Galactic structure and evolution

Transcription

1 and evolution Stellar populations concept Bulge and central regions Thin Disc Thick disc Halo and spheroid Kinematics and dynamics Slide 1 Galaxy content Stars (different stellar populations) Gas (neutral, ionised, molecular) Dust (range of grain size and composition) Dark matter (baryonic and non baryonic) Cosmic rays Slide 2

2 Slide 3 Galaxy content Stars (stellar populations) Gas (neutral, ionised, molecular) Dust (grains/various compositions) Dark matter (baryonic and non baryonic) Cosmic rays GAIA Slide 4

3 Stellar population concept Family defined by common characteristics : Abundance Age Kinematics Space distributions Age => time => history Tracers of :, potential Galactic evolution Slide 5 Slide 6

4 Bulge and central regions Central bulge : less than 0.5 deg from center high density, high mass clusters, young population, high metallicity central black hole Outer bulge : triaxial. Extension : degres. Bar-like Metallicity range : -1.6 to , alpha enhanced (Rich & Mc William, 2000) mean [Fe/H]=-0.3 Metallicity gradient : Minniti (1996): yes, Ibata & Gilmore (1995): No Kinematics : isotropic oblate rotator (Kent (1992), Kuijken (1995), Ibata & Gilmore (1995) or not (Rich 2007) Slide 7 Bulge star formation History of star formation Either : single burst (old stars) at epoch of halo formation (like elliptical galaxies) Or : longer star formation (range of ages) if formed from disc instability (bar) Or : discret distribution of ages (reliques of accretion) Probably a mix of the three (which proportion?) Slide 8

5 The bar First bar indication from gas dynamics (De Vaucouleurs, 1964, Kerr (1967),...) Stellar bar : IRAS (Nakada et al, 1991, Blitz & Spergel, 1991) AGB (Weinberg, 1992) COBE -DIRBE (Binney, Gerhard, Spergel, 1997) Microlensing experiments (events + variables) (Minniti, 1997,...) 3D models (Binney, Gerhard, Fux, Aguerri et al.,...) Bar towards us in the first quadrant (angle~ degres, corotation ~ 3-5 kpc, at the beginning of the spiral structure) Slide 9 Bar from MSX 5.7 microns Slide 10

6 Bar from 2MASS Slide 11 Bar angle Lopez- Corredoira et al external bar : Inside bar contaminated by the bulge Projection effects Extinction correction Slide 12

7 Bar : GLIMPSE a p 2.5 d(log n) /dm 1 GLIMPSE star counts at 4.5 microns. R=3, 4, 5 kpc (Benjamin et al, 2006) Bar angle and length R ~4.5 kpc Slide 13 Bar : secular evolution in the disc by instability Bulge : spheroidal structure old in center of disc galaxies similar to Spheroidal galaxies What do we see in the center of the Milky Way? Baade's window far from the plane (bulge...) In-plane observations difficult Need: combination of data in and out plane, and dynamical models for bulge+bar Slide 14

8 Bulge dynamics Hydrodynamic simulations Nakasato & Nomoto (2003) : Bulge stars are formed during accretions of sub-halos (formation time scale ~0.5 Gyr). A second component forms later in the central disc. Distribution of metallicities and velocitites compatible with Minniti observations Same results in simulations from : Ferreras, Wyse & Silk, 2003, MNRAS Slide 15 Slide 16

9 Simulation: Nakasato, Data : Miniti (1996) Slide 17 Bulge/Bar models self-consistent N-body model matched to a luminosity model from COBE-DIRBE deprojection (Bissantz, Debattista & Gerhard 2004) Test with Ogle 2 proper motions (Rattenbury et al. 2007) Slide 18

10 De Battista model good at low latitude deviates from data at high latitudes? Slide 19 Bulge kinematics Zhao96 Fux97 Sellwood93 Rich et al radial velocities of M giants and Pne (Beaulieu et al) Test of dynamical models The bulge does not rotate like a solid body No good dynamical model Slide 20

