Galaxies
Building Blocks of the Universe

April 12, 2017, jeb

Outline Discovery of Galaxies outside the Milky Way Hubble's Galaxy Classification Summary of Galaxy Types Frequency of Galaxy Types The Distribution of Galaxies in Space Hubble's Law Active Galaxies Seyfert Galaxies Radio Galaxies Quasi-stellar Objects The Central Engine of an Active Galaxy Summary

Discovery of Galaxies outside the Milky Way

• 1917
  • Harlow Shapley
    
    
  • Determines galactic center in the direction of Sagittarius and size of the Milky Way ( variables as standard candles in globular clusters)

• 1920
  • Shapley - Curtis Debate
    
  • Harlow Shapley (now famous) argues spiral nebulae are in the Milky Way
    • Shapley argued that these "spiral nebulae" were just nearby gas clouds, and that the Universe was composed of only one big Galaxy.
    
    
  • Heber Curtis argues spiral nebulae are "island universes" outside the Milky Way
    • Curtis argued that the Universe is composed of many galaxies like our own, identified as "spiral nebulae".

• 1923
  • Edwin Hubble
    
    
  • Modern photo of Andromeda (M31)
  • Cepheid variables in Andromeda are resolved
    • recall Cepheid variables are standard candles
  • M31 is about 2,500,000 light-years away
    • outside the Milky Way
      Curtis is proven right

• Re-examine the catalog of Messier Objects (which was created in the 18th century)
  • many turn out to be galaxies, outside the Milky Way

Hubble's Galaxy Classification

The Coma Cluster is one example of a collection of many galaxies, each consisting of hundreds of billions of stars
• Many different types of galaxies

The American astronomer Edwin Hubble was the first to categorize galaxies in a comprehensive way:
• Spirals
• Barred spirals
• Ellipticals
• Irregulars


Spiral and Barred-spiral Galaxies


classified by size of central bulge
• Sa have largest central bulge
  example is the Sombrero Galaxy
• Sb have mid size central bulge
• Sc have smallest central bulge

characteristic features:
• Two or more arms
• Much gas and dust
• O & B stars found
• Size typically 100,000 light year diameter
• Bluish disk, reddish halo & nucleus
• Young stars in the disk (population I)

variation of the spiral is the barred-spiral galaxy
• feature elongated "bar " of stellar and interstellar material

barred-spirals are also classified by size of central bulge
• SBa have largest central bulge
• SBb have mid size central bulge
• SBc have smallest central bulge

Recent findings that the bulge of our own Milky Way Galaxy suggest that it is a barred spiral of type SBb or SBc


Ellipticals


characteristic features:
• No arms
• Little gas or dust
• O & B stars absent
• Sizes range:
  • Some as big as a few million light-year diameter (giant)
    • Largest galaxies
    • trillions (> 10¹²) stars
  • Some only thousands of light-year diameters (dwarfs)
  • Dwarfs outnumber giants by 10 to 1
• Reddish in color
• Mostly old stars (population II)
• Many globular clusters, up to 13,000
  • For example, Milky Way is know to have about 150
• Shapes range from highly elongated to nearly circular
  • Most circular E0
  • Most elongated E7
  • Others E1, E2, E3, E4, E5, E6 (from slightly elongated on up)

S0 and SB0 Galaxies (also known as Lenticular Galaxies)
• Intermediate between E7 and Sa
• thin disk and a flattened bulge, but no gas and no spiral arms


Irregulars


• characteristic features
  • oddly shaped galaxies
  • rich in interstellar matter and young stars
  • no spiral arms
  • no central bulge
  • no regular structure
  
  
• Magellanic Clouds
  • Irregular galaxies orbiting the Milky Way Galaxy
  • Gravitationally bound to the Milky Way
  • Prominent features in the sky in the Southern Hemisphere
  • Large Magellanic Cloud was the site of the supernova SN1987A

