mpc
50-150 95%
Clusters 1-5 Mpc 200-500 99%
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Mass budget
Ωtotal = 1 (inflation) Ωdark energy = 0.73
Ωmatter = 0.27 Ωbaryons = 0.044
Ωnon-baryonic = 0.23
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Candidates for DM
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Mass budget
• In galaxies (Fukugita 2004, IAU
220): • Ω (stars) = 0.0025 • Ω (HI & He) =
0.00062 • Ω (H2) = 0.00016
• Ω (galaxies) = 0.0033 • Ωgal/Ωbaryons = 7.5% • DM in the
halos of galaxies baryons
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Conclusion DM in galaxies
• Models of the mass distribu)on
implies 50-‐90% of maXer is
dark in galaxies
• Luminous maXer (stars & gas)
represents 7.5% of the baryons
• Dark halos cons)tute at most
15-‐65% of the baryons • So,
not necessarily need non-‐baryonic
maXer to explain the dark halos
of galaxies (this doesn’t exclude
the possibility of non-‐baryonic
maXer)
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DM in clusters
• X observa)ons: DMclusters ~ 10
DMgalaxies • Not enough baryons so
need for non-‐baryonic maXer:
• HDM (hot dark maXer): par)cle
with no mass (or very liXle)
moving at ~c (e.g. neutrinos)
• CDM (cold dark maXer): par)cle
with enough mass so that they
move at non-‐rela)vis)c veloci)es
(e.g. neutralinos)
• Important difference on the Large
Scale Structure: large veloci)es of
HDM destroy small scale structures
(cf. simula)ons, ruled-‐out by COBE)
CDM favored.
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Cosmologic DM
• If infla)on is correct, the
Universe is flat, so Ω =
1 • Since Ωm ~
0.3, we need Ωλ =
0.7 = dark energy
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Cosmological DM
