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Milky Way (Home Sweet Home)

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Milky Way (Home Sweet Home) The Milky Way (home sweet home) Today: -Structure -Kinematics (incl. rotation & dark matter) -Death by (Supermassive) Black Hole Photo: R. Bell Visible Milky Way Lund Panorama - 1940’s Infrared Milky Way 2MASS (1990’s) Gamma Ray Milky Way (Why??) Compton Gamma Ray Obs. Multi-Wavelength Milky Way (mwmw) Southern MW Structure of MW:Historical .1785 - Herschel: attempted to determine shape and size of Galaxy Sun amoeba? What’s wrong with this picture?? Assumptions: .All stars have same intrinsic brightness .Stars are arranged uniformly throughout the MW .He could see to the edge of the system Herschel didn’t know about DUST & Dust obscuration... --Simple brightness mapping is NOT a measure of true distance >> Need map with true measure of distance! Structure of MW: Shapley Used Cepheid and RR Lyrae variables to How?? determine distribution of globular clusters and correct Herschel’s map 47 Tucanae (SUPER AWESOME, ALL POWERFUL) HR Diagrams of Clusters - MS turnoff gives age - Know L (and Mag) of AGB/HB stars - Use distance modulus 47 Tucanae Globular Clusters - Old (Population II) stars, 100,000+ - Spherical Distribution in Galaxy About Center - No massive stars or significant gas/dust --> no recent star formation 47 Tucanae Open (Galactic) Clusters - Young (Population I), 100-1000 - Spherical Distribution in Galaxy About Center - Massive stars/gas/dust present --> active/recent star formation Pleiades Open (Galactic) Clusters IC 1805 NGC 290 (Jewel Box) in LMC, Hubble image Open (Galactic) Clusters - Open clusters only appear in the Galactic Plane - Open clusters are unbound Pleiades Structure of Milky Way: Current - Globular clusters in spherical halo - Open clusters/Spiral Arms in disk - Molecular Clouds/Star formation in disk - High stellar density at center (Galactic bulge) - Sun is 8.4 kpc from center in Orion Arm Kinematics of MW Everything in the Galaxy orbits around the Galactic center Material closer to the center travels on faster orbits - DIFFERENTIAL ROTATION (WHY??) Orbital periods at different distances from GC tell us the distribution of mass in the Galaxy How?? Using Rotation to Measure Mass Kepler’s 3rd Law! Assume: Circular Motion & Centrally Condensed (Spherically) Symmetric M(R) Derive relation between v(R) and M(R) from first principles: F=ma M(R) = Ω(R)2R3/G v(R) or Ω(R) is the rotation curve of the galaxy So how do we measure v(R)??? Doppler Shift Due to Differential Motion Differential galactic rotation produces Doppler shifts in emission lines from gas in the Galactic disk **But Sun is NOT Static Doppler Shift Due to Differential Motion Consider material at a distance R from GC, moving with v(R) ... Radial velocity relative to Sun yields a Doppler shift. What are Ω0 (or v0) and R0? Velocity and Position of Local Standard of Rest Local Standard of Rest (LSR): Reference frame for measuring velocities in the Galaxy. Position of the Sun IF its motion were completely governed by its orbital motion around the Galaxy (The Sun (and most stars) are on slightly perturbed orbits) Sun is moving ~20 km/s towards RA=18h Dec=30 deg and lies 10-20 pc above Galactic plane... Ro = 8.5 kpc (8.0 kpc) Vo = 220 km/s (200 kpc) Finding Ω(R) - Galactic Rotation Curve Measure Gas Particle Speeds... Which wavelength? Finding Ω(R) - Galactic Rotation Curve Determined from HI 21-cm line Assume circular orbits and that there is at least some H all along any given line-of-sight Finding Ω(R) - Galactic Rotation Curve For planets in the Solar System, Minterior is dominated by Msun, so M does not change much with R - Keplarian rotation curve Inside the Galaxy, Minterior increases with radius, so velocity may stay constant or even increase with R. Outside the Galaxy, as in the Solar System, Minterior remains constant with increasing R. HUH?? Combined rotation curve shows no fall-off beyond edge of visible disk. Since luminous matter decreases beyond 15 kpc, some additional non- luminous material (i.e. Dark Matter) must exist in the galaxy! Dark matter - not necessarily confined to the disk, likely to be distributed in the Galactic Halo The Galactic Center Inner 500pc of Galaxy Extinction makes optical studies impossible - use radio or IR Observe ionized gas, line emission, dust, star clusters Resolution greatly improved with recent VLA and VLBI observatories, plus sensitive IR arrays Galactic Center Optical vs Radio observations •Radio emission shows bent arc of gas, filamentary structure •Also seen in IR •Thermal and synchrotron radiation •X-ray emission (produced when electrons from filaments collide with colder gas cloud) gives gas temperatures of T=107 to 108 K •Could result from past SN explosions Massive Black Hole in GC Radio image (80 pc Radio image (10 pc Investigate IR across) shows across) shows stellar motions in feature SgrA and feature known as region about 1pc radio filaments SgrA* - thought to across (~few ly) be position of to estimate BH SMBH mass SUPERMASSIVE BLACK HOLE •Measure proper motions of stars in GC •90 stars identified and proper motions centered about SgrA* to within 0.1” •Velocities consistent with Keplarian motion (all mass at center) 6 •M = 2.6 +/- 0.2 x 10 Msun Curvature of the paths near SgrA* constrain the volume of the mass to ~ Schwarzchild radius (few x 106 km), supporting SMBH theory. Additional evidence - x-ray emission •Chandra X-ray image of Sgr A* showing nucleus and several thousand other X-ray sources. •During 2-week observation period, several X-ray flares occurred. •Rapidity of flares indicates they originate near the Schwarzchild radius of the BH. •Even during the flares, X-ray emission from the nucleus is relatively weak. Suggests that Sgr A* is a starved black hole, possibly because explosive events in the past have cleared much of the gas from around it..
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