4. THE IONISED GAS AND ASSOCIATED OUTFLOWS

As mentioned in Section 1, a signature of Seyfert nuclei is the presence of ionised gas. This is believed to be either photoionised ambient galactic gas (Pedlar, Unger & Dyson 1985; Unger et al. 1987; Falcke, Wilson & Simpson 1998) or nuclear gas which is ionised and driven along the radio jets present in AGNs (e.g. Begelman, Blandford & Rees 1984; Schulz 1988; Colbert et al. 1996; Colbert et al. 1998). The ionised material and the observed photons are collimated by the dusty material (torus) obscuring the continuum source (Antonucci & Miller 1985; Wilson, Ward & Haniff 1988; Tadhunter & Tsvetanov 1989; Wilson & Tsvetanov 1994; Baker & Scoville 1998), causing them to exhibit a sharp linear edge so that the ionised gas is observed as a bi-conical cone in the narrow line region ⁽⁴²⁾ (Storchi-Bergmann, Wilson & Baldwin 1992; Dopita et al. 1998). In this model, ionised gas which passes through the sublimation radius (where hot dust radiates in the infra-red) is broken into clouds which are able to fall back closer to the nucleus, possibly being observed as the broad line region. Dust and ionised gas which have accreted onto the AGN are driven back outward in the direction of the jets by radiation pressure, thus maintaining the direction of the jet flow ⁽⁴³⁾ (Wilson & Tsvetanov 1994; Capetti et al. 1996), Table 2 ⁽⁴⁴⁾.

Figure 8. The hydrodynamical model of the accretion flow and dusty wind as proposed by Dopita et al. (1998). Courtesy of Mike Dopita.

Table 2. The Seyfert galaxies known to possess ionisation cones. Sy is the Seyfert type (obtained from the NASA/IPAC Extragalactic Database), r_ion, CA_ion are the extent (*for H₀ = 75 km s^-1 Mpc^-1) and opening angle of the ionisation cone, respectively, CA_radio is the opening angle of the nuclear radio structure and PA is the difference in the position angle of the radio structure and the ionisation cone. This table is adapted from Wilson & Tsvetanov (1994) but with NGC 2992 added from the results of Ulvestad & Wilson (1984); Márquez et al. (1998), NGC 4051 from Christopoulou et al. (1997), NGC 5929 from Su et al. (1996) and Circinus added from various other works (see Table 3 for details). **The result for NGC 1365 is added from the results of Sandqvist, Jörsäter & Lindblad (1995), who consider the galaxy as a Seyfert type 1.5 (Véron et al. 1980; Jörsäter, Lindblad & Boksenberg 1984; Jörsäter & Lindblad 1989).

Galaxy	Sy	rion [pc]	CAion [°]	CAradio [°]	PA [°]
NGC 1068	2	< 350	~ 60	< 30	+4±8
NGC 1365	1.8/1.5**	< 800	102±20	-	0**
NGC 2992	1.9	*~ 750	120	-	+10
NGC 3281	2	< 1300	75±10	-	-
NGC 4051	1.5	420	55	-	< 1 (NE)
NGC 4151	1.5	< 80	70±15	< 5	-3±7
NGC 4388	2	2200	50	< 12	- -9 (North)
		< 900	92±7		+6±5 (South)
NGC 5252	1.9	< 18000	74±4	< 0.3	+7±12
NGC 5728	2	< 270	60±10	20	-4±6 (NW)
		< 1200	56±10 &-	- (SE)
NGC 5929	2	*~ 90	60	-	15
NGC 7582	2	< 760	86±10	-	-
Mkn 6	1.5	3600	-	< 15	+12±10
Mkn 78	2	8600	50±10	25	+8±6
Mkn 573	2	< 1300	45±10	< 30	+8±7 (NW)
	< 1700	45±10		-2±6 (SE)
Circinus	2	< 520	100	15	20 (NW)

Table 3. The outflow properties in Circinus. The H outflow (Elmouttie et al. 1998c) is observed towards the NW only. *Note that Veilleux & Bland-Hawthorn (1997) (also) derive a position angle of 295° but an opening angle of ~ 100° for the ionised outflow. In the case of the radio continuum (Harnett et al. 1990; Elmouttie et al. 1995) and CO (Curran et al. 1999) observations, outflows are also measured towards the SE, although, unlike the molecular outflow, the radio lobe is strongest in the NW.

