Large Scale Structure

1
Large Scale Structure

• Physical Astronomy
• Professor Lee Carkner
• Lecture 24

2
Distance

• The biggest issue in determining the large scale
  structure of the universe is distance
• First stellar parallax measurement by Bessel in
  1862
• Distance methods build on each other
• Need accurate distances to near-by things to
  calibrate method used for more distant things

3
Distance Methods

• Parallax uses geometry for measurement
• An object for which luminosity is related to some
  observed property
• Extinction is a major factor in determination
• All methods have errors
• Usually about 0.3 mag or about 10

4
Distance Ladder

• Parallax and spectroscopic parallax
• For stars in our galaxy (1 Mpc)
• Pulsating stars
• For near-by galaxies (30 Mpc)
• Globular cluster or planetary nebula luminosity
  function
• For intermediate galaxies (50 Mpc)
• Tully-Fisher, D-s relation
• For distant galaxies (100 Mpc)
• Type Ia supernova
• Luminosity from light curve
• For largest distances (1000 Mpc)

5
Parallax

• Shift in position of star due to viewing from
  opposite ends of earths orbit
• d 1/p
• Does not depend on extinction or finding
  luminosity
• d 10(m-M5-A)/5
• Need extinction
• Very inaccurate (M /- 1 mag)

6
Cepheids

• Pulsating stars have a relationship between
  period and luminosity
• Discovered by Leavitt in 1912, calibrated by
  Hertzsprung in 1913
• There are multiple kinds of pulsating stars with
  different P-L relationships
• Can use to find absolute magnitude from P (days)
  and color
• MV -3.53 log Pd - 2.13 2.13(B-V)

7
Luminosity Function

• Rather than using a small sample of objects, we
  can create a luminosity function
• Can use with globular clusters or planetary
  nebula
• Calibrate so you know M for some point on the
  graph and compare to the observed m

8
Galaxy Rotation

• Brighter spiral galaxies have faster rotation
• Since spiral galaxies are bright, useful out to
  large distances
• For elliptical galaxies we can relate the
  diameter (D) to the velocity dispersion (s)
• Called D-s relation

9
Type Ia Supernova

• From the lightcurve of a supernova we can find
  the peak luminosity
• Since M -19.3, can be used to very large
  distances
• Best distance and accuracy

10
Clusters

• Galaxies are grouped in clusters and
  superclusters
• Groups
• Size 2 Mpc
• Clusters
• Size 10 Mpc
• Superclusters
• Size 50 Mpc

11
Local Group

• Dominated by Milky Way, M31 and M33
• About 20 other small groups are within 10 Mpc
• About 80 of all galaxies are in such groups

12
Virgo Cluster

• Nearest large cluster
• Bright galaxies dominated by spirals, faint by
  dwarf ellipticals

13
Intercluster Medium

• Space between galaxies in cluster is filled with
  hot, ionized gas
• T 108 K
• Free electrons fly past free protons and emit
  braking radiation
• Dominates visible mass

14
Cluster Dynamics

• Galaxies interact as they move through the
  cluster
• May form giant ellipticals at cluster core

15
Superclusters

• We are near the edge of the Local Supercluster
• Gravity of supercluster slows Local Group
  expansion

16
Great Attractor

• All near-by galaxies are moving in the direction
  of the constellation Centaurus
• May be due to an unobserved supercluster called
  the Great Attractor
• Mass 10000 mass of Milky Way

17
Largest Scales

• Redshift surveys can be conducted to produce a 3D
  map of structure
• Universe consists of large voids about 100Mpc
  across with galaxies clustered on the boundaries
  of the voids
• Voids too large for galaxies to have formed in
  them and then be pulled out
• Voids must be fundamental, seeded in the Big Bang

18
Next Time

• Read 27.2, 29.1, 30.1
• No Homework
• Final Exam, Monday Feb 25, 12-2pm
• 2/3 covers everything since test 2
• 1/3 covers rest of course
• 2 old 1 new equation sheet given
• 20 of grade