<<

110: SURVEY OF ASTRONOMY

13. THE REALM OF THE NEBULAE

1. Distances and Types of

2. Hubble’s Law and Evolution

3. Peculiar and Active Galaxies Galaxy History Revealed in This Colorful Hubble View 1. DISTANCES AND TYPES OF GALAXIES

a. An Extragalactic Distance Scale

b. Galaxy Morphology

c. Groups and Clusters Parallax Distances

Nearby appear to shift back and forth as we orbit the Sun. 2p Dec June The parallax angle p is inversely proportional to D the distance D: p 1 360° D ≃ AU 2π p AU

Using parallax, we can measure stellar distances out to a few hundred light-years. Distances

The same luminosity L must pass through each sphere.

A sphere of radius D has area A = 4πD2

So brightness is inversely proportional to (distance)2:

L L L B = = D = A 4πD2 √ 4πB An of known luminosity L is a standard candle. 106 Main Sequence Fitting

All stars in a cluster have the same distance, so plot HR diagrams for clusters using apparent brightness.

MS in appears 7.5 times brighter than MS in ; why?

Pleiades are √7.5 ≃ 2.7 times further away than Hyades! Distance Scale: Summary

1. Parallax measurements within the gave an accurate value for the : 1 AU = 1.496×108 km 2. Using the ’s orbit as a baseline, stellar parallax provides a distance to the Hyades cluster:

6 DHyades = 9.56×10 AU = 151 ly 3. Main sequence fitting yields the distance to other clusters in the galaxy in terms of DHyades. At each step, known distances are used to find unknown distances. Stars

3.5

4.0

Brightness (mag) 4.5 Period

2 4 6 8 10 12

Time (days) delta Cephei Massive stars become Cepheid Variables at one phase of their lives after leaving the main sequence.

During this phase, they vary in size, temperature, and brightness in regular ways with well-defined periods. Period-Luminosity Relationship. I

Cepheids in the were found to obey a relationship between period and apparent brightness. Large Magellanic Cloud

100

These stars are all at the 10 ightness same distance, so their 1.0

apparent brightnesses are absolute luminosity

e apparent br 0.1 proportional to their relativ absolute . 0.01

So Cepheids must obey a period-luminosity relationship! Period-Luminosity Relationship. II

To be useful for distance measurements, the P-L relationship must be calibrated in units of L⊙ by measuring absolute luminosities of some Cepheids.

Cepheids in star clusters are handy for this, since distances are available via main-sequence fitting.

Once this is done, a Cepheid’s luminosity can be found from its period. The Distance to (M31)

Is M31 another galaxy, or part of the ?

The luminosities of several Cepheids in M31 were determined from their periods via the P-L relationship.

Andromeda : Var!

Given their luminosities and brightnesses, distances to 6 these Cepheids could be computed: DM31 ≃ 2.4×10 ly.

M31 is far beyond the Milky Way!

M31: The White Dwarf Supernovae: Standard Bombs

These supernovae have a very narrow range of peak luminosities since they always occur in the same way.

To calibrate this peak, we must observe supernovae in galaxies with distances known from Cepheid variables. An Extragalactic Distance Scale

109 ly

106 ly

103 ly

1 ly

-3 y 10 l

Interlocking methods allow distances up to ~1010 ly to be measured fairly reliably. , Inclined

M63: The Sunflower Galaxy Spiral Galaxy, Edge-On

NGC 4565: Needle Galaxy

NGC 1365: A Nearby Barred Spiral Galaxy ‘Grand Design’ Spiral Galaxy

M51 Hubble Remix Disk Galaxy With Large Bulge

The from VLT With Dust

NGC 2787: A Barred Lenticular Galaxy Giant With Companions

Galaxies Away Companions to M31

M32: Blue Stars in an Elliptical Galaxy

M31: The Andromeda Galaxy Irregular Galaxy (Large Magellanic Cloud)

The Large Cloud of Magellan Peculiar Galaxy

The Colliding Galaxies of NGC 520 Hubble’s Galaxy Classification

Irregular and peculiar galaxies not included.

