Galactic and Extragalactic Astronomy AST6309 - PowerPoint PPT Presentation

Title: Galactic and Extragalactic Astronomy AST6309


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Galactic and Extragalactic Astronomy - AST6309

• MWF 8 period (3pm to 350pm)
• Textbooks
• Galactic Astronomy - Binney Merrifield
• Galactic Dynamics - Binney Tremaine
• Galaxies in the Universe - Sparke Gallagher
• 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
• History of Galactic Astronomy
• Galaxy classification
• Photometric Properties of Galaxies
• Stellar populations, ISM and kinematics of
  Galaxies
• Properties of the Milky Way Galaxy
• Gravitational potentials
• Stellar orbits in spherical, axisymmetric and
  non-axisymmetric potentials
• Stellar-dynamical systems (CBE and Jeans
  Equations)
• Galaxy morphology and environment
• Clusters, groups, collisions and mergers

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History of Galactic Astronomy

• 1610 -Galileo discovered the Milky Way is
  comprised of many stars
• 1755 - Immanuel Kant theorized that the galaxy
  has a planar structure, some nebulae might
  actually be entire other galaxies or island
  universes
• 1774-1781 - Messier catalog compiled including
  Andromeda galaxy as M31
• 1781-1802 - William and Caroline Herschel
  conducted first all-sky survey and cataloged
  5000 nebulae, resolving some into their
  individual stars
• Later (1845) William Parsons (Lord Rosse), using
  a 72-inch telescope, classified the nebulae into
  featureless ellipticals and whirlpool-like spiral
  nebulae
• Much later (1888) Dreyer would add to their list
  to create the New General Catalog (NGC) and Index
  Catalog (IC)

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• 1785 - Herschel attempted to determine the shape
  and size of Galaxy
• Assumptions
• All stars have same intrinsic brightness
• Star are arranged uniformly throughout the MW
• He could see to the edge of the system

Herschel hadnt considered the effects of DUST
(and of course that all stars are not the same
luminosity). 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 by dust.
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• Early 1900s - Kapteyn used stellar parallax to
  estimate the true size of the Galaxy ----Kapteyn
  Universe
• 10kpc diameter and 2kpc thick with the Sun less
  than a kpc from the center (rather heliocentric)
• Tried to estimate Rayleigh scattering due to ISM
  gas but determined it to be insignificant
  (because most obscuration is due to ISM 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 P-M relationship
• Concluded size to be 100kpc with Sun 15kpc from
  center
• Still wrongdidnt account for dust absorption
  which makes things look further away

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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 Suns location
not fully determined until 1950s
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• 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
Subsequent claims that the SMC and LMC are about
32-34 kpc away
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 300kpc -- well outside
of the MW (still off by a factor of 2 due to poor
Cepheid calibrations)
Note the date 6 Oct 1923
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• Also in the early 1900s, Lindblad was doing the
  first kinematic studies of the MW
• Estimated mass in MW from all stars in Kapteyns
  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 Suns
vicinity -These were actually slowly moving stars
with apparent high velocities due to the Suns
motions around the MW center. Such slow movers
would drift radially towards the center of the
galaxy explaining the central bulges that had
been observed in galaxies.
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• 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
  quite clumpy)
• Travels on circular orbits around the Galactic
  center
• Non-circular motion of gas near the Galactic
  center due to 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
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• 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