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Lecture 3: The Milky Way Galaxy 2Kinematics and the Galactic Center
● current astronomical events● kinematics of the Milky Way● the Galactic center
Current Astronomical Events2005 January 19th
Deep Impact on it's way: image from Palomar 200”
Huygens' descentinto Titan
Huygens' view of Titan:● only moon in solar system with significant atmosphere ● white areas = ground fog?● drainage channels?● shoreline?
Huygens' view of Titan's surface:● pebble-sized rocks/ice blocks● evidence of erosion● muddy?
Comet Machholz:near the Pleides currently
HST Cycle 14 DEADLINE:
January 21st -- 8 PM(Friday)
Chandra shows Galactic center chock full of variable objects, most likely either neutron star or black hole binaries
Kinematics of the Milky Way Galaxy
Celestial coordinate systems:● equatorial system: right ascension (R.A.; α ) and declination (Dec., δ)● ecliptic system● Galactic system: Galactic latitude (b) and Galactic longitude (l)● cylindrical system
Kinematics of the Milky Way Galaxy
Celestial coordinate systems:● equatorial system: right ascension (R.A.; α ) and declination (Dec., δ)● ecliptic system● Galactic system: Galactic latitude (b) and Galactic longitude (l) ● cylindrical system
(thru Galactic center)
● Galactic center●
● star
sun
z
R θ
Π = dR/dtΘ = Rdθ/dtZ = dz/dt
cylindrical coordinate system used for Galactic kinematics
Distance to the Galactic center
● centroid of globular cluster distribution● centroid of RR Lyrae distribution● centroid of Mira distribution
Harlow Shapley
Distance to the Galactic center
● kinematics of Cepheids● kinematics of OB stars● kinematics of HII regions
● G.C.
*
Sun *
*
Distance to the Galactic center
geometry of expanding H20 maser in Sgr B2 North:● measure radial velocity and change in angular size with
time; assume axisymmetry to get distance
expanding shell
Sun
Distance to the Galactic center
time delays of OH masers:● use variations in light curves of blue-/red-shifted
spectrum to derive size; compare with measured angular scale to derive distance
expanding shell
Sun
Distance to the Galactic center
● trigonometric parallax of Galactic center
Distance to the Galactic center
● orbits of stars around Galactic center
Distance to the Galactic center
● centroid of globular cluster distribution● centroid of RR Lyrae distribution● centroid of Mira distribution● kinematics of Cepheids ● kinematics of OB stars● kinematics of HII regions● geometry of expanding H20 maser in Sgr B2 North● time delays of OH masers
● trigonometric parallax of Galactic center● orbits of stars around Galactic center
R0 = 8.0 ± 0.5 kpc(old IAU standard of
8.5 kpc now outdated)
Terminology
● solar circle: perfect circle of radius R0 around Galactic center● LSR (local standard of rest): point which is instantaneously centered on the Sun and moving in a perfectly circular orbit around the Galactic center
● Galactic center●
● star
sun
z
R θ
Π = dR/dtΘ = Rdθ/dtZ = dz/dt
cylindrical coordinate system used for Galactic kinematics
ΠLSR= 0ΘLSR= Θ0ZLSR= 0
Terminology
● solar circle: perfect circle of radius R0 around Galactic center● LSR (local standard of rest): point which is instantaneously centered on the Sun and moving in a perfectly circular orbit around the Galactic center● peculiar velocity: velocity relative to LSR
V = (VR, Vθ, Vz) = (u, v, w)= (Π, Θ – Θ0, Z)
● solar motion: the Sun's peculiar velocity● apogalacticon: most distant point (from G.C.) in Galactic orbit● perigalacticon: closest point (to G.C.) in Galactic orbit
● Galactic center●
● star
sun
z
R θ
∏ = dR/dtΘ = Rdθ/dtZ = dz/dt
cylindrical coordinate system used for Galactic kinematics
⟨u⟩ = ⟨Π⟩ = 0⟨v⟩ = ⟨Θ – Θ0⟩ < 0
⟨w⟩ = ⟨Z⟩ = 0
relative to LSR:
● Galactic center●
● star
sun
z
R θ
∏ = dR/dtΘ = Rdθ/dtZ = dz/dt
cylindrical coordinate system used for Galactic kinematics
u(sun) = ⟨∆u⟩ = -9 km/sv(sun) = 12 km/s
w(sun) = - ⟨∆w⟩ = 7 km/s
measure the solar motion:
Oort's equations
better derivation thanthe textbook provides...
Jan H. Oort (1900-1992)
Rotation curve of Milky Way
(GMm)/R² = mv²/R
v ~ R-½
Rotation curve of Milky Way
Rotation curve of UGC 9242
Rotation curve of UGC 9242
Rotation curves
● Keplerian rotation: if most of the mass is interior
to R (as per solar system), v ~ R-½
● rigid-body rotation: like a spinning disk, v ~ R (i.e., ω = v/R = constant)
● flat rotation: implies M(R) ~ R, or ρ(R) ~ R-2
● better model is ρ(R) = C0/(a² + R²),where C0 = 4.6 x 108 M(sun)/kpc
a = 2.8 kpc
stellar distances revisited:moving cluster method
stellar distances revisited:moving cluster method
stellar distances revisited:moving cluster method
stellar distances revisited:secular parallax
overall solar motion (w.r.t. LSR) = 16.5 km/s = 3.5 A.U./yr
use Sun's motion over several years to increase baseline for parallax measurements
The Galactic Center
● 28 mag of extinction in the optical● solar motion of w =7 km/s will give us a good view in 15 Myr● forced to work at IR, X-ray, and gamma-ray
X-ray imagefrom Chandra(variable sources)
near-IR image from Gemini Observatory(JHK; 1.2-2.3 µm)
radio image from VLA
Sgr A East
shell-like appearance:young supernova remnant?
Sgr A West
● mini-spiral● includes Sgr A*, a strong, unresolved radio (point) source● no proper motion for Sgr A* -- very massive
infrared studies of the Galactic center:speckle imaging
infrared studies of the Galactic center:adaptive optics
Andrea Ghez (UCLA)using Keck telescopes
infrared studies of the Galactic center:adaptive optics
Reinhard Genzel(NTT + VLT)
infrared studies of the Galactic center:adaptive optics
Andrea Ghez(Keck)
M = 2.6 x 106 M(sun) in small area --- must be a black hole!
infrared studies of the Galactic center:fluctuations of Gal. center
~40 min
telescope resolution ~
lambda/diam.
infrared studies of the Galactic center:Sgr A* resolved
Bower et al. (2004)
infrared studies of the Galactic center:shadow of black hole
simulation of shadow cast by black hole
THE END
scenes from next class:● the Hubble sequence● spiral galaxies