The Hubble Tuning Fork — Classification of Galaxies The : Over 30 Galaxies

two large spirals with satellites

one smaller spiral

many dwarf elliptical and irregular galaxies Local Group The Virgo Cluster: Over 1000 Galaxies!

Distance: ~6 × 107 ly three massive elliptical galaxies many MW-sized galaxies

M86 in the Virgo Cluster A Rich Regular

Distance: ~2.5 × 108 ly mostly elliptical galaxies

Galaxies of the Perseus Cluster A Rich Irregular Galaxy Cluster

Distance: ~5 × 108 ly many disk galaxies some are colliding

The Hercules Cluster of Galaxies A Compact Group

Distance: ~6 × 107 ly one elliptical galaxy three spiral galaxies

Galaxy Group Hickson 44 2. HUBBLE’S LAW AND GALAXY EVOLUTION

a. The Expanding

b. Looking Back in Time

c. Class Survey The Doppler Shift

Doppler Effect

A stationary source sends If the source is moving, out waves of the same the waves bunch up in all ahead of its motion, and directions. spread out behind. The Doppler Shift: Light

We get a similar effect with light. The change in wavelength λ depends on the source’s velocity v red-shift blue-shift toward or away from us:

λ - λ v shift rest = Note: valid for v ≪ c λrest c where λshift is the observed (shifted) wavelength, λrest is the wavelength with the source at rest, and c is the . The

Text

Most galaxies have spectra systematically shifted toward the red, implying that they’re moving away from us. The Redshift: An Example

Define the redshift: λ - λ shift rest = z λrest

line λrest λshift z v = c z (nm) (nm) (km/s) Hβ 486.1 500.9 0.0304 9120 Hγ 434.1 447.3 0.0304 9120 Hδ 410.2 422.7 0.0304 9120

Arp 188 and the Tadpole's Tidal Tail “consta wher d v Plotting galaxyv The ExpansionoftheUniv , , againsttheirdistances, r e v e ealed ar H v 0

is nt” ≃

H Hubb : elationship: 0

d elocities, , le ’

s 1931ApJ....74...43H

erse The Expansion of the Universe

Plotting galaxy velocities, v, against their distances, d, revealed a relationship:

v ≃ H0 d, where H0 is Hubble’s “constant”:

H0 ≃ 22 km/s/Mly. Two consequences: (1) galaxy can be used to estimate distances; (2) the universe is expanding. The Cartoon History of the Universe The Universe Has No Center!

Observed from Galaxy A Observed from MW Observed from Galaxy B

MW MW MW

B B A B A A

All observers see other galaxies moving away from their position with speeds proportional to distances.

The expansion does not define a center! The Universe Has No Center!

The universe shows no sign of edges — it seems to be infinite in all directions. Cosmological Principle: The universe looks roughly the same everywhere.

• Matter is evenly distributed on very large scales. • There is no center and no edges. • Not proved but consistent with observations. The Universe Has An Age!

Assume that galaxies move apart at constant speeds; how long ago were they all ‘on top of each other’? A galaxy at distance d = 1000 Mly moves away at speed

v = H0 d = (22 km/s/Mly) × 1000 Mly = 22000 km/s The time required to travel this distance is d 1000 Mly 9.5×1021 km = = = 4.32×1017 s = 13.7 Gyr v 22000 km/s 22000 km/s (Note: d cancels out; you get the same time for any d!) 13.7 Gyr is a good estimate for the Universe’s age! High Redshift

Redshifts z bigger than one can’t be interpreted dtoday in terms of galaxy velocity:

v = c z then d A correct interpretation: then

dtoday 1 + z = dthen

Example: from redshift galaxy z = 2 to today, galaxy location distances have tripled. then Looking Back in Time

Light travels at finite speed, so when we look out into space we are also looking back in time!

Many of these galaxies are billions of lightyears away, so we’re seeing them as they were billions of years ago.

