The Milky Way Part 1 — Components of the MW Physics of Galaxies 2012 part 5 1 Boring introduction: the galactic coordinate system We use galactocentric coordinates to measure positions in the MW longitude direction l: the angle from the line between the Sun and the Galactic Center (GC) to an object, with the direction of the Sun’s rotation around the MW at l=90º latitude direction b: angle from the Galactic plane towards “North Galactic Pole” (NGP), the pole of the MW seen from the Northern hemisphere (note that the MW’s spin direction is 120º away from the Earth’s!) 2 Boring introduction: the galactic coordinate system If we need to specify three-dimensional positions of objects, we use Galactic cylindrical coordinates (R,ϕ,z) R is distance from GC ϕ is angle Sun-GC-object, positive towards l=90º z is distance away from Galactic plane, positive towards NGP 3 Boring introduction: the galactic coordinate system z y x ⊙ GC For motions near the Sun, we use Cartesian x, y, z coordinates x: radially outwards (away from GC) y: in direction of Sun’s rotation around MW z: out of Galactic plane, positive towards NGP 4 How can we determine the shape and structure of MW? We want to know the structure of the MW What are its components? We want to uncover the density distribution and characteristic scales in each component Need large number of stars in each component! How do these component relate to each other and the formation of MW? Couple density info to ages, compositions 5 A rough sketch of the MW 6 The Milky Way as seen by COBE in the infrared 7 8 The disk(s) of the MW The most obvious structure in all of these pictures is the thin disk of the MW How “big” and “thick” is the thin disk? For disk galaxies, we can approximate the stellar density in the disk as n(R, z, S)=n(0, 0,S) exp[ R/h (S)] exp[ z /h (S)] − R −| | Z where hR is the scale length of the disk – the length over which the density falls by a factor of e – and hz is the scale height of the disk – again, the height over which the density falls by e (BM call these Rd and z0, respectively) for some population S 9 By examining pictures like the COBE image and correcting for the extinction by dust (which is important even in the infrared), it is possible to estimate these sizes: on average, it appears that hR≈2.5–3.5 kpc and hz≈165 pc — so the ratio, hR/hz~18 — i.e., the disk is very thin! Using these numbers, you can show that the total 10 luminosity of the disk is Ld~1.5x10 LV,⊙ 10 Distribution perpendicular to the plane Even more interesting is the variation of scale height with stellar type in the disk Dynamically interesting! The motions of stars perpendicular to the disk tell us about the gravitational potential of the disk: how much mass is there? Is there more mass per unit light than we see in the stars? 11 We measure the distribution of stars perpendicular to the disk by first measuring trigonometric parallaxes of a subset of (nearby) stars to determine MV as a function of color – say, V–I Then we measure the apparent magnitudes V for stars as a function of V–I to determine their distances this is called “photometric parallax” works well if all the stars are main-sequence stars – not always true, but works well for stars earlier than late G and for late M dwarfs 12 Why is there this run of scale height with spectral type? Note the increasing mean age with spectral type: G dwarfs are typically much older than A stars This implies that there is an effect relating stellar age to scale height the Galactic disk is lumpy — giant molecular clouds scatter stars as theyn(R,z,S) pass = by,n(0,0,S) exp(-R/HR(S)) exp(-z/Hz(S)) increasing the scaleHR(S) is the scale-length of the disk height Hz(S) is the scale-height Strong dependence on spectral type 13 From Nordström et al. (2004), the Geneva-Copenhagen survey This effect can be seen in the distribution of the motions of (F and G) stars in the z direction older stars have a larger vertical velocity dispersion: σ2 v2 v 2 z ≡ z − z 14 The thick disk An interesting thing happens when measuring the density of stars far away from the Galactic plane Distribution cannot be fit by single exponential curve! Good fit with two exponentials: z0=300 pc (close to plane) and z0=1350 pc (farther from plane) The space density of stars cannot be fitted by a single exponential! Good fit from superposition of two exponentials with different scale- heights: z0 = 300 pc and z0' = 1350 pc. 15 The thick disk Two possibilities: 1. Functional form (exponential) is incorrect 2. There are two physically distinct components of the disk of the Galaxy: a thin and a thick