Astronomy 422 Lecture 7: The Milky Way Galaxy III
Key concepts: The supermassive black hole at the center of the Milky Way Radio and X-ray sources Announcements: Test next Tuesday, February 16 Chapters 12, 15.5 and 24
The Galactic Center Up to 30 mag extinction in the optical (only 1 in 1012 photons reaches us) Solar motion of w =7 km/s would give us a clearer view in 15 Myrs from a height of 100 pc So we are forced to work at radio, IR, X-ray and gamma ray wavelengths.
Galactic Center seen by IRAS
GC is about 8 kpc away, at a declination of -29, well in the southern hemisphere. It is very busy, as shown by this 1x1 kpc VLA map.
HESS (TeV) gamma-ray results HESS image HESS residual image
X-ray view of Galactic Center Deep Chandra Image - Diffuse emission - hot gas - Filaments -??? - X-ray binaries - SgrA*
X-ray image from Chandra, showing variable sources and SgrA* SgrA*
Chandra results
Chandra results
Chandra results Baganoff et al. 2001 0 10^4 2 x 10^4 3 x 10^4 Time (s)
Most information we have about the Galactic Center region is thus from IR and radio: 1 kpc: expanding gas at 100km/s (explosive event 10Myr ago?) 100 pc: Sgr A (strongest radio source on the sky). Synchrotron radiation, thus also magnetic fields. 10 pc: pinwheel gas structure <1 pc: Sgr A*, pointlike radio/x-ray/ir source Nucleus: Supermassive black hole ~3x10 6 M inferred from: IR, X-ray variability ~1min => size <1AU 10 6 M much more massive than e.g. a neutron star Stellar orbits
There are plenty of supernova remnants - the most obvious one is SNR359.1-00.5. Stars are very tightly packed in the central region, reaching densities of up to 10 7 M pc -3 near the Sun the density is only about 0.05 M pc -3 in globular clusters the central density reaches 10 4 M pc -3 => Stellar collisions probably frequent (1 every 10 6 yrs).
Emission from ionized gas is observed, heated from inside by hot stars, (e.g. Sgr D, Sgr B1 and B2, etc). There is a giant arc of radio emission showing filamentary structure. Likely this is associated with long, coherent magnetic field lines, much stronger than typical in the ISM.
Moving closer to the center, the picture gets more complex. The center-most source is called Sgr A, divides up into Sgr A West Sgr A East Sgr A * SgrA * is extremely compact, and is almost certainly due to the central black hole itself. Sgr A West appears like a mini spiral in an image at 6cm - complicated structure in the ionized gas.
Initial detection in IR started a search for energetic UV source (to heat the dust seen in IR). NIR from Gemini, 1.2-2.3µm. Stars detected close to SgrA*
Scale of central region about 0.3 pc, IR (1.6, 2.2 and 3.8 microns). Sgr A * located near the center of image (not bright in IR). Most of these stars are very young and massive and heavily reddened which is why they don't appear blue. Spectroscopic studies indicate that the stars are luminous super giants and only a few 10s of millions years old.
Consequently, IR stars can be observed very close to the center of the Galaxy. They move so fast (500 to 1000 km/s) that their proper motions are measurable even after a few years. In recent years even their orbits around the central massive source have been detected. 1992-2008 ESO 16 year time-elapse sequence
Animation showing the orbits 1995-2010 Note: S0-2 with period 15.56 yrs SO-16 which comes as close as 90AU from the dynamical center.
Enclosed mass in the center of the Milky Way as a function of distance from the center, supporting the presence of a black hole of mass 3.6x10 6 M at the Galactic center. The stars can be seen so close to the central source, that only a black hole is a reasonable explanation for so much matter in such a tight space.
Gas exists close to the Galactic Center Why doesn't plasma and gas fall into the black hole, making it a powerful source at many wavelengths? Measurements only show a very weak source at the center, perhaps 100 times weaker than predicted based on the black hole mass and the amount of material available to fall in. Apparently the black hole is quiet for some reason.
Modeling the accretion flow
Is the black hole the dynamical center? Supporting evidence comes from the proper motion of Sgr A *. At 8 kpc from us its proper motion is very small, about 7 mas/yr. It moves at a straight line across the sky. The amount turns out to exactly reflect the motion of the Sun around the Galaxy => Sgr A * is clearly the Milky Way's dynamical heart.
Where is the Schwarzschild radius? 43 GHz VLBA Resolution ~ 0.25 mas unpublished Imaged down to 215 GHz., Krichbaum et al, 2005.
Imaging the Event Horizon The event horizon can cause a 'shadow' due to bending of the light. Estimated size 30 microarcsec, which might be possible to image with VLBI. Measured size at 43 GHz is ~ 250 microarcsec or ~2AU Bower et al 2004.
Possible models for emission region Sgr A*
Problem: Derive the Schwarschild radius for the black hole at the center of the Milky Way Galaxy. Give the result in cm and in AU.
Next time: Normal galaxies The Hubble sequence Read chapter 25.1