Galaxy History Revealed in This Colorful Hubble View Lookback Time

Lookback time is related to redshift: longer times correspond to higher redshifts.

z tback (Gyr) then 1 7.73 2 10.3 3 11.5 13.7 ∞ galaxy location then z ≈ 2 High-Redshift Galaxies

200-million-year-old baby galaxies 200-million-year-old baby galaxies

These galaxies have redshifts z ≈ 7 to 7.5, implying lookback times of ~ 13 Gyr; back then, the was only 700 Myr. High-Redshift Galaxies

200-million-year-old baby galaxies 200-million-year-old baby galaxies

• Irregular shapes; no apparent symmetry • Very high rates of • Powerful outflows of gas 1. What do we need to know about a star before we can use it as a standard candle?

A. B. Diameter C. Age D. Luminosity E. Temperature 2. We compute the peak luminosity of a white-dwarf supernovae in another galaxy by determining its distance using ______and measuring its ______.

A. parallax; temperature from spectra B. Cepheids in the same galaxy; apparent brightness C. main-sequence fitting; apparent brightness D. Cepheids in the same galaxy; makeup from spectra E. parallax; mass using orbital motion 3. What kind of galaxy is this?

A. Irregular B. Elliptical C. Barred spiral D. Regular spiral E. High-Redshift

NGC 1365 4. Where do we see evidence of recent star formation?

D

A B

C

E

NGC 1365 5. Which of these is an elliptical galaxy?

A

D E

B C M86 in the Virgo Cluster 6. How can we tell another galaxy is moving away?

A. It appears smaller from year to year. B. It appears fainter from year to year. C. Its spectral lines are shifted toward the blue. D. Its spectral lines are shifted toward the red. E. Its parallax angle gets smaller over time. 7. If galaxy A has redshift zA = 0.05 and galaxy B has redshift zB = 0.1,

A. galaxy A is twice as far as galaxy B. B. both galaxies have the same distance. C. galaxy B is twice as far as galaxy A. D. galaxy B is four times as far as galaxy A. E. we cannot tell which galaxy is further. 8. Which statement is most likely to be correct?

A. Other galaxies are moving away from the Milky Way, but not from each other. B. Every galaxy in the universe is surrounded by other galaxies which are moving away from it. C. The universe is a finite sphere of galaxies expanding into empty space. D. All galaxies are orbiting the center of the universe. E. Galaxies are moving away from the Milky Way with speeds which do not depend on their distances. 3. PECULIAR AND ACTIVE GALAXIES

a. Galaxy Collisions

b. Starburst Galaxies

c. Active Galaxies Interacting and Merging Galaxies

Some galaxies don’t fit the elliptical/spiral/irregular classification.

Figure 1.4 Galaxy sample in this study. Top and middle row from left to right: Arp 256, NGC 7469, NGC 4676 and Arp 299. Bottom row from left to right: IC 883, NGC 2623 and NGC 7252. North is up, and east is to the left. Most colored images are restored from HST ACS/WFC images (courtesy of NASA, the Hubble Heritage, A. Evans and ESA, taken from http://hubblesite.org/newscenter/archive/releases/galaxy/interacting/2008/16/image/a/, and courtesy of NASA, H. Ford, G. Illingworth, M. Clampin, G. Hartig, and the ACS Science Team, taken from http://hubblesite.org/newscenter/archive/releases/2002/11/image/d/). Image of NGC 7252 is restored from B- and R-band images taken with CTIO 4m telescope from Hibbard et al. (1994).

12 How Can Galaxies Collide?

If galaxies move away from each other as the universe expands, how can they ever collide?

Interacting Galaxy UGC 9618

The gravitational attraction of two massive galaxy halos can locally reverse the expansion and cause a collision.

Most interacting pairs probably fell together “recently”. Tides between disk galaxies create filaments of stars.

