1. Chapter 15 Normal and Active Galaxies

2. M51 - Whirlpool Galaxy
Galaxies are among the grandest, most beautiful objects in the universe. They are colossal collections of typically a hundred billion stars, held together loosely by gravity. Despite their widespread existence in space, astronomers do not yet fully understand how galaxies originated. Here the graceful, winding arms of the majestic spiral galaxy, M51, also known as the Whirlpool Galaxy, resemble a curving staircase sweeping through space. This view, taken by the Hubble Space Telescope, shows the galaxy's yellowish central core, its reddish nebulae in the arms, and myriad stars throughout. (STScl)

3. Edwin Hubble Mt. Wilson observatory 100” (2.5 m) telescope
Categorized galaxies in 1924

4. Figure 15.1 Coma Cluster
(a) A collection of many galaxies, each consisting of hundreds of billions of stars. Called the Coma Cluster, this group of galaxies lies more than
100 million pc from Earth. (The blue spiked object at top right is a nearby star; virtually every other object in this image is a galaxy.) (b) A recent Hubble Space Telescope image of part of the cluster. (AURA; NASA)

5. Hubble classification scheme
Spirals
Barred spirals
Ellipticals
Irregulars

6. Spiral galaxies
Example: Milky Way

7. Spiral scheme Based on size of central bulge
Sa - largest bulge, tightly wrapped arms, least gas and dust
Sb - more open arms
Sc - smallest bulge, loose spiral structure, most gas and dust

8. Figure 15.2 Spiral Galaxy Shapes
Variation in shape among spiral galaxies. As we progress from type Sa to Sb to Sc, the bulges become smaller while the spiral arms tend to become less tightly wound. (R. Gendler; NOAO; D. Malin/AAT)

9. Tilted view Spiral structure can be hard to see on edge
Presence of disk with dust, gas and newborn stars signifies spiral

10. Figure 15.3 Sombrero Galaxy
The Sombrero Galaxy, a spiral system seen edge-on. Officially cataloged as M104, this galaxy has a dark band composed of interstellar gas and
dust. The large size of this galaxy’s central bulge marks it as type Sa, even though its spiral arms cannot be seen from our perspective. The inset
shows this galaxy in the infrared part of the spectrum, highlighting its dust content in false-colored pink. (NASA)

11. Barred spirals Elongated bar extending into disk
Spiral arms project from ends of bar
SBa, SBb, SBc
Milky Way intermediate between spiral and barred spiral

12. Figure 15.4 Barred-Spiral Galaxy Shapes
Variation in shape among barred-spiral galaxies. The variation from SBa to SBc is similar to that for the spirals in Figure 15.2, except that now the spiral arms begin at either end of a bar through the galactic center. In frame (c), the bright star is a foreground object in our own Galaxy; the object at top center is another galaxy that is probably interacting with NGC (NASA; AAT; ESO)

13. Elliptical galaxies No spiral arms No obvious disk
Dense central nucleus
E0 - most circular to
E7 - most elliptical
Classification depends on actual shape and orientation to earth

14. Figure 15.5 Elliptical Galaxy Shapes
Variation in shape among elliptical galaxies. (a) The E1 galaxy M49 is nearly circular in appearance. (b) M84 is a slightly more elongated elliptical galaxy, classified as E3. Both galaxies lack spiral structure, and neither shows evidence of cool interstellar dust or gas, although each has an extensive X-ray halo of hot gas that extends far beyond the visible portion of the galaxy. (c) M110 is a dwarf elliptical companion to the much larger Andromeda Galaxy. (AURA; SAO; R. Gendler)

15. Elliptical galaxy sizes
Sizes range from
Dwarf ellipticals - 1 kpc diameter, million stars
Giant ellipticals - few Mpc across, trillions of stars
Dwarfs more common by 10 to 1
Most of mass in ellipticals is in giants

16. Other elliptical properties
Little or no cool dust and gas
No young stars or star formation
Old reddish low mass stars
Large amounts of very hot interstellar gas

17. Intermediate between E7 & Sa
Thin disk and flattened bulge
No spiral structure
No gas and dust
S0 - no bar
SB0 - if bar present

18. Figure 15.6 S0 Galaxies
(a) S0 (or lenticular) galaxies contain a disk and a bulge, but no interstellar gas and no spiral arms. They are in many respects intermediate between E7 ellipticals and Sa spirals in their properties. (b) SB0 galaxies are similar to S0 galaxies, except for a bar of stellar material extending beyond the central bulge. (Palomar/Caltech)

