1. (From Seeds Ch )
Galaxies

2. The Milky Way
Almost everything we see in the night sky belongs to the Milky Way.
From outside, our Milky Way might very much look like our cosmic neighbor, the Andromeda Galaxy.
We see most of the Milky Way as a faint band of light across the sky.

3. First Studies of the Galaxy
First attempt to unveil the structure of the galaxy by William Herschel (1785), based on optical observations.
The shape of the Milky Way was believed to resemble a grindstone, with the sun close to the center

4. Strategies to explore the structure of our Milky Way
I. Select bright objects that you can see throughout the Milky Way and trace their directions and distances.
II. Observe objects at wavelengths other than visible (to circumvent the problem of optical obscuration), and catalog their directions and distances.
III. Trace the orbital velocities of objects in different directions relative to our position.

5. Determining the Structure of the Milky Way
Galactic Plane
Galactic Center
The structure of our Milky Way is hard to determine because:
1) We are inside.
2) Distance measurements are difficult.
3) Our view towards the center is obscured by gas and dust.

6. Measuring Distances: The Cepheid Method
The more luminous a Cepheid variable, the longer its pulsation period.
Instability Strip
Observing the period yields a measure of its luminosity and thus its distance!

7. The Cepheid Method
Allows us to measure the distances to star clusters throughout the Milky Way

8. Exploring the Galaxy Using Clusters of Stars
Two types of clusters of stars:
1) Open clusters = young clusters of recently formed stars; within the disk of the Galaxy
Open clusters h and c Persei
Globular Cluster M 19
2) Globular clusters = old, centrally concentrated clusters of stars; mostly in a halo around the galaxy

9. Globular Clusters Dense clusters of 50,000 – a million stars
Globular Cluster M80
Dense clusters of 50,000 – a million stars
Old (~ 11 billion years), lower-main-sequence stars
Approx. 200 globular clusters in our Milky Way

10. Locating the Center of the Milky Way
Distribution of globular clusters is not centered on the sun,
but on a location which is heavily obscured from direct (visual) observation.

11. The Structure of the Milky Way
75,000 light years
Disk
Nuclear Bulge
Sun
Halo
Open Clusters, O/B Associations
Globular Clusters

12. Infrared View of the Milky Way
Near-infrared image
Galactic plane
Interstellar dust (absorbing optical light) emits mostly infrared.
Nuclear bulge
Infrared emission is not strongly absorbed and provides a clear view throughout the Milky Way
Far-infrared image

13. Orbital Motions in the Milky Way (I)
Disk stars:
Nearly circular orbits in the disk of the galaxy
Halo stars:
Highly elliptical orbits; randomly oriented

14. Orbital Motions in the Milky Way (II)
Differential Rotation
Sun orbits around galactic center at 220 km/s
1 orbit takes approx. 240 million years.
Stars closer to the galactic center orbit faster.
Stars farther out orbit more slowly.

15. Mass determination from orbital velocity:
The more mass there is inside the orbit, the faster the sun has to orbit around the galactic center.
Combined mass:
M = 400 billion Msun
M = 100 billion Msun
M = 11 billion Msun
M = 4 billion Msun
M = 25 billion Msun

16. The Mass of the Milky Way
If all mass was concentrated in the center, Rotation curve would follow a modified version of Kepler’s 3rd law.
Rotation Curve = orbital velocity as function of radius.

17. The Mass of the Milky Way (II)
Total mass in the disk of the Milky Way:
Approx. 200 billion solar masses
Additional mass in an extended halo:
Total: Approx. 1 trillion solar masses
Most of the mass is not emitting any radiation:
dark matter!

18. The History of the Milky Way
The traditional theory:
Quasi-spherical gas cloud fragments into smaller pieces, forming the first, metal-poor stars (pop. II);
Rotating cloud collapses into a disk-like structure
Later populations of stars (pop. I) are restricted to the disk of the galaxy

19. Modifications of the Traditional Theory
Ages of stellar population may pose a problem to the traditional theory of the history of the Milky Way.
Possible solution: Later accumulation of gas, possibly due to mergers with smaller galaxies.
Recently discovered ring of stars around the Milky Way may be the remnant of such a merger.

