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
Compton Gamma Ray Obs.
(Why??)
Multi-Wavelength Milky Way (mwmw)
Southern MW
Structure of MW:Historical
Sun
Assumptions:All stars have same intrinsic brightnessStars are arranged uniformly throughout the MWHe could see to the edge of the system
1785 - Herschel: attempted to determine shape and size of Galaxy
What’s wrong with this picture??
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!
amoeba?
Structure of MW: Shapley
Used Cepheid and RR Lyrae variables to determine distribution of globular clusters and correct Herschel’s map
47 Tucanae
How??
(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
NGC 290 (Jewel Box) in LMC, Hubble image
IC 1805
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 MWEverything in the Galaxy orbits around the
Galactic centerMaterial 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 MassKepler’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)???
Differential galactic rotation produces Doppler shifts in emission lines from gas in the Galactic disk
Doppler Shift Due to Differential Motion
**But Sun is NOT Static
Consider material at a distance R from GC, moving with v(R) ...
Radial velocity relative to Sun yields a Doppler shift.
Doppler Shift Due to Differential Motion
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?
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??
Finding Ω(R) - Galactic Rotation Curve
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 CenterOptical 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 across) shows feature SgrA and radio filaments
Radio image (10 pc across) shows feature known as SgrA* - thought to be position of SMBH
Investigate IR stellar motions in region about 1pc across (~few ly) to estimate BH mass
•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)
•M = 2.6 +/- 0.2 x 106 Msun
SUPERMASSIVE BLACK HOLE
Curvature of the paths near SgrA* constrain the volume of the mass to ~ Schwarzchild radius (few x 106 km), supporting SMBH theory.
•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.
Additional evidence - x-ray emission