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Components of Galaxies: Dark Matter
Dark Matter: “Any Form of matter whose existence
is inferred solely through its gravitational effects.”
-B&T, pg 590
• Nature of Major
Component of Universe
• Galaxy Formation
• Fate of the Universe
Evidence for Dark Matter
• Rotation curves of spiral galaxies - convincing
• X-ray halos in elliptical galaxies - convincing
• Clusters of galaxies - convincing
• Local group infall - cute, but not very convincing
• Vertical velocities in galactic disk - not very convincing(LM = DM)
• Disk stability - convincing only for a subset of spiralgalaxies (Sc)
• Dwarf ellipticals - convincing if anisotropies aren’taffecting the results
• Inflationary Model - mostly convincing in light of theWilkinson Microwave Anisotropy Probe (WMAP)
Evidence for Dark Matter:
1. Rotation Curves of Spiral Galaxies
• HI Rotation Curves do not “turn over” at Large Radii (where
there is negligible luminous matter)
Spiral Galaxies Rotation Curves (cont’)
• Rotation curves:
v(r) ~ r at small radii
v(r) ~ constant at large radii
• Central Bulge: distribution of stars is homogeneous &
spherical. For a spherical distribution of mass of density !(r),
Since the distribution is homogeneous, then,
Thus,
Substituting in the following equation for M(r),
leads to,
• Beyond the radius where luminous matter is seen, rLM,
the rotation curve should fall off like a Keplerian rotation
curve,
• But what is seen is V(r) = constant. The discrepancy is
believe to be due to a significant amount of dark matter
in the outer part of spiral galaxies.
How much Dark Matter is there in Spiral
Galaxies Relative to Light Matter?
• In terms of the critical density of the universe of the
universe, !crit,
and
Density Profile of Dark Matter
V(r) = constant. Thus,
Can be differentiated with respect to r,
Substituting the above differential equation into,
the density of the dark matter halo must be,
Many groups have tried to deconvolve the contribution to
the rotation curve of the halo & disk components by
modeling the disk as a constant M/L exponential disk,
where I0 is the central intensity, and by modeling the halo by
using the (physically unmotivated) fitting function,
where !halo(0) is the central density of the halo component.
In the example to the
right,
" ~ 2.1 – 2.25.
•For r >> a,
which is slightly different
from r -2, but is due to
the non-negligible disk
contribution at larger r.
(Van Albada et al. 1985 ApJ, 295, 305)
2. X-ray Halos in Elliptical Galaxies
• Elliptical Galaxies and Galaxy Clusters have X-ray Halos
• The equation of hydrostatic equilibrium + perfect gas law# M(r)
• The equation of hydrostatic equilibrium is,
From the ideal gas law,
Taking the equation of H.E. & substituting in for P,
Solving for M(r) yields,
Note that for stellar systems that do not rotate,
Thus,
I.e., velocity dispersion & gas temperature are equivalent.
An Example: M87
• For M87,
T(r) = constant = 107 K.
Thus,
• For an X-ray Halo size of 200 kpc, the M(r) traced by the
halo is 1.5x1013 Msun.
• The B-band luminosity of M87 us LB = 6.2x1010 Lsun, and
Thus M / LB ~ 250 Msun / Lsun.
3. Clusters of Galaxies
For a cluster of galaxies, the virial theorem can be written,
This method typically yields values of
An Example
Early determination of the mass of the Coma cluster –
For 21 galaxies,
The velocity dispersion is thus,
Given a cluster size of 1.7x106 ly = 1.6x1022m, the total
mass is
The total # of Brightest Cluster Members (BCMs) is 670, thus
Each BCM has a mass of
The luminosity range of the BCDs is 0.08-2x109 Lsun,
thus M / L ~ 500 Msun / Lsun.
Possible Sources of Error in Determining Cluster
Masses…
• Assuming the cluster is virialized when it’s not
• Non-cluster members affecting the determination of the
cluster velocity dispersion
Also Note: One has to keep in mind scale of matter that is
being sampled!
E.g., only 10% of galaxies are in clusters, so the $
of clusters may not be representative of the density
of DM in the universe.
4. Local Group Timing Argument
The MW & the M31 are observed to be approaching
each other at Vr = -125 km/s.
Assumption: MW & M31 formed near each other at not
much greater than the distant they are apart now.
Thus, during ~ 1010 years, they must have completed a
substantial fraction of one orbit. The orbit is thus less
than 15 billion years.
The Period, P, of the Local Group is taken to be,
where a = semi-major axis radius, and M* is the reduced
mass of the MW & M31, i.e.,
Assuming no angular momentum (i.e. to provide the
smallest minimum mass), the total energy of the
LG is
where,
D = present MW-M31 separation (480 kpc)
KE = kinetic energy per unit mass
Solving for reduced mass yields,
Which is six times larger than the reduced mass of
MW & M31.
5. Galactic DiskVertical velocity dispersion %z of stars & to the disk
determines how high star can climb out of the disk
%z depends on vertical gas density of the disk + putative dark
matter halo component, Scale height
Result: there is as much DM as LM.