11 The disc Is exponential radially at the solar position (R,z) = 0 x exp (-(R-R 0 )/h R )x exp (-(z-z 0 )/h z ) h R ~ kpc, h Z depends on age( pc) Inside cutoff? (Paczynski: yes, Minniti:No, Picaud: yes, Lopez- Corredoira: yes) Outside cutoff : yes (Djorgovski & Sosin, 1989, Robin et al. 1992, Freudenreich, ) R=10-15 kpc Inhomegeous (clusters, associations, groups...) in young structures (age< 1 Gyr?) Slide 21 Disc structure Ring (gaseous but also stellar), continuous? Stable? Related to the bar? Spiral structure (2 or 4 arm pattern + a local arm) Warp : stellar warp may not follow the gas Flare : some indications in stars Other local structure : Gould belt Slide 22

12 Spiral structure Georgelin & Georgelin (1977) Ortiz & Lépine (1994) Slide 23 Warp Warp in Gas in Dust in stars Slide 24

13 HI Warp Slide 25 Jozsa (2007) Tilted ring model Slide 26

14 Marshall et al, 2006 Slide 27 Warp in stars 2MASS star counts at K<12 (*/deg2) Slide 28

15 2MASS-Model Slide 29 Warp model slope 0.18 Slide 30

16 warp model slope 0.09 Slide 31 Slide 32

17 Warp parameters Node angle : about center-anticenter +/- 10 deg Start position : 8.5 to 11 kpc warp : 0.06 to 0.31 (2MASS fitting => 0.09) HI North/south asymmetry: z c max : North :4 kpc South : -1.5 kpc Slide 33 Warp origin External torque exerted on the disc : Mis-aligned or flattened halo Tidal interaction with small companion galaxies, for example LMC (Weinberg & Blitz 2006) Quite long-lived structure (old stars) Gas / dust /stars respond differently Slide 34

18 Flare CO: hz from 100 pc at R=10.5kpc to 180pc at 12.5kpc (Grabelsky et al, 1987) Molecular clouds : hz from 200 to 800 pc with R from 9 to 19 kpc (May et al, 1997) HI : hz=3 kpc at R=24 kpc (Diplas & Savage, 1991) COBE/DIRBE : hz from 134 to 188 pc with R=0.5 to 1. R0 (Drimmel et Spergel, 2001) Flare kinematical signature in GAIA : variation of Kz with R Slide 35 Disc inhomogeneities The local disc is not well mixed : A-F Hipparcos sample : less than 7% in clusters, but 63% in (Eggen's) superclusters of large velocity dispersion (6 kms/s) (Chereul et al., 1998) Stars form in groups, from bursts separated by quiescent periods. A fraction of initial groups are gravitationally bounds and forms open clusters. Slide 36

19 Thick disc Thick disc existence well established More and more evidence for a scenario of accretion Probable characteristics : Scale height pc, scale length similar or larger than disc Local density ~ 2-6% of disc Kinematics : Asymetric drift ~35-40 km/s, Z ~40 kms/s Abundances : [Fe/H] ~ /- 0.3, alpha enhanced, no vertical gradient Slide 37 Thick disc formation Top-down scenarios : Star formation during collapse halo=> disc Bottom-up scenarios : Posterior to the disc formation. secular evolution (heating) => orbit diffusion rapid evolution (merger) => rapid heating, re-formation of the thin disc Slide 38

20 thick disc formation Top-down scenarios : Gradients (kinematics, metallicity) Distinct from thin disc Continuity with halo? Intermediate rotation Bottom-up scenarios No gradients Discontinuity with halo (Dis)-continuity with disc Intermediate kinematics abundances ratios like old disc Slide 39 Thick disc Low metallicity tail -1.6<[Fe/H]<-1. Age : a bit younger than the spheroid (~12 Gyr), short period of formation (< 1 Gyr) In situ abundances and kinematical measurements limited to the solar neighborhood (~2 kpc in R) Is the thick disc homogeneous? Where is the encounter? GAIA : Large scale studies, thick disc in central regions, measurement of flaring, kinematics vs abundances, necessary for a detailed scenario of formation Slide 40

