milky-way-part2

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Unformatted text preview: erial from the course can be tested for 30% of your grade Homework #8 assigned Course evaluations are. online this semester—you should have received an e-mail about this. Potential observing opportunity Friday night—weather permitting. '- = ‘ NASA, NOAO, ESAand The Hubble Heritage Team (STSCI/ The Milky Way (Kulner, Ch. 16; also Bennett et al. Ch. 19, Shu Ch. 12) AST 203 (Spring 2011) Galactic Coordinates Galactic coordinates: (Bremhamipedia) Galactic longitude, l: angle measured within the galactic plane. The galactic center is taken to be I = 0°. Galactic latitude, b: angle above or below the galactic plane. Disc AST 203 (Spring 2011) http://antwrp.gsfc.nasa.gov/apod/ap090212.html Credit 1; Copyright: Daniel LOpez. mo AST 203 (Spring 2011) Local Standard of Rest We measure relative motion—we need to know the Sun's motion to get absolute numbers. Kinematic measurement: take average velocity of stars in the solar neighborhood. This is a difficult measurement Best estimate: Distance to galactic center: R0 = 8.5 kpc Orbital velocity at the Sun's galactic radius: ’00 : 220 km s71 AsT 203 (Spring 2011) Mass of the Milky Way We can compute the period of the Sun's orbit around the galaxy. _ 27mg _ 27r(8.5 x 103 pc)(3.08 x 1018 cm pc_1) P 710 220 x 105 cm 8’1 : 7.7 X 1015 s : 240 million yrs. We can also estimate the mass (assuming spherical): Mm) : U3}: 2 (220 x 105 cm 8—1)2(8.5 x 103 pc)(3.08 x 1018 cm pc-l) G 6.67 X 10’8 dyn cm2 g’2 = 2.0 x 1044 g = 1011 MG AST 203 (Spring 2011) Rotation Curve Material a distance R from the GC, with orbital velocity U(R): relative radial velocity: "UT. : 22(R) cos(90° — 0) — v0 cos(90° — l) Now cos(90° — 6’) : sine so "UT. : U(R) sinQ — 220 sinl Dlfierentral reunion and radial velocitiesfhc sun is a disume Ra from me galactn: (emeriwc observe an 1 object at poinl P. along a time making an angle 1 with [he Inns of sight tram us to the galactic center. P is a distance R {rum the galactic center. sin(180° — 0) i sinl _ O i _ R0 — R and 8111(180 — 6) — Slnél or : %U(R) sinl — 210 sinl : R0 — $20] sinl AST 203 (Spring 2011) Orbital vs. Line of Sight Velocity The orbital velocity shown here is the “circular velocity" orbital velocity (tangential to circular orbit) Elliptical orbits are also allowed radial velocity (projection of orbital velocity onto line of sight) What would the escape velocity be? R line of sight A {6; V A Observer (us) AST 203 (Spring 2011) Rotation Curve . _. my) r-.l.‘.v. v.7. mm”Inn-terminalu-lncily quadrants. 5"— Always some material _ inside Sun's orbit in U quads 1 and IV. E St} A m Galactic quadrants. Hm _....d._d. Ami... . .V t) .‘I ll) l5 Ztl radius (Apr! Consider a line of at Constant l : Radialvelucltyasafunction oldistannelmrn ‘ the Sun. ddlong a given line of sight.The upper curve is ‘ typical of f between Cl and 90".The maximum it, corresponds l I < V to the point at Which the line of sight passes closest to the ntl galactic center.The close point Wilh v, 0 corresponds to local materialdnd the far point with v, " 0 corresponds to O o I our line of sight crossing the Sun‘s orbit. Inside the Sun's orbit. I I < l < each Ur (except for the maximum) occurs twice.ThE Inwer curve is Kypkal of values off between 90 and 180". All of these points are nutsuie the Sun‘s OTblL so each Circle is decreases. crossed only onceand each vr is reached only once. AST 203 (spring 2011) {F rancorse Combes. Observatoiru dc Meudon] Milky Way Rotation Curve Inside