20. Exploring the structure of the Milky Way with O/B Associations
O/B Associations
Perseus arm
Orion-Cygnus arm
Sun
Sagittarius arm
O/B Associations trace out 3 spiral arms near the sun.
Distances to O/B Associations determined using Cepheid variables

21. Neutral hydrogen concentrated in spiral arms
Radio Observations
21-cm radio observations reveal the distribution of neutral hydrogen throughout the galaxy.
Distances to hydrogen clouds determined through radial-velocity measurements (Doppler effect!)
Sun
Neutral hydrogen concentrated in spiral arms
Galactic center

22. The Structure of the Milky Way Revealed
Distribution of stars and neutral hydrogen
Distribution of dust
Sun
Bar
Ring

23. Star Formation in Spiral Arms (I)
Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts forming spiral arms trigger star formation.
Spiral arms are stationary shock waves, initiating star formation.

24. Star Formation in Spiral Arms (II)
Spiral arms are basically stationary shock waves.
Stars and gas clouds orbit around the galactic center and cross spiral arms.
Shocks initiate star formation.
Star formation self-sustaining through O/B ionization fronts and supernova shock waves.

25. Self-Sustained Star Formation in Spiral Arms
Star forming regions get elongated due to differential rotation.
Star formation is self-sustaining due to ionization fronts and supernova shocks.

26. Grand-Design Spiral Galaxies
Grand-design spirals have two dominant spiral arms.
Flocculent (woolly) galaxies also have spiral patterns, but no dominant pair of spiral arms.
NGC 300
M 100

27. Grand-design galaxy M 51 (Whirlpool Galaxy):
The Whirlpool Galaxy
Grand-design galaxy M 51 (Whirlpool Galaxy):
Self-sustaining star forming regions along spiral arm patterns are clearly visible.

28. The Galactic Center (I)
Our view (in visible light) towards the Galactic center (GC) is heavily obscured by gas and dust:
Extinction by 30 magnitudes
 Only 1 out of 1012 optical photons makes its way from the GC towards Earth!
galactic center
Wide-angle optical view of the GC region

29. Radio View of the Galactic Center
Many supernova remnants; shells and filaments
Arc
Sgr A
Sgr A
Sgr A*: The center of our galaxy
The galactic center contains a supermassive black hole of approx. 2.6 million solar masses.

30. Measuring the Mass of the Black Hole in the Center of the Milky Way
By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to be ~ 2.6 million solar masses.

31. X-Ray View of the Galactic Center
Galactic center region contains many black-hole and neutron-star X-ray binaries.
Supermassive black hole in the galactic center is unusually faint in X rays, compared to those in other galaxies.
Chandra X ray image of Sgr A*

32. Distance Measurements to Other Galaxies (I)
Cepheid method: Using period – Luminosity relation for classical Cepheids:
Measure Cepheid’s period  Find its luminosity  Compare to apparent magnitude  Find its distance
b) Type Ia supernovae (collapse of an accreting white dwarf in a binary system):
Type Ia supernovae have well known standard luminosities  Compare to apparent magnitudes  Find its distances
Both are “Standard-candle” methods:
Know absolute magnitude (luminosity)  compare to apparent magnitude  find distance.

33. Cepheid Distance Measurement
Repeated brightness measurements of a Cepheid allow the determination of the period and thus the absolute magnitude.
 distance

34. Distance Measurements to Other Galaxies (II): The Hubble Law
E. Hubble (1913):
Distant galaxies are moving away from our Milky Way, with a recession velocity, vr, proportional to their distance d:
vr = H0*d
H0 ≈ 70 km/s/Mpc is the Hubble constant.
=> Measure vr through the Doppler effect  infer the distance.

