Galaxies What is a galaxy?

How many stars are there in an average galaxy?

About how many galaxies are there in the universe?

What is the name of our galaxy?

Galaxy

Gravitationally bound stars, gas, dust, neutron stars, black holes, etc.

Billions of stars in a galaxy and billions of galaxies in the universe

Milky Way: 1,000 LY thick and 100,000 LY across!

Types of Galaxies

Elliptical:

Spiral:

Irregular:

Elliptical Galaxies

Spiral Galaxies

Note: There is no direct or simple evolutionary connection between the types of Galaxies. However, there is strong evidence that interactions BETWEEN Galaxies may drive galaxy evolution.

X

Active Galactic Nuclei (AGN)

• Normal Galaxies – one million to one trillion solar luminosities

• AGN - several hundreds of trillions of solar luminosities (seriously bright) AND:– Spectrum is not black

body! What does this mean?

A huge amount of energy, spanning much of the electromagnetic spectrum, is generated within—and usually also emitted from—a central compact nucleus.

AGN

What is the power source that generates such enormous luminosities in AGNs?

AGN Energy SourceRapidly varying intensity => small size

High luminosity and small size => supermassive black hole

Billions of solar masses!

Consumes entire stars!

(ANIMATION)

Cosmic Distance Scale

Closest galaxy: Andromeda – 2.5 million light years away“Standard Candles”

Extremely bright events with well-known luminosities

Can you think of an example of such an event?

Cosmic Distance Scale

“Tully-Fisher” - galactic rotation used to determine rotation rate gives us distance to galaxy. “Standard Candles”

Extremely bright events with well-known luminosities

Ex. Type Ia (carbon-detonation) supernovae Works correctly out to ~ 3

billion light years

Recall: Apparent Brightness = Luminosity/Distance^2

Structures of Galaxies

GroupsA few to a few dozen galaxies bound together by their combined gravity.

On intermediate scales, galaxies form identifiable structures.

Local Group

Clusters

Largest (known) gravitationally bound structures - typically containing thousands of galaxies.

The Virgo Cluster of about 2500 galaxies (central part shown).

The center of the Hercules Cluster

Superclusters

Larger structures made up of clusters and groups. 10,000's of galaxies.

The Local Supercluster consists of the Virgo Cluster, the Local Group and several other groups.

Galaxy Evolution

1) Density fluctuations in the primordial matter

2) galaxies grew by repeated merging of smaller objects

- evidence: galaxies at large red shifts (-> large distances -> far back in time) are smaller and more irregular

3) Quasars, most luminous objects known, supermassive balck hole with lots of fuel -> less fuel becomes AGN -> runs out of fuel becomes “normal galaxy”. Black hole remains at center, but no longer giving off vast amounts of energy.

Basically, the difference between a normal galaxy and an active one (or a Quasar) seems to be mainly a matter of fuel supply.

Many, many galaxies are not alone -> mergers change galaxies, this is how they may evolve.

Computer simulations:

Two large Spirals can merge to become an Elliptical

Small galaxy passing through a galaxy can cause it to become a Spiral

AND, merging galaxies are active galaxies:

- food for BHs-star formation

Cosmological Structure?

We have seen that galaxies are organized into clusters and superclusters on intermediate scales in the universe.

How is matter distributed in the universe on the very largest scales?

No Structure on Largest Scales (Galaxies distributed fairly uniformly)

Surprising given structure on smaller scales

Cosmological Principle:Universe is homogeneous

and isotropicHomogeneity => universe

has no edgeIsotropy => universe has no

center