SPIN

–0.5 0 0.5 1

1.5 2 2.5 3

Galactic Bridges and Tails A Simulated Interaction A Simulated Interaction

Tidal Interaction The Whirlpool Nebula

M51 Hubble Remix The Mice: Two Colliding Spirals

Colliding Galaxies

"The Mice":

NGC 4676: True-Color RGB Image Simulation of the Mice

The Mice at Play Why do Galaxies Merge?

Tidal forces transform the organized orbital motion of galaxies into random motions of stars and .

• This is a form of friction — it slows galaxies down.

• Dark matter plays critical role — absorbs momentum. What Kind of Galaxy is Produced?

Random stellar orbits can naturally account for the oval shapes and slow rotation of elliptical galaxies.

• Merger hypothesis: spiral galaxies merge to form elliptical galaxies.

• Estimated merger rates can produce right number of elliptical galaxies.

• Need additional star formation in mergers to form cores of elliptical galaxies. The Antennae

Super Star Clusters in the Rapid star formation is common in merging spiral galaxies! NGC 4038/4039 Antennae Simulation With Star Formation Starburst Galaxies

Galaxy Wars: M81 versus M82 Starburst Galaxies

Starburst Galaxy M82

Star formation rate: ~10 × Milky Way’s.

Gas outflow driven by supernovae

Galaxy Wars: M81 versus M82 Arp 299: Factory

First encounter ~700 Mry ago

Ultra-Luminous Galaxy 12 (L > 10 L⊙)

Interacting Galaxy NGC 3690 Arp 220: Merger Remnant

Core contains as much gas as entire Milky Way!

A Collision In The Heart Of A Galaxy

Star formation rate: ~100 × Milky Way’s!

Active nucleus as well as stars.

Interacting Galaxy Arp 220 If the center of a galaxy is unusually bright, we call it an .

Quasars are the most luminous examples.

Active Nucleus in M87

Copyright © 2009 Pearson Education, Inc. Galaxies around sometimes appear disturbed by collisions.

Copyright © 2009 Pearson Education, Inc. Radio galaxies contain active nuclei shooting out vast jets of plasma that emit radio waves coming from electrons moving at near light speed.

Copyright © 2009 Pearson Education, Inc. Characteristics of Active Galaxies

12 • Luminosity can be enormous (>10 LSun). • Luminosity can rapidly vary (comes from a space smaller than solar system). • They emit energy over a wide range of (contain matter with wide temperature range). • Some drive jets of plasma at near light speed.

Copyright © 2009 Pearson Education, Inc. What is the power source for quasars and other active galactic nuclei?

Copyright © 2009 Pearson Education, Inc. The accretion of gas onto a supermassive appears to be the only way to explain all the properties of quasars.

Copyright © 2009 Pearson Education, Inc. Energy from a Black Hole

• The gravitational potential energy of matter falling into a black hole turns into kinetic energy. • Friction in the accretion disk turns kinetic energy into thermal energy (heat). • Heat produces thermal radiation (photons). • This process can convert 10–40% of E = mc2 into radiation.

Copyright © 2009 Pearson Education, Inc. Jets are thought to come from the twisting of a magnetic field in the inner part of the accretion disk.

Copyright © 2009 Pearson Education, Inc. Do supermassive black holes really exist?

Copyright © 2009 Pearson Education, Inc. Orbital speed and distance of gas orbiting center of M87 indicate a black hole with mass of 3 billion MSun.

Copyright © 2009 Pearson Education, Inc. Orbits of stars at center of Milky Way indicate a black hole with mass of 4 million MSun.

Copyright © 2009 Pearson Education, Inc. Galaxies and Black Holes

• The mass of a galaxy’s central black hole is closely related to the mass of its bulge.

Copyright © 2009 Pearson Education, Inc. Galaxies and Black Holes

• The development of a central black hole must somehow be related to galaxy evolution.

Copyright © 2009 Pearson Education, Inc. Galaxy Mergers With Gas

Transformations of Galaxies II: Gas Only Transformations of Galaxies II: Final Encounter Mergers With Gas and Black Holes

Galaxy Collisions Awaken Dormant Black Holes