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

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 ObservatoryT. Oka – University of Chicago

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

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 thatin 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: H2 2.12μm on CO 2-1 contours

(Muller-Sanchez et al. 2009)

100 pc

NNGC 1068

H2O - VLBAGreenhill & Gwinn (1997)

UKIRT 2006R=2,000 (150 km/s); 0.2”x1.0” NS slit

Unsuccessful searches for CO fundamental band absorption lines

UKIRT 1998R=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)

slitCapetti et al. (1997)

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

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

Galactic dense (n ≥ 103 cm-3) vs. diffuse (n < 103 cm-3) clouds(✔ = consistent with NGC 1068 IR data)

PROPERTY DENSEDIFFUSE

fraction of H in H2: ~100% ✔ ~50% ✔

presence of silicate feature YES ✔YES ✔presence of 3.4μm feature NO YES ✔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)

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.

SILICATEHYDRO-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)~1045 ergs/s

L(Sgr A*)~1037 ergs/s

Sightlines

Expanding Molecular RingCentral Molecular Zone

Milky Way - COBE

NGC 1068

GCS 3-2

Oka et al. (2005)

Foregroundspiral arms

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 108 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

H3+ 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

TROUGH

Column length of H3+ producing

trough (CMZ) absorption is 10-100 pc

(dependent on assumed c-r ionization rate)

(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.