Lecture Five: The Milky Way: Structure

Transcription

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. 4th May 2015 Sparke & Gallagher, chapter 1,2 1 2 Mapping the Milky Way Mapping the Milky Way - seeing through the dust Lund Observatory The Milky Way as seen by COBE satellite in the infrared 3 4

2 The Galactic coordinate system 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 5 6 The Galactic coordinate system z x An overview of the Milky Way y GC 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 7 8

3 The distribution with age Stellar Density Open circles show stars with less than a quarter of the Sun s iron abundance 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 mid-plane: hz ~ pc for K-dwarfs From Nordström et al. (2004), the Geneva-Copenhagen survey This is caused by giant molecular clouds scattering stars as they pass by, increasing the scale height, and this effect increases with time. for more massive shorter lived A-dwarfs, hz < 200pc The distribution of the motions of (F and G) stars in the z direction, shows that older stars have a larger vertical velocity dispersion: 2 z vz2 vz 2 the Sun moves upwards at ~ 7 km/s 9 10 Good Tracers of spiral arms: The Milky Way gas disc(s) Structure in the Galactic plane. - known to be associated with spiral arms in external galaxies - young: not drifted away from their birthp lace - intrinsically The locations of young stellar and lumino us: associations seen at large distan clusters andces the HII regions that surround hot, massive young stars trace three (or four) spiral arms-inintrins the disc. ic brightness known: to derive for neutral HI gas, hz < 150pc for molecular gas, hz < 60-70pc reddening and true spatial structure 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. Assuming the M/L~2, it can be shown that the total luminosity of the disc is Ld~1.5x1010 L corresponding to Md ~ 3x1010 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. 11 =O-B associations, HII regions, Cepheids, young clusters... The most recent studies suggest that the Milky Way has four spiral arms 12

4 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 wobble 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 The Milky Way disc(s) The thick disc stars have distinctly different chemical properties from thin disc stars. Thin disc stars make up ~90% of the stars near the Sun. Integrated over the z direction, the thick disk has ~1/3 the surface density of the thin disc! One idea is that there was originally a thin disc that was puffed up by a collision with a small satellite galaxy; this became the thick disc, while remaining gas settled into a new thin disc Another possibility is that radial migration of stars from the inner disc, combined with scattering off of giant molecular clouds, may cause an apparent thick disc that is really just the central thin disc extending outwards The Youngest stars in the Milky Way disc The Galactic stellar halo (r) 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 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 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 0.2% of the thin disk s central density in the Solar neighborhood very concentrated: half-light radius ~3 kpc 15 16

5 The mapping the Galactic halo The Galactic stellar halo Numbers of stars at each B V color with apparent V magnitude 19 < m V < 20, per square degree near the north Galactic pole. The solid line shows the prediction of a model where 0.15% of stars near Sun belong to metal-poor halo: thin-disk stars (triangles) are red, halo stars (stars) are blue, and thick-disk stars (squares) have intermediate colors N. Reid. Looking in a range of directions shows a slightly flattened but basically round stellar halo Interestingly, the metal-poor globular clusters trace the halo distribution! 17 The Field of Streams 18 Disentangling the Milky Way NGP Credit: S. Koposov and the SDSS-III collaboration G.C. U Rot. V W T=(U 2 + W 2 ) 1/2 Venn et al

6 Properties of different components The Bulge of the Milky Way The other obvious feature in the COBE image is the central light concentration, which is called the bulge. The size of the bulge helps determine the type of galaxy (Sa, Sb, Sc...) Can we tell if there is a bar? Venn et al The Bar of the Milky Way 23 24

7 The Galactic nucleus The very centre 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 ~3x10 7 M 10 arcsec M halo M The Structure of our Galaxy M bulge 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. M disc M M gas M 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±0.3x10 6 M from Sparke & Gallagher 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 27