Summary of Galaxy Types

Hubble's "tuning fork" diagram



TABLE 24.1 Basic Galaxy Properties by Type

SPIRAL/BARRED SPIRAL (S, SB) ELLIPTICAL (E) IRREGULAR (IRR) Shape and structural properties Highly flattened disk of stars and gas, containing spiral arms and thickening to a central bulge; Sa and Sba galaxies have largest bulges, the least obvious spiral structure, and roughly spherical stellar halos. SB galaxies have an elongated central "bar" of stars and gas No disk Stars smoothly distributed through an ellipsoidal volume ranging from nearly spherical (E0) to very flattened (E7) in shape No obvious substructure other than a dense central nucleus No obvious structure; Irr II galaxies often have "explosive" appearance Stellar content Disks contain both young and old stars; halos consist of old stars only Contain old stars only Contain both young and old stars Gas and dust Disks contain substantial amounts of gas and dust; halos contain little of either Contain little or no gas and dust Very abundant in gas and dust Star formation Ongoing star formation in spiral arms No significant star formation during the last 10 billion years Vigorous ongoing star formation Stellar motion Gas and stars in disk move in circular orbits around the galactic center; halo stars have random orbits in three dimensions Star have random orbits in three dimensions Stars and gas have very irregular orbits.

Frequency of Galaxy Types

One survey of galaxies in the Universe reports the following distribution:



Types of Galaxies

Spiral 77% Elliptical 20% Irregular 3%


From this table it would appear that irregular galaxies are not very common
However, irregular galaxies tend to be small, and not very bright

We can remove the bias against faint galaxies by looking at the galaxies "nearby", where even faint galaxies are visible

A survey of galaxies within 30,000,000 light years of the Milky Way reports the following distribution:



Types of Galaxies within 30,000,000 light years

Spiral 34% Elliptical 13% Irregular 54%



Conclusion:
The majority of galaxies in the universe are low luminosity, irregular galaxies
• they don't show up in the full Universe survey above because of their low luminosity
• but nearby, where they are visible, they dominate

The Distribution of Galaxies in Space

Extending the Distance Scale
• Current estimate for the number of galaxies in the observable universe:
  40 billion (4 x 10¹⁰)
• Measuring the distances
  • Cepheid variable technique
    • works out to 25 Mpc, or so (about 80 million light-years)
  • Other standard candles
    • novae
    • nebulae
    • globular clusters
    • supernovae
    • entire galaxies
  • Tully-Fisher
    • determine absolute luminosity of galaxy
    • rotational velocity (measured by broadening) related to total mass, and therefore, luminosity


Galaxy Clusters
• Nearly 50 galaxies populate the local neighborhood of the Milky Way
  • Major astronomical objects within about 1 Mpc (1,000,000 pc = 3,300,000 light-years) of the Milky Way Galaxy
  • These galaxies are bound together by their combined gravity
  • Such a group of galaxies is called a galaxy cluster
  • Our cluster is called the Local Group
  
  
• Composition of the Local Group (nearly 50 galaxies)
  • 3 spirals
    • the Milky Way, Andromeda (M31), and M33
  • irregulars
    • Large Magellanic Cloud and Small Magellanic Cloud
  • dwarf irregulars or dwarf ellipticals
    • M32 is a dwarf elliptical near Andromeda (M31)
  
  
• The Local Cluster is a galaxy "poor" cluster
  • this means there are a relatively small number of galaxies in the Local Group
  
  
• Rich clusters (such as Virgo and Coma) - thousands of galaxies
  • Central region of the Virgo Cluster, about 17 Mpc from Earth
  • Distant rich galaxy cluster, Abell 1689, located roughly two billion light years from Earth

Hubble's Law

• Galaxies in all directions are moving away from us
  • The light they send us is redshifted when we see it
    • Hubble discovered this redshift in the 1920's
• Their velocities are proportional to distance from us
  • farthest galaxies are moving away fastest
  • This is Hubble's Law
    • The rate at which a galaxy recedes is directly proportional to its distance from us
  • This was determined from Doppler effect
    • A redshift resulting from Hubble flow is called the cosmological redshift

• The whole universe is expanding
  • Galaxies are moving away from all points in space - not just us

• The light from the most distant observed galaxies has traveled for about 13 billion years before reaching Earth.
  • Look-back time - 13 billion years.
  • Consequence -> The universe is at least this old.

