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