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Galaxy Dynamics
Lab 11
The areas of the sky covered by various surveys
Redshift Surveys
• Redshift surveys are ways of mapping the distribution of galaxies
around us
• We use redshift as the measure of the radial coordinate in a
spherical coordinate system centered on the Milky Way
• These are spectroscopic observations of about 1100 galaxies in a
strip on the sky 6 degrees wide and about 130 degrees long
• We are at the apex of the wedge
• The radial coordinate is redshift measured in kilometers per second
with the current best Hubble constant conversion of about 20
kilometers per second per million light years.
• That means the outer arc of the plot is at a distance of about 700
million light years
• The angular coordinate is the right ascension, or the celestial
equivalent of longitude
Distribution of Galaxies
• This initial map showed that the
distribution of galaxies in space was NOT
random
• That with galaxies actually appear to be
distributed on surfaces, almost bubble like,
surrounding large empty regions, or
``voids''
Red V < 3000 km/s
Blue V 3000- 6000 km/s
Magenta 6000-9000 km/s
Cyan 9000-12000 km/s
Green >12000 km/s
a plot of the sky distribution of the galaxies, each point represents a galaxy in the
northern celestial hemisphere that is brighter than an apparent blue magnitude of 15.5
and with a measured redshift inside 15,000 km/s. The nearest galaxies are shown in
red, followed by blue, magenta, cyan and green.
CfA Redshift Survey Map
• The large red area at the center of the map (12 hours,
+10 degrees) is the dense central region of the Virgo
Cluster of galaxies which is the core of the Local
Supercluster
• The dark blue points which dominate the RHS of the
map (0-4 hours +30 to +40 degrees) show the location of
the Pisces-Perseus Supercluster
• The cyan points at 15 hours running almost from the
equator to +40 degrees declination are the Hercules
Supercluster.
• The blank areas running N-S and looping over the north
celestial pole are the regions of the sky inaccessible to
optical surveys due to dust extinction when looking
through the Milky Way, aka the "Zone of Avoidance"
The Great Wall
• perhaps the largest single structure yet
detected in any redshift survey
• dimensions are ~ 600x250x30 million light
years, sort of like a giant quilt of galaxies
across the sky
Details of The Great Wall
• The Great Wall itself can be viewed in this CfA Survey
with velocities between 6,500 and 11,500 km/s in the
north galactic cap
• Galaxies with velocities of 6,500-8,500 km/s are plotted
as red points and those 8,500-11,500 km/s are blue
• Because the Great Wall is slightly tilted in velocity space,
the higher velocity end dominates the eastern portion of
the figure
• The Great Wall is a surface with both low and high
density regions with considerable 2-D structure --embedded clusters and groups of galaxies as well as
some filaments and other more amorphous structures
• The Coma Cluster is the dense region right near the
center of the plot (13h and +29d)
The Local Group
• The Local Group of galaxies consists of:
– 2 large spiral galaxies (Milky Way and M31)
– the small Sc spiral M33
– 2 large satellites of M31 (the E2 galaxy M32
and the dE NGC 205)
– 2 large satellites of the Milky Way (the Large
and Small Magellanic Clouds)
– And ~30 dIrr, dE and dSph dwarf galaxies,
mostly satellites of the Milky Way or M31
Differences in Galaxies
• The individual galaxies seem to have
widely differing star formation histories
• The main difference in galaxy properties seems
to be between large and dwarf galaxies
• In both spiral and elliptical galaxies, there is a
difference in properties like total mass, surface
brightness (both ↓ in dwarfs), and gas content of
spirals (↑ in dwarfs)
Elliptical galaxies
• In large elliptical galaxies the main distinction is
between “boxy” (blunt) and “disky” (pointed)
ellipticals
• boxy ellipticals are larger and more massive,
pressure-supported, and sometimes radio-loud
• disky ellipticals are smaller, more flattened,
rotationally supported and always radio-quiet
• Disky ellipticals have much in common with S0
galaxies.
Hubble Constant
• The constant H is the rate of recession of
distant astronomical objects per unit distance
away
• This single number describes the rate of the
cosmic expansion, relating the apparent
recession velocities of external galaxies to their
distance
• The more distant objects are receding more
rapidly than closer ones seems to imply
expansion of the universe, and is the main
observation which led to the Big Bang theory
Not constant????
• The Hubble constant changes as a function of time
depending on the precise cosmological models as the
expansion of the universe slows due to gravitational
attraction of the matter within it
• The current value of the Hubble constant is hotly
debated, with two opposing camps generally getting
values near the high and low ends of 50-100 km s-1/Mpc
• Using observations of Cepheid variables, Hubble
constant has been reported as 83±13 km s-1/Mpc or
81±8 km s-1/Mpc
• Using supernovae, results favor a relatively small Hubble
constant (slow expansion rate) of ~ 55 kilometers per
second per megaparsec, which means that galaxies one
megaparsec (3 million lightyears) distant appear to
recede from us at a speed of 55 kilometers per second
Kinematic density waves
(a) nested orbits forming a bar
(b) orbits offset to form two-armed spiral
(c) 3:2 resonance (instead of 2:1) makes a 3-armed spiral
(d) 4:1 resonance makes a 4-armed spiral.