Galactic and Extragalactic Astronomy - AST6309
Now called “Galaxies and Cosmology”
•MWF 6 period (12:50 to 1:40pm)
•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 - 10 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
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)
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
Sun
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)
•
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
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%.
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
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
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.
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”
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