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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