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