	Radio	H	CO
Position angle	115° and 315°	292±5°*	120° and 300±20°
Inclination angle	-	-90 to 40°	-12° and 168±10°
Opening angle	15°	66°*	90±5°
Inferred length	±1 kpc	400 to 520 pc	±500 pc
Outflow velocity	-	150 to 200 km s-1	190±10 km s-1

Due to the depletion of the accreting gas, the opening angle of the cone will increase as the system evolves, thus making Sy1s more readily observable over a wider range of angles in mature sources (Dopita 1998). Worth mentioning is that Pogge (1989) finds that extended ionised gas structures occur more frequently in Sy2s (as in Table 2), although this result is from an admittedly small sample. Ionisation cones are expected to have a dusty layer form along their inner edge (Dopita et al. 1998), thus permitting the presence of molecules along the surface of the outflow (Curran et al. 1999; Curran, Johansson & Rydbeck 2000). ⁽⁴⁵⁾ In the model of Dopita et al. (1998), the ionisation cone and the radio jet (Fig. 8) have different origins, i.e. from the dusty torus and from the black hole, respectively ⁽⁴⁶⁾ (Whittle et al. 1988). It should be noted, however, that the generally small scales and wide opening angles of the cones, in comparison with the jets, can also be explained by a simple wide ionised outflow in which the radio jet is simply a central high velocity component (Wilson et al. 1993). In Circinus the radio jets close to the nucleus have been inferred from the observations of Davies et al. (1998) ⁽⁴⁷⁾ and the ionisation cone ⁽⁴⁸⁾ in the form of a unipolar (to the north-west only) V-shaped outflow, Fig. 9. The highly ionised state of the highly excited (Oliva et al. 1994) low density (n_e ~ 40 cm^-3, Marconi et al. 1994) supersonic (Veilleux & Bland-Hawthorn 1997) gas is confirmed by the presence of the [NeIII, V, VI], [SIV], [MgV,VII,VIII], [OIV] and [SiIX] species (Moorwood et al. 1996a). The various outflow features in Circinus are summarised in Table. 3, and the results appear to support the hypothesis that the jet drives the ionisation cone, together with an envelope of molecular gas, out along the rotation axis of the molecular ring ⁽⁴⁹⁾ (Curran et al. 1998; Curran et al. 1999; Figure 9).

Figure 9. Left: The ionisation cone in Circinus (Marconi et al. 1994) scaled and superimposed upon the molecular outflow of Curran et al. (1999). [OIII] / (H + [NII]) image courtesy of Ernesto Oliva. Right: The CO distribution within the centre of Circinus (Curran et al. 1999). The possible high velocity outflow component (which would coincide with the jet) is shown.

This geometry is also evident in Mrk 231, where the ~ 100 pc scale gas disk appears to be centred on the AGN and perpendicular to the radio lobe axis (Carilli, Wrobel & Ulvestad 1998), Fig. 10. So like Circinus, this suggests a continuous alignment of the disk/ring/torus. This scenario, however, may be exceptional according to the results of Schmitt & Kinney (1996); Schmitt et al. (1997) who find that, although the larger scale molecular ring is expected to be coplanar with the disk of the galaxy (McLeod & Rieke 1995), it will not in general be coplanar with the obscuring torus. Although, admittedly from a very simple model, Curran (2000) finds that there is tendency for the torus, ring and large scale disk to be approximately aligned (within 30°). In fact the situation may be somewhat more complicated than this, as Maiolino & Rieke (1995) suggest that the molecular ring will cause some obscuration of both the narrow and broad line regions. This will have the consequence of dimming the luminosity of the narrow lines, which are more extended in Sy2s (Schmitt & Kinney 1996), above inclinations of 50°, thus making these galaxies appear as type 1.8 and 1.9 Seyferts. This idea is somewhat supported by Schmitt & Kinney (1996) who find that galaxies of inclinations as high as 60° will still appear as Sy1s, although this may merely indicate the large opening angle of the torus (CA_ion in Table 2). Also, Kohno et al. (1996) propose that the large scale (~ 100 pc) molecular disk contributes significantly to the obscuration of X-rays from the weak Seyfert nucleus in M51.