19. Irregular galaxies Rich in interstellar matter and young stars
Lack regular structure
Irr I - look like misshapen spirals
Irr II - often explosive or filamentary appearance
Usually smaller than spirals, larger than dwarf ellipticals
Between 108 and 1010 stars

20. Magellanic Clouds Pair of Irr I galaxies Orbit our galaxy
50 kpc from center of our galaxy
Visible from southern hemisphere
Lots of dust, gas and blue stars
Also old stars and globulars

21. Figure 15.7 Magellanic Clouds
The Magellanic Clouds are prominent features of the night sky in the Southern Hemisphere. Named for the sixteenth-century Portuguese explorer Ferdinand Magellan, whose around-the-world expedition first brought word of these fuzzy patches of light to Europe, they are dwarf irregular (Irr I) galaxies, gravitationally bound to our own Milky Way Galaxy. They orbit our Galaxy and accompany it on its trek through the cosmos. (a) The Clouds’ relationship to one another in the southern sky. Both the Small (b) and the Large Magellanic Cloud (c) have distorted, irregular shapes, although some observers claim they can discern a single spiral arm in the Large Cloud. (F. Espenak; Harvard Observatory)

22. Figure 15.8 Irregular Galaxy Shapes
Some irregular (Irr II) galaxies. (a) The strangely shaped galaxy NGC 1427A is probably plunging headlong into a group of several other galaxies (not shown), causing huge rearrangements of its stars, gas and dust. (b) The galaxy M82 seems to show an explosive appearance, probably the result of a recent galaxy-wide burst of star formation. (NASA; Subaru)

23. Dwarf irregulars Most common irregular
Dwarf ellipticals and irregulars most common galaxies in universe
Often found close to a larger “parent” galaxy

24. Figure 15.9 Galactic “Tuning Fork”
Hubble’s tuning fork diagram, showing his basic galaxy classification scheme. The placement of the four basic types of galaxies—ellipticals, spirals, barred spirals, and irregulars—in the diagram is suggestive, but the tuning fork has no physical meaning.

25. Table 15.1 Basic galaxy Properties by Type

26. Measuring galactic distance
Cepheid variables to 25 Mpc
Need “standard candles” - astronomical objects of known luminosity
Luminosity + apparent brightness  distance

27. Tully-Fisher relation
Correlation between rotation speed and luminosity of spiral galaxies
Can be used out to about 200 Mpc
Other related correlations for ellipticals

28. Figure 15.10 Galaxy Rotation
A galaxy’s rotation causes some of the radiation it emits to be blueshifted and some to be redshifted (relative to what the emission would be from an unmoving source). From a distance, when the radiation from the galaxy is combined into a single beam and analyzed spectroscopically, the redshifted and blueshifted components combine to produce a broadening of the galaxy’s spectral lines. The amount of broadening is a direct measure of the rotation speed of the galaxy, such as the one at the right, NGC 4603, about 100 million light-years away. (NASA)

29. Type I supernovae Peak luminosity can be used as standard candle
Can be used out to 1 Gpc

30. Figure 15.11 Extragalactic Distance Ladder
An inverted pyramid summarizes the distance techniques used to study different realms of the universe. The techniques shown in the bottom four layers—radar ranging, stellar parallax, spectroscopic parallax, and variable stars—take us as far as the nearest galaxies. To go farther, we must use new techniques—the Tully–Fisher relation and the use of standard candles—based on distances determined by the four lowest techniques.

31. Local Group 45 galaxies in a local cluster Gravitationally attracted
3 spirals - Milky Way, Andromeda, M33
Remainder dwarf irregulars and ellipticals

32. The Local Group is made up of some 45 galaxies within approximately 1 Mpc of our Milky Way Galaxy. Only a few are spirals; most of the rest are dwarf-elliptical or irregular galaxies, only some of which are shown here. Spirals are colored blue, ellipticals pink, and irregulars white, all of them depicted approximately to scale. The inset map (top right) shows the Milky Way in relation to some of its satellite galaxies. The photographic
insets (top left) show two well-known neighbors of the Andromeda Galaxy (M31): the spiral galaxy M33 and the dwarf elliptical galaxy M32 (also visible in Figure 14.2a, a larger-scale view of the Andromeda system). (M. BenDaniel; NASA)
Figure Local Group

33. Virgo cluster
17-18 Mpc from Milky Way
2500 galaxies
3 Mpc across

34. Figure Virgo Cluster
The central region of the Virgo Cluster of galaxies, about 17 Mpc from Earth. Many large spiral and elliptical galaxies can be seen. The inset shows several galaxies surrounding the giant elliptical known as M86. An even bigger elliptical galaxy, M87 noted at bottom, will be discussed later in this
chapter. (M. BenDaniel; AURA)

