the milky way · 2011-05-03 · structure of milky way: current - globular clusters in spherical...

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