ASTR 113 – 003 Spring 2006Lecture 10 April 5, 2006

Review (Ch4-5): the Foundation

Galaxy (Ch 25-27)

Cosmology (Ch28-29)

Introduction To Modern Astronomy II

Star (Ch18-24)

1. Sun, Our star (Ch18)2. Nature of Stars (Ch19)3. Birth of Stars (Ch20)4. After Main Sequence (Ch21)5. Death of Stars (Ch22)6. Neutron Stars (Ch23)7. Black Holes (Ch24)

Extraterrestrial Life (Ch30)

1. Our Galaxy (Ch25)2. Galaxies (Ch26)3. Active Galaxies (Ch27)

1. Evolution of Universe (Ch28)2. Early Universe (Ch29)

Galaxies

Chapter Twenty-Six

ASTR 113 – 003 Spring 2006Lecture 10 April 5, 2006

Guiding Questions• How did astronomers first discover other galaxies?• How did astronomers first determine the distances to

galaxies?• Do all galaxies have spiral arms, like the Milky Way?• How do modern astronomers tell how far away galaxies

are?• How do the spectra of galaxies tell astronomers that the

universe is expanding?• Are galaxies isolated in space, or are they found near

other galaxies?• What happens when galaxies collide with each other?• Is dark matter found in galaxies beyond the Milky Way?• How do astronomers think galaxies formed?

First Discovery of Other Galaxies • Spiral “nebulae” were thought to be inside the Milky

way

Hubble proved that the spiral nebulae are beyond the Milky Way

• Edwin Hubble used Cepheid variables to show that the “nebula” were actually immense star systems far beyond our Galaxy

• Cepheid variables obey the period-luminosity law

Classifying Galaxies • Hubble classification: classification is based on appearance

only

• Four major types of galaxies:1. Spiral galaxies (S) (Type 1+2: 77%)2. Barred spiral galaxies (BS) 3. Elliptical galaxies (E) (20%)4. Irregular galaxies (I) (3%)

Spiral galaxies (S)• Spiral arms are active star-forming region• The stars in the spiral arms are mainly metal-rich

population I star• Sub-classification based on the smoothness of spiral

arms and size of bulge: Sa, Sb, Sc– Sa: broad arms, relatively large bulge– Sc: narrow arms, relatively small bulge– Sb: intermediate

Barred Spiral galaxies (SB)• A bar shaped region running through the nucleus.• Spiral arms originate at the end of the bar rather than

the nucleus itself• Sub-classifications: SBa, SBb, SBc (same as S

galaxies)

Elliptical galaxies (E)• Elliptical shape, have no spiral arms• Devoid of gas and dues• Consists of old, red and metal-poor population II stars• Sub-classifications: E1, E2…E7 (based on flatness)

– E1: roundest– E7: flattest

Irregular galaxies

They are often found associated with other galaxies

• Irregular galaxies have ill-defined, asymmetrical shapes• They are rich in interstellar gas and dust

Hubble’s Tuning Fork Diagram• E, S, Sb and I types• Lenticular galaxies are intermediate between spiral and

elliptical galaxies

Summary Table of Classification

Determine Distances to Galaxies

•Use Standard Candle, an object that lies within that galaxy and for which we know the luminosity

•Standard candles include Cepheid variables, supernovae (Type Ia).

The Distance Ladder• Parallax: 500 pc• Spectroscopic parallax: 10 kpc• RR Lyrae variable: 100 kpc• Cepheid variable (104 Ls): 30 Mpc • Type 1a Supernovae (109 Ls): 1000 Mpc

Redshift of Galaxies•Hubble found the spectrum of galaxies have redshift

z = (λ – λ0)/λ0

z = (Δλ)/λ0

z: value of redshift

λ: wavelength of shifted spectral line

λ0: wavelength of unshifted spectral line

•According to Doppler’ law, redshift means the galaxies are receding from us•e.g., z=0.20

V (velocity)=0.20 C (speed of light)

V = 75000 km/s

The Hubble law•The Hubble law: the more distance a galaxy, the greater its redshift and the more rapidly it is receding from us.

v = H0 dV: velocity in unit of (km/s)D: distance in init of Mpc

H0, Hubble constant, ~ 71 km/s/Mpc, but not certain

•The Hubble constant indicates how fast our universe is expanding

ASTR 113 – 003 Spring 2006Lecture 11 April 12, 2006

Review (Ch4-5): the Foundation

Galaxy (Ch 25-27)