21 Spheroid (stellar halo) Oldest known population (12-14 Gyr) Non rotating (or so) V < 50 km/s (partly retrograde?) Pressure supported. Velocity ellipsoid orientation uncertain at high z Lowest abundances : [Fe/H] -4 to -1.5, alpha enhanced (relative to solar), gradients? Short period of star formation Density law R -n, n=2.5-3, slightly flattened (0.5 to 0.8) Local density */pc 3 (a few of the thin disc) Slide 41 Spheroid homogeneity How much of the spheroid formed from accretion of hierarchical fragments? Scenarios : Dissipationnal collapse (ELS) and/or Accretions (Searle & Zinn) Large scale homogeneous surveys : Many debris streams recently found (Vivas & Zinn, 2000AAS, Yanny et al, 2000, Ivezic et al, 2000,..., Belokurov, et al. 2006) GAIA: Large scale inhomogeneities in phase space. True distances for a part of it. Abundances. Constraints on formation and merging history. Slide 42

22 Distribution (R,A, g) of halo turn-off stars G=19.4=> r=11 kpc g=22.5=> r=45 kpc CG Newberg et al 2002 SDSS Slide 43 Distribution of M giants 2MASS in the orbit plane of Sgr dwarf galaxy Majewski et al, 2003, ApJ Slide 44

23 Large external ring? INT WFC 5 fields 122 <l<221 15<Rg<20 kpc Ibata et al, 2002 l=150, b=20 Slide 45 Ibata et al, 2002 l=180, b=30 Slide 46

24 Halo streams Belokurov et al 2006, SDSS data Slide 47 Halo formation "Hierarchical cosmologic model predicts that Milky Way has accreted and distroyed hundred satellites in the last 12 Gyr " (Bullock et al, 2004) Simulations : Helmi et al Observations 2MASS BHB : Brown et al (2004) : Internal halo shows few streams at b >50 (<5%) External halo may contains more visible relics of accretions Slide 48

25 Simulation of halo accretions in -CDM : Accretion at : dark grey: z>2.4, light grey: 0.83<z<2.4, black z< % of the mass accreted at z=2.4 Slide 49 Disc evolution Age : Individual ages difficult to estimate - coronal activity X, H (young stars) - narrow and intermediate band photometry stars > 1 Mo Age of a population : turn-off, clump, white dwarfs, actinides Correlations between age and abundances age and kinematics Slide 50

26 Venn et al 2004 Slide 51 Abundances : main features Metallicity distribution (G dwarf problem? probably not) Radial gradient : dex/kpc Gradients explained by formation scenario inside-out Vertical gradients explained by orbital diffusion and Age/ z relation GAIA : Detailed description of kinematics/abundances/age relations far away from the solar neighborhood Slide 52

27 Abundances [Fe/H] vs Age [Fe/H] vs [O/Fe] local late type dwarfs Rocha-Pinto et al 2004 Slide 53 Tsujimoto et al, 1996 Slide 54

28 [O/Fe] vs [Fe/H] SNII : Massive single stars => reject alpha elements and a bit of iron. Evolution time : short SNIa : binaries (WD + giant) : instability of the accreting WD when M>1.4 Mo => explosion Reject : Iron, and iron peak elements Evolution time : longer Slide 55 Notions of kinematics and dynamics Disc kinematics : Dominated by differential rotation (circular orbits, nearly flat rotation curve) Can be described by a velocity ellipsoid of orthogonal vectors ( U,, ) U towards galactic center, V towards rotation, W towards NGP The distribution can be assumed to be gaussian (to a first approximation) on each vector Velocity dispersion increases with age (disk heating due to orbit diffusion) Origin of orbit diffusion : spiral structure, interaction with GMC,... Slide 56

29 Nordstrom et al 2004 Slide 57 Orbit diffusion + Rocha-Pinto et al (2004) tr Cayrel de Strobel (1974) W Wielen (1974) S Stromgren (1987) o Meusinger et al. (1991) Rocha-Pinto et al 2004 Slide 58

