Part1 - Dark Matter Flashcards
(57 cards)
what do we need to know to answer what universe we live in?
Ω0, which depends on the matter density of the Universe, ρ
ways to determine estimates of the matter density
visible stars in the milky way
galaxy rotation curves
application of the virial theorem
gravitational lensing
hubble diagram of standard candles
galaxy redshift and redshift-distance surveys
estimating matter density - visible stars in milky way
If we assume that all stars in the galaxy are of one solar mass, M⊙, then we can estimate the matter density to be:
p=NM⊙/volume of milkyway
In the early 1930s, Oort and Linblad showed that the Milky Way does not rotate as a rigid body, but…
differentially – i.e., the angular speed of stars around the galactic centre depends on their distances from it
The inner part of the disc rotates like
a rigid body: the speeds of stars are proportional to their distances from the galactic centre
The outer part of the disc is known as the
Keplerian part since the orbits approximately obey Kepler’s laws
The transition from rigid-body to Keplerian motion occurs at a distance
just inside the Sun’s distance from the galactic centre.
A rotation curve is a plot of
rotation speed as a function of distance from the centre of the galactic disc
The mass outside the distance of a star from the centre of
the galactic disc has no effect on this star’s orbit hence
the total mass of the Galaxy
interior to, e.g., the Sun’s distance from the galactic centre can be estimated using Kepler’s third law
the observed rotation velocities are greater than those expected from the gravitational influence of the luminous stars alone, indicating
the presence of a dark matter halo surrounding the galaxy
what does it mean for a galaxy cluster to be virialised
reached a state of balance between their kinetic energy and potential energy such that 2K+U=0
virial mass estimate for a clutser
M=<v^2>Re/G
<v^2> is the 3D mean square peculiar velocity but in practice we measure
only the radial component of the peculiar velocity (deduced from the galaxy redshift)
assuming a spherical cluster with an isotropic velocity distribution, <v^2>=
3<v_radial^2> = 3sigma^2
The use of the Virial theorem assumes that
clusters are spherically symmetric and in a relaxed equilibrium state. It is further assumed that galaxies can be seen as being in a fluid in hydrostatic equilibrium (HE) with random orbits acting as outward
pressure.
Various methods exist to solve for M(< r) given the observables (σ^2
and the projected galaxy surface density), including:
- Assuming that light traces mass, and estimating M/L.
- Observing hot gas in X-rays, knowing that X-ray emissivity scales as ρ^2
these methods also depend on knowledge of Hubble’s constant
what is gravitational lensing
A massive object acts as a lens that
bends the passage of light from a source object behind it. The result is the generation of images of the source object that can be distorted, magnified and / or brightened.
2 ways in which we can use lensing to deduce estimates of the matter density
- strong and weak lensing events
- microlensing
strong and weak lensing events
light from distant galaxies is distorted by passage through an intervening cluster.
what is microlensing
where the mass of the bending object is too low for easy detection
of its effect on the light of the source object.
In microlensing, the expectation
is that the bending object will change position over
a relatively short timescale,
cosmologically speaking.
(so photometry over a period of time may be used to monitor changes in the source’s apparent brightness)
For relatively nearby objects the relation between apparent magnitude and log redshift is
linear
relationship between apparent magnitude and log redshift for more distant objects
begins to curve (amount of curvature indicates the curvature of the universe)
The large-scale distribution of galaxies in the Universe is far from uniform, due to
the influence of gravity causing structures in the galaxy distribution to grow as the Universe evolves.
