Molecular absorption in Cen A on VLBI scales Huib Jan van Langevelde, JIVE Ylva Pihlström, NRAO...

Molecular absorption inMolecular absorption inCen A on VLBI scalesCen A on VLBI scales

Huib Jan van Langevelde, JIVEHuib Jan van Langevelde, JIVE

Ylva Pihlström, NRAOYlva Pihlström, NRAO

Tony Beasley, CARMATony Beasley, CARMA

Dense Molecular GasWorkshop Zwolle

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Goals & Goals & QuestionsQuestions

• 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 ACentaurus 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?

Cen A at 2.2 GHzTingay & Murphy 2001 ApJ 546 210

1pc

MolecuMoleculesles……

Seaquist & Bell 1990 ApJ 364 L94Israel et al, 1991 A&A 245 L13


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Absorption: HIAbsorption: 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 & IRMolecules & IR

• Molecular ring at ±1.5 kpc •Warped or barred?

• And a circumnuclear disk•Warmer gas at 100pc

Wild et al 1997 A&A 322 419Leeuw et al 2002 565 131

1 kpc

100 pc


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ModelsModels

•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 A&A 371 865


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Previous ATCA Previous ATCA result on OHresult on OH

Van Langevelde et al 1995 ApJ 448 L123

OH 1667 absorption at 550 km/s

1 kpc


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Excitation Excitation modelmodel

• 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”: n=104 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

red

systemicblue red

systemicblue


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VLBA observationsVLBA observations

•1995 – 1997•10 Antennas, but

8 useful•Limited coverage

• δ = -43o

•Hard to image•Good spectral resolution

•Confirms previous funny OH result


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OH absorption OH absorption on VLBI scaleson VLBI scales

1667 absorption at 550 km/s 1667 absorption at 600 km/s

1 pc

Systemic Red-shifted

jet sidecounter jet side

3σ upper limit on τ

τ detected (>2σ)

No 15 GHz detection

No redshifted H2CO

3 narrow components


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HH22CO CO absorption absorption

1 pc


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Where is the Where is the core?core?

Tingay & Murphy 2001 ApJ 546 210


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ContinuumContinuum

Imaging very difficult due to low elevation and limited mutual visibility…

18 cm 6 cm 2 cm

1 pc 0.1 pc


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HH22CO CO absorption absorption

1 pc

systemic

|τ| 6cm maser


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InterpretationInterpretation

• 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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ModelsModels

• 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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ConclusionsConclusions

• 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 continuumOH continuum

1609 MHz 1716 MHz

1661 MHz 1663 MHz

• 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 mapsOH ATCA maps

OH 1667 absorption at 550 km/s OH 1720 emission at 600 km/s

Centaurus ACentaurus A

• NGC 5128 picture• Dust band• Recent merger

Molecular absorption in Cen A on VLBI scales Huib Jan van Langevelde, JIVE Ylva Pihlström, NRAO...

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Transcript of Molecular absorption in Cen A on VLBI scales Huib Jan van Langevelde, JIVE Ylva Pihlström, NRAO...