35. Figure 15.14 Distant Galaxy Cluster - 2 Gpc distant
The galaxy cluster Abell 1689 contains huge numbers of galaxies and resides roughly 2 billion parsecs from Earth. Virtually every patch of light in this photograph is a separate galaxy. Thanks to the high resolution of the optics onboard the Hubble Space Telescope, we can now discern, even at this great distance, spiral structure in some of the galaxies. We also see many galaxies in collision—some tearing matter from one another, others merging into single systems. Despite the frenzied activity of individual galaxies, though, all off them recede from us—like buzzing fireflies in a moving jar. (NASA)

36. Universal Recession
Slipher found almost all spiral galaxies are redshifted
All galaxies, except some local ones, are receding
Motion of galaxies in a cluster is random
Clusters are receding, as are galaxies not part of a cluster

37. Analogy 15-1 Like fireflies in a moving jar

38. Figure Galaxy Spectra
Optical spectra, shown at left, of several galaxies named on the right. Both the extent of the redshift (denoted by the horizontal red arrows) and the distance from the Milky Way Galaxy to each galaxy (numbers in center column) increase from top to bottom. The vertical yellow arrow in each spectrum highlights a particular spectral feature (a pair of dark absorption lines). The horizontal red arrows indicate how this feature shifts to longer wavelengths in the spectra of more distant galaxies. The many white vertical lines at the top and bottom of each spectrum are laboratory references. (Palomar/Caltech)

39. Hubble’s Law
1920’s - Hubble plotted recessional velocity versus distance for galaxies
Hubble diagrams
Discovered rate at which a galaxy recedes is directly proportional to its distance from us - Hubble’s Law

40. Figure Hubble’s Law
Plots of recessional velocity against distance (a) for the galaxies shown in Figure and (b) for numerous other galaxies within about 1 billion pc of Earth.

41. Hubble’s Law Universal recession known as Hubble flow
Distances separating clusters and superclusters is expanding
Universe (space itself) is expanding
Individual objects are not expanding

42. Hubble’s Constant - H0 Recessional velocity = H0 X distance
Some measurements give H0 between km/s/Mpc
Other types give km/s/Mpc
We will use H0 = 70 km/s/Mpc

43. Top of distance ladder Use Hubble’s law to find distances
Redshift  recessional velocity  distance

44. Figure 15.17 Cosmic Distance Ladder
Hubble’s law tops the hierarchy of distance-measurement techniques. It is used to find the distances to astronomical objects all the way out to the limits of the observable universe.

45. Redshifts Largest redshifts greater than 6
Means wavelengths shifted 7X
UV spectral lines shifted to infrared
Such objects 9000 Mpc away

46. Redshifts and look-back time
Redshift of 6.0 means galaxy is receding at 96% of the speed of light
It is now 8420 Mpc = 27.5 billion light-years away
It was 12.7 billion light-years away when it emitted the light we see today
Its look-back time is 12.7 billion years
Light traveled 12.7 billion years to us

47. Table 15-2 Redshift, Distance, and Look-Back Time

48. Active galaxies More than 90% of all galaxies are normal
Few percent of all bright galaxies are active galaxies
Overall active are brighter than normal, and at more wavelengths
Normal galaxy - accumulated light of stars
Active galaxy - most of radiation nonstellar
Violent events in galactic nucleus

49. Figure 15.18 Galaxy Energy Distribution
The energy emitted by a normal galaxy differs significantly from that emitted by an active galaxy. This plot illustrates the general run of intensity for all galaxies of a particular type and does not represent any one individual galaxy.

50. Seyfert galaxies Discovered in 1943 by Carl Seyfert
Resemble normal spiral galaxies except Seyfert nucleus emits the most energy
Brightest Seyfert nuclei 10X brighter than entire Milky Way

51. Figure Seyfert Galaxy
The Circinus galaxy, a Seyfert with a bright compact core, lies some 4 Mpc away—it is one of the closest active galaxies. (NASA)

52. Seyfert spectrum Some produce from infrared to X-ray
75% emit most of their energy in infrared
Broad spectral lines - rapid internal motion
Varies in time within fraction of year - small energy producing region

53. Figure 15.20 Seyfert Time Variability
This graph illustrates the irregular variations of a Seyfert galaxy’s luminosity over two decades. Because this Seyfert, called 3C 84, emits strongly in the radio part of the electromagnetic spectrum, these observations were made with large radio telescopes. The optical and X-ray luminosities vary as well. (NRAO)