30 Asymmetric drift (rotational lag) V lag = 1./(2. V LSR ) [(R( d /dr) + R(d U /dr) + (1- ( / U ) 2 + (1-( / U ) 2 )] Non axisymmetries and inhomogeneities : Presence of spiral structure, bar, local structure (Gould belt), local streams (thin disc, thick disc or halo) Slide 59 Slide 60

31 Venn et al 2004 Slide 61 Asymmetric drift (rotational lag) V lag = 1./(2. V LSR ) [(R( d /dr) + R(d U /dr) + (1- ( / U ) 2 + (1-( / U ) 2 )] Non axisymmetries and inhomogeneities : Presence of spiral structure, bar, local structure (Gould belt), local streams (thin disc, thick disc or halo) Slide 62

32 Famaey et al 2004 Hipparcos K,M gia 3D velocities Kinematical groups have heterogeneous ages (formation history) =Spiral effects? Slide 63 Oort's limit Total surface mass density at the solar position Stars : M o /pc 3 Gas : 0.05 M o /pc 3 Dark matter halo : M o /pc 3 Total : M o /pc 3 Dynamical mass determined by the Kz (force exerted by the potential over stars, perpendicularly to the Galactic plane) Slide 64

33 Boltzmann equation Link between the vertical density law of a given population (isothermal, relaxed, plan parallel)and its velocity dispersion and the potential (z)/ (0) = exp (- / W2 ) W velocity dispersion potential difference between z and the plane (from Poisson equation and a mass model) Slide 65 Crézé et al 1998 Hipparcos selected sample 125 pc Slide 66

34 Determination of Oort's limit Determined on selected samples of isothermal populations (F dwarfs, K giants) Oort (1960) 0.18 Mo/pc3 Turon-Lacarrieu (1971) Hill et al (1979) Bahcall (1983) 0.21 Bienaymé et al (1987) Kuijken et Gilmore (1989) Crézé et al (1998) Bienaymé et al (2006) 0.10 Van Leeuwen (2007) 0.12 Visible dynamical density => Dark matter Slide 67 Future Large scale surveys very useful to understand Galactic structure and evolution Multivariate parameters linked with Galactic evolution : abundances and kinematics, tools to imagine a scenario of formation and evolution of the Galaxy Gaia : With accurate distances, accurate proper motions and reasonnably accurate radial velocities and abundances, a crucial role in our understanding of the Milky Way (and other galaxies) Slide 68

35 Besancon Galaxy Model A model of population synthesis Inversion : deduce one or several parameters from observations (ex a velocity dispersion for a given set of stars, a luminosity function...) With surveys : complex multivariate analysis When a reasonable scenario points out : synthesis approach Allow comparisons of hypothesis/scenario with observations (even large data sets) all different types (multiwavelength...) Very powerful test method Require a priori knowledge (or guess) Slide 69 Population synthesis Gas => Stars (IMF, SFR) Stars evolve and populate the HR diagram F(M, Teff, g, age) Slide 70

36 Simple dynamical principles Stellar populations Interstellar matter Dark matter Mass model Potential Boltzmann constraint (isothermal and relaxed thin disc population) z 0 exp 2 Thin disc : = f(age) Scale height =f(age)) Slide 71 Observables Evolutionary tracks Model atmospheres Kinematics Velocity ellipsoid = f(age) Asymmetric drift Extinction distribution (3D) Observational errors (Mv, Teff, Age, Z) Apparent magnitude, colourss Proper motions, Vrad Star counts Simulated catalogues Equation of stellar statistics A(m)= M r r2d Slide 72

37 Population synthesis Main inputs : Star formation history Initial Mass Function (IMF) Stellar models (evolutionary tracks) Stellar atmosphere models (grid) Density distribution Chemical evolution (Dynamical consistency) Slide 73 Products and usage Simulations of surveys (multiwavelength) Decontamination of data Milky Way stars on top of external galaxies Field stars on clusters Tests for galaxy evolution scenarios Slide 74