# Lecture Five: The Milky Way: Structure

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1 Lecture Five: The Milky Way: Structure The Celestial Sphere We use equatorial coordinates to determine the positions of stars in the sky. A stars declination (like latitude on the Earth) is the angle between its position and the celestrial equator. 2nd May 2014 Sparke & Gallagher, chapter 1,2 1 2 How we see the Milky Way Mapping the Milky Way Lund Observatory 3 4

2 The Milky Way in Neutral Hydrogen Mapping the Milky Way - seeing through the dust The Milky Way as seen by COBE satellite in the infrared 5 6 Milky Way in different wavebands The Milky Way - Far Infrared Cirrus Far-Infrared: concentration of old stars in the bulge Near-IR: sensitive to giant stars and dust Gamma-rays: neutron stars and X-ray binaries Optical :dark nebulae X-rays: hot supernovae remnants The Milky Way as seen by IRAS/COBE satellite in the far infrared 7 8

3 The Galactic coordinate system The Galactic coordinate system We use Galacto-centric coordinates to measure positions in the Milky Way Galactic longitude l is measured in the plane of the disc from the Sun-GC line, towards the direction of the Sun's rotation. The Galactic latitude b is the angle of the star from the Galactic plane towards North Galactic Pole (NGP) To specify the positions of stars in three-dimensional space, we use Galactic cylindrical coordinates (R,ϕ,z) The radius, R, is the distance from the Galactic Centre in the disc-plane of the Galaxy The azimuthal angle ϕ is angle from the Sun-Galactic Centre line. The height above the midplane, z, is positive towards North Galactic Pole 9 10 The Galactic coordinate system x z y GC How can we determine the shape and structure of Milky Way? It has more than one component, and so we need to determine the density distribution and characteristic scales of each component. This means we need to observe the properties of a large number of stars in each component. We can then ask - how do these components relate to each other and the formation of Milky Way? This requires the coupling of density info with ages, compositions, etc. For motions near the Sun, we use Cartesian x, y, z coordinates x: radially outwards (away from Galactic Centre) y: in direction of Sun s rotation around Milky Way z: out of Galactic plane, positive towards North Galactic Pole The Milky Way as seen by COBE in the infrared 11 12

4 Stellar Density An overview of the Milky Way For disc galaxies, we can approximate the stellar density in the disk as double exponential: n(r, z, S) = n(0, 0, S) exp[ R/hR (S)] exp[ z /hz (S)] 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 for some population S Can measure the spatial distribution of stars by first measuring trigonometric parallaxes of nearby stars to determine MV as a function of color. Then measure the apparent magnitudes V for stars as a function of colour to determine their distances. This is called photometric parallax. mid-plane: hz ~ pc for K-dwarfs for more massive shorter lived A-dwarfs, hz < 200pc The distribution with age Thickness of the disc From Nordström et al. (2004), the Geneva-Copenhagen survey There is an effect relating stellar age to scale height. This is probably caused by giant molecular clouds scattering stars as they pass by, increasing the scale height, and this effect increases with time. Age z(s)) n(r,z,s) = n(0,0,s) exp(-r/hr(s)) exp(-z/h The distribution of the motions of (F and G) stars in the z direction, shows that older stars have a larger vertical velocity dispersion: HR(S) is the scale-length of the disk Hz(S) is the scale-height 2 z vz2 vz 2 Strong dependence on spectral type the Sun moves upwards at ~ 7 km/s 15 16

5 The Milky Way gas disc(s) Structure in the Galactic plane. HI gas (mixed with dust) in the disc is even thinner than young stellar distribution near the Sun hz < 150pc. For cold molecular clouds this even less, hz < 60-70pc. The scale length is typically in the range hr kpc. Assuming the M/L~2, it can be shown that the total luminosity of the disc is Ld~1.5x10 10 L corresponding to Md ~ 3x10 10 M If stars are produced with the standard IMF then to build the disc over 10Gyr the Milky Way must produce 3-5 M of new stars every year Structure in the Galactic plane. Disentangling the Milky Way The locations of young stellar associations and clusters and the HII regions that surround hot, massive young stars trace three (or four) spiral arms in the disc. Slow, uniform! Fast, chaotic! Eggen, Lynden-Bell & Sandage Disc Halo 19 20

6 Stellar Orbits in the Milky way Stars in the disk all orbit the Galactic center: in the same direction in the same plane (like planets do) they bobble up and down this is due to gravitational pull from the disk this gives the disk its thickness Stars in the bulge and halo all orbit the Galactic center: in different directions at various inclinations to the disk they have higher velocities they are not slowed by disk as they plunge through it nearby example: Barnard s Star Motions in the Solar Neighbourhood: Geneva- Copenhagen survey F & G dwarfs with Hipparcos parallaxes, Tycho-2 proper motions Nordstrom et al The Milky Way disc(s) Disentangling the Milky Way The vertical distribution of stars in the Milky cannot be fit by single exponential curve. There is only a good fit with two exponentials: z0=300 pc (close to plane) and z0=1350 pc (farther from plane) Two possibilities: Functional form (exponential) is incorrect 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! NGP G.C. U Rot. V W T=(U 2 + V 2 ) 1/2 Venn et al