• To establish Hubble's Law we must:
  1. Measure velocity of galaxies --> Doppler Effect
  2. Measure distance to galaxy
     • Use "Standard Candle"

recessional velocity = H0 × distance

• H₀ is Hubble's constant
• It is the slope of the straight line on the plot of velocity vs. distance
• H₀ = 70 km/s/Mpc

Active Galaxies

Some galaxies are more luminous (and more active) than normal galaxies
• These are known as Active Galaxies
  • M87
    • Central galaxy in the Virgo cluster (Virgo A)
      • 60 Mly away
    • more
  • Centaurus A
    • Closest radio galaxy
      • 15 Mly away
    • more
  • Active Galaxy - NGC 7742
    • ring of blue star-forming region surrounding a very bright yellow core spanning about 1 kpc
      • 24 Mpc away
    • Hubble Image


The most energetic active galaxies can emit hundreds or thousands of times more energy per second than the Milky Way

The nature of the energy emitted from a normal galaxy differs from that of an active galaxy
• Plot of wavelength dependence
• A normal galaxy has a spectrum that looks like a collection of stars following the blackbody curve
  • The sum of blackbody curves of many stars of different temperatures

An Active Galaxy will have large emission of electromagnetic radiation in the infrared, radio, ultraviolet, and X-ray
• That is large compared to a normal galaxy like the Milky Way

An Active Galaxy will be characterized by
1. higher than normal luminosity
2. non-thermal emission
3. rapid variability
4. peculiar appearance
   • nucleus and large structures
5. jets from nucleus
6. emission lines broad or narrow
7. polarized emission

Synchrotron Radiation
• When charged particles accelerate they emit electromagnetic radiation (Synchrotron radiation)
  • Radio transmitter
  • Magnetic fields bend the paths of charged particles
    => acceleration => radiation
• Synchrotron radiation is more intense at low frequency and independent of temperature of the emitting object

Types of Active Galaxies
• Seyfert Galaxies
• Radio Galaxies
• Quasars (Quasi-stellar Objects)
  • very distant objects

A small percentage of all galaxies are active galaxies
• Most of the active galaxies are radio galaxies

The fraction of galaxies which are active galaxies increases with increasing distance from the Milky Way
• galaxies were generally more active in the past

Seyfert Galaxies

Seyfert Galaxies are spiral galaxies with extremely bright nuclei
• Nuclei 10,000 times as bright as the Milky Way

Seyfert Galaxies emit a lot of infrared and radio energy
• This is not "star-like"
  • That is, not like blackbody radiation

Seyfert Galaxies have broad emission lines
• Atoms emit photons of specific colors
  • The emission spectra of some well-known elements
• If the atoms are moving rapidly, the lines will be broadened by Doppler Effect
• The atoms can be moving rapidly if the galaxy is hot, or if it is rotating rapidly
• Therefore, we conclude, the Seyfert Galaxies are hot or rotating rapidly

Seyfert Galaxies have small nuclei ( few light years)
• The irregular variations of a particular Seyfert galaxy's luminosity over a period of two decades.
  • Because this Seyfert, called 3C 84, emits most 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

Radio Galaxies

Most active galaxies are Radio Galaxies
1. Compact radio galaxies
   • radio emission is same size or smaller than the optical image
   • core-halo radio galaxies
     • Radio contour map of a typical core-halo radio galaxy superimposed on an optical image
       • The radio emission from such a galaxy comes from a bright central nucleus, or core, surrounded by an extended, less intense halo
     
