Normal Galaxies

Hubble Classification Scheme

Classification has nothing to do with evolution of an isolated galaxy from one type to another

Spirals, Ellipticals, Irregulars - Fig. 24.9

Properties of a Spiral

Flattened disk, central bulge, spiral arms

Subdivided on basis of size of bulge and tightness of arms

Halos have old stars, gas-rich disks are sites if ongoing star formation

Some spirals have bars extending from bulge

Properties of an Elliptical

No disk, contains little gas and dust

Large range in size from dwarf elliptical (much smaller than Milky Way) to giant elliptical (trillions of stars)

SO and SBO are intermediate between spiral and elliptical- have halos and disks, but little or no gas and dust

Most common galaxy - dwarf elliptical

Most of the mass in the Universe is in giant ellipticals

Properties of an Irregular

Galaxies that do not fit Spiral, Elliptical, or SO types

Many have gas and dust and have vigorous ongoing star formation

May be result of galactic collisions or close encounters

Thought to be younger than other types of galaxies

Magellanic Clouds are prototype irregulars

Thousands of Cepheids and RR Lyraes in LMC and SMC

LMC and SMC

Large and Small Galactic Clouds - Fig. 24.8

Naked Eye Objects!

Hubble Deep Field

Hubble pointed toward a previously blank piece of sky- viola!

What types of galaxies do you see?

Fig. 24-23

Cepheid in M100

Galaxy in Virgo cluster

18 Mpc

Fig. 24-10

Tully - Fisher Relation

Correlation between luminosity and rotational speed in a galaxy, very tight relation

Rotation speed is a measure of mass

Doppler broadening: Fig 24-11

Can be used out to 200 Mpc

Type Ia Supernovae

Type Ia supernovae (mass overloading of a white dwarf) always occur in the same type star (white dwarf) with the same mass (1.4 solar masses)

The explosion energy is the same for each supernovae

Thus, the absolute brightness is the same for each Type Ia Supernova, with the apparent brightness- we can know the distance!

Type Ia Supernovae are "standard candles"

Distance Ladder

Fig. 24-12

Distribution of Galaxies

Need distance to know

Local Group

Fig. 24-13

Hubble's Relation

Hubble found the following relation: V=H₀ d

H=Hubble constant, v= recession velocity of galaxy, d=distance

Empirical observation, not a theory

Fig. 24-28

Implication of Hubble's Law

V=H₀ d

Best estimate:

H₀ =65 km/s/Mpc

What happens to the velocity for galaxies that are farther away?

What is the implication for the Universe of Hubble's Law?

Cosmological Redshift

V=H₀ d

Current experiments yield

H₀ =50-80 km/s/Mpc

The Hubble constant measures the recessional velocity at a certain distance

Best estimate:

H₀ =65 km/s/Mpc

Fig. 24-27 shows the greater redshift at greater distance

Cosmic Distance Ladder

Fig. 24.29

Galaxy Formation

Galaxies DO NOT evolve from one type to another

Smaller galaxies may merge into larger ones

Tidal interactions and close encounters can change the character and composition of a galaxy

Simulated galaxy interaction, Fig. 24.25

Galaxy Merger

Possible formation of Milky Way

Several smaller systems merge together

Rotation starts, disordered velocities

Rotation causes gas and dust to fall into disk, get spinning disk

New stars forming in disk inherit ordered rotation of disk

Fig. 23-14

Evidence in Early Universe

Fig. 23-22

Galaxy Mass

Galactic rotation curves imply missing mass- dark matter

Studies of galactic clusters imply missing mass- dark matter

Fig. 24-18 and 24-19

Clusters of Galaxies

Galaxies are gravitationally bound in clusters

Virgo Cluster

Inset shows M86 and surrounding galaxies

Fig. 24-15

Superclusters

Clusters of Galaxies cluster together

Form Superclusters

Local Supercluster

Fig. 24-16

Large Scale Structure of Universe

Drs. Gregory and Thompson mapped out galaxies in 1978 and found enormous voids and filamentary structure. What causes this?

Local Universe: Fig. 24-30 and 24-31

What is the Missing Mass in Clusters

On left, the Virgo cluster, the fuzzy blue glow is X-ray emission from hot cluster gas, Fig. 24-20

On right is the central region of the Virgo cluster M87, where the cluster gas is shown in false color, Fig. 24-21

Dark Matter in Clusters

If you add up all the hot gas (T= 10⁷ K) will it add up to the mass determined using gravity?

Gas is so hot, that you actually need even more mass than normal to bind it all to M87

Inter-cluster gas cannot account for missing mass

Cluster gas is to low by a factor of 10-100 times

Summary

Basic properties of main 3 types of galaxies:

Spiral, Elliptical, Irregular

Distance ladder

Know techniques that allow astronomers to measure solar system to edge of Universe

Large scale distribution of galaxies

Voids, clusters

How galaxies form

Why most of the Universe is dark matter

Know Hubble's Law and how it used for distance