EANAM5 (Oct. 29 , 2012 @Kyoto) Structures,,y Dynamics, and Formation of the Milky Way: A Narrow Perspective Juntai Shen (Shanghai Astro. Obs.) 沈俊太 (上海天文台) 1 只 不 緣 識 身 廬 在 山 y . 此 真 山 面 蘇 東 中 目 坡 Outline There is no place like home! y Global properties y Components y Spiral structure y Bars in general y MW’s Bulge/Bar y Classical vs. pseudo bulges y Milkyyy Way’s bulg e y New developments y X-shaped structure 3 Components of the MW y Disk y Thin disk y Thick disk y Bulgg/e / Bar y Halo y Dark matter y Stellar y Gas y Spiral structure 5 Global properties of the MW y Rd ~ 2.5±0.5 kpc 10 y Ltot ~ 330X10 L⊙ y MI = -22.7 y Disk 85% vs. Bulge 15% 10 y Mdisk ~ 5X10 M⊙ y (Flynn et al 2006) 10 y Mtot (R) ~ R(kpc) X 10 M⊙ y Disk stellar (()M/L)R~ 2 (()M/L)⊙ 6 y SMBH ~ 4 X 10 M⊙ y Hubble type: SBbc 6 Solar neighborhood properties y R⊙ ~ 8.0-8.5 kpc y Vcirc ~ 220-245 km/s ? -3 y Disk ρ ~ 0.1 M⊙pc -2 y Disk Σ ~ 50 M⊙/pc y Disk thickness ~ 500 pc y Rotation period ~ 220 Myr y Vertical period ~ sqrt(4*pi*Gρ)~90 Myr 7 Component: thin disk y Scale-height ~ 300pc y Continuous on-going star formation for 10Gyr y Wide range of ages y Metal-rich y >~ solar metallicity 8 Component: thick disk y Scale-height ~ 1 kpc y Stars older; metal-poorer; alpha elements enhanced y Surface density ~ 7% of that of the thin disk y At midp lane, thin /thick stars ~ 50:1 y Created by thickening by an encounter with a smaller galaxy? y Quillen & Garnett 2001 y Is it really a distinct thick disk, z or just a thicker disk component? (Bovy et al. 2012) Gilmore & Reid (1983) 9 Component: stellar halo y Spherical y About 1% of the total stellar mass y Very metal-poor; 0.02Z⊙ y Little mean rotation y ρ~r-3 y Debris of disrupted stellar systems, globular clusters and small satellites? y Inner (<30kpc): in situ star formation? (Font et al. 2011) 10 CtComponent: gas hhlalo y Massive gaseous halo? y Ionized hydrogen at >106K, extends for 100s kpc y Shull et al (2009); Gupta et al (2012) y The missing baryons? 10 y Mass comparable to stars? >10 M⊙ y Reservo ir of gas to cool and fa ll in to the Gal axy y Observed high-speed clouds (HVCs)? y Stuff in and out: same things? y To be confirmed in follow-ups 11 Component : dark matter ha lo y The least well understood y Shape: spherical or triaxial? y Mass & Size y Kinematics of distant GCs and nearby galaxies 12 y Wilkinson & Evans (1999): 2X10 M⊙; rhalf ~ 100 kpc y Xue et al. (2008): blue horizontal-branch halo stars; 12 1X10 M⊙ 12 y Bovy et al. (2012): 0. 85X10 M⊙ 10 y Mtot (R) ~ R(kpc) X 10 M⊙ 12 Component: spiral arms y Very uncertain y How many arms y only two major stellar arms?: the Perseus arm and the Scutum- Centaurus arm (Benjamin et al. 2008) y BeSSel project 13 Spiral structure Contrast global and flocculent spiral structures M 51 NGC 5055 M 83 NGC 2841 14 Flocculent spiral structure y Easy to understand y Differential rotation; strong shear y “Spiral structures are quite natural. Every structural irregularity is likely to be drawn out into a part of a spiral” (Oort 1962) 15 Glo ba l “gran d-didesign” spiilrals y FtFact: observed spiilral arms are always trailing y The winding problem y Not material arms y They are density waves y Lin & Shu 1964 y Some of them are clearly driven by tidal interactions or perhaps bars 16 Self-excited global spirals y Quasi-steady modes y Lin & Shu y Persistence problem: group vel. not 0 y Anti-spiral theorem (Lynden- Bell & Ostriker 1967): y non-steady y dissipational y Short-lived, transient, recurrent y Today’s grand-design spirals have not been in M74 place in HDF. If a steady-state solution of a time- y reversible set of equations has the Swing amplifying of form of a trailing spiral, then there unsmooth distribution must be an identical solution in the func. (see Sellwood 2010) form of a leading spiral 17 Testing spiral structure theories y EtExpect to fifidnd a progression: y cold HI gas and CO y H2 and 24-μm emission from stars forming in clouds y the UV emission of fully formed and unobscured stars. y Both theories can have density waves surviving much longer than the TSF y Foyle et al. 2011: no systematic offsets y Observational evidence against long-lived spiral arms in galaxies? 