disk For the second possibility to be correct, need to conclusively demonstrate that they have different and distinct properties! The space density of stars cannot be fitted by a single exponential! Good fit from superposition of two exponentials with different scale- heights: z0 = 300 pc and z0' = 1350 pc. 16 The Galactic Plane, in depth and across the spectrum XXVIIth IAU General Assembly, August 2009 !c 2009 International Astronomical Union Janet Drew, Melvin Hoare, Nicholas Walton, eds. DOI: 00.0000/X000000000000000X Abundance structure and chemical evolution of the Galactic disc Thomas Bensby1 and Sofia Feltzing2 1European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile email: [email protected] 2Lund Observatory, Box 43, SE-221 00 Lund, Sweden email: [email protected] Abstract. We have obtained high-resolution, high signal-to-noise spectra for 899 F and G dwarf stars in the Solar neighbourhood. The stars were selected on the basis of their kinematic properties to trace the thin and thick discs, the Hercules stream, and the metal-rich stellar halo. A significant number of stars with kinematic properties’inbetween’thethinandthick discs were also observed in order to in greater detail investigate the dichotomy of the Galactic disc. All stars have been homogeneously analysed, using the exact same methods, atomic data, model atmospheres, etc., and also truly differentially to theSun.Hence,thesampleislikelyto be free from internal errors, allowing us to, in a multi-dimensional space consisting of detailed elemental abundances, stellar ages, and the full three-dimensional space velocities, reveal very small differences between the stellar populations. Keywords. stars: abundances, stars: kinematics, Galaxy: disk, Galaxy: evolution, Compared to our previous studies of the Galactic thin and thick discs (Bensby et al. 2003, 2005) the current stellar sample is larger by a factor of ∼ 8. The figure above shows the thin and thick disc abundance trends based on kinematicalselectioncriteriaonly.The red fullIt lineappears in each plotthat is thethis running is the median case! from the thick disc stars, and the dashed blue line the running median from the thin disc stars. It is clear that there is separation between the two discs up to at least solar metallicities, signaling the dichotomy of the Galactic stellarThick disc, disk and (red that thelines two above) discs have stars had very have diff erentdistinctly chemical histories. First results,higher based [α on/Fe] this enlargedat a given sample, [Fe/H] regarding and the extendorigin of to the lower Hercules stream and the metal-rich[Fe/H] than limit ofthe the thin thick discdisk were (blue published lines) in Bstarsensby et al. (2007a,b). The full data set will be published in the fourth quarter of 2009 where we in great detail will arXiv:0908.2445v1 [astro-ph.GA] 17 Aug 2009 investigate the abundance structure and chemical evolutionoftheGalacticstellardisc. References Bensby, T., Feltzing, S., & Lundstr¨om, I. 2003, A&A,410,527 17 Bensby, T., Feltzing, S., Lundstr¨om, I., & Ilyin, I. 2005, A&A,415,155 Bensby, T., Oey, M.S., Feltzing, S. & Gustafsson, B. 2007a, ApJ,655,L89 Bensby, T., Zenn, A.R., Oey, M.S., & Feltzing, S. 2007b, ApJ,663,L13 1 Thin disk stars make up ~90% of the stars near the Sun integrated over the z direction, the thick disk has ~1/3 of the surface density of the thin disk 18 Where do the disks come from? One idea is that there was originally a thin disk that was puffed up by a collision with a small satellite galaxy; this became the thick disk, while remaining gas settled into a new thin disk Another possibility is that radial migration of stars from the inner disk, combined with scattering off of giant molecular clouds, may cause an apparent thick disk that is really just the central thin disk extending outwards 19 Finally, the young stars (<30 Myr) – and the associations and clusters in which they’re born – appear to live in a (partial) ring called “Gould’s Belt” that is tilted by ~20º from the Galactic plane, with stars closer to the center lying off the plane 20 WhatGood Tracer s ofabout spiral arms: the structure in - known to be associated with spiral arms in external galaxies the-young: notGalactic drifted away from their plane? birthplace - iIfntrinsically we map luminous: the locations seen at large of distances young stellar associations and - intrinsicclusters brightness and the know HIIn :regions to derive reddeningthat surround and true spatial hot, massivestructure =O-Byoung associations, stars, HIIwe regions,find that Cepheids, they youngtrace clusters..