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MOLECULAR GAS IN THE CORES OF AGN

MOLECULAR GAS IN THE CORES OF AGN. Violette Impellizzeri (NRAO) Alan Roy ( MPIfR ), Christian Henkel ( MPIfR ). The unified scheme of AGN. Diversity of AGN classes explained by a single unified scheme :

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MOLECULAR GAS IN THE CORES OF AGN

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  1. MOLECULAR GAS IN THE CORES OF AGN Violette Impellizzeri (NRAO) Alan Roy (MPIfR), Christian Henkel (MPIfR)

  2. The unified scheme of AGN • Diversity of AGN classes explained by a single unified scheme : • The nuclear activity is powered by a supermassive black hole (∼106–1010 M⊙) and its accretion disk. • This central engine is surrounded by dusty clouds, which are individually optically thick, in a toroidal structure (Krolik & Begelman 1988). Populated by molecular clouds accreted from the galaxy. • Observed diversity is the result of viewing this axisymmetric geometry from different angles.

  3. The unified scheme of AGN

  4. The torus: size • To determine an actual size one must rely on the torus emission. • Pier & Krolik (1992) approximate the density distribution with a uniform one : outer radius R ∼ 5 pc – 10 pc. • Pier & Krolik (1993) : later speculated that compact structure might be embedded in a much larger, and more diffuse, torus with R ∼ 30 pc –100 pc. • Granato and Danese (1994) elaborate toroidal geometries, concluded that the torus must have an outer radius R ~ 300 pc – 1000 pc. • Granato et al (1997) settled on hundreds of pc. Subsequently, R > 100 pc became common lore. • High-resolution IR observations brought unambiguous evidence in support of compact torus dimensions. VLTI interferometry shows that the 10 μm flux comes from a hot (T > 800 K) central region of ∼ 1 pc and its cooler (T ∼ 320 K) surrounding within R ∼ 2 pc and 3 pc (for NGC 1068; Jaffe et. al 2004).

  5. The dusty torus: MIR interferometry • T1 = 334 K • T2 = 298 K T1 Tristram et al. 2007

  6. The dusty torus: MIR interferometry • T1 = 334 K • T2 = 298 K Tristram et al. 2007

  7. Sy1 Sy2 The torus : molecular NH 1022 cm-2 ; OH opacities << 1 NH 1024 cm-2 ; OH opacities > 1 Molecular abundances in tori: N(OH)/N(HI) ≤ 10-3 (Krolik & Lepp 1989) Dusty AGN torus is molecular gas rich

  8. OH surveys • Expect to observe molecular absorption against the compact, flat spectrum radio cores of NLRs. • Not only confirm the existence of a torus, but also derive valuable physical andkinematical information. Observed ~ 300 galaxies Low detection rates (absorption towards two Seyferts maser emission in five). OH surveys at 1.6 GHz • Schmelz et al. 1986, • Staveley-Smith et al. 1992, • Baan et al. 1992 • ... (sensitivities of few percent)CO, HCN and HNC searches • Drinkwater et al. 1997Surprisingly few detections!OH abundance lower than predicted ? No torus ?

  9. No CO absorption in Cygnus A • Of all AGN Cygnus A is expected to have a molecular torus • NLR • Large X-ray absorbing column (1023.5 cm-2) • Search for CO (1-0) absorption yielded non detection! • Search for OH (1.6 GHz) non detection H2CO non detection HI detected (Coway & Blanco 1995)

  10. Radiative excitation I • Gas in a hot (5000 -10000 K) mainly atomic phase. • Radiative excitation of CO, OH and NH3 due to the central radio source causes the non detection of molecular absorption (Maloney, Begelman & Rees 1994).

  11. Radiative excitation I • Gas in a hot (5000 -10000 K) mainly atomic phase. • Radiative excitation of CO, OH and NH3 due to the central radio source causes the non detection of molecular absorption (Maloney, Begelman & Rees 1994).

