Astronomy 110: SURVEY OF ASTRONOMY
13. THE REALM OF THE NEBULAE
1. Distances and Types of Galaxies
2. Hubble’s Law and Galaxy 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 stars 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. Luminosity 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 star 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 Hyades appears 7.5 times brighter than MS in Pleiades; why?
Pleiades are √7.5 ≃ 2.7 times further away than Hyades! Distance Scale: Summary
1. Parallax measurements within the solar system gave an accurate value for the astronomical unit: 1 AU = 1.496×108 km 2. Using the Earth’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. Cepheid Variable 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 Large Magellanic Cloud 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 luminosities. 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 Andromeda (M31)
Is M31 another galaxy, or part of the Milky Way?
The luminosities of several Cepheids in M31 were determined from their periods via the P-L relationship.
Andromeda Nebula: 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 Andromeda Galaxy 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. Spiral Galaxy, Inclined
M63: The Sunflower Galaxy Spiral Galaxy, Edge-On
NGC 4565: Needle Galaxy Barred Spiral Galaxy
NGC 1365: A Nearby Barred Spiral Galaxy ‘Grand Design’ Spiral Galaxy
M51 Hubble Remix Disk Galaxy With Large Bulge
The Sombrero Galaxy from VLT Lenticular Galaxy With Dust
NGC 2787: A Barred Lenticular Galaxy Giant Elliptical Galaxy With Companions
Galaxies Away Dwarf Elliptical Galaxy 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 Local Group: 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 Galaxy Cluster
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 Universe
b. Looking Back in Time
c. Class Survey The Doppler Shift
Doppler Effect Doppler Effect
A stationary source sends If the source is moving, out waves of the same the waves bunch up wavelength 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 speed of light. The Redshift
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 redshifts 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 age of the universe 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 star formation • Powerful outflows of gas 1. What do we need to know about a star before we can use it as a standard candle?
A. Mass 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 dark matter.
• 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 Antennae Galaxies 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: Supernova Factory
First encounter ~700 Mry ago
Ultra-Luminous Infrared 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 active galactic nucleus.
Quasars are the most luminous examples.
Active Nucleus in M87
Copyright © 2009 Pearson Education, Inc. Galaxies around quasars 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 wavelengths (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 black hole 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