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
Transcript
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