54. Radio galaxies
Active galaxies emitting large amounts of energy at radio wavelengths
Radio energy from huge radio lobes

55. Centaurus A radio galaxy
Visible light E2 galaxy with band of dust
Perhaps merger of spiral and elliptical
4 Mpc from earth
Radio lobes span half a Mpc
Lobes 10X size of Milky Way
Two lobed - roughly symmetrical

56. Figure 15.21 Centaurus A Radio Lobes
Radio galaxies, such as Centaurus A shown here optically in (a), often have giant radio-emitting lobes (b) extending a million parsecs or more beyond the central galaxy. The lobes cannot be imaged in visible light and are observable only with radio telescopes. The lobes are shown here in false color, with decreasing intensity from red to yellow to green to blue. (ESO; NRAO)

57. Figure 15.22 Centaurus A, Close Up
The main image (a) shows an optical photograph of Centaurus A, one of the most massive and peculiar galaxies known. The pastel false colors mark the radio emission shown in Figure 15.21; the data in this case were more recently acquired and have higher resolution. (b) Although the radio jets emit no visible light, they do emit X rays, as shown in this Chandra image. (c) Increasingly high-resolution optical views of the galaxy’s core region, taken by the Hubble Space Telescope. (NASA; SAO; J. Burns)

58. Figure Cygnus A
(a) Cygnus A also appears to be two galaxies in collision. (b) On a much larger scale, it displays radio-emitting lobes on either side of the optical image. The optical galaxy in (a) is about the size of the small dot at the center of (b). Note the thin line of radio-emitting material joining the right lobe to the central galaxy. The distance from one lobe to the other is approximately a million light-years. (NOAO; NRAO)

59. Radio galaxies Radio lobes emit roughly 10X energy of entire Milky Way
Nucleus emits up to 100X energy of radio emission
Total emission up to 1000X of Milky Way
Not all have radio lobes - depends on view

60. Figure 15.24 Core-Dominated Radio Galaxy
On this radio contour map of the radio galaxy M86, we can see that the radio emission comes from a bright central nucleus, which is surrounded by an extended, less-intense radio halo. The radio map is superimposed on an optical image of the galaxy and some of its neighbors, a wider-field
version of which was shown previously in Figure (Harvard-Smithsonian Center for Astrophysics)

61. Figure 15.25 Radio Galaxy - view determines lobe visibility
A central energy source produces high-speed jets of matter that interact with intergalactic gas to form radio lobes. The system may appear to us as either radio lobes or a core–dominated radio galaxy, depending on our location with respect to the jets and lobes.

62. Common active features
Huge energy generation in compact nucleus
Evidence of interacting galaxies
Many contain jets
If view jets end-on, see intense doppler shifted radiation
This is a blazar - X or gamma rays

63. Figure M87 Jet
The giant elliptical galaxy M87 (also called Virgo A) is displayed here in a series of zooms. (a) A long optical exposure of the galaxy’s halo and embedded central region. (b) A short optical exposure of its core and an intriguing jet of matter, on a somewhat smaller scale. (c) An infrared image of M87’s jet, examined more closely compared with (b). The bright point at left in (c) marks the bright nucleus of the galaxy; the bright blob near the center of the image corresponds to the bright “knot” visible in the jet in (b). (NOAO; NASA)

64. Quasars
In 1960, faint blue star like object located with radio source 3C48
C spectral lines found to be redshifted 16% - moving at 48,000 km/s
3C 48 redshifted 37%

65. Figure Quasar 3C 273
a) The bright quasar 3C 273 displays a luminous jet of matter, but the main body of the quasar is starlike in appearance. (b) The jet extends for
about 30 kpc and can be seen better in this high-resolution image. (AURA)

66. Quasars Large redshift - enormous distance 3C 48 is 1400 Mpc away
3C 273 has luminosity of 1040 W
Comparable to 20 trillion suns or 1000 Milky Way galaxies
Quasars range from 1038 W to 1042 W

67. Figure 15.28 Quasar Spectrum
Optical spectrum of the distant quasar 3C 273. Notice both the redshift and the widths of the three hydrogen spectral lines marked as Hβ, Hγ, and Hδ. •(Sec. 2.6) The redshift indicates the quasar’s enormous distance. The width of the lines implies rapid internal motion within the quasar. (Adapted from Palomar/Caltech)

