Molecular absorption in Cen A on VLBI scales

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Molecular absorption inCen A on VLBI scales

• Huib Jan van Langevelde, JIVE
• Ylva Pihlström, NRAO
• Tony Beasley, CARMA

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Goals Questions

• The physical and chemical properties of starburst
  circumnuclear disks
• What are good tracers of circumnuclear disks?
• That allow high resolution VLBI measurements?
• To weight black holes and measure accretion rates
• Is the Cen A absorption associated with the
  nucleus or outer galaxy?
• Geometric constraints (VLBI imaging)
• Excitation constraints (modeling)
• Chemical signatures, altered abundances
• Physics of interstellar medium
• In starburst/AGN connection

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Centaurus A

• In NGC5128, nearest AGN
• D 3.4 Mpc, 1 16pc
• Large double lobed radio source
• Modest black hole ( 107 M?)
• VLBI source, subluminal
• Strong mm core
• Elliptical with dust band
• Recent merger (few 108 109 yr)
• Star formation in dust band
• Gas settling in potential of Elliptical
• Feeding the central engine?

1pc
Cen A at 2.2 GHz Tingay Murphy 2001 ApJ 546 210
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Molecules
Israel et al, 1991 AA 245 L13
Seaquist Bell 1990 ApJ 364 L94
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Absorption HI

• Absorption against nuclear radio source
• Several narrow components on VLA scales
• Red-shifted absorption only seen against core

Sarma et al, 2002 ApJ 564 696
Peck 1999, NMIMT thesis
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Molecules IR

• Molecular ring at 1.5 kpc
• Warped or barred?
• And a circumnuclear disk
• Warmer gas at 100pc

100 pc
1 kpc
Wild et al 1997 AA 322 419
Leeuw et al 2002 565 131
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Models

• Molecular ring circumnuclear disk
• Tilted and precessing thin rings
• Or even a bar
• Constrained by far-IR and CO imaging
• Co-rotating high latitude gas may explain
  absorption

Liszt 2001 AA 371 865
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Previous ATCA result on OH
OH 1667 absorption at 550 km/s
Van Langevelde et al 1995 ApJ 448 L123
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Excitation model

• Simple escape probability calculation
• But does include OH hyperfine transitions
• fOH (2-6) 10-7 (bit high compared to Galactic)
• fform 10-10 (low compared to Galactic)
• 2 components
• local molecular cloud n104 cm-2, Tkin 10K
• And 43 K IR radiation field
• Additional radiation from AGN components
• 150 K IR from circumnuclear component
• Important for OH excitation
• Flat spectrum radio source
• Important for H2CO excitation
• Parameterized as distance from nucleus

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blue
red
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systemic
red
systemic
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red
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systemic
systemic
blue
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VLBA observations

• 1995 1997
• 10 Antennas, but 8 useful
• Limited coverage
• d -43o
• Hard to image
• Good spectral resolution
• Confirms previous funny OH result

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OH absorption on VLBI scales
Systemic
Red-shifted
1 pc
1667 absorption at 600 km/s
1667 absorption at 550 km/s
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3s upper limit on t
t detected (gt2s)
jet side
counter jet side
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H2CO absorption
1 pc
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Where is the core?
Tingay Murphy 2001 ApJ 546 210
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Continuum
Imaging very difficult due to low elevation and
limited mutual visibility
1 pc
0.1 pc
2 cm
18 cm
6 cm
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H2CO absorption
1 pc
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systemic
t 6cm maser
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Interpretation

• Absorption gradient over source
• 20 mas 0.34pc scale !?
• Could this be of nuclear origin?
• Must be very thin, and still unrealistically
  close
• Excitation, and velocity width seem to rule this
  out
• Or in outer dust band?
• Must have completely different scale size than
  OH, HCO, CO
• Small clump at large distance
• Chance alignment?
• Note H2CO does not show red-shifted component
• Size OK for small scale structure

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Models

• Molecular ring circumnuclear disk
• Tilted and precessing thin rings
• Or even a bar
• Constrained by far-IR and CO imaging
• Co-rotating high latitude gas may explain
  absorption

Eckart et al 1999 ApJ 516 769
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Conclusions

• Favour location in molecular ring
• OH observations do not constrain location much
• Confirming the ATCA conjugate line results
• Notably traces different line of sight than most
  molecules
• Excitation anomalous, but not excited by nuclear
  source
• H2CO seems more complex
• Absorption gradient over source
• 20 mas 0.34pc scale!?
• Still favour location in molecular ring 200
  2000 pc
• In which H2CO is confined to the densest parts of
  the molecular ring (low z)
• Excitation shows no sign of central radio source
• Still need to work out abundances
• Can probably compare H2CO to millimeter lines
  directly, distinctly different from OH

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OH continuum
1609 MHz
1716 MHz
1661 MHz
1663 MHz
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• Competition of satellite transitions
• Depends critically on optical depth in far IR
• Only in modest IR field and Galactic conditions
• Allows a direct measure of OH column in
  individual clump
• By comparing total column in absorption
• And local column in excitation

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OH ATCA maps
OH 1720 emission at 600 km/s
OH 1667 absorption at 550 km/s
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Centaurus A

• NGC 5128 picture
• Dust band
• Recent merger