7 arxiv: v1 [astro-ph.ga] Properties of different components 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, The Milky Way disc(s) Compared previous studies of thechemical Galactic thin and thick (Bensby et al. The thick to discour stars have distinctly different properties from thindiscs disc stars. 2003,Thin 2005) current stellar is larger by asun. factor of 8. The figure above shows discthe stars make up ~90%sample of the stars near the the thin and thick disc abundance trends based on kinematical selection criteria only. The Integrated z direction, the thickmedian disk hasfrom ~1/3 the the surface density of the thin disc red full line in over eachthe plot is the running thick disc stars, and the dashed blue line the running median from the thin disc stars. It is clear that there is separation between theidea two discs up was to at least solar signaling dichotomy is that there originally a thin metallicities, disc that was puffed up by the a collision with a of the One Galactic and thethe two discs very different smallstellar satellitedisc, galaxy; thisthat became thick disc,have whilehad remaining gas settledchemical into a newhistories. thin First disc results, based on this enlarged sample, regarding the origin of the Hercules stream and the metal-rich limit of the thick disc were published in Bensby et al. (2007a,b). The possibility is that radial migration of stars from the inner disc, combined with Another full data set will be published in the fourth quarter of 2009 where we in great detail will scattering off of giant molecular clouds, may cause an apparent thick disc that is really just investigate the abundance structure and chemical evolution of the Galactic stellar disc. the central thin disc extending outwards Venn et al References Bensby, Bensby, Bensby, Bensby, T., T., T., T., 26 Feltzing, S., & Lundstro m, I. 2003, A&A, 410, 527 Feltzing, S., Lundstro m, I., & Ilyin, I. 2005, A&A, 415, 155 Oey, M.S., Feltzing, S. & Gustafsson, B. 2007a, ApJ, 655, L89 Zenn, A.R., Oey, M.S., & Feltzing, S. 2007b, ApJ, 663, L13 The Bulge of the Milky Way The Youngest stars in the Milky Way disc The other obvious feature in the COBE image is the central light concentration, which is called the bulge. 1 The size of the bulge helps determine the type of galaxy (Sa, Sb, Sc...) Can we tell if there is a bar? Unfortunately, the extinction in the direction of the bulge is large, and so studies of the bulge as a whole need to made in the IR. LK~1010 L with a scale length Rb~0.7 kpc. The youngest stars (<30 Myr) and the associations and clusters in which they form are found in a (partial) ring called Gould s Belt that is tilted by ~20º from the Galactic plane, with stars closer to the center lying further off the plane The MW bulge from Spitzer in the Infrared 27 Luckily, a few low-extinction patches can be found to observe bulge fields in the optical (still with some extinction). The most famous and important is Baade s Window at (l,b)=(1º, 3.9º), which has 1.3<AV<2.8, which is low enough to observe stars in the optical. Most of our understanding of the age and chemical composition of the bulge comes from optical studies through Baade's window. 28

8 The Bar of the Milky Way 29 The Galactic nucleus 30 latest results on cloud falling into black hole in centre of MW The very center of the Milky Way the nucleus is invisible in the visible: the extinction AV=31 mag There is a stellar cluster of very young stars (2 7 Myr old) with a total mass of ~3x107 M By studying the motions of these stars with infrared adaptive optics observations, it is possible to measure the distance and the mass of the central object which is almost certainly a black hole. Between 1995 and 2009, the star S2 was seen to make a complete orbit Using the observed motions and angular scale, Gillessen et al. (2009) inferred R0=8.38±0.15 kpc and MBH=4.3±0.3x106 M 31 32

9 The Galactic stellar halo (r) The Galactic stellar halo Looking even farther out from the plane of the Galaxy, it appears that there is a sort of truncation of the disk into a much more tenuous stellar halo these stars are all metal-poor The density profile of these stars is n(r) =n 0 (r/r 0 ) 3.5 Properties of the Galactic stellar halo: extends to (at least) 80 kpc L~10 9 L about 1% of the total luminosity of the MW 0.2% of the thin disk s central density in the Solar neighborhood very concentrated: half-light radius ~3 kpc Interestingly, the metal-poor globular clusters trace the halo distribution! The Galactic stellar halo The stellar halo is very important for understanding the formation of the Milky Way, as the stars are generally very old and metal-poor. The stellar halo preserves the early formation history of the Galaxy, and we can see similar processes at work today. The Galactic stellar halo These are the remnants of objects (satellites, GCs) being disrupted by the gravitational potential of the Milky Way. Credit: S. Koposov and the SDSS-III collaboration 35 36

10 The Structure of our Galaxy M halo M! M bulge M! M gas M! M disc M! The Milky Way Disc younger generation of stars, large age and metallicity range contains gas and dust Bulge very old but metal rich stars Halo Old stars no gas or dust location of the globular clusters from Sparke & Gallagher 37 38

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