the Sun: use subcentral point Maximum Doppler shift occurs there Given galactic longitude, l, corresponds to a specific distance from the galactic center Lots of H l in the ISM—observe in 21 cm in quadrants l and IV Highly likely to be H l at the subcentral point Maximum Doppler shift tells us orbital velocity at subcentral point We can map out v(R) vs. R Assumes: Circular orbits 0(R) monotonically decreases with R AST 203 (Spring 2011) Milky Way Rotation Curve The radius of the subcentral point is Rmin : R0 sinl , velocity there is vmax so Ur : Utnax : [9(Rmin) _ 90] R0 Sinl 9(R0 sinl) : Um.“ + 90 Orbital velocity flattens out near Sun. Galactic center does not contain all the mass. ROTATION SPEED (KM S") AST 203 (Spring 2011) (Clemens 1985) Milky Way Rotation Curve Different technique for radii outside the Sun's orbit. Observe molecular clouds with H II regions. Orbital velocity from C0 Doppler shifts. Distance from spectroscopic parallax of central star The curve continues to be flat—maybe even increases! What's going on? ROTATION SPEED {KM S") s s in 12 u 16 R A D l u s ( K P c ) .MMM m is WWW 1 m. Wm" 1“, 71m 1 AST 203 (Spring 2011) (Clemens 1985) Maximum Radial Velocity To be clear: ’UT. : R0 M(R) — S20] sinl For a given line of sight, everything on the RHS is constant except (MB). The maximum 2),. occurs at the maximum (MR), so if (MR) decreases with R then this is at the subcentral point. 105307"“ ,r'E/Lac) ll {m > data picked from Clemens 7 x (1985) plot and converted to Q. AST 203 (Spring 2011) 5 H (km) Dark Matter Flat rotation curve —> M(R) must increase Not enough stars to explain this far from the galactic center Luminosity decreases quickly as a function of R. Some non-luminous but gravitationally interacting material is needed—dark matter. When we look at other galaxies, we see the same behavior. AST 203 (Spring 2011) Spiral Structure Does the MW have spiral arms? Spiral galaxies have lots of dust and gas—we do too. Molecular clouds, H II regions, and OB associations are trace spiral structure in other galaxies Do they show a pattern in the MW? (NASA, ESA, S. Beckwilh (STScI), and The Hubble Heritage Team (STScI/AURA» AST 203 (Spring 2011) Milky Way Spiral Structure Inside the solar circle, we can map out molecular clouds in radio Strong evidence for spiral arms exists. The best evidence comes from observations outside the solar circle LDCEIlDl’lS of Mapped Clouds a axln‘dmeMa.‘ lx‘lerax 105m. a o axlflsrixlUEMan E o mu‘raxiufiMa. 3 “2., 7 7 3 o 9% \ 15 Z , Oks ‘ ($33? 0 \ W o \ I W e \ *\ l ' lo \I G K l e i; so l f r‘ .l Mucus MJM / fl cum ’ ‘ - Cum» ' 5 . > 10 5 v», (x * 1 5 Ha / l2 \,1 I 3x1o' l0 p I. a 4 Fl —13 kpc a; m 10 —a x10 - o a x 103 — m“ saalrmims Flg IA. :4. Splnl structure or the galaxy as determined from yarn: molecular clnud complexes Inside the solar :lrtle. The sizes ol me circles indicate the masses or (he tomplues, (“l III] as indicated m m upper rlgmne 4 kp: and Scutum mm m Molecular clouds uuuldethe sun‘s orb“ and m (Comma) M The ,amh qulmm in a are drawn lmm me 2l rm mapsth Sagltlafl'us arm ls spiral struuul‘e [a] Fm and snmnd gzlinx quadnthe I I d 250a no” Th 2 (in d! Wm drawn as u would best fitthe CO am mamas Dame gum" “"3"” 3 mm m ' E‘ u - - cloud masses are denoted bythe symbol; sham at the m aim! ‘5 called [he Carma m“ [(3) Kathy" N Mm V CFNDamelTMxI aMstmplvyslu 105‘ 592- W86. Fig 9] my InfLThc min at I} kpc .5 drawn m for Mlunn(2 ‘ ' ‘ (bl Yasua Fukul‘ Nabeyam: Radla Dbsemmry] Recent observations of our galaxy by the Spitzer Space Telescope