Galaxies and Cosmology - AST6309

1. Galaxies and Cosmology - AST6309 • MWF 8 period (3pm to 3:50pm) • Textbooks • Galactic Astronomy - Binney & Merrifield • Galactic Dynamics - Binney & Tremaine • Galaxies in the Universe - Sparke & Gallagher • Cosmology: The Origin and Evolution of Structure - Coles & Lucchin • Exams - 1 mid-term and 1 final exam • Homeworks - 5 homeworks will be assigned throughout the semester • Talk - 15 minutes during the last week of the semester • Topics: • Galaxy classification and photometric properties of galaxies • Gravitational potentials and stellar orbits for axisymmetric and non-axisymmetric systems: Virial theorm, Jean’s equation, Epicycle Theory for disks, Boltzmann equation, Jean’s instability • Observed Kinematics & ISM of Spiral galaxies and Ellipticals/Spheriods • Galaxy Luminosity Functions • Active Galaxies and Supermassive Black Holes • Classical Physical Cosmology: Friedmann Equations, Robertson-Walker Metric, Le Maitre Universes • Thermal History of the Universe • Nucleosynthesis • Structure Formation and Evolution, Large Scale Structure • Cosmic Microwave Background

2. History of Galactic and Extragalactic Astronomy • 1609 - Galileo discovered the Milky Way is comprised of many stars • 1755 - Immanuel Kant theorized that the galaxy has a planar structure, some nebulae may be separate galaxies or island universes • 1774-1781 - Messier catalog compiled (109 objects in Northern sky) including Andromeda galaxy - M31 • 1781-1802 - William and Caroline Herschel conducted first “all-sky survey” and cataloged 5000 nebulae, resolving some into their individual stars • 1845 - William Parsons (Lord Rosse), using a 72-inch telescope, classified the nebulae into featureless ellipticals and whirlpool-like spiral nebulae • 1888 - Dreyer would add to the Herschel’s list to create the New General Catalog (NGC) and Index Catalog (IC)

3. Sun • Determining the size and shape of our Galaxy • 1785 model developed by William Herschel used star gauging in 683 sky regions Assumptions: • All stars have same intrinsic brightness • Star are arranged uniformly throughout the MW • He could see to the edge of the system Herschel didn’t take into account the effects of dust. More dust along the disk causes the distribution of stars to drop-off artificially – objects more than a few kpc from the Sun are obscured at optical wavelengths. (1 kiloparsec = 3 x 1016 km = 3300 lightyears)

4. Early 1900’s - Kapteyn used stellar parallax (proper motion) to estimate the true size of the Galaxy  Kapteyn Universe • Tried to estimate Rayleigh scattering due to ISM gas but determined it to be insignificant (because most obscuration is due to dust absorption which has a smaller  dependence) • Shapley (1919) noted that globular clusters are distributed asymmetrically in the sky and that if one assumes they are distributed about the center of the galaxy, this implies the Sun in not near the center • Estimated distances to GCs using variable stars and period-magnitude relationship • Concluded size to be 100kpc with Sun 15kpc from center • Still wrong…didn’t account for dust absorption which makes things look further away

5. Magnitudes, Fluxes, Distances…. An object’s apparent brightness, m, compared to its absolute brightness, M Distance Modulus Dust extinction causes apparent magnitude to be fainter (i.e adds on to distance modulus) If the globular clusters were 0.2 mag fainter than they would have been in the absence of dust, by how much would the distance be in error? m1 - M = 5 log r1 - 5, m2 - M = 5 log r2 - 5, so Δm = 0.2 = 5 log r1/r2 r1/r2 = 100.2/5 = 1.096 so distance is overestimated by 9.6%.

6. Globular Clusters in our Galaxy Shapley realized that the GCs map out the true extent of our galaxy Galactic Halo The hub of the galaxy is the Galactic Center - about 8 kpc from the Sun Real size of the Galaxy and the Sun’s location not fully determined until 1950’s

7. In 1920, the National Academy of Science hosted the Great Debate concerning the nature of the Spiral Nebulae: were they island universes outside of the MW? • Shapley had MW size too big and therefore argued “NO”; they are part of MW • Curtis (and many others at that time) believed the Kapteyn model of a much smaller MW and argued “YES”, they are separate galaxies beyond the extent of the MW. His notes about a variable star In 1922-1924 Edwin Hubble resolved the controversy using the superior 100-inch telescope at Mount Wilson. He observed Cepheid variables in Andromeda and, using the P-m relation, determined its distance at 300 kpc -- well outside of the MW (still off by more than a factor of 2 due to poor Cepheid calibrations) Note the date: 6 Oct 1923

8. Also in the early 1900’s, Lindblad was doing the first kinematic studies of the MW • Estimated mass in MW from all stars in Kapteyn’s model • Determined velocities of GCs to be as high as 250 km/s - much higher than escape velocity of Kapteyn model • Lindblad (1927) developed first detailed kinematic model of MW • Spherical component with random motions - HALO • Flattened component with rotational motion - DISK • Measured disk to rotate at 200 to 300 km/s near Sun • Oort (1927,1928) developed a complete theory of Galactic stellar kinematics, including an explanation of High Velocity Stars in the Sun’s vicinity • - slow moving stars with apparent high velocities due to the Sun’s motions around the MW center. These drift towards the center of the galaxy explaining the central bulges that had been observed in galaxies.

9. In 1932, Karl Jansky discovered that the MW produced a broad range of radio emission. Later in 1951, several groups detected the 21-cm hyperfine transition of atomic hydrogen which allowed for precise line-of-sight velocities to be determined without the hindrance of dust absorption. • Gas is confined to the disk • Distributed roughly uniformly (actually it is somewhat clumpy) • Travels on ~ circular orbits around the Galactic center • Non-circular motion of gas near the Galactic center -galactic bar From Oort, Kerr and Westerhout (1958) Early radio observations led to the first galaxy mass determination in the 70s and the dynamical evidence for “dark matter”

10. In 1944, Baade used the 100-inch Mount Wilson telescope to resolve stars in the inner regions of nearby spirals and elliptical galaxies. • Spiral spheroids and Ellipticals contain red giant stars • Spiral arms in disks contain blue supergiants • Population I: blue stars and open clusters accompanied by gas and dust in the disks of spiral galaxies • Population II: red stars and globular clusters in spheroids and elliptical galaxies • Plotting stars on HR diagrams showed that the populations also differed in age and it was subsequently determined that they differed in metallicity: • Pop I young and metal rich • Pop II old and metal poor