Searching for supermassive black holes

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The Milky Ways Supermassive Black Hole
The Milky Ways Supermassive Black Hole

• Dr. Benjamin Bromley
• Department of Physics
• University of Utah

Dr. Benjamin Bromley Department of
Physics University of Utah
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Overview

• Astrophysical black holes Static
  background metrics
• No waves, yet
• Extragalactic evidence
• X-rays from MGC-6-30-15
• The Galactic Center Sgr A the hole in the
  middle

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Black holes
An observational wish list

• Mass M
• Size the gravitational radius Rg
  GM/c 2 (1.5 km / Msolar)
• Angular momentum J spin a J
  x c/GM (max a M)
• Maps of particle orbits
  (stable circular orbits inside 3 Rg if agt0)

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A trip toward a black hole
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Astrophysical black holes

• Stellar-mass -- a few Solar masses --
  end-products of supernovae
• Middleweight -- (nucleus of M33, Globular
  clusters)
• Supermassive -- millions to billions of Solar
  masses -- centers of many (most) galaxies

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Quasars

• Small size (gt 1 pc)
• Enormous brightness (1013 Lsolar )
• Black hole inside accretion disk
  relativistic jets MBH 107 - 1010
  Msolar

3C296
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M87
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M87
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Seeking strong gravity

• Mapping curved spacetime Distance scales
  Rg GM/c2
• Preferred Reference clock (known
  frequency) kinematical tracer (known
  orbit)
• Iron Ka fluorescence in accretion disk (6.4
  keV Xray emission line)

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A Relativistic accretion disk
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Viewing a relativistic disk

• Doppler shift
• Gravitational redshift
• Gravitational lensing
• Disk emissivity

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Relativistic line profile

Flux (arbitrary units)
4 6
8 Energy (keV)
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Relativistic line profile MGC-6-30-15

Flux (arbitrary units)
4 6
8 Energy (keV)
Tanaka et al. 1995
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Relativistic line profile MGC-6-30-15

• Suggestion of black hole spin? Iwasawa et
  al. 1997
• Red edge of line Inner edge of disk 3 Rg
  Bromley, Miller Pariev 1998
• Warm absorber, intervening dust Clouding
  the interpretion Fabian et al. 2002, 2003

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The SMBH next door

• Many galaxies host SMBHs why not our Galaxy?
• The candidate Sgr A -- polarized radio
  source -- dynamical center of the Galaxy --
  variable X-ray emitter

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The center of the Milky Way
2MASS
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Sgr A Milky Ways Supermassive Black Hole
X-ray (Chandra)
Radio (VLA)
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Stellar motion near Sgr A
From Eckart et al. (2003)
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Sizing up Sgr A

• Stellar orbits ( 10 light-days)
• X-ray flares ( 10 light-minutes)
• 1013 cm (Rg 4 x 1011 cm)

(3 million Solar masses)
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Clues in the Radio

• Peak signal at 300 GHz
• Sub-mm excess
• Linear polarization
• Polarization flip

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The Radio Spectrum
I Jy 10-23 erg/s/cm2/Hz (Melia
Falcke 2001)
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Flow near the horizon

• Overall picture Infalling gas forms rotating
  disk Differential rotation winds up B-field
• Details MHD model (Melia, Coker Liu, 2001) B
  10 G, T 1010 K, r 10-17 g/cm3
• Goal Explain the radio excess/polarization

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Schematic of the flow
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The near-horizon radio signal

• Thermal synchrotron plasma
• emission, absorption along light rays
• spacetime curvature
• interstellar scattering (Bromley, Melia Liu,
  2001)

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Polarimetric map
400 GHz 250 GHz 150 GHz
combined scattering vertical
horizontal
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Sub-millimeter bump
Negative vs Positive polarization 90o change
in orientation (Aitken et al. 2000)
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Sub-millimeter bump
Negative vs Positive polarization 90o change
in orientation (Aitken et al. 2000)
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The polarization flip

• Low frequency multiple scattering,thermal
  equilibrium, no polarization
• Intermediate frequency geometric projection of
  disk
• High frequency beaming from blueshifted part of
  disk

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Disclaimer

• Model explains the sub-mm bump (and X-ray flux)
• Does not account for longer wavelengths
• Small jet? Pre-circularized gas?
• Unsteady flow?

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Black hole spin

• GR effects maximum spin frame
  dragging orbit stability
• Seek periodic signal! 20 min for Sgr A

(Melia, Bromley, Liu Walker 2001)
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Black hole spin detected?

• IR flares within 10 AUof BH
• (quasi) periodic signal! 17 min for Sgr A
• Spin at 50 max

(Genzel et al. 2003, Nature)
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Conclusions

• Strong evidence for a supermassive black hole in
  Galactic Center
• Radio polarization indicates circular flow near
  the horizon
• Future interferometric telescopes will map the
  black hole region!
• Outstanding questions Spin? Formation?

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Relativistic line profile

0.8 0.9 1 1.1
Observed/Rest-frame Energy