GalaxiesGalaxies

Tuesday, February 26Tuesday, February 26

What is a galaxy?Examples of galaxies:

It took centuries for astronomers to deduce the true nature of galaxies.

Andromeda Galaxy as seen by naked eye

(Al Sufi, AD 964)

Whirlpool Galaxy as seen with telescope

(Rosse, AD 1845)

What is a galaxy?

A gravitationally bound assembly of

many stars (+ associated planets)

interstellar gas (+ dust)

and dark matterdark matter.

“many” ≥ 10 million

The Milky Way Galaxy (our galaxy!) is surrounded by orbiting dwarf galaxies.

The dwarf galaxies (even distant ones) are orbiting rapidly.

Andromeda

This shows that our dark halo is (1) massive & (2) large in radius.

500,000 parsecs

50,000 parsecs

Dark halo

Luminous galaxy

Luminous Luminous part of a galaxy (made of electrons, protons, & neutrons)

is much smaller than the darkdark part (made of Weakly

Interacting Massive Particles).

What’s special about electrons, protons, & neutronselectrons, protons, & neutrons that concentrates them at the

center of dark halos?

“Tootsie pop” hypothesis: central luminous galaxy forms 1st,

then is “dipped” in dark matter.

“Twinkie” hypothesis: outer dark halo forms 1st, then luminous galaxy is

“injected”.

During the first years of the universe, Dark matter (WIMPs) & ordinary

matter (electrons, protons, neutrons) were mixed together.

What can ordinary matter do that dark matter cannot?

Emit light!

Consider a gas of electrons, protons, helium nuclei, and WIMPs all mixed

together…

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… and all moving in random directions.

Initially, the particles move rapidly. They have a high temperaturetemperature…

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… and therefore a high pressurepressure.

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However, the ordinary particles emit photons, which carry away energy…

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…so ordinary particles (but not WIMPs) slow downslow down. .

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Ordinary particles, no longer supported by pressure, flow where gravity takes

them…

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…to the densest clumps of dark matterdark matter

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Astronomy jargon: “falling down the gravity well”

Since ordinary stuff, made of electrons, protons, & neutrons,

can easily dump its excess energy, it falls toward dense

regions.

Modified Twinkie Hypothesis:Modified Twinkie Hypothesis: originally (dark matter) sponge cake & (ordinary matter) creme filling coexist.

Gravity “injects” the ordinary matter to the center of the dark matter.

Computer simulation of galaxy formation: ordinary matter on left, dark matter on right.

small box = eventual size of galaxy

We expect luminous ordinary matter (which loses energy by radiating light)

to form denser knots than dark matter.

Galaxies have been seen that have a redshift z z ≈ 10≈ 10.

The light we see from these galaxies was emitted at t t ≈ 500 million years≈ 500 million years.

The time from t ≈ 350 thousand years to t ≈ 500 million years is known as the

“Dark Ages” (no galaxies yet).

From t = 500 million years to t = NOW is the “Age of Galaxies”.

All we can see of a galaxy is the luminous part (duh!)

dwarf galaxy

giant galaxygiant galaxy

Galaxies have a range of luminosities. (Milky Way Galaxy, with a luminosity of

1011 Suns, is a bright galaxy.)

Very few galaxies are brighter than twice the Milky Way’s luminosity.

Why is there an upper limitupper limit on the size of galaxies?

Why is there an upper limit on the size of land animals?

Animals bigger than 200 tons can’t support their own weight & still be flexible enough to move.

Why is there an upper limit on the size of superclusters?

Dense regions more than 50 Mpc across haven’t had time to collapse.

Why is there an upper limit on the size of galaxiesgalaxies?

Bigger clumps of gas haven’t had time to cool down.

Coma Cluster of galaxies

It takes time for an object to cool off by radiating away photons.

A huge turkey cools off more slowly than a small french fry.

It takes time for an object to cool off by radiating away photons.

A huge gas cloud cools off more slowly than a small gas cloud.

Bright galaxies tend to have one of two shapes.

1) SpiralSpiral galaxies, like the Milky Way Galaxy and the Andromeda Galaxy

Stars in a spiral galaxy go around on neat (almost) circular orbits.

2) Elliptical Elliptical galaxies, like the galaxy known as M87.

Spiral galaxies are more photogenic, so they appear in all the “coffee table” books.

Stars in an elliptical galaxy are on disordered, randomly oriented orbits.

Spiral galaxy:Spiral galaxy: stars are “good citizens”, traveling on orderly orbits,

all moving in the same direction.

Elliptical galaxy:Elliptical galaxy: stars are “individualists”, traveling on chaotic

orbits, all in different directions.

Why are some galaxies orderly (spiral) & others chaotic (elliptical)?

Let’s see what happens when the Milky Way and

Andromeda galaxies collide.

When 2 orderly spiral galaxies collide, they become a chaotic elliptical galaxy.

(When 2 orderly cars collide, they don’t become an orderly truck: they become

a chaotic heap of metal.)

Spiral galaxies exist mainly in lower-density regions

(such as the Local Group).

Elliptical galaxies exist mainly in high-density clusters

(such as the Coma Cluster).

Thursday’s Lecture:

Reading:

none

Formation and Evolution of StarsStars