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“Evidence from the motions of old stars that the Galaxy collapsed” • Eggen, O.J., Lynden-Bell, D., & Sandage, A.R. 1 - PowerPoint PPT Presentation

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02/20/1962: John Glenn becomes 1 st American to orbit Earth “Evidence from the motions of old stars that the Galaxy collapsed” • Eggen, O.J., Lynden-Bell, D., & Sandage, A.R. 1962, ApJ, 136, 748 (ELS62) • • • 01/28/1962: Ranger 3 misses Moon by 22,000 miles

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02/20/1962: John Glenn becomes 1st American to orbit Earth

“Evidence from the motions of old stars that the Galaxy collapsed”•Eggen, O.J., Lynden-Bell, D., & Sandage, A.R. 1962, ApJ, 136, 748 (ELS62)• • •

01/28/1962: Ranger 3 misses Moon by 22,000 miles

04/26/1962: Ranger 4 impacts Lunar surface without returning any scientific data

about the authors

Olin J. Eggen (1919-1998): Renowned observer. In addition to ELS62, known for work on moving groups and pioneering work on discovering white dwarfs. 346 refereed publications on ADS, ~90% single author.

Donald Lynden-Bell (1935-): Famous for theoretical work on dynamics, black holes, accretion disks, and more. Two appearances on list of eligible papers for this seminar.

Allan R. Sandage (1926-): Famous observer. Known for galaxy atlases, work on quasars, and insistence that H0≈50 km/s/Mpc. 337 refereed publications on ADS, 220 first author.

About the Authors• • •
Overview• • •
  • Structure of ELS62:
    • Toy model of the Galaxy
      • Stellar dynamics in a steady potential
      • Dynamics in a contracting galaxy
    • Correlations among observed stellar properties and orbital parameters
    • Interpretation: the Galaxy collapsed during or after initial star formation
  • Putting ELS62 in a modern context
toy model steady potential i

Two-dimensional motion with cylindrical symmetry:

angular momentum


(R,) are coordinates,  is potential

Parametric orbit equations:

Toy Model: Steady Potential I• • •
toy model steady potential ii

Seek a potential in which (,t) equations may be integrated using trig. functions, with bdry. conditions:


M is the total mass of the Galaxy

Solutions for potential and circular velocity:


b is a characteristic scale

Toy Model: Steady Potential II• • •
toy model steady potential iii
Toy Model: Steady Potential III• • •

Evaluate b using Oort constants A=15 km/s/kpc and B=–10 km/s/kpc, and assuming R0=10 kpc and vc0=250 km/s  b=2.74 kpc.

toy model steady potential iv
Toy Model: Steady Potential IV• • •

For stars in the solar neighborhood, use observed space motions ( U´,V´ ) to calculate orbit. Define:

eccentricity (arbitrary)

toy model steady potential v
Toy Model: Steady Potential V• • •
  • Motion perpendicular to the Galactic plane (Z direction, or W´ velocity) has been neglected
    • Small perturbation for nearly coplanar orbits
    • Orbits for stars with large W´ velocities are inaccurate

Calculated eccentricity becomes simply a measure of non-coplanarity

toy model contracting potential i
Toy Model: Contracting Potential I• • •


(1) Galactic potential was always axisymmetric

(2) Stars do not exchange angular momenta with each other or with gas

Stars conserve their angular momenta

Two cases:

(1) Slow: little change during one orbital period

(2) Rapid: large change during one orbital period

toy model contracting potential ii
Toy Model: Contracting Potential II• • •
  • Slowly changing potential of the form
    • Can define an adiabatically invariant eccentricity e*
    • e*≈e to within about 0.1

small change in eccentricity

  • ELS62 assume e*=e
toy model contracting potential iii
Toy Model: Contracting Potential III• • •
  • Rapidly changing potential
    • Circular orbits become eccentric
    • Stars near apocenter: eccentricity increases
    • Stars near pericenter: eccentricity may decrease

