24. Normal Galaxies• The discovery of other galaxies• Edwin Hubble proved galaxies are very distant• Edwin Hubble classified galaxies by shape• Methods for determining distances to galaxies• The Hubble Law & the Hubble constant• Clusters & superclusters of galaxies• Colliding galaxies produce spectacular features• Most matter in the Universe is undiscovered• Theories of galaxy formation
The Discovery of Other Galaxies• An historical perspective
– Immanuel Kant suggested “island universes”
1755• Vast collections of stars beyond the Milky Way
– Milky Way was thought to be the only galaxy• No clear evidence of anything at really great distances
– William Herschel discovered & cataloged “nebulae”• Thought to be distant gas & dust clouds
– Lord Rosse built the largest telescope to date
1845• Mirror was 6 feet in diameter with a 60 foot focal length• Discovered that some “nebulae” had a spiral structure• Made many sketches of celestial objects he saw
Lord Rosse’s “Leviathan” (1845)
The National Academy of Sciences• The “Great Debate” of 26 April 1920
– Harlow Shapley Measured the Milky Way’s size• Thought that “spiral nebulae” were forming solar systems
– Heber Curtis Studied solar eclipses• Thought that “spiral nebulae” were separate galaxies
• The resolution of the debate– Edwin Hubble photographs the “Andromeda nebula”
• He discovered Cepheid variables– Average luminosities ~ 2.0 . 104 . L☉
• This revealed the ~ 2 million ly distance to Andromeda– Implication: Universe contains billions of galaxies
Table 24-1: Properties of Galaxies
Hubble Classified Galactic Shapes• Spiral galaxies
– Grand design Spiral arms are narrow & distinct– Flocculent Spiral arms are broad & hazy
• Barred spiral galaxies– Nucleus varies in relative size– Bars may be short & wide or long & narrow
• Elliptical galaxies– Complete absence of spiral arms– Some appear spherical but may be seen end-on
• Depends entirely on our perspective from Earth• Irregular galaxies
– No obvious structure
Some Interesting Galaxy Facts• Spiral galaxies
– Barred are ~ 2 x as common as classical spirals– Distinctly different amounts of gas & dust
• Sa ~ 4% of mass is in the form of gas & dust• Sb ~ 8% of mass is in the form of gas & dust• Sc ~ 25% of mass is in the form of gas & dust
• Elliptical galaxies– Universe’s smallest & largest galaxies are elliptical
• Dwarf elliptical galaxies Quite common• Giant elliptical galaxies Quite rare
– Stellar motion in elliptical galaxies• Isotropic Typical in flattened ellipticals• Anisotropic Typical in nearly round ellipticals
– These are virtually devoid of gas & dust• Minimal new-star formation
Three Kinds of Spiral Galaxies
Sa Sb Sc
Modern View of Spiral Galaxy M51
3 Kinds of Barred Spiral Galaxies
Sba SBb SBc
Three Kinds of Elliptical Galaxies
E0 E3 E6
Giant Ellipticals: The Virgo Cluster
Hubble’s Tuning Fork Diagram
Hubble’s Diagram: Another Look
http://skyserver.fnal.gov/en/proj/advanced/galaxies/images/TuningFork.jpg
Large & Small Magellanic Clouds
Large Magellanic Cloud Small Magellanic Cloud
Bright Stars as Standard Candles• Distance < 60 Mpc
– Cepheid variables are reliable standard candles• Visible as far as ~ 60 Mpc (~ 200 Mly)
– Average luminosities ~ 2.0 . 104 . L☉
• Luminosity is correlated to their period
• 60 Mpc < Distance < 150 to 250 Mpc– Supergiant stars are reliable standard candles
• Red supergiants Maximum luminosities ~ 1.0 . 105 . L☉– Visible to distances of ~ 150 Mpc
• Blue supergiants Maximum luminosities ~ 2.0 . 105 . L☉– Visible to distances of ~ 250 Mpc
Clusters & Nebulae as Standard Candles• 250 Mpc < Distance < 400 Mpc
– Globular clusters• Brightest are about ~ 1.0 . 106 . L☉
– Visible as far as ~ 400 Mpc (~ 1.3 billion ly)
• 400 Mpc < Distance < 900 Mpc– H II nebulae
• Brightest are about ~ 6.0 . 106 . L☉
– Visible as far as ~ 900 Mpc (~ 3.0 billion ly)
Reliability of Standard Candles• Reasonably reliable standard candles
– Cepheid variables & supergiant stars– Globular clusters & H II nebulae– Type Ia supernovae
• Reasons for their reliability– Very bright Visible at great