     
2. Extended radio galaxies
   • radio emission is larger than the optical image
   • lobe radio galaxies
     • Centaurus A with giant radio-emitting regions 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
       • In this image, the lobes are filled with false color to indicate decreasing intensity from red to yellow to green to blue
     
     

The giant elliptical galaxy M87 (also called Virgo A)
• image is different at different wavelengths
• A long optical exposure of its halo and embedded central region
• An expanded short optical exposure of its core and an intriguing jet of matter
• An expanded infrared image of the jet
  • The red dot at left marks the bright nucleus of the galaxy; the red and yellow blob near the center of the image corresponds to the bright "knot" visible in the jet image above

Centaurus A
• One of the most massive and peculiar galaxies known
• Believed to be the result of a galaxy collision some 500 million years ago
• superposition of the radio emission on the optical image, and inset image of X-ray emission from the inner part of the lobe
• Note that the radio lobes emit no visible light

Cygnus A
• also appears to be two galaxies in collision
• On a much larger scale, this cosmic object displays radio-emitting lobes on each side of the optical image
• The optical galaxy is about the size of the small dot at the center of the "radiograph"
• A thin line of radio-emitting material joins the right lobe to the central galaxy
• The distance from one lobe to the other is approximately a million light years

Two radio images of the active "head-tail" galaxy NGC 1265
• A radiograph, in false color
• and the same radio data, in contour form, superposed on the optical image of the galaxy
• This object is likely moving rapidly through space, trailing a "tail" behind as it goes
  • a little like a comet, but on a vastly larger scale

Perhaps the core-halo and the lobe galaxies are largely a matter of perspective
• 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 a lobe or a core-halo radio galaxy, depending on our location with respect to the jets and lobes

Quasi-stellar Objects

Quasars are radio sources not associated with an obvious visible source
• Source appears to be "innocent" star-like object
• But the star-like object has a very large redshift
  • The emission lines are shifted to the red
• This redshift comes from the expansion of the universe
  • cosmological redshift
• Hubble's Law then tells us that the quasars are at great distances

Redshifts
• If light is emitted with a true wavelength but appears to have a different observed wavelength due to the cosmological redshift, we say it has
  • redshift = ( observed wavelength - true wavelength ) / true wavelength

TABLE 25.1 Redshift, Distance, and Look-Back Time REDSHIFT v/c PRESENT DISTANCE LOOK-BACK TIME (millions of years) (Mpc) (106 ly) 0.000 0.000 0 0 0 0.010 0.010 42 137 137 0.025 0.025 105 343 338 0.050 0.049 209 682 665 0.100 0.095 413 1350 1290 0.200 0.180 809 2640 2410 0.250 0.220 999 3260 2920 0.500 0.385 1880 6140 5020 0.750 0.508 2650 8640 6570 1.0 0.600 3320 10,800 7730 1.5 0.724 4400 14,400 9320 2.0 0.800 5250 17,100 10,300 3.0 0.882 6460 21,100 11,500 4.0 0.923 7310 23,800 12,100 5.0 0.946 7940 25,900 12,500 6.0 0.960 8420 27,500 12,700 10.0 0.984 9660 31,500 13,200 50.0 0.999 12,300 40,100 13,600 100.0 1.000 12,900 42,200 13,700 infinite 1.000 14,600 47,500 13,700		These are estimates of look-back times, which depend on the value of the Hubble constant and the nature of the universe. These estimates assume the best understanding of Hubble's constant (70 km/s/Mpc) and a FLAT universe, in which matter (mostly dark) contributes just over one-quarter of the total density.



