1 / 28

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

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

Albert_Lan
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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

  2. 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• • •

  3. 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

  4. Two-dimensional motion with cylindrical symmetry: angular momentum energy (R,) are coordinates,  is potential Parametric orbit equations: Toy Model: Steady Potential I• • •

  5. Seek a potential in which (,t) equations may be integrated using trig. functions, with bdry. conditions: and M is the total mass of the Galaxy Solutions for potential and circular velocity: and b is a characteristic scale Toy Model: Steady Potential II• • •

  6. 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.

  7. Toy Model: Steady Potential IV• • • For stars in the solar neighborhood, use observed space motions ( U´,V´ ) to calculate orbit. Define: eccentricity (arbitrary)

  8. 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

  9. Toy Model: Contracting Potential I• • • Assumptions: (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

  10. 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

  11. 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

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

  13. (U-B) and Eccentricity• • • Dark circles: stars with accurate space motions Light circles: stars with large space motions No bias Bias No bias Age correlates with eccentricity.

  14. (U-B) and Angular Momentum• • • Age anti-correlates with angular momentum. No bias

  15. Upper Envelope (U-B) and W´ Velocity• • • Young stars formed in the disk. Old stars formed at a range of heights above the plane.

  16. 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

  17. 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

  18. Angular Momentum and Eccentricity• • • Two effects: • Most of time spent near apocenter. • Galactic density gradient.

  19. 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

  20. 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

  21. 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

  22. 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

  23. Monolithic or Hierarchical?• • • CDM theory  hierarchical structure formation Observational evidence: -Sagitarrius dwarf galaxy -Observed galaxy interactions and mergers

  24. 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 (0.9,-2)

  25. 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

  26. “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

More Related