18 BdBarred galilaxies y Bars: elongated cigar- shaped features in disk y CdComposed of old stars y Easier to detect bars in near infrared y Ubiquitous: ~ 2/3 of disk galaxies are barred y Latest NIR survey (Eskridge et al. 2000) y More normal than “normal” disk galaxies! y Bar pattern rotates rapidly 19 Impact of bars on galaxy evolution y Strongest internal disturber: influence disk, bulge, and dark matter halo y Drive gas flow inward y Ignite circum-nuclear starbursts y Build up pseudo-bulges y Different from classical bulges y Secular evolution (Kormendy + Kennicutt 2004) y Bars exchange angular momentum with dark matter halos via dynamical friction y Understanding bars is an integral part of understanding galaxy formation and evolution. 20 Making bars in simulations y Spontaneous bar instability y if ordered rotation dominates over random motion in the initial disk (dynamically “cold”) (Hohl 1971, Sellwood 1981) y Tidally-induced (e.g. Noguchi 1996) y Formation of real bars may be more sophisticated y Still lots of questions y HlHalo properties are itimportan t y Why 2/3 are barred? 21 Dynamics of bars y Bars are made of elongated x1 orbits y x1 are analogous to z-axis tubes y but veryyg elongated y They are nearly resonant (ILR) orbits that would precess exactly together like if Ωp= Ω –κ/2 y Since Ω –κ/2 is not qqjuite constant with R, it is the job of self-gravity to force the orbits precess exactly together. 22 Bars & ellipticals: fundamentally different! y Bars are tri-axial because of too much angular momentum y Giant ellipticals are tri-axial because of too little angular momentum y Supported by random motions y Mostly boxy orbits y Anggpular momentum is provided in z-axis tube orbits (not very elongated) Boxy O Boxy z -axis t u r bits bes Credit: J. Barnes23 BlBulges We do not fully understand them yet! y Classical bulges y Pseudo-bulges y ≈ Mini-elliptical y Extra light at small R; y Merger-made central thick comp. y Sersic n > 2 y Formed from disk by itinterna l secular y not rotation-dominated processes y Retain some memory of their disk origin y Rotation dominated y Young stars, gas, ddtust y Sersic index 1-2 y Including “boxy bulges” Kormendy & Kennicutt (ARAA, 2004) 24 B/Boxy/peanu t-shdhaped blbulges COBE Near IR image of the Milky Way y Most of bulge stars are old (>5 Gyr, Clarkson et al. 2008) y A wide range of metal abundances (McWilliam & Rich 1994; Fulbright et al. 2006; Zoccali et al. 2008) B/Boxy/peanu t-shdhaped blbulges HCG 87 y ~45% edge-on disks have peanut-shaped bulges (Lutticke et al. 2000) y Comparable to the fraction of bars 26 Boxy peanut-shaped bulges: side-on bars? y Simulation of bended/thickened bars y Buckling/firehose instability (Toomre 1966) σ Z ≤ 0.3σ R y Bar formation Æ Buckling instability Æ saturation Æ B/PS bulges (e.g. Raha, Sellwood et al. 1991) Buc kling inst abilit y and ellipti cal s y Possible explains why there are no ellipticals more elongated than E6 or E7, corresponding to a maximum axis ratio of about 3:1. y Sufficiently thick Æ low κz Æ disturbances damped Merritt & Hernquist 1991 Stellar Kinematics of the Bulge y BRAVA ((gBulge Radial Velocity Assay) y)y survey y ~10,000 M giants as targets y Stellar kinematics coveringgg the whole Bulge y Build a simple dynamical model to explain it The Milky Way in IR (COBE) 29 Modeling the Milky Way Bulge • A simple model of the Galactic bulge matches the BRAVA data extremely well in almost all aspects: – b = ‐4o major axis – b = ‐8o degree major axis – l = 0o degree minor axis – Surface density Shen, J., et al 2010, ApJL 30 Power of simplicity y High resolution N-body simulations with millions of particles y Cold massive disk, initial Q ~ 1.2 y A pseudo-isothermal rigid halo with a core which gives a nearly flat rotation curve of ~220 km/s from 5 to 20 kpc y Inside solar circle, Mdisk/Mhalo ~ 151.5, max disk y A good starting point 31 Modelinggyyg the Milky Way Bulge --- Surface Brightness Map Sun DIRBE Composite map – The bar ang le from kinema tic cons tra in t is a bou t ~ 20o – The bar’s axial ratio is about 0.5 to 0.6, andShen, its half-length J., et al 2010,is ~4kpc ApJL 32 Mo de ling the Milky Way Bu lge --- Match stellar kinematics in all strippgys strikingly well Black line = model; it is not a fit to data points Shen, J., et al 2010, ApJL 33 Modeling the Milky Way Bulge --- Constraining the Bar Angle • Bar angle consistent with other studies of star counts and surface brightness (Stanek et al.