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The nature of the dust and gas in the nucleus of NGC 1068.

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Presentation on theme: "The nature of the dust and gas in the nucleus of NGC 1068."— Presentation transcript:

1 The nature of the dust and gas in the nucleus of NGC 1068

2 The nature of the dust and gas in the nucleus of NGC 1068 – comparison to the Galactic center’s “Central Molecular Zone” T. R. Geballe & R. E. Mason - Gemini Observatory T. Oka – University of Chicago

3 Type II view UNIFIED MODEL OF AGN Type I view Type II : View to the central engine is obscured by a torus of dusty molecular material. Can only see BLR in scattered light. Type I: torus does not block line of sight to the central region. BLR is directly observed. Appearance of AGN depends on orientation of nuclear region w.r.t. our line of sight NGC 1068: the prototypical Seyfert II galaxy

4 Structure and composition of obscuring material Extends outward to r ~ 100pc Extends inward to r < 1 pc Must contain dust to obscure AGN If dust is present, then molecules too. Generally thought to be sufficiently high density and cool to be ~fully molecular. If it is molecular then one might expect that in a Type II AGN with bright central IR continuum source it would be possible to detect molecular gas absorbing (IR) radiation along the line of sight to the source + gas (molecules) NGC 1068: H 2 2.12μm on CO 2-1 contours (Muller-Sanchez et al. 2009) 100 pc NNGC 1068 H 2 O - VLBA Greenhill & Gwinn (1997)

5 UKIRT 2006 R=2,000 (150 km/s); 0.2”x1.0” NS slit Unsuccessful searches for CO fundamental band absorption lines UKIRT 1998 R=15,000 (20 km/s) Mason et al. (2006) Geballe et al. (2009) CO fundamental band CO should be easiest interstellar molecule to detect via IR absorption spectroscopy; either as a broad band or as individual lines. similar to Lutz et al. (2004) slit Capetti et al. (1997)

6 Where is the CO not being detected? (i.e., how close into the center are we looking?)  Average 3-5μm continuum “surface” is at r ~5-10 pc. Our absorption data do not address the nature of the torus inside of that radius. undeconvolved PSF AO at L’ (Gratadour et al 2004) shows a bright central core of diameter ~11 pc (0.16”) and complex of sources oriented NS. For UKIRT R~2000 spectrum, the deconvolved FWHM at 3-5μm along slit was 0.3” (d~20 pc). τ= 1 at 3-5μm No CO

7 9.7μm Silicate 3.4μm hydrocarbon NGC 1068 0.4” x 0.4” Gemini N / MICHELLE Mason et al. (2006) Silicate (9.7μm) / hydrocarbon (3.4μm) absorptions occur at longer / shorter wavelength than CO (4.7μm). Silicate feature is found in dense and diffuse Galactic clouds. Hydrocarbon feature has only been found in diffuse Galactic clouds. Is the explanation for no CO that all of the cool ISM outside of r~5-10 pc and in our sightline is diffuse in nature? Detected absorption features

8 Galactic dense (n ≥ 10 3 cm -3 ) vs. diffuse (n < 10 3 cm -3 ) clouds ( ✔ = consistent with NGC 1068 IR data) PROPERTYDENSEDIFFUSE fraction of H in H 2 :~100% ✔ ~50% ✔ presence of silicate featureYES ✔ YES ✔ presence of 3.4μm featureNOYES ✔ percent of C in CO~100%~1% ✔ * * consistent with upper limit on CO column density … suggests diffuse cloud environment in outer portion of torus (r > 5-10 pc)

9 Supporting evidence that hydrocarbon and silicate features are formed close to the nucleus in the same location : Hydrocarbon: UKIRT/UIST – Geballe et al. (2009) Silicate: Gemini/MICHELLE – Mason et al. (2006) (1)Variations of silicate and hydrocarbon features across the nucleus are similar. Variations across the small nuclear continuum source imply proximity to nucleus. (2) The carriers of the hydrocarbon and silicate features coexist in diffuse clouds.

10 SILICATE HYDRO- CARBON CO Lutz et al. (1997) What does that have to do with the Galactic center? A similar diffuse cloud environment appears to make up a significant fraction of the Galaxy’s Central Molecular Zone. Lutz et al. (1996) L(AGN)~10 45 ergs/s L(Sgr A*)~10 37 ergs/s

11 Sightlines Expanding Molecular Ring C entral Molecular Zone Milky Way - COBE NGC 1068 GCS 3-2 Oka et al. (2005) Foreground spiral arms

12 Significance? GC has no BLR, no coronal lines (i.e., no strong far UV source). NGC 1068 has all of these and its AGN is 10 8 times more luminous. Yet both appear to have similar diffuse interstellar environments filling significant fractions of their central ~100 pc. - a result of the obscuration of N1068’s AGN Are the outer portions of AGN toroids typically diffuse cloud environments? Some type II AGNs show the silicate feature and some are known to have the hydrocarbon feature; don’t know if features are co-located CO has not been detected toward several other Type II AGN (Lutz et al. 2004), but upper limits are weak. MORE OBSERVATIONS NEEDED

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14 H 3 + and CO spectra of GCS3-2 B. (1,1): Note similarity to CO, except for the broad absorption trough, on which narrow absorption lines from (dense) clouds in external spiral arms are superimposed. ==> trough is formed in diffuse clouds. C. (3,3): Overall absorption profile crudely approximates the (1,1) trough. ==> same gas as R(1,1) trough ==> trough gas is warm (250 K) No narrow features ==> trough gas is in rapid motion; gas is close to the GC. D. (2,2): No absorption ==> non-LTE population distribution / low density (confirms comparison with CO). A. CO R(1) (2.34 μm): Only narrow absorption features – from known spiral arms. No broad absorption trough. Spiral arms TROUGH Column length of H 3 + producing trough (CMZ) absorption is 10- 100 pc (dependent on assumed c-r ionization rate)

15 (2,2) (3,3) (1,1) (2,2) (3,3) (1,1) All eight observed stars within 30 pc of Sgr A* show the H3+ absorption due to warm and diffuse gas Goto et al. (2008) This gaseous environment appears to pervade the CMZ, but we need to observe on more sightlines. We are currently engaged in a program to find suitable probe stars.


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