35. The Extragalactic Distance Scale
Many galaxies are typically millions or billions of parsecs from our galaxy.
Typical distance units:
Mpc = megaparsec = 1 million parsecs
Gpc = gigaparsec = 1 billion parsecs
Distances of Mpc or even Gpc  The light we see has left the galaxy millions or billions of years ago!!
 “Look-back times” of millions or billions of years

36. Galaxy Sizes and Luminosities
Vastly different sizes and luminosities:
From small, low-luminosity irregular galaxies (much smaller and less luminous than the Milky Way) to giant ellipticals and large spirals, a few times the Milky Way’s size and luminosity

37. Rotation Curves of Galaxies
From blue / red shift of spectral lines across the galaxy
 infer rotational velocity
Plot of rotational velocity vs. distance from the center of the galaxy:
rotation curve
Observe frequency of spectral lines across a galaxy.

38. Determining the Masses of Galaxies
Based on rotation curves, can use Newtonian Gravity to determine the
masses of galaxies

39. Supermassive Black Holes
From the measurement of stellar velocities near the center of a galaxy:
Infer mass in the very center  Central black holes!
Several million, up to more than a billion solar masses!
 Supermassive black holes

40. Dark Matter
Adding “visible” mass in
stars,
interstellar gas,
dust,
etc., we find that most of the mass is “invisible”!
The nature of this “dark matter” is not understood at this time.
Some ideas:
Brown dwarfs, small black holes, exotic elementary particles.

41. Clusters of Galaxies
Galaxies do not generally exist in isolation, but form larger clusters of galaxies.
Rich clusters:
1,000 or more galaxies, diameter of ~ 3 Mpc, condensed around a large, central galaxy
Poor clusters:
Less than 1,000 galaxies (often just a few), diameter of a few Mpc, generally not condensed towards the center

42. Hot Gas in Clusters of Galaxies
Space between galaxies is not empty, but filled with hot gas (observable in X rays)
That this gas remains gravitationally bound, provides further evidence for dark matter.
Visible light
X rays
Coma Cluster of Galaxies

43. Gravitational Lensing
The huge mass of gas in a cluster of galaxies can bend the light from a more distant galaxy.
Image of the galaxy is strongly distorted into arcs.

44. Our Galaxy Cluster: The Local Group
Milky Way
Andromeda Galaxy
Small Magellanic Cloud
Large Magellanic Cloud

45. Ultraluminous infrared galaxies
Starburst Galaxies
Starburst galaxies are often very rich in gas and dust; bright in infrared:
Ultraluminous infrared galaxies

46. Interacting Galaxies
Cartwheel Galaxy
Particularly in rich clusters, galaxies can collide and interact.
Galaxy collisions can produce
ring galaxies and
tidal tails.
NGC 4038/4039
Often triggering active star formation: Starburst galaxies

47. Tidal Tails Example for galaxy interaction with tidal tails: The Mice
Example for galaxy interaction with tidal tails:
The Mice
Computer simulations produce similar structures.

48. Simulations of Galaxy Interactions
Numerical simulations of galaxy interactions have been very successful in reproducing tidal interactions like bridges, tidal tails, and rings.

49. Galactic Interaction Simulations
Joshua Barnes:
John Dubinksi:
Chris Mihos:
Bob Berrington (Wyoming):
There are others…

50. Mergers of Galaxies
Radio image of M64: Central regions rotating backwards!
NGC 7252: Probably result of merger of two galaxies, ~ a billion years ago:
Small galaxy remnant in the center is rotating backwards!
Multiple nuclei in giant elliptical galaxies

51.  “active galactic nuclei” (= AGN)
Active Galaxies
Galaxies with extremely violent energy release in their nuclei (pl. of nucleus).
 “active galactic nuclei” (= AGN)
Up to many thousand times more luminous than the entire Milky Way; energy released within a region approx. the size of our solar system!

52. Line Spectra of Galaxies
Taking a spectrum of the light from a normal galaxy:
The light from the galaxy should be mostly star light, and should thus contain many absorption lines from the individual stellar spectra.