On average, eccentricity increases

Observations• • •
  • Sample of 221 dwarf stars from Eggen (1961, 1962) catalogs
  • Ultraviolet excess: (U-B)
    • For a given stellar type:
    • Metallicity indicator  age indicator
u b and eccentricity
(U-B) and Eccentricity• • •

Dark circles: stars with accurate space motions

Light circles: stars with large space motions

No bias


No bias

Age correlates with eccentricity.

u b and angular momentum
(U-B) and Angular Momentum• • •

Age anti-correlates with angular momentum.

No bias

u b and w velocity



(U-B) and W´ Velocity• • •

Young stars formed in the disk.

Old stars formed at a range of heights above the plane.

evidence of collapse
Evidence of Collapse• • •
  • Upper envelope in the (U-B), W´ plane suggests:
    • Maximum height above plane for star formation at epoch corresponding to (U-B)
    • Collapse of factor ~25 along Z direction during/after first epoch of star formation
  • Angular momenta of oldest stars
    • Comparable to young stars in inner disk, ~5 kpc
    • Apocenter distances ~50 kpc  scale of radial collapse ~10
angular momentum and r max
Angular Momentum and RMax• • •

Dark circles: large (U-B)

X’s: intermediate (U-B)

Light circles: small (U-B)

Makes sense if stars form preferentially near apocenter

angular momentum and eccentricity
Angular Momentum and Eccentricity• • •

Two effects:

  • Most of time spent near apocenter.
  • Galactic density gradient.
timescale of collapse
Timescale of Collapse• • •
  • Long compared to rotation period ~2Myr
    • Requires vR << v

Stars form with low eccentricities

    • Eccentricities are approximately preserved

Large eccentricities never occur

  • Short compared to rotation period
    • Large eccentricities can be imparted at birth or result from collapse
    • Conclusion: collapse timescale ~108 yr
summary of collapse model i
Summary of Collapse Model I• • •
  • Two-body relaxation time is long compared to the age of the Galaxy
    • Barring large scale catastrophe, stars “remember” the dynamical conditions under which they formed
  • First generation stars have eccentric, not necessarily co-planar, orbits
  • Second generation stars have nearly circular orbits
summary of collapse model ii
Summary of Collapse Model II• • •
  • Scale of collapse ~25 in vertical direction, ~10 in radial direction
  • Collapse time ~108 years, similar to protogalaxy dynamical time
  • Successfully explains contemporary observations of spatial distribution and kinematics of metal poor globular clusters and RR Lyrae stars
summary of collapse model iii
Summary of Collapse Model III• • •
  • Predicts narrow distribution of age, metallicity for halo stellar population
  • In this picture, Hubble type is determined by initial angular momentum of protogalaxy
  • Predicts elliptical galaxies form in luminous starbursts at high redshift
monolithic or hierarchical
Monolithic or Hierarchical?• • •

CDM theory  hierarchical structure formation

Observational evidence:

-Sagitarrius dwarf


-Observed galaxy

interactions and


reprise fe h and eccentricity

Chiba, M. & Beers, T.C. 2000, AJ, 119, 2843

Reprise: [Fe/H] and Eccentricity• • •

1203 stars in the Solar vicinity with [Fe/H]<-0.6 and no kinematic bias

Clump near


legacy of els62
Legacy of ELS62• • •
  • Combination of ground-breaking work and provocative conclusion
    • Stimulated interest in galaxy formation
    • Use of kinematics and metallicities of halo stars as a probe of Galactic dynamical history
  • Scientifically exemplary for its internal consistency and testable predictions

“Science may benefit very much (or only very little) from what we do, but since we do it for ourselves, for the satisfaction of our own curiosity, we should be thankful for the circumstances that permit a life in the dark.”

-Olin J. Eggen, “Notes from a Life in the Dark”, 1993, ARA&A, 31, 1