Dark Matter in Galaxies and Clusters

AST 112

Matter

• Galaxies appear to be made up of stars, gas and dust

• Reasonable to think that’s the end of the story…

• …but there are some glaring inconsistencies.

Matter

• We know what “everyday” matter is made of:– Atoms and molecules– Can study their spectra, etc.

• Evidence points to an abundance of a type of matter that we don’t understand– Can’t see it!

Dark Matter in Galaxies

• Something’s speed vs. how far it is from the center

• Curve for merry-go-round is shown

Dark Matter in Galaxies

• Kepler’s 3rd Law:

– Planets in our solar system orbit at slower speeds farther from the Sun

Dark Matter in the Milky Way

• Can get rotation curve of Milky Way

• Use 21-cm line to measure Doppler shift of H clouds

Dark Matter in the Milky Way

• Can use Kepler’s Laws to calculate mass “inside of an orbit”

• For the curve to fall:– Mass must be

concentrated at the center

Dark Matter in the Milky Way

• Mass clearly not concentrated at the center

• Sun is 2/3 of the way to the edge– Shouldn’t it start to fall?

• Milky Way “ends” at 50k LY!– Really, shouldn’t it be

falling by now?

Dark Matter in the Milky Way

• All of this points to dark matter

• Radius may be 10x as large as our halo of stars, 10x as massive as disk

Dark Matter in Other Galaxies

• Maybe it’s just the Milky Way?

• Measure Doppler shift of arms in spirals

Dark Matter in Other Galaxies

• The mass of this galaxy is clearly concentrated at the center

• Yet the rotation curves of similar galaxies do not fall off away from center

Dark Matter in Other Galaxies

• Their rotation curves flatten out just like the Milky Way

Mass - To - Light

• Measure a galaxy’s luminosity

• Leads to estimate of star count– Leads to estimate of mass

• Orbits of stars indicate total mass in the galaxy– Does this mass agree with the star count?

Is it “ordinary stuff” that we just can’t see?

• Is dark matter simply ordinary matter that we can’t see? Or is it a new form of matter that we are unfamiliar with?

Dark Matter in Galaxy Clusters

• Fritz Zwicky measured Doppler shifts of galaxies in clusters

• Came up with average orbital speeds– Gives total cluster mass

• Measured luminosity of cluster– Gives total star mass

Dark Matter in Galaxy Clusters

• Velocities of galaxies in clusters such that the cluster would not remain bound given the mass measured by starlight.

• The mass contributed by stars is much less than the total mass that the cluster appears to contain.

Dark Matter in Galaxy Clusters

Bullet Cluster

This is a collision between two galaxyclusters!

Bullet Cluster

The pink regions areregions of hot gas thatis glowing in x-rays.

Dark Matter in Galaxy Clusters

• Clusters of galaxies are embedded in hot gas– So hot it glows in x-rays, not visible

• So is this what dark matter is? Hot gas?

Dark Matter in Galaxy Clusters

• Gas temperature is an indicator of cluster mass

– Higher temperature means faster molecules, so more gravity is required to keep them from leaving

– So we can estimate the mass of the x-ray gas• The mass of the gas exceeds that of all the galaxies in a cluster put

together by 7x!

– Even accounting for mass of gas and galaxies, still not massive enough to account for galaxies’ motions and temperature of intergalactic gas

Dark Matter in Galaxy Clusters

• Don’t trust the orbits?

• Don’t trust the gas-cloud-mass method?

• A third opinion: astronomers use gravitational lensing to measure galaxy cluster masses

Gravitational Lensing

Dark Matter in Galaxy Clusters

• Galaxy clusters are LARGE– They bend space-time

• Light from background galaxies would normally miss us– But the curved

spacetime bends it toward us

(Multiple lensed image)

Bullet Cluster

The blue regions aredark matter as mappedusing gravitationallensing.

Notice that they overlapwith the visible matter.

Dark matter does not interact with light. Doesit interact with itself?

Dark Matter in Galaxy Clusters

• To measure the mass of a galaxy cluster, we can:

– We can look at orbits

– We can use the temperature of the gas clouds

– Use gravitational lensing to map massive areas

• These methods all agree! And they all point to dark matter!

Abell 520: Train Wreck Cluster

Orange is starlight, blue is (mapped) dark matter.

Does dark matter interact with itself?

Is this a contradiction?

Abell 520: Train Wreck Cluster

Another team tookbetter data and foundmany dim galaxies inthe “empty” dark matter cloud.

Their results are not yet confirmed.

Ordinary Dark Matter

• Some dark matter is regular stuff that’s simply too cold to emit much radiation– Brown dwarfs, faint red MS stars

• Some dark matter is in the form of black holes

• Some dark matter is stuff that’s just too small to observe (e.g. asteroids)

Extraordinary Dark Matter

• But most dark matter is a form of matter that is unknown to us– “Extraordinary” dark matter– Our models for the Big Bang explain element

abundances accurately– If the “mass deficits” that we see are ordinary

matter:• Big Bang nucleosynthesis would have produced way

less deuterium than we see

What is it?

• Ask the particle physicists!

• The Standard Model is like the periodic table of subatomic particles

• They do not know of a dark matter particle

Alternate Theories

• Quantum gravity– We don’t have a working quantum gravity theory!

• Modified gravity– Doesn’t always predict everything correctly• Might get spiral rotation curves right but not cluster

lensing

Dark Matter: Summary

Gravity from the objects that we can observe is not enough to explain rotation

rates of galaxies and bound galaxy clusters.

“Needs more gravity!”