Cosmology (Ch28-29)

Introduction To Modern Astronomy II

Star (Ch18-24)

1. Sun, Our star (Ch18)2. Nature of Stars (Ch19)3. Birth of Stars (Ch20)4. After Main Sequence (Ch21)5. Death of Stars (Ch22)6. Neutron Stars (Ch23)7. Black Holes (Ch24)

Extraterrestrial Life (Ch30)

1. Our Galaxy (Ch25)2. Galaxies (Ch26)3. Active Galaxies (Ch27)

1. Evolution of Universe (Ch28)2. Early Universe (Ch29)

Galaxies are grouped into clusters and superclusters

• Galaxies do not evenly or randomly distributed throughout the Universe

• Galaxies tend to be grouped into clusters• Rich cluster has far more number of galaxies than

poor cluster• A poor cluster is often called group• Local Group is the galaxy cluster containing the Milky

Way Galaxy; Local Group is a poor cluster of about 40 galaxies

Local Group• A poor, irregular cluster of about 40 galaxies• The diameter is about 1 Mpc (mega parsec)• The largest is M31, the Andromeda Galaxy• The Milky Way is in the second place• Both Milky Way and M31 are surrounded by a number of small satellite

galaxies

• A rich cluster contains hundreds or even thousands of galaxies

• The Coma cluster, a rich and regular cluster is about 90 Mpc (300 million light year) from the Earth

• It has as many as 10000 galaxies

An example of Rich Cluster of Galaxies

Coma Cluster of Galaxies

Supercluster of Galaxies• A supercluster of galaxies is a huge association of

clusters of galaxies• A typical supercluster contains a dozen of individual

clusters• It spans up to 50 Mpc

Distribution of Galaxies in the Universe• This map shows 1.6 million galaxies from the 2MASS

(Two-Micron All-Sky Survey) survey• Supercluster of Galaxies lie along filaments• There are large dark voids that contain few galaxies

Distribution of Galaxies in the Universe• This map shows 60000 galaxies in two wedges extending up to

redshift z=0.25 from 2dfGRS (Two Degree Field Galactic Redshift Survey)

• It also show filements and voids• The voids are roughly spherical, 30 to 120 Mpc in diameter

Galaxy CollisionThe gravitational tidal force deforms the galaxies: stars are hurled into intergalactic space along arching streams

Galaxy Collision•Two galaxies can merge into a bigger galaxy•Galactic cannibalism•Interstellar gas can be compressed, triggering star formation

Dark Matter inferred from Rotation Curve• If mass distribution follows the luminosity distribution, the rotation

curve would fall off according to Neuton’s and/or Kepler’s Law• The flat rotation curve at large distance indicates the presence of

extended halo of no-luminous matter, or dark matter

Dark Matter inferred from Gravitational Lensing•Gravitational Lensing: a massive galaxy deflects light rays like a lens so that an observer sees multiple distorted images of a more distant galaxy•Gravitational lensing is predicted by Einstein’s general theory of relativity: space is curved due to gravity

Dark Matter inferred from Gravitational Lensing• Examples of Gravitational lensing•The mass of galaxies calculated from gravitational lensing is much larger than the visible mass; again, 90% dark matter

One More Example of Gravitational Lensing

Dark Matter Candidates• Massive neutrons, called WIMPs (weakly interacting

massive particles)• MACHOS (massive compact halo objects), e.g., small black

holes or brown dwarfs

Galaxies formation

• A full-size galaxy is formed by the merger of smaller objects (or sub-galactic unit)

• These small objects (less than 1 kpc) (numbered in the figure) are seen when the Universe is young (3400 Mpc away, or 11 billion lys ago)

Formation of Spiral or Elliptical Galaxies• It depends how fast the gas is used up to form galaxies• If star formation is fast, no gas is left elliptical galaxy• If star formation is slow, gas forms disk spiral galaxy

Key Words• anisotropic• barred spiral galaxy• clusters (of galaxies)• dark-matter problem• distance ladder• dwarf elliptical galaxy• elliptical galaxy• fundamental plane• galactic cannibalism• giant elliptical galaxy• gravitational lens• groups (of galaxies)• Hubble classification• Hubble constant• Hubble flow• Hubble law• intracluster gas

• irregular cluster• irregular galaxy• isotropic• lenticular galaxy• Local Group• maser• poor cluster• redshift• regular cluster• rich cluster• spiral galaxy• standard candle• starburst galaxy• supercluster• Tully-Fisher relation• tuning fork diagram• void