Transcript Max_BHs_in_Nearby_Gals.ppt

Black Holes in Nearby Galaxies
Claire Max
NGAO Team Meeting
March 7, 2007
Outline
• General principles of black hole mass
measurements
• Potential benefits of NGAO, and science
requirements (first cut)
General Principles
• Equate kinetic energy of rotation with gravitational
potential energy:
mgas v
2

M black  hole 
G M black  hole mgas
R
R v 2
G
• Spatial resolution matters: need to resolve R’s as small
as possible. Width of PSF gives an upper limit on black
hole mass (can’t “see” any closer).
• PSF stability matters: R must be well determined
Estimate radius of black hole’s
“gravitational sphere of influence”
• Equate kinetic energy, gravitational potential energy
aBH 
2GM BH
2
 M BH   100 km/sec 
 9

 kpc


 10 M sun 
2
• Example: 200 km/sec, 2 x 108 solar masses, aBH= 50 pc
• Within this distance from the black hole, gas and stars “feel”
the black hole’s gravity
• To measure the black hole’s mass, must be able to resolve aBH
• Keck AO today resolves 30 pc at z = 0.02, 50 pc at z = 0.05.
• NGAO will resolve even smaller distances (e.g. using Ca triplet
at 8498 Å, 8542 Å, 8662 Å ).
Two general types of measurements:
1) Keplerian circular velocities
• Assume material is in isotropic, circular Keplerian rotation around
the black hole. Measure radial velocities (component of velocity in
and out of plane of the sky).
• Examples: water masers in NGC 4258 (mm wave); orbits of
individual stars in Galactic Center
Two general types of measurements:
2) Velocity dispersions
• Measure velocity dispersion as function of position (e.g. Sauron IFU)
M black  hole 
R v 2
G
R 2

G
NGC 3377, Sauron IFU
• Issues:
– Stellar velocity dispersions in elliptical galaxies: velocities are not
isotropic. Need to model the orbits in some detail. Challenging.
– Gas velocity dispersions: gas velocity fields are often quite disordered
due to non-gravitational forces.
– Increased spatial resolution helps in both cases. Ca triplet helps a lot.
Additional Considerations
• May need to use IFU together
with set of long-slit spectra, to
unravel gravitational field of
the larger scale galaxy
• Alternatively, can use a wider
field mode of the IFU
• Necessary for non-ideal
galaxies:
– Kinematically decoupled cores
– Bars
– Warps
– Merger remnants
40 arc sec
NGAO benefits and science requirements
• Benefits of using Calcium
Triplet (8500 - 8660 Å) with
decent wavefront error
– At a given distance from us,
can measure lower black hole
masses
– At given black hole mass, can
detect BH in more distant
galaxies
• Key performance metrics:
– Low wavefront error
– Operation at 8500 Å
– Stable PSFs
Science and Instrument Requirements
• AO system:
– Wavefront error low enough to give “good” performance at I band
(needs to be quantified in simulations, work is in progress)
– Stable PSF in near IR (needs to be quantified further)
• Instruments: single IFUs
– I band out to K band
– Field of view: Narrow mode (a few arc sec) to resolve gravitational
sphere of influence. Possibly a wide mode as well (up to 30 arc sec) to
map galaxy kinematics on larger scales.
– Velocity resolution not a key driver: a few 10’s of km/sec (OSIRIS today
can do this). (Need to quantify requirement.)
• Instruments: long slit spectroscopy
– Key if wide field mode of IFU is not available
– Slit length up to 30 arc sec, configurable at arbitrary angles