68. Quasars Quasars look star-like because of great distance
Quasi-stellar radio sources, shortened to quasars
Quasi-stellar object or QSO is more common today
More than 30,000 quasars known
250 Mpc to 9000 Mpc away

69. Quasars Only seen at great distances Means long ago in time
Perhaps early phase of galaxy formation
Variable over short periods (days or hours)
Jets
Bright cores of distant galaxies too faint to see

70. Figure Quasar Jets
Radio image of the quasar 3C 175, showing radio jets feeding faint radio lobes. The bright (white) central object is the quasar, some 3000 Mpc
away. The lobes themselves span approximately a million light-years. (NRAO)

71. Active galactic nuclei features
High luminosities, mostly nonstellar
Highly variable (small region)
Jets and explosive activity
Optical spectra with broad emission lines

72. Active galaxy energy production
Supermassive black hole M
Accretion disk - infalling matter heated by friction
Emits large amounts of radiation
Jets - ejected material in magnetic fields along axis of accretion disk

73. Figure 15.30 Active Galactic Nucleus
The leading theory for the energy source in active galactic nuclei holds that these objects are powered by material accreting onto a supermassive black hole. As matter spirals toward the hole, it heats up, producing large amounts of energy. At the same time, high-speed jets of gas may be ejected perpendicular to the accretion disk, forming the jets and lobes seen in many active objects. Magnetic fields generated in the disk are carried by the jets out to the radio lobes, where they play a crucial role in producing the observed radiation.

74. Active galaxy energy production
10% to 20% of infalling mass-energy converts to radiation
One M consumed per decade can power 1038 W active galaxy

75. Figure 15.31 Giant Elliptical Galaxy
(a) A combined optical/radio image of the giant elliptical galaxy NGC 4261, in the Virgo Cluster, shows a white visible galaxy at center, from which blue-orange (false color) radio lobes extend for about 60 kpc. (b) A close-up photograph of the galaxy’s nucleus reveals a 100-pc-diameter disk surrounding a bright hub thought to harbor a black hole. (NRAO; NASA)

76. Figure M87 Disk
Recent imaging and spectroscopic observations of M87 support the idea of a rapidly whirling accretion disk at its heart. (a) An image of the central region of M87, similar to that shown in Figure 15.26c, shows its bright nucleus and jet. (b) A magnified view of the nucleus suggests a spiral swarm of stars, gas, and dust. (c) Spectral-line features observed on opposite sides of the nucleus show opposite Doppler shifts, implying that material on one side of the nucleus is coming toward us and material on the other side is moving away from us. The strong implication is that an accretion disk spins perpendicular to the jet and that at its center is a black hole having some three billion times the mass of the Sun. (NASA)

77. Energy Emission Emitted energy at broad range of wavelengths
Some radiation absorbed and reemitted by dust in surrounding disk

78. Figure Dusty Donut
The accretion disk surrounding a massive black hole, drawn here with some artistic license, consists of hot gas at many different temperatures (hottest nearest the center). When viewed from above or below, the disk radiates a broad spectrum of electromagnetic energy extending into the X ray band. However, the dusty infalling gas that ultimately powers the system is thought to form a rather fat, donut-shaped region outside the accretion disk (shown here in dull red), which effectively absorbs much of the high-energy radiation reaching it, reemitting it mainly in the form of cooler, infrared radiation. Thus, when viewed from the side, strong infrared emission is observed. The appearance of the jets, radiating mostly in radio and X-rays, also depends on the viewing angle (see Figure 15.25). (Adapted from D. Berry)

79. Synchrotron radiation
Ejected charged particles in jet spiral around magnetic field at high speeds
This is nonthermal radiation
Jet slowed by intergalactic medium, magnetic field tangled, forms radio lobes

80. Figure 15.34 Nonthermal Radiation
(a) Charged particles, especially fast-moving electrons (red), emit synchrotron radiation (blue) while spiraling in a magnetic field (black). This process is not confined to active galaxies. It occurs, on smaller scales, when charged particles interact with magnetism in Earth’s Van Allen belts •(Sec. 5.6), when charged matter arches above sunspots on the Sun •(Sec. 9.4), in the vicinity of neutron stars •(Sec. 13.2), and at the center of our own Galaxy. •(Sec. 14.7) (b) Variation of the intensity of thermal and synchrotron (nonthermal) radiation with frequency. Thermal radiation,
described by a blackbody curve, peaks at some frequency that depends on the temperature of the source. Nonthermal synchrotron radiation, by contrast, is more intense at low frequencies and is independent of the temperature of the emitting object. Compare this figure with Figure