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

Dense Molecular Gas Workshop Zwolle 18/2/2004 2/28 huib 16/2/2004 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

Dense Molecular Gas Workshop Zwolle 18/2/2004 3/28 huib 16/2/2004 Centaurus A In NGC5128, nearest AGN D= 3.4 Mpc, 1” ~ 16pc Large double lobed radio source Modest black hole (~ 10 7 M  ) VLBI source, subluminal Strong mm core Elliptical with dust band Recent merger (few 10 8 – 10 9 yr) Star formation in dust band Gas settling in potential of Elliptical Feeding the central engine? Cen A at 2.2 GHz Tingay & Murphy 2001 ApJ pc

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

Dense Molecular Gas Workshop Zwolle 18/2/2004 5/28 huib 16/2/2004 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 Peck 1999, NMIMT thesis

Dense Molecular Gas Workshop Zwolle 18/2/2004 6/28 huib 16/2/2004 Molecules & IR Molecular ring at ±1.5 kpc Warped or barred? And a circumnuclear disk Warmer gas at 100pc Wild et al 1997 A&A Leeuw et al kpc 100 pc

Dense Molecular Gas Workshop Zwolle 18/2/2004 7/28 huib 16/2/2004Models 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

Dense Molecular Gas Workshop Zwolle 18/2/2004 8/28 huib 16/2/2004 Previous ATCA result on OH Van Langevelde et al 1995 ApJ 448 L123 OH 1667 absorption at 550 km/s 1 kpc

Dense Molecular Gas Workshop Zwolle 18/2/2004 9/28 huib 16/2/2004 Excitation model Simple escape probability calculation But does include OH hyperfine transitions f OH = (2-6) (bit high compared to Galactic) f form = (low compared to Galactic) 2 components “local molecular cloud”: n=10 4 cm -2, T kin = 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 H 2 CO excitation Parameterized as distance from nucleus

systemic red blue

red systemic bluered systemic blue

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004 VLBA observations 1995 – Antennas, but 8 useful Limited coverage δ = -43 o Hard to image Good spectral resolution Confirms previous funny OH result

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004 OH absorption on VLBI scales 1667 absorption at 550 km/s 1667 absorption at 600 km/s 1 pc Systemic Red-shifted

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

No 15 GHz detection No redshifted H 2 CO 3 narrow components

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004 H 2 CO absorption 1 pc

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004 Where is the core? Tingay & Murphy 2001 ApJ

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004Continuum Imaging very difficult due to low elevation and limited mutual visibility… 18 cm 6 cm 2 cm 1 pc 0.1 pc

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004 H 2 CO absorption 1 pc

systemic |τ| 6cm maser

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004Interpretation 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 H 2 CO does not show red-shifted component Size OK for small scale structure

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004Models 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

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004Conclusions 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 = 0.34pc 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

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004 OH 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

Dense Molecular Gas Workshop Zwolle 18/2/ /28 huib 16/2/2004 OH ATCA maps OH 1667 absorption at 550 km/s OH 1720 emission at 600 km/s

Centaurus A NGC 5128 picture Dust band Recent merger