  12. Radiative excitation effects • Maloney, Begelman, & Rees (1994)postulate that lack of CO absorption is due to radiative excitation by non-thermal radio source: High Tex depletes lower rotational levels, decreasing optical depthis OH, too, radiatively excited ? • For torus located 10 pc from AGN: • opacity in 1.6 GHz transition suppressed by factor 103 • opacity in 6.0 GHz transition suppressed by factor 102 • opacity in 13.4 GHz transition will increase slightly by factor 2. • Black (1998) • Thus, absorption will strongly depend on transition one chooses to observe. • Have we been looking at the right transitions ?

  13. A new survey for OH • Search for highly excited rotational states of OH at 6.035 GHz & 6.031 GHz, 4.750 GHz and 13.4 GHz. The sample: • 31 Seyfert 2 galaxies • High X-ray absorbing columns (> 1022 cm-2 ) • Continuum flux density at 6 GHz > 50 mJy • HPBL Effelsberg observations: • 3-10 hours per source at 4.7 GHz and 6.0 GHz, PSW • Velocity coverage 2000 km/s (4 km/s per channel) • Sensitivity 3.5 mJy (5 σ) = line opacity of 0.002 to 0.07

  14. Effelsberg Spectra 6.0 GHz Best targets due to high X-ray columns and high background continuum at 6.0 GHz.

  15. Effelsberg Spectra 6.0 GHz Too strong continuum - Large standing wave ripple! 5 new detections - detection rate 18 %

  16. Effelsberg Spectra 6.0 GHz Lets look at NGC 3079 & NGC 5793.

  17. NGC 3079 NGC 5793 •Width ~ 800 km s-1 •Line opacity τ ~ 0.055 •4.7 GHz non-detection •1.6 GHz abs (Baan et al. 1995) Tex = 30 K NOH = 1.5 1018 cm-2 from X-rays: NH = 1.0 1025 cm-2 •Width ~ up to 1000 km s-1 •Line opacity τ ~ 0.036 •4.7 GHz non-detection •1.6 GHz abs (Hagiwara et al. 2000) Tex = 67 K NOH = 2.2 1017 cm-2

  18. Conclusions from OH survey • A survey for 6.0 GHz and 4.7 GHz with Effelsberg found highly excited, broad absorption lines in 5 out of 28 sources, i.e. 18%. • Line widths are 100s-1000 km/s (close to nuclear region?). • Selection criteria improved detection rate c.f. other OH surveys (< 7%). • No new detections at 6.0 GHz were made where there was no previous OH absorption/emission at 1.6 GHz. • Results do not support radiative excitation models alone to explain the lack of detections. • Still some sources with high column density didn’t show absorption. But still the non detections must be explained !!

  19. Some open questions • Bimodal distribution of absorptions –> very compact clouds ? • Broad lines in infall/outflow ? Not a simple rotating torus ? • X-ray absorber may be non-molecular in most galaxies ??

  20. => 0.9 mas beam VLBI Observations • VLBA observations carried out with interferometricobservations to detect OH at 13.4 GHz towards the core of NGC 1052 and Cygnus A. Observing time: NGC 1052 – 7 hours Cygnus A – 8 hours Bandwidth 16 MHz (256 channels) per IF. Velocity coverage ~ 560 km/s.

  21. NGC 1052 • Nearby radio galaxy classified as LINER (z=0.0049) • Hosts well-defined double-sided radio jet w/ P.A. 650 • Jet extends up to kpc scale, angle of jet axis is > 760.

  22. NGC 1052 • Nearby radio galaxy classified as LINER (z=0.0049) • Hosts well-defined double-sided radio jet w/ P.A. 650 • Jet extends up to kpc scale, angle of jet axis is > 760. • Multi-frequency VLBI observations have revealed the presence of a dense circumnuclear structure.