Figure 10. A schematic model of the Sy1 Mrk 231 (adapted from Carilli, Wrobel & Ulvestad 1998) showing the orientation of the gas disk components with respect to the radio lobes, which are shown by the centre contours. The gas disk is also known to have such a structure in Circinus (Elmouttie et al. 1998a; Curran et al. 1998; Elmouttie et al. 1998b).

Returning to the molecular outflows, although it is as yet uncertain whether they are common to all Seyferts, Irwin & Sofue (1992) derive the presence of a such an outflow directed along the VLBI jet in the Sy2 NGC 3079 ⁽⁵⁰⁾, and their presence cannot be ruled out in other galaxies (Curran 2000) ⁽⁵¹⁾. A molecular outflow has also been observed in the previously mentioned star-burst galaxy M82 (Nakai et al. 1987) and, like the outflow in Circinus ⁽⁵²⁾, this extends to 500 pc in a direction normal to the galactic disk at a velocity of ~ 200 km s^-1. Unlike Circinus, however, this outflow takes the form of hollow cylinder rather than a cone surrounding the ionised gas, thus indicating the absence of the compact dense obscuring torus which would collimate the supernova driven outflow into a conical shape (Chevalier & Clegg).

⁽⁴²⁾ The cone is often referred to as the extended narrow line region, although the mechanism behind the narrow line region (Section 1) may not be the same (Morris et al. 1985; Unger et al. 1987; Storchi-Bergmann, Wilson & Baldwin 1992; Hjelm & Lindblad 1996). Back.
⁽⁴³⁾ If this were directed along the axis of the galaxy (Wilson 1991), it would place the axis of the outflow perpendicular to the large scale disk. Back.
⁽⁴⁴⁾ Note that the opening angles of the ionisation cones suggests that the obscuration allows only 1/5 of the sky to be visible from the central engine. This is consistent with the fact that there appears to be only one Sy1 for approximately every five Sy2s (Maiolino & Rieke 1995). Note also that the large-scale radio structures in Seyfert galaxies may be weaker versions of those observed in radio-loud AGNs and that pc-scale jets aligned with these have been observed (Murray et al. 1999; Kukula et al. 1999), although ionisation cones have yet to be detected in radio-loud sources (e.g. Peterson 1997). Back.
⁽⁴⁵⁾ Research Papers B and F. Back.
⁽⁴⁶⁾ Pedlar et al. (1998) propose a model where the torus is a consequence of the weak radiation emitting from the equator of the continuum source, whereas the cone arises from gas ionised by the strong polar radiation. Back.
⁽⁴⁷⁾ Elmouttie et al. (1995) also observe large scale radio lobes. Back.
⁽⁴⁸⁾ In Circinus the IR luminosity from the ionising radiation is greater than that from the star-burst (Moorwood et al. 1996a; Siebenmorgen et al. 1997). Back.
⁽⁴⁹⁾ This indicates that the ring axis is coincident with that of the obscuration collimating the outflow. Back.
⁽⁵⁰⁾ Like Circinus, their CO 1 -> 0 observations also show the presence of a 750 pc molecular ring, see Table 1. Back.
⁽⁵¹⁾ Quillen et al. (1999) have recently observed excited molecular hydrogen ( = 1.9750 µm) coincident with the ionised gas in NGCs 2110, 5643 and Mkn 1066 and we plan to search for molecular outflows in other Seyfert galaxies, Chapter 3/Appendix D. Back.
⁽⁵²⁾ A molecular ring is also present in M82, Section 2.2. Back.