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Transcript
High velocity clouds (v > 90 km/s), up to 108 M_sun in total! Seen at 21 cm, with high velocities up to 500 km/s. Mixed metallicity. ! Many partially ionized, and can contribute up to 1 M_sun/year (Lehner & Howk 2011)! ! Galactic fountain! However, many HVCs have subsolar metallicity suggesting a more primordial orig Magellanic stream! 21cm emission, about 180 deg across. Tidal debris tail. Gas falling into the Milky Way! Could be as much as 0.4 Msun/year(van Woerden et al. 2004)! Coronal gas! Observed in highly ionized lines, e.g. far-UV OIV (absorption).! Astronomy 422! Lecture 6: The Milky Way Galaxy II! Term paper topics:! ! ! Outline is due on March 8, make sure you have started your research before then.! ! Avery, Montie ! !CMB and cosmology! Dike, Veronica ! !Fast Radio Bursters ! Jackson, Kathryn! !Black hole formation and growth ! Leyba, Kirtus ! !Dark Matter vs MOND ! Lopez, Jessica ! !Cluster magnetic fields ! Quintana, Chris ! !Finding Supermassive Black Holes ! Sansistevan, Isiah !Galaxy Mergers ! Tallbrother, Andrea !Cosmic Dawn ! Trapp, Cameron ! !Modeling Galaxy Formation/Evolution ! Vaitkus, Austin ! !Large Scale Structure and Motions of galaxies ! ! ! Galactic bulge, as observed by COBE (1.2 to 3.4 micrometers).! Vela pulsar! LWA Reveals Giant Radio Bubbles? ! Key concepts:! ! !Milky Way kinematics! ! !Galactic coordinate system! ! !Rotation curve! ! ! Galactic coordinate system! Equatorial coordinate system is inconvenient when talking about Galactic structure and kinematics.! Galactic equator: midplane of Galaxy on the sky! Galactic latitude (b): angle north or south of equator! Galactic longitude (l): angle east along Galactic equator, 0º at Galactic center.! Through Galactic Center! What is the α, δ of the center of the Galactic coordinate system?! ! l = 180º! l = 0º, b = 0º => !α = 17h45m37.s20! ! ! δ = -28º56´9.˝6! ! l = 270º! l! l = 0º! anti-center direction! There are, of course, spherical trigonometric conversions between equatorial and Galactic coordinates (see C&O 24.3).! l = 90º! To investigate motions, we want a coordinate system with the Galactic center at the origin, a cylindrical coordinate system.! ! ! ! ! Radial coordinate R increases outward, angular coordinate θ pointed in direction of rotation, and vertical coordinate z increases to the north. ! Corresponding velocity components are:! Local Standard of Rest (LSR)! ! The dynamical LSR is defined to be a point instantaneously centered on the Sun. This point is moving on a perfectly circular orbit along the solar circle (= circle of radius R0). Then:! ! Why isn't solar motion = LSR motion? ! The Sun is slowly drifting inward, and north; it has a peculiar velocity. ! Note:! ! The kinematical LSR is defined to be a point where the velocity is equal to the average stellar velocity for stars close to the Sun. ! ! This is the convention used by observational astronomers!! ! The peculiar velocity of a star is its velocity relative to the dynamical LSR.! ! ! ! ! ! ! ! For the Sun, ! For a large sample of stars:! <u> = 0 and <w> = 0 for stars in the solar neighborhood - as many going ! ! in as out, up or down.! ! BUT: <v> ≠ 0! ! Why? Stars near us (and near LSR) are at apogalacticon or perigalacticon.! From Kepler's laws: ! Thus, there will be more stars on orbit A.! Then, <v> = negative = asymmetric drift! How to measure Θ0! Measure solar motion relative to group of objects with no net rotation around Galactic center:! ! • Globular clusters, RR Lyrae stars! • External galaxies! Will appear that they are streaming toward us at -Θ0! Θ0! Find Θ0 = 220 km/s! ! Q: How many times has the Sun revolved around the Galactic center? Take the age of the Sun to be 4.5 billion years:! 9! Relation peculiar velocity and age! The older a star is, the more its motion is departing from the LSR in general.! Halo stars with no rotation should reflect a negative rotational motion (-220 km/s). ! What is Θ at other radii?! ! Θ( R) vs R is called rotation curve.! If we observe Doppler shift of star or gas cloud at S,! then:! ! ! ! GC! Θ! Θ 0! Tangent Point method! ! What if we don't know R?! In practice, we measure Vr of HI clouds along line of sight through Galaxy.! ! HI profile caused by several, distinct clouds along line of sight.! The cloud with the maximum radial velocity is the one at Rmin. ! Here, α=0°, cos α=1, and the LOS is tangent to its orbit.! If R0, Θ0 known, we get Θ(Rmin). Repeat for other l's, gives Θ(R ). ! This works in inner Galaxy only, within ±90° of Galactic Center. Does NOT work for 90°<l<270°, since there are no tangent points!! Result: rotation curve is flat from R ~ 4kpc to at least R ~ 16 kpc.! ! Other spiral galaxies similar (Rubin et al 1978)! Implication of Θ(R )=constant:! Recall the Solar system, central mass M, which is in Keplerian rotation:! ! ! Keplerian! V! r! Rigid body rotation: like a spinning disk,! Rigid body! V! r! For a flat rotation curve, v is constant with r.! Flat! V! r! Estimate of mass! ! Assume spherical distribution of mass, define M(r ) as mass inside a radius r.! ! Acceleration of star/gas clump at radius r is! ! ! Acceleration produced by gravitation! ! ! Yielding an expression for the interior mass ! In general, , where M (r ) is the mass interior to r. ! ! This is how V should fall off with r as long as all of the mass is interior to the orbits being considered. Now, consider a spherical distribution of mass of uniform density, in which particles (stars) orbit inside the mass distribution. The mass interior to the orbit is then ! ! ! ! ! Measuring rotation curve gives info about mass distribution! ! Flat rotation curve, for the velocity to be constant means:! ! 3! and M α r so total Galaxy mass increases with radius A more appropriate form for the density distribution is:! ! ! ! where C and a are constants.! However, halo star counts have shown that! ⇒ there is extra unseen mass with shallower r dependence.! Crude lower limit on dark matter mass! Model of MW rotation curve.! Milky Way rotation curve! Dark Matter halos! AT large radii there is little starlight. There is 5-10 times as much dark matter associated with galaxies, as ordinary matter.! Dark Matter candidates! ! • MACHOs (Massive compact halo objects)! • Brown dwarfs (low mass stars)! • White dwarfs (burn-out stars)! • Neutron stars (dead stars)! • Stellar black holes (dead stars)! • mini (primordial) black holes! • massive (primordial) black holes! • WIMPS (Weakly interacting massive particles: neutrons, axions, etc).! • other?! Sample Problem! ! How much dark matter is in this room?! ! How much dark matter is in the Sun?! Next time: ! ! !The Galactic Center and its super massive black hole (SMBH)! !Mass distribution! !Radio and X-ray sources! !! ! Read chapter 24.4!