distances– Well-known luminosities Inverse square intensity– Easily identified Unlike all other objects– Relatively common Distance to many
galaxies
One More Distance Measure• The Tully-Fisher relation
– Basic observation• The 21 cm hydrogen line width depends on galaxy mass
– High-mass spiral galaxies have wide 21 cm lines– Low-mass spiral galaxies have narrow 21 cm lines
– Basic physical process• The greater the mass, the faster the rotation speed
– This results in greater Doppler shifts of the 21 cm line• The greater the mass, the greater the compression
– This results in more & brighter stars• The basic strategy
– Estimate the mass of distant spiral galaxies• The 21 cm line width is directly proportional to brightness
• The basic problem– Poor correlation with standard candle distances
A Supernova In Galaxy NGC 4526
Six Steps on the Distance Ladder
Masers: Microwave Lasers• Basic physical process
– Nearby stars cause H2O to emit microwave l’s• Microwave amplification by stimulated emission of radiation
• Basic observations– Use of the VLBA
1990s• Ten 25-meter radio antennas between Hawaii & Caribbean• Extreme detail possible because of synthetic aperture
– Observed masers in the spiral galaxy M106• Some approaching, some receding, some tangential
– Measure blue & red Doppler shifts to determine orbital speed– Measure proper motion of tangentially moving masers
• Calculated a distance of ~ 7.2 Mpc (~ 23 Mly)• Basic problem
– The technique is still in its infancy• Method is completely independent of other approaches
Masers As Standard Candles
The Hubble Law & Hubble Constant• An historical perspective
– Slipher starts spectral study of “spiral nebulae”1914
• 11 of 15 “spiral nebulae” had substantial redshifts• Noted by Curtis during the 1920 Shapley-Curtis debate
– Evidence that these features were far beyond the Milky Way– Hubble & Humason extend spectral analyses
1920s• Also analyzed Cepheid variables in these features
– The farther away they are, the faster they are moving away
• The Hubble flow– Redshift is directly proportional to recessional speed
• Definition of redshift– z = (l – l0) / l0 = Dl / l0
• Definition of the Hubble law– v = H0 . d where H0 = the Hubble
constant
The Hubble Constant• Current understanding
– H0 = 69.32 ± 0.80 km . sec–1 . Mpc–1
• At 100 Mpc, galaxies recede at 6,932 km . sec–1
• At 1,000 Mpc, galaxies recede at 69,320 km . sec–1
• Current problems– Different distance methods yield different H0 values
• Supernova H0 values from 40 to 65 km . sec–1 .
Mpc–1
• Tully-Fisher H0 values from 80 to 100 km . sec–1 .
Mpc–1
• HST Cepheid H0 values ~ 73 km . sec–1 . Mpc–1
– Precision is + 20%
Redshifts In Spectra of 5 Galaxies
Hubble Law: Redshift & Distance
Galaxy Clusters & Superclusters• Basic observations
– Galaxies are not uniformly distributed in space• Some regions of space have few galaxies• Other regions of space have many galaxies
• High-density regions of space– Poor clusters
• One example is our Local Group of galaxies– Rich clusters
• One example is the nearby Virgo cluster of galaxies– Superclusters
• Great Wall is one example of a supercluster
• Southern Wall is another example of a supercluster
The Local Group of Galaxies• Contains ~ 40 galaxies
– Uncertainty due to sparseness of many galaxies• Sagittarius Dwarf Discovered in 1994• Antilla
Discovered in 1997– Dust in the Milky Way plane obscures some galaxies
• Most Local Group galaxies are dwarf ellipticals• Two large spiral galaxies ~ 2.2 Mly apart
– Andromeda galaxy M31• Largest galaxy in the Local Group
– Milky Way galaxy• Second largest galaxy in the Local Group
Some Galaxies in the Local Group
The Andromeda Galaxy• The largest galaxy in the Local Group
– Diameter of ~ 125,000 ly• Covers ~ 3° of area in the night sky• We see only the small core as a fuzzy patch of light
– Apparent magnitude of ~ 3.0 spread out over a large area– Most distant object visible to the unaided eye
– An Sb spiral inclined ~ 15° to our line of sight• Rather tightly wound spiral arms
– Contains ~ 2 times as many stars as the Milky Way• Unusual properties
– HST images suggest Andromeda may have 2 cores• Possibly the result of ancient collision with another galaxy
