1

Astr 2310 Tues. April 28, 2009

Today’s Topics

• Chapter 21: Galaxies Beyond the Milky Way– General Properties

• Classification

• Photometric Properties

• Spectral Properties

– Radio, Infrared, and X-Ray Emission• Radio

– 21-cm Emission

– Radio Continuum Emission

• Infrared– Dust Emission

– PAH Emission

• X-Ray– Supernoave Remnants

– X-ray Binaries

– Mass of Galaxies• Mass Distribution

• Evidence for Dark Matter

• Chapter 22: Hubble’s Law and the Distance Scale– Period-Luminosity Relation for Cepheids

– Hubble’s Law• Redshift, Distance and the Age of the Universe

– Calibrating Hubble’s Law

• Chapter 23: Large-Scale Structure in the Universe– Clusters of Galaxies

– The Local Group of Galaxies

– Luminosity Function of Galaxies

– Superclusters and Peculiar Motions

2

Chapter 21, 22, 23 Homework

Chapter 21: #1, 3, 11

Chapter 22: #2, 9, 11

Chapter 23: #11

(Due Thurs. April 30)

3

Chapter 21: Galaxies Beyond the Milky Way:

General Properties of Galaxies

• Classification of Normal Galaxies

– Spiral Galaxies: gas-rich disk, spiral arms, current star formation, barred and

unbarred forms

– Elliptical Galaxies: spheroidal, no current star formation, no gas

– Irregular Galaxies: irregular shape, gas-rich, vigorous star formation

4

Chapter 21: Galaxies Beyond the Milky Way

• Observational Properties

– Photometry of Galaxies

• Apparent magnitudes

• Angular sizes

• Surface Brightness Profiles

• Colors

• Importance of Distances

– Observed Properties Cannot be Compared without Distances

• We need a way to Determine the Distances of Galaxies

– Period-Luminosity Relation of Cepheid Variables

• Leavitt Found a Relation Between Pulsational Period and Luminosity (seeChapter 17)

– Cepheids are Bright Enough to be Seen in Nearby Galaxies

– Pulsation Period Luminosity (Absolute Mag)

– Apparent + Absolute Distance of Galaxy

• Hubble Found Cepheids in Nearby Galaxy and Showed They WereDistant

– The universe is vast and filled with galaxies!

5

Chapter 21: Galaxies Beyond the Milky Way

• Spiral Galaxies

– Bulge and Disk Components

Similar to Milky Way’s.

• Large bulge: Sa

• Small bulge: Sc

– Old, metal-rich stars in

bulge, star formation in

disk.

– Dust and gas in a thin disk.

HI, HII

– Spiral arms are waves in the

disk which compress gas

and form stars. O-B stars

trace arms.

– About 1/3 have bars of old,

metal-rich stars

6

Chapter 21: Galaxies Beyond the Milky Way:

• Elliptical Galaxies

– Spherical or ellipsoidal systems

– Light dominated by old (1010 yrs

old) stars

• H-R diagrams of nearby

ellipticals reveal range in

metallicity

– Little gas or dust

– Constitute both the largest

galaxies (1013 Msun) and the

smallest (106 Msun) Largest have

highest metallicity

• Smallest have low metallicity

(see image at right)

– Random, internal velocities

measured via velocity dispersion

– Likely Form via Mergers of

Smaller Systems

7

Chapter 21: Galaxies Beyond the Milky Way:

• Irregular Galaxies

– About 25% of all

galaxies

– Biggest have about 1/10

mass of Milky Way

– No real structure but

larger ones are disk-like

– Large amounts of gas

but little dust

– Metallicities about 1/10

Solar

– Vigorous to intense star

formation

8

Radio Properties of Galaxies

• HI is typically much

more extended than

the optical light (stars)

and concentrated to

the spiral arms. HI

spectra are ideal for

measuring rotation

curves

• Molecular gas traced

via CO is more

clumped than HI

• Non-thermal Radio

emission traces

synchrotron emission

from electrons in mag.

Fields (from

supernovae)

9

Radio Properties of Galaxies

• Discovered as Strong

Radio Sources

– Central compact

source

10

Infrared Properties of Galaxies

• Space-based infraredobservations can probewavelengths notaccessible from Earth.

• Spectral energydistribution of spiralgalaxies shows strongemission from PAHfeatures at 8 μm.

– PAH trace warmmolecular gas

• Emission at longerwavelengths ( ~ 20 μm) trace warm dustfrom star formation.

• Cold dust traced via farinfrared emission

11

Masses of Galaxies

• Spectra Provide Rotation Curves of Spiral Galaxies

• M(R) = V2R/G

• Rotation curves

obtained for hundreds of

spiral galaxies.

• All are flat as far out as

can be measured.

• Dark matter is present

in all spiral galaxies!

•Alternative theories of

gravity have been

proposed but have failed.

