Molecular absorption in Cen A on VLBI scales Huib Jan van Langevelde, JIVE Ylva Pihlström, NRAO Tony Beasley, CARMA 18/2/2004 huib 16/2/2004
Goals & Questions Dense Molecular Gas Workshop Zwolle 18/2/2004 • 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 2
Centaurus A Dense Molecular Gas Workshop Zwolle 18/2/2004 3 • In NGC 5128, nearest AGN • D= 3. 4 Mpc, 1” ~ 16 pc • 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? huib 16/2/2004 1 pc Cen A at 2. 2 GHz Tingay & Murphy 2001 Ap. J 546 210
Molecules… Israel et al, 1991 A&A 245 L 13 huib 16/2/2004 Dense Molecular Gas Workshop Zwolle 18/2/2004 4 Seaquist & Bell 1990 Ap. J 364 L 94
Absorption: HI Dense Molecular Gas Workshop Zwolle 18/2/2004 • Absorption against nuclear radio source • Several narrow components on VLA scales • Red-shifted absorption only seen against core huib 16/2/2004 Sarma et al, 2002 Ap. J 564 696 Peck 1999, NMIMT thesis 5
Molecules & IR Dense Molecular Gas Workshop Zwolle 18/2/2004 6 • Molecular ring at ± 1. 5 kpc • Warped or barred? • And a circumnuclear disk • Warmer gas at 100 pc 100 pc 1 kpc huib 16/2/2004 Leeuw et al 2002 565 131 Wild et al 1997 A&A 322 419
Models Dense Molecular Gas Workshop Zwolle 18/2/2004 7 • 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
Previous ATCA result on OH Dense Molecular Gas Workshop Zwolle 18/2/2004 8 1 kpc OH 1667 absorption at 550 km/s Van Langevelde et al 1995 Ap. J 448 L 123 huib 16/2/2004
Excitation model Dense Molecular Gas Workshop Zwolle 18/2/2004 • Simple escape probability calculation • But does include OH hyperfine transitions • f. OH= (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= 10 K • 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 H 2 CO excitation • Parameterized as distance from nucleus huib 16/2/2004 9
Dense Molecular Gas Workshop Zwolle 18/2/2004 10 huib 16/2/2004
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Dense Molecular Gas Workshop Zwolle 18/2/2004 13 red blue huib 16/2/2004 systemic
VLBA observations Dense Molecular Gas Workshop Zwolle 18/2/2004 14 • 1995 – 1997 • 10 Antennas, but 8 useful • Limited coverage • δ = -43 o • Hard to image • Good spectral resolution • Confirms previous funny OH result huib 16/2/2004
OH absorption on VLBI scales Systemic Dense Molecular Gas Workshop Zwolle 18/2/2004 Red-shifted 1 pc 1667 absorption at 550 km/s huib 16/2/2004 1667 absorption at 600 km/s 15
Dense Molecular Gas Workshop Zwolle 18/2/2004 16 3σ upper limit on τ τ detected (>2σ) counter jet side huib 16/2/2004 jet side
Dense Molecular Gas Workshop Zwolle 18/2/2004 17 No redshifted H 2 CO 3 narrow components No 15 GHz detection huib 16/2/2004
H 2 CO absorption 1 pc huib 16/2/2004 Dense Molecular Gas Workshop Zwolle 18/2/2004 18
Where is the core? Dense Molecular Gas Workshop Zwolle 18/2/2004 19 Tingay & Murphy 2001 Ap. J 546 210 huib 16/2/2004
Continuum Dense Molecular Gas Workshop Zwolle 18/2/2004 20 Imaging very difficult due to low elevation and limited mutual visibility… 1 pc 18 cm huib 16/2/2004 0. 1 pc 6 cm 2 cm
H 2 CO absorption 1 pc huib 16/2/2004 Dense Molecular Gas Workshop Zwolle 18/2/2004 21
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Dense Molecular Gas Workshop Zwolle 18/2/2004 systemic |τ| 6 cm maser huib 16/2/2004 25
Interpretation Dense Molecular Gas Workshop Zwolle 18/2/2004 • Absorption gradient over source • 20 mas = 0. 34 pc 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 H 2 CO does not show red-shifted component • Size OK for small scale structure huib 16/2/2004 26
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 Ap. J 516 769 huib 16/2/2004 Dense Molecular Gas Workshop Zwolle 18/2/2004 27
Conclusions Dense Molecular Gas Workshop Zwolle 18/2/2004 • 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 • H 2 CO seems more complex • Absorption gradient over source • 20 mas = 0. 34 pc scale!? • Still favour location in molecular ring 200 – 2000 pc • In which H 2 CO 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 H 2 CO to millimeter lines directly, distinctly different from OH huib 16/2/2004 28
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OH continuum 1609 MHz 1661 MHz huib 16/2/2004 Dense Molecular Gas Workshop Zwolle 18/2/2004 30 1716 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 huib 16/2/2004 Dense Molecular Gas Workshop Zwolle 18/2/2004 31
OH ATCA maps OH 1667 absorption at 550 km/s huib 16/2/2004 Dense Molecular Gas Workshop Zwolle 18/2/2004 OH 1720 emission at 600 km/s 32
Centaurus A • NGC 5128 picture • Dust band • Recent merger huib 16/2/2004 Dense Molecular Gas Workshop Zwolle 18/2/2004 33
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