The Discovery of Other Galaxies. 24. Normal Galaxies

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1 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 ~ 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

2 Some Interesting Galaxy Facts Spiral Three Kinds of Spiral Galaxies galaxies Barred are ~ 2 x as common as classical spirals Distinctly different amounts of gas & dust Sa Sb Sc ~ 4% of mass is in the form of gas & dust ~ 8% of mass is in the form of gas & dust ~ 25% of mass is in the form of gas & dust Elliptical galaxies Universe s smallest & largest galaxies are elliptical Dwarf elliptical galaxies Giant elliptical galaxies Quite common Quite rare Sa Stellar motion in elliptical galaxies Isotropic Anisotropic Sb Sc Typical in flattened ellipticals Typical in nearly round ellipticals These are virtually devoid of gas & dust Minimal new-star formation Modern View of Spiral Galaxy M51 3 Kinds of Barred Spiral Galaxies Sba Three Kinds of Elliptical Galaxies E0 E3 E6 SBb SBc Giant Ellipticals: The Virgo Cluster

3 Hubble s Tuning Fork Diagram Hubble s Diagram: Another Look Large & Small Magellanic Clouds Bright Stars as Standard Candles Distance < 60 Mpc Cepheid variables are reliable standard candles Visible as far as ~ 60 Mpc (~ 200 Mly) Average luminosities ~ L Luminosity is correlated to their period 60 Mpc < Distance < 150 to 250 Mpc Large Magellanic Cloud Small Magellanic Cloud Supergiant stars are reliable standard candles Red supergiants Maximum luminosities ~ L Visible to distances of ~ 150 Mpc Blue supergiants Maximum luminosities ~ L Visible to distances of ~ 250 Mpc Clusters & Nebulae as Standard Candles 250 Mpc < Distance < 400 Mpc Globular clusters Brightest are about ~ L Visible as far as ~ 400 Mpc (~ 1.3 billion ly) 400 Mpc < Distance < 900 Mpc H II nebulae Brightest are about ~ 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

4 One More Distance Measure A Supernova In Galaxy NGC 4526 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 Six Steps on the Distance Ladder Masers: Microwave Lasers Basic physical process Nearby stars cause H2O to emit microwave λ 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 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 = (λ λ0) / λ0 = Δλ / λ0 Definition of the Hubble law v = H0. d where H0 = the Hubble constant

5 The Hubble Constant Current understanding Redshifts In Spectra of 5 Galaxies H 0 = ± 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 H 0 values Supernova H 0 values from 40 to 65 km. sec 1. Mpc 1 Tully-Fisher H 0 values from 80 to 100 km. sec 1. Mpc 1 HST Cepheid H 0 values ~ 73 km. sec 1. Mpc 1 Precision is + 20% 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

6 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 Large-Scale Distribution of Galaxies 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

7 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

8 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 Important Concepts The Hubble Law & Hubble constant Totally consistent galactic redshift Proportional to distance H 0 = 65 km. sec 1. Mpc 1 Estimates of H 0 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