Caution: measurements of %z require use of halo stars,
Which are very distant and thus faint
z
6. Stability of Galactic Disks vs. Bar Instabilities
• Self-gravitating “cold” disks, i.e., disk with high KErotation/|W|(>0.14), are unstable to bar formation, but only 50% of spiralgalaxies have bars
• Solution: there exists a massive spherical halo that controls,in part, the potential well & much of the self-gravity. I.e., thehalo increases the KErandom, which “heats” the disk.
Disk Stability(cont’)
Note that other things can heat the disk as well, thus
stabilizing it against bar formation:
• large bulges can stabilize disks
• large random motions (velocity dispersion), which lowers
KErotation/|W|
• Thus, DM is needed for Sc galaxies
• But not S0 & Sa galaxies
(see Ostriker & Peebles 1973 ApJ, 186, 467
Sellwood 1981 A&A, 99, 362)
7. Dwarf Elliptical Galaxies
• The velocity dispersion of dwarf
elliptical galaxies have been
used to determine M / L.
• M / L = 10 – 100 Msun / Lsun.
We’ll talk more about these in a
few weeks…
8. The Inflationary Universe
Many believe that the universe went through a period of
exponential expansion.
Consider the Newtonian approximation of the expanding
universe. Let the universe be an expanding sphere oftotal mass M, radius R, velocity R, & density !.
The total energy of the universe is,
•
Substituting the following equation for mass,
The energy equation becomes,
Setting H0 = R / R and solving for density yields,•
For E = 0, the universe has critical density,
In terms of the critical density, $ can be expressed as,
If the universe underwent a period of exponential expansion,
then,
and thus,
Based on Big Bang Nucleosynthesis, WMAP observations
of the angular size of the background fluctuations, & high-z
supernovae measurements, the present thinking is that -
The Inflationary Universe theory solved a couple of problems.
Among them,
1) Flatness Problem: why $ is so close to 1.
2) Isotropy Problem: why the universe looks the same in
every direction.
The luminosity density of the universe is
Thus, for $ = 1,
With a cosmological constant, the value is 30% of this
number.
Summary of $ Determinations Based on
Various Techniques
WMAP
What is the Dark Matter?
• Non-baryonic &/or baryonic
• The value of $ is important for making this distinction.
Present Day Abundances of Light Elements
• In the present universe, 4He, 3He, Deuterium (D), &
Lithium (7Li) have observed abundances that cannot be
accounted for by stellar nucleosynthesis
• Helium fraction, Y ~ 0.25 – 0.3, but only 'Y ~ 0.04 can
be due to stellar nucleosynthesis.
• Thus, most He must have a primordial origin.
Big Bang Nucleosynthesis(see Kolb & Turner: The Early Universe)
• First 100 seconds: conditions suitable for fusing the
above mentioned nuclei out of available n & p
• Important: expansion rate must be less than Rx rate
• Rx rate = (cross section)(number density)(velocity [T])
• Thus determining primordial abundances puts aconstraint on what $baryon can be
Before tuniverse ~ 1 second (T > 1010 K)
The following reactions occured
The ratio of p to n around this time was, 1.5x1010 K
Also note that the D abundance was kept low #
Photodissociation rate via blackbody photons > formation
rate via p + n.
At T < 1010 K
The above listed Rx could only go into one direction.
Result:
• Neutrinos decouple from Baryonic matter
• n / p ratio freezes out
• Blackbody photons energy < D binding energy• Thus, D # 3He, 4He, 3H, & 7Li
Primordial Abundance Models
Baryon
density
Primordial Abundance Models (cont’)
Baryons/Photon
Number density
Ratio
For Sun, ( ~ 100
For SN core, ( ~ few
Helium
fraction
4He Abundance
• Because np ~ 7 nn
• And 2p + 2n # 4He
• This is similar to the present day abundance of 4He in
the universe.
• Most 4He was created in the first 100 seconds of the
Universe
Constraining Abundances
•D & 7Li measured in
atmosphere of Jupiter, in Halo
stars, & in gas at cosmological
distances
Abundance of Light Elements are Used toConstrain $baryon
• The best estimate based on these analyses yield,
• From dynamical estimates of dark matter in galaxies,
• Thus, ~10% of DM is probably baryonic in nature, and~ 90% is something else
$baryon ~ 0.04
Baryonic Dark Matter Candidates
• Stellar remnants (white dwarfs, black holes, neutralstars)
# Would overproduce metals, inconsistent with MACHO
• Brown dwarfs or Jupiters
# Inconsistent with MACHO
• Intermediate mass black holes (102-6 Msun)
# Possible candidate (high mass/low density), but X-rayand MACHO could eventually rule this out
• Large black holes (>106 Msun)
# Would heat the disk too much
• Note that Microlensing toward LMC, SMC, M31 havetentatively concluded that ~20% of halo DM is in 0.5 Msun
objects. Better data will yield a tighter constraint
Cold dark matter
• The rest of the DM is Cold Dark Matter. We have no
clue what it might be.
Hot dark matter
• Note that for a long time it was speculated that Hot
Dark Matter (neutrinos) might make up the bulk of DM.
Two conditions that must be true are that
1) Neutrinos have mass, and
2) Structures form in a top-down manner: Large scale
structures (superclusters) would form first because
neutrinos, which are weakly interacting and initially
possess random relativistic velocities, would stream
out of high density regions into low-density ones. Thus,
small density perturbations that were Jeans unstable
would be smoothed out.