Galaxy Dynamics

1
Galaxy Dynamics

Lab 11
2
The areas of the sky covered by various surveys
3
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

4
(No Transcript)
5
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''

6
Red V lt 3000 km/s Blue V 3000- 6000 km/s
Magenta 6000-9000 km/s Cyan 9000-12000 km/s
Green gt12000 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.
7
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"

8
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

9
(No Transcript)
10
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)

11
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

12
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)

13
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.

14
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

15
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 8313 km s-1/Mpc or
  818 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

16
Kinematic density waves

• nested orbits forming a bar
• orbits offset to form two-armed spiral
• 32 resonance (instead of 21) makes a 3-armed
  spiral
• (d) 41 resonance makes a 4-armed spiral.