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3D Spectrography: II - The tracers

3D Spectrography: II - The tracers. Morphology : distribution of each component Dynamics : kinematics via the emission or absorption lines Line strengths : allow to study stellar populations. The different tracers: Gas. 90% H, 10% He Neutral, ionized, molecular. H. He. Dust. Mass.

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3D Spectrography: II - The tracers

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  1. 3D Spectrography:II - The tracers • Morphology: distribution of each component • Dynamics: kinematics via the emission or absorption lines • Line strengths: allow to study stellar populations

  2. The different tracers: Gas • 90% H, 10% He • Neutral, ionized, molecular H He Dust Mass Cloud T Density Orion HI HII H2 Dust Msun Msun (K) cm-3

  3. HI Gas • Hyperfine transition line at 21 cm Aligned poles (higher energy) Opposed poles (lower energy) • Rare transition, but very abundant gas

  4. HI Gas = Radio astronomy

  5. HI Gas - Cartography

  6. HI Gas • Velocity profiles Sofue et al.

  7. HI Gas • Position-Velocity diagram

  8. HI Gas - Kinematics • NGC 253 – HI Observations Koribalski et al.

  9. Ionized gas: Ha • Spectrum in the visible

  10. Ionized gas: Ha • Comparison HI / Ha

  11. Ionized gas: Ha • Velocity map Khoruzhii et al.

  12. Stars galaxy • Absorption lines template Calcium triplet • Deconvolution: G = S*  LOSVD  G = S*LOSVD LOSVD : Line Of Sight Velocity Distribution l [ang] LOSVD V [km/s]

  13. Stars • Problems due to population differences (template mismatching) Different populations = Different kinematics • Deconvolution: G = Siai Si*  LOSVDi  G= Siai Si* LOSVDi

  14. LOSVDs and kinematics • Many different methods for deconvolving: • Direct pixel fitting • Fourier fitting • Cross-correlation techniques • Fourier Quotient Correlation method • Others… • Fittings LOSVD moments: • Gauss-Hermite moments (van der Marel & Franx 93, ApJ 407, 525 Gerhard 93, MNRAS 265, 213)

  15. LOSVDs and kinematics • LOSVDs of NGC 5582 Halliday et al., 2001, MNRAS, 326, 473

  16. LOSVDs and kinematics Halliday et al., 2001, MNRAS, 326, 473

  17. How to determine the age and composition of a galaxy? • Assume1 age and uniform composition. • Assume same laws of physics as in • a globular cluster. • Stellar evolution: artificial HR diagram • Findmatching spectra • Add these spectra composite galaxy spectrum • Repeat previous steps for different ages/metallicities • Determinebest fit

  18. Determining age and metallicity in practice The Lick System of Indices • Determine strengths of absorption features • Correct them for velocity broadening of the galaxy • Compare them with theoretical line strengths

  19. Stellar population models Vazdekis (1999) models at Lick resolution (~9 Å FWHM) based on Jones (1999) library [MgFe52]=(Mgb x Fe5270)^0.5

  20. Age & metallicity for Fornax galaxies Kuntschner 2000, MNRAS, 315, 184

  21. Aperture spectroscopy Velocity, velocity dispersion …

  22. Long-slit spectroscopy Kinematical profiles

  23. Integral field spectroscopy We obtain a spectrum at each position

  24. IFU spectroscopy And each spectrum gives: Flux Line Strength Dispersion Velocity

  25. NGC 3384 S0 (cluster) V s Hb Mgb Fe5270

  26. Line-strength maps – N3384 No H gradient Strong Mgb in centre Fe peaks in centre Restricted wavelength range de Zeeuw et al. 2002, MNRAS, 329, 513

  27. 3D Spectrography:Adaptive 2D Binning

  28. Photometry binning NGC4342 WFPC2 Cappellari 2001: Efficient MGE fitting method

  29. Spectroscopy 1D-binning IC1459 Major axis kinematics Cappellari, Verolme et al. 2002

  30. The SAURON test data:NGC 2273 Reconstructed image S/N map • Result of multiple pointings: • irregular domain • vertical S/N jumps Barred Sa galaxy

  31. 2D-binning requirements • Topological: partition the plane without holes or overlapping bins • Morphological: bins as compact or “round” as possible • Uniformity: minimal S/N scatter

  32. Tiling of the plane Towle 2000 Penrose tiling

  33. 2x 2D-binning by QuadTree decomposition • Regular cells but: • large S/N scatter • border problems SatisfiesTopologicalandMorphologicalrequirements

  34. Voronoi Tessellation Definition: each point in a bin is closer to its generator than to any other point SatisfiesTopological requirement ONLY

  35. Taking pixels into account 1D case: growing bins along the slit 2D analog: growing bins around the bin baricenter

  36. Centroidal Voronoi Tessellation It is the perfect solution in the case of Poissonian noise and many pixels. AllTopological, Morphological and Uniformityrequirements satisfied! Cappellari & Copin 2002

  37. Voronoi Tesselation2D-binning forNGC 2273 • Small S/N scatter • Compact bins • No border problems

  38. NGC 2273 stellar mean velocity field 2D-binned velocity Not binned

  39. What to keep in mind • Ionized gas and stars are (easily?) traceable via emission and absorption line spectra. • Derivation of the distribution, kinematics and line strengths. • Again, a two-dimensional spatial coverage is often critical for the scientific interpretation • More importantly: it is the link between all these tracers which allows us to really understand the physical status of these objects, leading to a theory of their formation and evolution. • Further readings: • Galactic Astronomy, Binney & Merrifield, Cambridge University Press

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