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 origin! 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 = 0º, b = 0º => α = 17h45m37.s20 l = 180º δ = -28º56´9.˝6 l = 270º l = 90º
l!
l = 0º anti-center direction
There are, of course, spherical trigonometric conversions between equatorial and Galactic coordinates (see C&O 24.3). 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: = 0 and
Thus, there will be more stars on orbit A. Then,
Will appear that they are streaming Θ0 toward us at -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?! GC Θ( R) vs R is called rotation curve. If we observe Doppler shift of star or gas cloud at S,! then: Θ
Θ0 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° V! Keplerian! r! Rigid body rotation: like a spinning disk, V! Rigid body! r! For a flat rotation curve, v is constant with r. V! Flat! 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