Important properties of Quasars
1. Very large redshifts
   • Some larger than 4
2. Very short period brightness changes
   • can be just days, or even hours

These properties imply
1. Very great distances
   • From Hubble's Law
2. Very small size
   • Like the size of the Solar System

3C 273
• 273rd object in the Third Cambridge Catalogue of radio sources
• Strong radio source identified with a faint starlike object in 1963
• emission lines puzzled astronomers (could not identify)
  • Optical spectrum of the distant quasar 3C 273
• Then it was realized this was a red shifted spectrum
• The redshift indicates the quasar's enormous distance
• The width of the three hydrogen spectral lines (H-beta , H-gamma, and H-delta) implies rapid internal motion within the quasar itself

3C 275 and QSO 1229+204
• Optical photograph of 3C 275 (one of the first quasars discovered) and a field of quasars
  • 3C 275's starlike appearance shows no obvious structure and gives little outward indication of this object's enormous luminosity
  • However, 3C 275 has a much larger redshift than any of the other stars or galaxies in this image; it has a look-back time of about 7 billion years.
  • The optical image of the field of quasars includes QSO 1229+204, one of the most powerful quasars yet discovered, shown enlarged
  • The starlike appearance of QSO 1229+204 shows only a hint of structure and gives little outward indication of the object's enormous luminosity
    • Like 3C 275, QSO 1229+204's look-back time is large, about 7 billion years. (2000 Mpc)

Although quasars are the most luminous objects in the universe, they are often rather unimpressive in appearance
• Optical image of a distant quasar seen close (on the sky) to a nearby spiral galaxy
• The quasar's much greater distance makes it appear much fainter than the galaxy

The bright quasar 3C 273 displays a luminous jet of matter, but the main body of the quasar is starlike in appearance
• The jet extends for about 100,000 light years (30 kpc)

Radio image of the quasar 2300-189 showing radio jets feeding faint radio lobes
• This quasar is some 10 billion light years (3000 Mpc) away
• The lobes span about 1,000,000 light-years

More than 30,000 quasars are known
with distances ranging from 240 Mpc to 9000 Mpc,
mostly over 1000 Mpc, or in the distant past.

The big mystery of Quasars
• How can so much power (10,000 times that of a normal galaxy) be created in such a small space (light year or less)
  • The volume is less than a trillionth of the volume of the nuclear bulge of the Milky Way Galaxy

The Central Engine of an Active Galaxy

The leading theory for the energy source in active galactic nuclei (and quasars)
• 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 beams 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

A combined optical/radio image of the giant elliptical galaxy NGC 4261, in the Virgo Cluster,
• A white visible galaxy at center, from which red-orange (false color) radio lobes extend for about 200,000 light years (60 kpc)
• The close-up photograph of the galaxy's core reveals a 300 light year diameter disk surrounding a bright hub that presumably harbors a black hole

Imaging and spectroscopic observations of M87 support the idea of a rapidly whirling accretion disk at its core
• An image of the central region of M87 shows its bright core and jet
• The magnified view of the core suggests a spiral swarm of stars, gas, and dust
• Spectral-line features observed on opposite sides of the core show opposite Doppler shifts
  • implying that the material there is coming toward us on one side and moving away from us on the other
• The strong implication is that an accretion disk spins perpendicular to the jet
  • and that at its center is a black hole having some 3 billion times the mass of the Sun

A network of radio telescopes has probed the core of the spiral galaxy NGC 4258
• NGC 4258 in the light of mostly hydrogen emission
  • Within the innermost 0.7 light year observations of Doppler-shifted molecular clouds (designated by red, green, and blue dots) show
    • They obey Kepler's third law perfectly
    • This reveals a slightly warped disk of rotating gas
    • At the center of the disk presumably lurks a huge black hole

Energy Emission
Appearance depends on viewing angle due to different emission processes
• Charged particles, especially fast electrons, emit synchrotron radiation while spiraling along magnetic field lines
  
  The nature of synchrotron radiation is much different from blackbody radiation
  • Variation in 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 most intense at low frequencies
  • It is independent of the temperature of the emitting object

Summary

• Hubble's galaxy classification scheme divides galaxies into classes based on appearance Spiral galaxies
• Barred spiral galaxies
• Elliptical galaxies
• S0 and SB0 galaxies
• Irregular galaxies