– Andromeda is approaching us at ~ 68 mi . sec–1
• This may portend a future collision with the Milky Way
The Andromeda Galaxy (M31)
Clusters of Galaxies• The Virgo cluster
– Covers an area ~ 10° by 12° in the sky– Located ~ 15 Mpc (~ 50 Mly) away– A moderately rich irregular cluster
• Dominated by 3 giant ellipticals ~ 750 Kpc across• Diameters are ~ 5% their distance from the Local Group
• The Coma cluster– Located ~ 90 Mpc (~ 300 Mly) away– A rich regular cluster
• Dominated by 2 giant ellipticals• Telescopic images show ~ 1,000 galaxies• Probably 10 times that many dwarf ellipticals
The Hercules Cluster of Galaxies
Superclusters of Galaxies• Usually include dozens of clusters
– Spread over ~ 30 Mpc (~ 100 Mly) of space
• Delicate filamentary patterns at the largest scale– The Great Wall in the northern sky– The Southern Wall in the southern sky
Large-Scale Distribution of Galaxies
Spectacular Colliding Galaxies• Galaxy collisions are relatively common
– Galaxies orbit one another like planets orbit stars– Occasionally they actually hit each other
• Possible interactions– Most of the volume of galaxies is empty space
• Stars seldom hit one another– Gas & dust are much more widespread than stars
• Interactions are far more common– Substantial compression occurs– Substantial star formation ensues
– Cores may orbit each other or even merge• Possible double core of the Andromeda galaxy• Possible future for the Andromeda & Milky Way galaxies• Discovery of 2 supermassive black holes in 1 galaxy
Simulated Collision of Two Galaxies
Colliding Galaxies With “Antennae”
Most Matter Remains Undiscovered• Dark matter is evident only from gravity effects
– Coma cluster should have disintegrated long ago• Abundant “dark matter” keeps the cluster bound together
• A partial solution– X-ray emitting gas
1930s• Comparable to the mass of stars in typical rich clusters• Can only account for ~ 10% of the dark matter
• The distribution of dark matter– May be deduced from galactic rotation curves
• Orbital speeds nearly constant to visible edge of galaxy– Gravitational lensing by massive galaxies
• Dark matter constitutes ~ 90% the mass of galaxies• Dark matter is distributed much like visible matter
Rotation Curves of 4 Spiral Galaxies
“Double” Quasar: Gravitational Lensing
Theories of Galaxy Formation• Basic observations
– The greater the distance, the farther back in time• The most distant galaxies are seen in their earliest stages
– Galaxies were bluer long ago than they are now• Vigorous formation of OB associations
– HST work by Dressler, Oemler, Gunn & Butcher• Two rich clusters with z = 0.4
~ 4 billion years old– About 30% of galaxies in distant rich clusters are spirals– About 5% of galaxies in nearby rich clusters are spirals
• Galactic collisions probably consume spirals– Ellipticals were probably the end product of these collisions
• Basic theories– Gravitational collapse of huge nebulae
1960s– Gravitational merging of smaller nebulae
1977 – Gravitational merging of many tiny nebulae
Stellar Birthrates In Galaxies
• The problem of “spiral nebulae”– The Great Debate of 1920
• They are forming solar systems• They are distant separate galaxies
– Cepheid variables solve the debate• The classification of galaxies
– Ellipticals• From giants to dwarfs
– Normal spirals• Grand design & flocculent spirals
– Barred spirals– Irregular
• Determining distance to galaxies– Normal & spectroscopic parallax– Standard candles
• Cepheids & supergiants• Globular clusters, & H II nebulae
– Additional techniques• Tully-Fisher relationship & masers
• The Hubble Law & Hubble constant– Totally consistent galactic redshift
• Proportional to distance• H0 = 65 km . sec–1 . Mpc –1
– Estimates of H0 vary by factor of 2• Uneven distribution of galaxies
– Groups• Irregular & regular
– Clusters• Poor & rich
– Superclusters• Colliding galaxies
– Spirals more common in old clusters• Blue color of many OB associations
– Collisions form ellipticals• Formation of galaxies
– Three common hypotheses• Collapsing and/or merging nebulae
Important Concepts