indicate that it has a prominent bar at the center. Credit: NASA/JPLrCa Itech AST 203 (Spmg 2°11) http://www.spitzer.caltech.edu/Media/releases/SSCZOOB-1 0/ssc2008-10b.smml Galactic Center (from Bennett et al.) IR image near Sgr A* Radio image showing gas and SgrA* IR image ~ 1000 Radio image ofthe Iy on a side center. Circular ““““““““““““““““““““““““““““““““ structures are Sne remnants We can see the center of the Milky Way in radio and IR lots of hot gas and SNe remnants are seen A large cluster and very large GMCs are also observed The main source of radio at the center is called Sgr A* AST 203 (Spring 2011) Galactic Center Visible wavelength image of the galactic center. AST 203 (Spring 2011) Galactic Center Radio image of the center of the Galaxy. Note the large # of SNRs, and the (unexplained) filamentary structures. http://antwrp.gsfc.nasa.gov/apod/ap990128.html AST 203 (Spring 2011) SC UTU_M I ‘ISAGITTARIUS’. ; QR-is-l _' O ' . - M70 ' sack-Plus Credit: w. Kel (u. Alabama in Tuscaloosa), erro Tololo, Chile http://antwrp.gsfc.nasa.gov/apod/ap971 1 11 .html l-‘l‘ida—Fr'c’fc Radio Irv-urge aft/re (J mar 0 ('m iii-LassimJ ' ' {3.1mm [Willi Sllflkt‘ .-\f.m.i-r_-' - Six-W 35911.13: Galactic Center IR image of the inner 2 ly of our Galaxy. GC is indicated by the arrows. The Centre of the Milky Way om YEPUN + NACO) M0 I’ll lem MW (9 0mm Mill lam llllt'illl Smilum Olm‘i \ um 4» Credit : Rainer Schudel (MPE) et al.. NAOS-CONICA. ESO AST 203 (Spring 2011) http://antwrp.gsfc.nasa.gov/apod/ap051023.htm| Galactic Center Credit: NASNCXC/CaltecthMuno et al. http://chandra.harvard.edu/photo/2007/gcle/ “This set of Chandra images shows evidence for a light echo generated by the Milky Way's supermassive black hole, a.k.a. Sagittarius A* (pronounced "A-star"). Astronomers believe a mass equivalent to the planet Mercury was devoured by the black hole about 50 years earlier, causing an X-ray outburst which then reflected off gas clouds near Sagittarius A*.” AST 203 (Spring 2011) Galactic Center Credit: NASNUMass/D.Wang et al. http://chandra.harvard.edu/photo/ZOOZ/gcenter/ “This 400 by 900 light-year mosaic of several Chandra images of the central region of our Milky Way galaxy reveals hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The supermassive black hole at the center of the Galaxy is located inside the bright white patch in the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high).” AST 203 (Spring 2011) Galactic Center (NACONLT, Lucy-Richardson) “E o 0.15 0.1 Offset from SgrA*. Dec. ['1 0.05 CA (1.05 CI —B.C5 —C.1 unset rmm SgnM. RA ["1 Observations of a starjust 17 light hours from Sgr A* allow us to map out its orbit and determine the mass of the system SgrA* has a mass of ~ 3 million Me It is a massive blackhole AST 203 (Spring 2011) The Local Group The Milky Way is in a loose cluster of galaxies called the Local Group. ~ 30 galaxies ~ 10 Mly across MW and Andromeda are the most massive ~ 1012 MO total mass (Richard Powell) AST 203 (Spring 2011) The Local Group Stream at. the {arms hllr-aiorgalaxtl The Canis Major Galaxy (closest Galaxy to MW— 42kly) and its associated tidal stream. Illustration Credit Ii Copyright: R. lbata (Strasbourg observatory. ULP) et al., ZMASS, NASA AST 203 (Spring 2011) http://antwrp.gsfc.nasa.gov/apod/ap071104.html ...
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This note was uploaded on 05/04/2011 for the course AST 203 taught by Professor Simon,m during the Spring '08 term at SUNY Stony Brook.

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