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Yan-Ji, 2004/8

The disk and halo population stars Chenggang Shu cgshu@center.shao.ac.cn. Yan-Ji, 2004/8. Outline. 1. Introduction 2. Observations 2.1. Galactic halo 2.2. Galactic disk 3. Studies on the MW by LAMOST. 1. Introduction.

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Yan-Ji, 2004/8

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  1. The disk and halo population stars Chenggang Shu cgshu@center.shao.ac.cn Yan-Ji, 2004/8

  2. Outline 1. Introduction 2. Observations 2.1. Galactic halo 2.2. Galactic disk 3. Studies on the MW by LAMOST

  3. 1. Introduction • How a galaxy forms and evolves is the main goal for modern astrophysics; • As a typical disk galaxy, the Milky Way is the “zero-point” of the theory of galaxy formation and evolution; • A realistic model MUST physically explain the observed global properties of the Milky Way; • More and more data are available now: SDSS, RAVE, The Century Survey Galactic Halo Project, …………………..

  4. 2. Observations2.1. Galactic Halo The circular velocity is Vc ~130km/s; Mh = Vc3 / 10 G H ~ 5 x 10 11 M◉ Rh = Vc / 10 H = 130 h -1 kpc λ ~ 0.035 – 0.065 * WMAP:Ωtotal = 1.02 ± 0.02 Ωm,0 = (0.135 ± 0.009) h -2 Ωb,0 = (0.0224 ± 0.0009) h -2 h = 0.71 ± 0.04

  5. High resolution simulations of the halo density profile:NFW profile (Hayashi et al 2004)

  6. Metallicity : [F/H] < -1; [O/H] ~0.3 the star formation timescale ~ 10 8 yr; t d ~ R h / Vc ~ 0.1 t h Satellite galaxies : 17 LMC : d ~ 55 kpc; M ~ 2 x 10 10 M◉ ; SMC: d ~ 65 kpc; M ~ 2 x 10 9 M◉ ; dwarf Sgr ω is descending into the Galactic center due to dynamic friction

  7. Moore et al (1999) Substructure : Simulations show ~ 500 satellites ; mass > 10 8 M◉; Like Draco & Unsa-Minor Size  kpc Observations do not ! warm dark matter? Jing et al (2001) feedback effects of early reionization ? Susa & Masayuki 2004, ApJL 609, 467

  8. Globular clusters:

  9. Metallicity distribution: (Chen et al 2003) disk globular clusters halo globular clusters

  10. Open questions for GCs: • The Gaussian form of GCLFs with peak at ~ - 7.2; • The widths of GCLFs are different; • The dynamic evolution of MF; • No correlations between: M_gc vs R_gc M_gc vs [Fe/H] • No obvious metallicity gradient; • The origin of field halo stars ; • ……..

  11. eccentricity vs metallicity If yes ! ---- ELS If no ! ----- accretion Chiba & Beers, (2002, AJ 119, 2843) do not support ELS, no correlation ; Brook et al (2003 ApJL 585 125) -- simulation Only a clump of stars (halo stars) show this correlation –modern galaxy formation paradigm (mergers)

  12. Gerhard astro-ph/0307057

  13. Gerhard astro-ph/0307057

  14. SDSS data : substructure ? V_sun (circular) = 245.9 ±13.5 km/s; V_halo(rotational) = 23.8 ±20.1 km/s ΔV = 222.1 ±7.7 km/s (Sirko et al 2004, AJ, 127, 899 & 914 ) Velocity ellipsoid( r, θ,φ) = ( 101.4 ± 2.8, 97.7 ± 16.4, 107.4 ± 16.6) km/s

  15. 1. 2.

  16. Surface and volume densities in the solar neighborhood based on kinematicsZeker (astro-ph/0401185, but this is old version)

  17. 1476 stars in Hipparcos with measured radial velocities within z < 0.4kpc Korchagin et al (2003, AJ, 126, 2896) Siebert et al (2003, A&A, 399, 531 ): < 800 pc, ~ 76 M_o /pc^2

  18. There is an AMR in the thick disk based on 229 stars ?? (Bensby et al 2004, A&A, 421, 969) implied: star formation could have been an ongoing process for some Gyr ??? Thick disk ??

  19. Mashonkina et al (2003 A&A 397, 235) thin disk, thick stars and halo stars --- open, filled circle and * thick disk stars formed on a timescale ~ 1.5Gyr from the beginning of protogalactic collapse ??? Chen, Nissin, Zhao (2004)

  20. Chemodynamical model (Samland & Gerhard 2003) can match observations (Koch & Grebel astro-ph/0407212)

  21. From specific angular momentum :non-negligible stars originated from satellites now are in thin or thick disks. (Navarro astro-ph/0405497) That disk heating due to substructures within a halo can result : the thickness to be 20-30% of the scale length for an L* spiral galaxy. (Benson et al, 2004, MNRAS, 351, 1215)

  22. 3. Studies on the MW By LAMOST (personal) • 3.1. Kinematics: • map the large-scale kinematics of the MW, and fit a model- determine best-fit rotation curve; • do the same for the velocity dispersions; • search in the data for unusual velocity features- search for streams/ disrupted galaxies/ warps/ and other features of interest, e.g the velocity structure of the spiral arms in the MW. There may be many ways which we could search for unusual features. One simple method would be to subtract off the best-fit model calculated in 1a, and examine parts of the dataset where the residualsare very high. Other search algorithms might be developed which could seek out features with particular characteristics; • calculate the mass of the MW to high accuracy. There are a number of methods of obtaining a mass for the MW, but because of thesize of the LAMOST data set, the statistics might be able to give much more accurate numbers.

  23. 3.2. Density Structure: • characterise the density structure of the milky way to high accuracy, in 3 dimensions. we could go further than previous studies, and look at the detailed shape and structure; • combine with the velocity data to identify the distinctive components-- disk, thick disk, bulge, halo. Are these components really distinct? do they give a fair representation of the MW's structure? are there other components to be found? • …….

  24. 3.3 Metallicity data: • an extension of 2b. having analyzed the components of the MW based on density and velocity, what is the correlation with age/metallicity? is the bulge older? how much older? what about the thick disk? • (b) the metallicity data will allow us to date the matter in the milky way as a function of position and velocity. Effectively we will be able to calculate the distribution function for the MW as a function of age. From this we can gain insight into how the MW formed. Perhaps we will be able to answer for certain whether the milky way formed inside-out, or in a more messy fashion; and furthermore, quantify these issues; • (c ) redo the analysis of ELS (1962). this is the classic analysis were they found the correlation between old stellar matter and high angular momentum, implying that the MW formed by collapse. We would be able to repeat a similar analysis in much greater detail.

  25. 3.4. Density inversion via Poisson's equation: • given the velocity and density data at our disposal, we can solve Poissons equation to get the potential of the MW; • by adding up the mass in stars, and the mass in gas (from other data sets), we can then investigate the distribution of dark matter in the MW; • fit the MW potential with a NFW profile, king profile, r^1/4 profile, etc., and see which fits best-- the MW is the only galaxy in which we can measure the potential in such a direct manner, and with such accuracy; and the only opportunity we have to compare observational data with these theoretical potentials; • can we measure the potential accurately enough to answer the question whether or not the core of the milky waypotential has a cusp or a power law?

  26. The main reasons for studies on the MW to be in the frontier again are : • more data avaible; • galaxy formation theory fast developed; • more and more numerical simulations; --- modern studies should be within the current framework of galaxy formation and evolution! Have we prepared both observationally and theoretically?

  27. Thank you !

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