The Structure of the Universe

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Galaxy Groups and Clusters

• A few galaxies are all by themselves

• Most belong to groups or clusters

• Galaxy Groups:– Loose collections of about 5-50 galaxies– Many spirals found in groups

• Galaxy Clusters:– Hundreds or thousands of galaxies– Dominated by ellipticals

Galaxy Groups

Hickson CompactGroup 87

170,000 LY indiameter

Local Group

• Two heavyweights– Milky Way– Andromeda

• One intermediate– Triangulum Galaxy

• 30-40 dwarf galaxies

Galaxy Clusters

Abell 1689

2 million LY across

Galaxy Clusters

• Galaxies being drawn into clusters

• The structure of the Universe is still evolving!

Nearby Groups and Clusters

Nearby or part of something larger?

• Measurements on all of these groups and clusters show that they are interacting, but not gravitationally bound

This suggests that separate groups and clusters interact as a larger structure:

Supercluster

The Local Supercluster

Superclusters

• Superclusters sometimes have a large central concentration

• They have filaments of galaxies that branch off

• These filaments are like connections between superclusters

Superclusters

• We don’t see superclusters through telescopes.

• We make velocity measurements on separate clusters– See if they interact gravitationally

• They show up in large-scale maps

Large-Scale Motion

• If we look at velocities of galaxies nearby, we see some variation due to:

– Random motion within group / cluster

– Large scale motion of groups / clusters

• Groups and clusters often seem to be “flowing” somewhere!

The Great Attractor

• The Local Group and all matter within 150 million LY appear to be moving toward the “Great Attractor”

• Calculated mass of tens of thousand of Milky Ways!

• What is it?

• There was no obvious cause for this!

The Great Attractor

• It’s behind the band of the Milky Way!

• One cluster (Abell 3627) near the center of the “Great Attractor”

• Probably extremely large supercluster

Structure of the Universe

Are galaxy superclusters distributedin any particular way?

Redshift Surveys

• Large surveys attempt to answer that.

• We can make a map of observable galaxies and clusters, including their distances

• Dark matter plays a significant role– Holds galaxies and clusters together

Redshift Surveys

• Maps of galaxies: very time / resource / data intensive

– We need long exposures with large telescopes to even see distant galaxies

– We need their distance too• Determines distance using Hubble’s Law

CfA Redshift Survey

• They surveyed “wedges”

Carroll and Ostlie, 3rd Edition

CfA Redshift Survey

• Galaxies not distributed randomly

• Chains and filaments with superclusters at intersections

• Voids live in betweenCarroll and Ostlie, 3rd Edition

CfA Redshift Survey

• Another CfA wedge

Carroll and Ostlie, 3rd Edition

CfA2 Survey

• Another CfA survey (different declinations)– Great Wall – South Wall

• “Walls” are large superclusters

2dF Survey

Sloan Digital Sky Survey (SDSS)

• Outer circle is 2 billion LY

• Spectra for 800,000galaxies

Great Wall

• 1 billion LY from end to end

• Largest known structure in the Universe

• Sub-structure– Filaments– Voids

Deep Sky Surveys

Cosmological Principle

• At some scale, universe should start to look the same everywhere– 1 billion LY

• “The End of Greatness”

Structure of the Universe

The structure of the Universe consistsof walls of galaxies (superclusters) connected

together by filaments of galaxies.

Structure Formation

Simulations show that dark matter forms the structure ofthe Universe and it “corrals” light matter.

Structure Formation

• Light matter is thought to “map out” dark matter

• Dark matter structure forms a “framework”, light matter follows and collects where dark matter is most dense

Structure of the Universe

• Combine:

– Our deep sky surveys

– Our knowledge of dark matter

– Physics

• Millenium Simulation models what the Universe looks like (as though we can see dark matter).

Structure of the Universe