  23. NGC 1052 • Nearby radio galaxy classified as LINER (z=0.0049) • Hosts well-defined double-sided radio jet w/ P.A. 650 • Jet extends up to kpc scale, angle of jet axis is > 760. • Multi-frequency VLBI observations have revealed the presence of a dense circumnuclear structure.

  24. NGC 1052 • Nearby radio galaxy classified as LINER (z=0.0049). • Hosts well-defined double-sided radio jet w/ P.A. 650. • Jet extends up to kpc scale, angle of jet axis is > 760. • Multi-frequency VLBI observations have revealed the presence of a dense circumnuclear structure. • Plasma condensation covers 0.1 pc and 0.7 pc of approaching and receding jet (Kameno et al. 2001). • X-ray column is 1022 – 1023 cm-2 (e.g. Kadler et al. 2004). • H2O megamasers found towards the centre redshifted by 50-350 km/s wrt systemic (e.g. Braatz et al. 2003).

  25. NGC 1052 • H2O detected where free-free opacity large. Other lines have been detected : • HI, OH, HCO+, HCN and CO… Sawada-Satoh et al. 2008

  26. NGC 1052 Vermeulen et al. 2003 Sawada-Satoh et al. 2008

  27. NCG 1052 – VLBA 13.4 GHZ 0.2 pc Broad Width: FWHM ≈ 200 km/s Optical depth (counter jet) τOH ≈ 0.264 Vsys Obscuration in the inner jet region (< o.3 pc), coincident with free-free absorption.

  28. NGC 1052 – where is absorption located? Optical depth map

  29. NGC 1052 – where is absorption located? Optical depth map

  30. NCG 1052 – VLBA 13.4 GHZ Approaching jet Receding jet

  31. Are we seeing the torus in NGC 1052? Approaching jet Receding jet

  32. Fitting into the picture Sawada-Satoh et al. 2008

  33. Fitting into the picture OH Sawada-Satoh et al. 2008

  34. Cygnus A Continuum peak flux 453 mJy per beam Optical depth (core) τOH ≈ 0.12 Absorbing gas is diffuse around the inner jets. Profile strongest over the entire continuum.

  35. Cygnus A Continuum peak flux 453 mJy per beam Optical depth (core) τOH ≈ 0.12 Absorbing gas is diffuse around the inner jets. Profile strongest over the entire continuum. Conway & Blanco 1995

  36. Conclusions from VLBA observations • 13.4 GHz OH was detected for the first time in the core of an AGN (NGC 1052 and Cygnus A). • Evidence for molecular gas in the region we would expect to see a torus. • In Cygnus A, if real the detection is consistent with radiative excitation models proposed for this source. • In NGC 1052, redshifted OH gas cloud detected toward the counterjet. • Part of rotating torus/infalling material ? • Detection consistent with previous H2O observations.

  37. Future outlook • Detections will strongly depend on which transition one chooses to observe! • Future surveys for the molecular component of AGN should include the 13.441 GHz and 13.434 GHz lines of OH. • New instruments like the VLBI, EVLA, and ALMA (justaroundthe corner) with their increased sensitivity will hugely contribute to a high resolution, sensitive study of the molecules in AGN: perhaps a new era also for the Torus?

  38. Future outlook For a sourceat the distance ofNGC 1052: VLBA resolution : 1mas at 13.0 GHz : 0.1 pc EVLA resolution : 4 arcsec at 5.0 GHz : 400 pc ALMA resolution: 18mas at 1mm : 2 pc (extended configuration 17 km) ALMA resolution : 0.5 arcsec at 1mm: 50 pc (early science)

  39. ALMA

  40. Spiral Galaxy in Ursa Major Distance: 15 Mpc (50 millionlight-years) Dimensions: Thefull image 4.9´( 21 kpc) Sy2 galaxy Nh = 1.6 1022 cm-2 FIR excess: Lfir = 19 109Lsun CO (1-0) emissiontracingdisk (Nh = 1.4 1024 cm-2) Most luminous H2O megamaserknown

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