1. (AO) Observations of the Galactic Center
Seth Hornstein (UCLA)
CfAO Summer School on Adaptive Optics

2. Why Study the Galactic Center?
Is there a super massive black hole?
What are the interactions of the black hole with its environment?
Lensing
Modified stellar formation
Accretion of material
August 6, 2001
CfAO Summer School on Adaptive Optics

3. Courtesy of the Keck Observatory

4. Courtesy of the Keck Observatory

5. CfAO Summer School on Adaptive Optics
Proof of a Black Hole
Proper Motion Studies
If point mass exists, objects should follow Keplerian (elliptical) orbits
Velocities yield enclosed mass as a function of radius
August 6, 2001
CfAO Summer School on Adaptive Optics

6. CfAO Summer School on Adaptive Optics
Proof of a Black Hole
High resolution is required to get to stars closest to the black hole (possibly with the shortest periods)
Ghez et al., 2000
August 6, 2001
CfAO Summer School on Adaptive Optics

7. More Proof of a Black Hole
Rschwarzschild≈3km*M
For a 3x106 M black hole = 9x106km=0.05 AU!!
In order to prove there is a black hole at the Galactic Center, we must constrain the mass to a very small area. Again, this requires very high resolution.
Enclosed Mass Plot
Ghez et al., 1998
August 6, 2001
CfAO Summer School on Adaptive Optics

8. Effects of the Black Hole on Its Environment
Variability
Stellar population
Accretion disk
August 6, 2001
CfAO Summer School on Adaptive Optics

9. Variability

10. CfAO Summer School on Adaptive Optics
Variability
Lensing
Excellent probe of black hole mass & location.
Modified Stellar formation
How does a 2.6x106 M object effect formation of nearby stars?
Accretion Rate increase
Stationary variability at the position of Sgr A* could be coming from the black hole itself.
August 6, 2001
CfAO Summer School on Adaptive Optics

11. Current Work Variability
August 6, 2001
CfAO Summer School on Adaptive Optics

12. Current/Future Work High resolution spectroscopy
Previous low-res spectroscopy was able to establish the lack of CO bandhead absorption in several of the stars.
However, when looking at features intrinsic to early-type stars, it was unable to distinguish between absorption from the stellar sources and emission from background gas.
Lack of bandheads means not late type. Possibly Early type.
Doppler shifts from the stellar emission lines will allow calculation of the radial velocities of the central cluster of stars. Thus allowing an actual estimate of the enclosed mass.
Gezari et al., 2001
August 6, 2001
CfAO Summer School on Adaptive Optics

13. Current/Future Work Proper Motion
Radial velocities & accelerations
Radial velocities will allow 3-D orbits to be fit. Thus giving estimates of both the positional location of the central dark matter and the enclosed mass.
Acceleration provides another method of confining the center of mass.
Ghez et al., 2000
August 6, 2001
CfAO Summer School on Adaptive Optics

14. Problems With Adaptive Optics at the Galactic Center
Natural guide stars are severely lacking
Keck guide star:
Very far off axis (30” from central cluster)
Dim (Vmag=13.2)
Gemini guide star:
Off axis (18” from central cluster)
Very dim (Vmag=13.8)
Anisoplanatism
August 6, 2001
CfAO Summer School on Adaptive Optics

15. Ohio State image - guide stars
Courtesy of OSU (OSIRIS)

16. CfAO Summer School on Adaptive Optics
Solutions
Infrared Wavefront Sensor
This would allow the use of IRS 7 as the guide star
Kmag=6.6
Distance from GC = 6”
Laser guide star
Larger target audience.
Would be disabled by even a small amount of clouds.
August 6, 2001
CfAO Summer School on Adaptive Optics