Normal Galaxies
The Large-Scale Structure of the Universe

posted Apr 11, 2001, jeb
Apr 17, 2001, jeb
Apr 18, 2001, 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
• Galaxy Masses
• Galaxy Formation and Evolution
• Hubble's Law
• Supplemental Readings and References

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 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
  • M31 is about 2,000,000 light-years away
    • outside the Milky Way
      Curtis is proven right

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

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Hubble's Galaxy Classification

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

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

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:
  • few million light year diameter (giant)
    • trillions (10¹²) stars
  • thousands of light years (dwarfs)
  • dwarfs outnumber giants by 10 to 1
• reddish in color
• mostly old stars
• 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
• Intermediate between E7 and Sa
• thin disk and a flattend 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
  • 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 15 Mpc, or so (50 million light-years)
  • Other standard candles
    • novae
    • nebulae
    • globular clusters
    • supernovae
    • entire galaxies
  • Tully-Fisher


Galaxy Clusters
• Some 20-30 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
  • 3 spirals
    • the Milky Way, Andromeda (M31), and M33
  • 4 irregulars
    • Large Magellanic Cloud and Small Magellanic Cloud
  • 4 ellipticals
  • 16 dwarf irregulars or dwarf ellipticals
    • photos of M33(a spiral galaxy) and M32(a dwarf elliptical), both close neighbors of 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 18 Mpc from Earth
    • schematic illustration of the relative locations of several galaxy clusters in our part of the universe


Clusters of Clusters
• The galaxy clusters appear to be clustered into superclusters
  • The Local Supercluster
• clustering goes on deep into space
  • the Coma Cluster more than 100 Mpc from Earth
  • the galaxy cluster CL1358+62 about a billion parsecs from Earth, photographed by the Hubble Space Telescope
  
  
  

Galaxy Masses

Mass Measurement
• Rotation curves of galaxies are measured by observing the Doppler shift of light versus distance from the galactic center
• As with the the Milky Way, these curves show an anomaly from what is expected based on the distribution of visible mass
• the Binary galaxy systems and galaxy clusters also reveal mass by observing the motion using Doppler shifts


Dark Matter in the Universe
• The rotation curves of spiral galaxies suggest an invisible dark halo
• Velocities within galaxy clusters suggest a lot more mass than we can see is needed to bind the galaxies together
• Upward of 90 percent of the universe appears to be composed of dark matter and dark energy


• What is the dark matter?
  • stellar remnants?
  • exotic subatomic particles?
  • we do not yet know
  
  
• What is dark energy?
  • not understood
  • accelerating expansion of the universe


Intracluster Gas
• Intergalactic Matter (between galaxies)
  • Interstellar medium (between stars within a galaxy)
    • about 1 atom per cubic cm
  • Intergalactic medium (between galaxies within the cluster)
    • about 10^-6 atoms/cm³, or about 1 atom per cubic meter
  • Intercluster medium (between galactic clusters)
    • very little matter
• Hot gas is observed from X-ray emission ( see Abell 85 and Abell 2256, for example) and extends into the space between galaxies, but it does not add up sufficiently to solve the missing matter problem above
• The amount of mass required to bind this gas to the clusters the ( see the Virgo Cluster) greatly exceeds the total visible mass of the galaxies
• There is no evidence for such gas beyond the clusters, in the "extracluster" spaces

Galaxy Formation and Evolution

Motion of galaxies in clusters often leads to close encounters or collisions.
• Over time, it is thought that large galaxies are built up from smaller ones through collisions and mergers
• The Hubble Deep Field image shows numerous small, irregularly shaped galaxies at distances of thousands of megaparsecs from Earth
• Images reveal galactic mergers were common ten billion years ago

These close encounters and collisions lead to:
• Starburst Galaxies
  • Rapid star formation
  • NGC 1275 is an example of globular cluster formation initiated by the collision of two galaxies
• Ejection of Stars
  • Cartwheel galaxy
  • The antennae
    • computer simulation
  • High resolution image from Hubble Space Telescope clearly shows star formation
• Galactic Cannibalism
  • Brightest galaxies devour weaker ones
• Shape distortion
  • The Whirlpool Galaxy, M51
  • computer simulation

Why is a given galaxy elliptical or spiral?
• Nature vs. Nurture
(Genes vs. Environment)


1. Is it in the inherent Nature of a given galaxy?
   • A plot of the star formation in time shows stars are formed earlier in an elliptical galaxy than in a spiral galaxy
   • So, elliptical uses up gas before it can collapse into disk.
   • Spirals use gas slower --> a disk can form
     
     This suggests early differentiation of ellipticals vs. spirals
     Suggesting the Nature of the galaxy is key
   
   
   
2. Is is that Nature and Nurture can be important?
   • Rich regular galactic clusters dominated by ellipticals.
   • Poor irregular galactic clusters, many spirals.
   • WHY?
     Dense region of galaxies could aid either Nature or Nuture effects
     • Rapid star formation (Nature)
     • Many collisions, cannibalism (Nurture)
   
   
   
3. Is it caused by the Nurture of the given galaxy?
   • Largest galaxies are Elliptical
     • show telltale signs of mergers - ripples, tails, shells
     So this suggests the Nurture of the galaxy is key

Ellipticals vs. spirals


Nature, Nurture, or Both


Probably both are important

Hubble's Law

• Galaxies in all directions are moving away from us
  • The light they send us is red-shifted when we see it
• 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
    • This 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

• Most distant observed galaxies about 15 billion light years away.
  • Look-back time - 15 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 x distance

• H₀ is Hubble's constant
• It is the slope of the straight line on the plot of velocity vs. distance
• H₀ = 45-90 km/s/Mpc
  • this is hard to measure precisely
  • best guess = 65 km/s/Mpc

• On very large scales the galaxies are not distributed randomly
  • "Bubbles" of galaxies surround regions of little matter ( voids)
  • the Great Wall is one of the largest known structure in the universe

Need to explain observation of giant voids between clusters
1. Bottom-up Theory
   • Galaxies were formed first - then gravity pulled them together (leaving voids)
2. Top-down Theory (pancake model)
   • Flat gargantuan clouds of gas (pancakes) emerged from Big Bang.
   • Then - galaxies formed on these pancakes of matter

Age of the Visible Universe

Supplemental Readings and References

Galaxies
The Galaxy Catalog
Dark Matter in the Universe
Cepheid Variables