12

Structural Properties of Galaxies

• Galaxies Follow “Simple”

Scaling Relations

– Size, Rotational Velocity,

Luminosity, etc. are Corrolated

– Tully-Fisher Relation for

Spirals

• Suggests Underlying Similarity

of Formation

• Powerful Tool for Measuring

Distances

• Should be an Ideal Probe of

Galaxy Evolution

– Fundamental Plane of Elliptical

Galaxies

• Used to measure distances

• Should Probe Galaxy Evolution

13

Chapter 22: Hubble’s Law and the

Distance Scale

• Period Luminosity Relation for Cepheids– Leavitt (1910) Found a Relation Between Pulsational Period and

Luminosity (see Chapter 17)

• Cepheids are Bright Enough to be Seen in Nearby Galaxies

• Pulsation Period Luminosity (Absolute Mag)

• Apparent + Absolute Distance of Galaxy

– Hubble Found Cepheids in Nearby Galaxies (1923) and Showed They

Were Distant

• The universe is vast and filled with galaxies!

– Hubble Space Telescope has Surveyed About 35 Nearby Galaxies for

Cepheids

• Cepheids used to calibrate structural scaling relations of galaxies

• Distances of distant galaxies measured via these relations

– Extragalactic Distances Based on Galactic (Milky Way) Cepheids

• Definitive Calibration Being Done Here at RBO (Monson and Pierce)

14

Extragalactic Distance Scale

• Distances to Galaxies and the Distance Scale

– Methods for Determining Extragalactic Distances

• Cepheids

– Period-Luminosity Relation

» Used to Calibrate the Following Methods

• Globular Clusters

– Luminosity Function of Globular Clusters in Distant Galaxies Compared with

Milky Way’s

• Type Ia Supernovae

– Luminous, Exploding Stars with Small Dispersion in Luminosity

• Tully-Fisher Relation

– Luminosity of Spirals Correlates with Rotational Velocity

• Planetary Nebulae Luminosity Function

– Luminosity Function has Sharp Cut-off at Bright End

• Fundamental Plane of Elliptical Galaxies

– Luminosity, Size, Velocity Dispersion are Strongly Correlated

• Surface Brightness Fluctuations

– Graininess of Galaxy due to Stars Depends on Distance

15

Hubble’s Law

• Redshift, Distance and the Age of the Universe

– Hubble found that the Velocity of a Galaxy Correlated with its Distance

– Hubble’s Law: V = HoD (Ho – Hubble Constant)

• Hubble’s Law

- Universe is Expanding

- In the Past Galaxies were Closer

- In Past Distances Between Galaxies

was Zero! (Big Bang)

- Each Galaxies Appears to be Center of

Expansion

- Modern Data (right)

- Various Methods Agree

Ho = 72 km/sec/Mpc

- For every Mpc Universe Expanding 72

km/sec

16

Chapter 23: Large-Scale Structure of the

Universe

• Structure Exists on Small Scales

– Something of a Surprise

• Hierarchical Clustering over Broad Range of

Scales

– Small groups (~ 1 Mpc in size)

– Clusters (few Mpc in size)

– Superclusters (~ 50 Mpc in size)

– Filaments (~ hundreds of Mpc in size)

– Voids (empty regions ~ 50 Mpc in size)

Cosmological Principle: On the Largest Scales the Universe is Homogeneous

(uniform) and Isotrpoic (looks same in all directions).

17

The Local Group of Galaxies

• Local Group– Two Large Spirals

• Milky Way

• Andromeda (M31)

– Medium Spiral• Triangulum (M33)

– Many Dwarf Galaxies• Milky Way

Satellites

• M31 Satellites

• Isolated Dwarfs

18

Clusters of Galaxies

• Clusters of galaxies (size ~ few Mpc) can contain 103

galaxies

– Gravitationally bound, strongly interacting, galaxies of all types.

– Usually filled with hot, x-ray emitting gas and intracluster stars.

19

Luminosity Function of Galaxies

• Distribution of GalaxyLuminosities

– Describes the number ofgalaxies as a function ofluminosity

– Flat, power-law

– Sharp cutoff at brightend

– Used to characterize thestatistical properties ofgalaxies

– Rich clusters show excessat bright end due tomerging

– Distant universe showsall galaxies moreluminous

– Most distant surveysreveal absence ofluminous galaxies.

20

Superclusters and Peculiar Motions

• Structure is present on thelargest scales

– Redshift surveys of distantgalaxies reveal large-scalestructure

– Not gravitationally bound

• Amplified by gravity

• Origin is not fully known

• Peculiar Motions resultfrom gravitationalinteraction of galaxies.

– Amplitude of peculiarvelocities used to map themass distribution on largescales

21

Chapter 21, 22, 23 Homework

Chapter 21: #1, 3, 11

Chapter 22: #2, 9, 11

Chapter 23: #11

(Due Thurs. April 30)