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Molecular absorption in Cen A on VLBI scales

Molecular absorption in Cen A on VLBI scales. Huib Jan van Langevelde, JIVE Ylva Pihlström, NRAO Tony Beasley, CARMA. Goals & Questions. The physical and chemical properties of starburst & circumnuclear disks What are good tracers of circumnuclear disks?

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Molecular absorption in Cen A on VLBI scales

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  1. Molecular absorption inCen A on VLBI scales Huib Jan van Langevelde, JIVE Ylva Pihlström, NRAO Tony Beasley, CARMA

  2. 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 huib 16/2/2004

  3. 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 huib 16/2/2004

  4. Molecules… Israel et al, 1991 A&A 245 L13 Seaquist & Bell 1990 ApJ 364 L94

  5. 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 huib 16/2/2004

  6. Molecules & IR • Molecular ring at ±1.5 kpc • Warped or barred? • And a circumnuclear disk • Warmer gas at 100pc 100 pc 1 kpc huib 16/2/2004 Leeuw et al 2002 565 131 Wild et al 1997 A&A 322 419

  7. 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 A&A 371 865 huib 16/2/2004

  8. Previous ATCA result on OH 1 kpc OH 1667 absorption at 550 km/s Van Langevelde et al 1995 ApJ 448 L123 huib 16/2/2004

  9. 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”: 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 huib 16/2/2004

  10. red blue red systemic systemic blue

  11. VLBA observations • 1995 – 1997 • 10 Antennas, but 8 useful • Limited coverage • δ = -43o • Hard to image • Good spectral resolution • Confirms previous funny OH result huib 16/2/2004

  12. OH absorption on VLBI scales Systemic Red-shifted 1 pc 1667 absorption at 600 km/s 1667 absorption at 550 km/s huib 16/2/2004

  13. 3σ upper limit on τ τ detected (>2σ) jet side counter jet side

  14. No redshifted H2CO 3 narrow components No 15 GHz detection

  15. H2CO absorption 1 pc huib 16/2/2004

  16. Where is the core? Tingay & Murphy 2001 ApJ 546 210 huib 16/2/2004

  17. Continuum Imaging very difficult due to low elevation and limited mutual visibility… 1 pc 0.1 pc 2 cm 18 cm 6 cm huib 16/2/2004

  18. H2CO absorption 1 pc huib 16/2/2004

  19. systemic |τ| 6cm maser

  20. 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 huib 16/2/2004

  21. 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 huib 16/2/2004

  22. 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 huib 16/2/2004

  23. OH continuum 1609 MHz 1716 MHz 1663 MHz 1661 MHz huib 16/2/2004

  24. 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

  25. OH ATCA maps OH 1720 emission at 600 km/s OH 1667 absorption at 550 km/s huib 16/2/2004

  26. Centaurus A NGC 5128 picture Dust band Recent merger

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