Observations to be explained

• Galaxy shapes: disks, spirals, bars, elliptical, irregular.
• Percentages of different types.
• Galactic halos: spherical shape, globular clusters, older stars.
• Neutral (i.e. not ionised) hydrogen clouds.
• Supermassive compact objects near centres of larger galaxies.
• Star populations, dark matter, rotation curves.
• The galaxy colour-magnitude diagram

Colour-magnitude diagram

Source: Joshua Schroeder CC BY-SA 3.0

Colour-magnitude interpretation

• The red sequence comprises galaxies where few young stars are found - usually elliptical galaxies.
• The blue cloud galaxies contain more young stars - usually spiral galaxies.
• The distribution is bimodal - there is a gap between them - the green valley.
• Unlike the HR diagram for stars, there is no simple interpretation of this in terms of mass and evolution.

Jeans mass

• Given the temperature and density of a gas cloud in space, the Jeans mass is the mass above which the cloud will be prone to collapse.
• A higher temperature gives a higher pressure, so a larger Jeans mass.
• A greater density gives a larger Jeans mass.
• Gas clouds that are cool and dense are more likely to collapse.
• But this process is relevant for star formation.
• Another process is believed to be responsible for galaxy formation.

Top-down or bottom-up?

• Originally it was thought that galaxies collapsed from single, large gas clouds - top-down formation.
• But now it is believed that many smaller objects coalesce to form galaxies - bottom-up formation.
• The latter is favoured by the evidence of dark matter halo.

Dark matter

• Dark matter was first proposed to account for the missing mass suggested by the galactic rotation curve.
• This dark matter was later discovered to be more consistent with a halo that extended well above and below the galactic disk.
• Current estimates require that there is 10 times as much mass in dark matter as in matter we can see in our galaxy.
• Dark matter interacts gravitationally but cannot be observed conventionally, i.e. it doesn't emit, absorb or scatter electromagnetic radiation (light, radio waves etc)

Dark matter - unseen gas clouds?

We know the Universe is full of gas clouds of mainly H and some He. Could dark matter be such clouds, perhaps spread very thinly (low density) throughout the galaxy so they're hard to see? The arguments against are many, including:

• If these gas clouds were H atoms, then we'd see them at the 21cm line.
• Even if low density, we do observe along very long lines of sight, so that emission or absorption should be detected.
• Molecular hydrogen (H₂) can't be detected easily (no 21cm line) but where we do find it, we'd expect to see carbon monoxide which has spectral lines that we can see - but we don't.
• Dark matter extends significantly out of the disk, but why are there no stars forming in those parts?
• Cosmological models predict the ratio of H, He and Li we see, but they do not predict a sufficient mass to account for the dark matter.

Dark matter - MACHOs

Could the mass be hidden in many massive but compact objects, such as white dwarfs, neutron stars and black holes? Referred to as MAssive Compact Halo Objects (MACHOs).

• Searches for gravitational lensing by such objects have found some, but not enough to account for the amount of dark matter required.
• Also, the cosmological model problem still needs addressed.

Dark matter - however improbable!

"You will not apply my precept," he said, shaking his head. "How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth? We know that he did not come through the door, the window, or the chimney. We also know that he could not have been concealed in the room, as there is no concealment possible. When, then, did he come?"

Sherlock Holmes in The Sign of the Four, Chapter 6. (1890)

Non-baryonic matter

• Normal matter has its mass composed mainly of protons and neutrons, which are baryons, so we call it baryonic matter.
• Baryonic matter interacts with gravity and interacts with electromagnetic radiation.
• The evidence for dark matter suggests it interacts with gravity, but not with electromagnetic radiation, hence it must be non-baryonic.
• We need to hunt for WIMPs - Weakly Interacting Massive Particles.

WIMPs

• We know of only one WIMP: the neutrino.
• It can pass through a planet or a star without interacting at all.
• There are plenty of them - estimated at 330 million per cubic metre.
• But the mass (and energy) of each one is so small that they cannot account for the total mass of dark matter needed.
• Also, these neutrinos move near the speed of light, which makes it hard to use them to build a model of how galaxies evolve.
• Astronomers look to particle physicists for ideas of other possible WIMPs.

Dark matter - hot or cold

Cold Dark Matter (CDM) refers to dark matter that has speeds much less than the speed of light.

• Simulations show that CDM collapses to form structures that are typically about a million solar masses.
• These structures would then merge to form the dark matter galactic halos: bottom-up formation.
• This model is currently favoured.

Hot Dark Matter (HDM) refers to dark matter that moves close to the speed of light, such as the neutrino.

• Simulations show that very large structures would form.
• Those structures are much bigger than any we see, and it's not clear how they would then fragment into the galaxies we currently see.

Disk formation

• So CDM models predict that both baryonic matter and dark matter build up into galaxies through merger.
• As these structures undergo gravitational collapse, they will heat up but only the baryonic structures can emit light and so lose energy.
• This is one possible reason why baryonic matter forms a thin disk, while the dark matter does not.
• The disk does not collapse into a central bulge because the disk material is moving with a speed, and so continues to orbit the centre.

Elliptical galaxies and central bulges

• The central bulges of spiral galaxies resemble small elliptical galaxies.
• They have a similar shape and similar star types, with a larger number of older stars than the disk of a galaxy.
• This is consistent with the CDM merger hypothesis.

Galaxy interactions

• There are numerous observations of galaxy interactions.
• We see these in "freeze-frame" because they take place over hundreds of millions of years.
• But we can simulate the interactions on computers to find out what's possible.

Antenna galaxies - NGC 4038 and 4039

Source: NASA Public Domain

NGC 4038 and 4039 - Hubble

Source: ESA/Hubble CC BY-SA 3.0

Galaxy interactions - simulations

Try running the simulation on the galaxy crash website to produce the following from the two disk galaxies (that start with no spiral):

• spiral arms
• an elliptical-like galaxy
• a galaxy with a larger disk
• antennae