53. Seyfert Galaxies Unusual spiral galaxies: Very bright cores
Unusual spiral galaxies:
Very bright cores
Emission line spectra.
Variability: ~ 50 % in a few months
Most likely power source:
Accretion onto a supermassive black hole (~107 – 108 Msun)
NGC 1566
Circinus Galaxy
NGC 7742

54. Often: gas outflowing at high velocities, in opposite directions
Interacting Galaxies
Seyfert galaxy NGC 7674
Seyfert galaxy 3C219
Active galaxies are often associated with interacting galaxies, possibly result of recent galaxy mergers.
Often: gas outflowing at high velocities, in opposite directions

55. Cosmic Jets and Radio Lobes
Many active galaxies show powerful radio jets
Hot spots:
Energy in the jets is released in interaction with surrounding material
Radio image of Cygnus A
Material in the jets moves with almost the speed of light (“relativistic jets”).

56. Radio Galaxies
Centaurus A (“Cen A” = NGC 5128): the closest AGN to us.
Jet visible in radio and X-rays; show bright spots in similar locations.
Infrared image reveals warm gas near the nucleus.
Radio image superposed on optical image

57. Radio Galaxies (II)
Visual + radio image of 3C31
Radio image of 3C75
Radio image of NGC 1265
Evidence for the galaxy moving through intergalactic material
3C75: Evidence for two nuclei  recent galaxy merger

58. Formation of Radio Jets
Jets are powered by accretion of matter onto a supermassive black hole.
Black Hole
Accretion Disk
Twisted magnetic fields help to confine the material in the jet and to produce synchrotron radiation.

59. The Jets of M87
Jet: ~ 2.5 kpc long
M87 = Central, giant elliptical galaxy in the Virgo cluster of galaxies
Optical and radio observations detect a jet with velocities up to ~ 1/2 c.

60. The Dust Torus in NGC4261
Dust torus is directly visible with Hubble Space Telescope

61. Model for Seyfert Galaxies
Seyfert I:
Strong, broad emission lines from rapidly moving gas clouds near the black hole
Gas clouds
Emission lines
UV, X-rays
Seyfert II:
Weaker, narrow emission lines from more slowly moving gas clouds far from the black hole
Supermassive black hole
Accretion disk
dense dust torus

62. Other Types of AGN and AGN Unification
Observing direction
Cyg A (radio emission)
Radio Galaxy:
Powerful “radio lobes” at the end points of the jets, where power in the jets is dissipated.

63. Other Types of AGN and AGN Unification
Quasar or BL Lac object (properties very similar to quasars, but no emission lines)
Emission from the jet pointing towards us is enhanced (“Doppler boosting”) compared to the jet moving in the other direction (“counter jet”).
Observing direction

64. The Origin of Supermassive Black Holes
Most galaxies seem to harbor supermassive black holes in their centers.
Fed and fueled by stars and gas from the near-central environment
Galaxy interactions may enhance the flow of matter onto central black holes

65. Quasars
Active nuclei in elliptical galaxies with even more powerful central sources than Seyfert galaxies.
Also show strong variability over time scales of a few months.
Also show very strong, broad emission lines in their spectra.

66. Spectral lines show a large redshift of
The Spectra of Quasars
The Quasar 3C273
Spectral lines show a large redshift of
z = Dl / l0 = 0.158

67. Quasar Red Shifts z = Dl/l0 Our old formula Dl/l0 = vr/c
Quasars have been detected at the highest redshifts, up to
z ~ 6
z = 0
z = 0.178
z = 0.240
z = Dl/l0
z = 0.302
Our old formula
Dl/l0 = vr/c
is only valid in the limit of low speed, vr << c
z = 0.389

68. Studying Quasars
The study of high-redshift quasars allows astronomers to investigate questions of
1) Large scale structure of the universe
2) Early history of the universe
3) Galaxy evolution
4) Dark matter
Observing quasars at high redshifts
 distances of several Gpc
Look-back times of many billions of years
 Universe was only a few billion years old!

69. Probing Dark Matter with High-z Quasars: Gravitational Lensing
Light from a distant quasar is bent around a foreground galaxy
 two images of the same quasar!
Light from a quasar behind a galaxy cluster is bent by the mass in the cluster.
Use to probe the distribution of matter in the cluster.

70. Gravitational Lensing of Quasars
Gravitational Lensing of Quasars

71. Gallery of Quasar Host Galaxies
Elliptical galaxies; often merging / interacting galaxies