Astron. Astrophys. 360, 49-56 (2000)

4. Discussion

4.1. Nuclear continuum structure

We derive basic morphological and spectral properties of the parsec-scale radio structure of NGC 5793 from the 18 cm and 6 cm continuum maps and discuss the implications of these results. The continuum radio source has a core-jet structure that is typical for those seen in Seyfert galaxies and radio active galaxies. The VLBA data at both wavelengths suggest that the three components C1(C), C1(NE), and C2(W) have steep spectra of -1.0 -0.7 (using )(see Table 2). The component C2(E) in the 6 cm map could not be clearly identified in the 18 cm map. Estimating an 18 cm flux density of 16.1 mJy at the position of C2(E), its spectral index is 0.13 and is definitely inverted ( 0) or flat between these two wavelengths.

One might ask which component is the `true' radio core of the galaxy. If we assume that the strongest continuum peak C1(C) is the core, the steep spectrum of C1(C) would likely be the result of a mixture of spectra from several components including a flat-spectrum radio core. Steep spectra in the range -1.3 -0.5 are generally observed in extragalactic extended radio sources (Barvainis & Lonsdale 1998), similar to those observed in the components in NGC 5793, suggesting that the continuum structure at C1(C) may still not be resolved on a scale of a few parsecs.

In contrast to the structure at 6 cm, the 18 cm image at the position of C2(E) does not show any counterpart corresponding to C2(E) at 6 cm. There are three possible explanations for this. We can account for the relative faintness of C2(E) at 18 cm by invoking a model of free-free absorption by an optically thick intervening gas disk/torus, which is highly inclined to the line of sight. In this case, the relative flatness of the C2(E) spectrum can be explained by there being a radio continuum core lying behind the obscuring gas along our line of sight. Therefore, the other possible candidate for the `true' radio core is C2(E) in the 6 cm image. In that case, the central continuum structure containing C1(C), and the western extended emission containing C2(C) and C2(W), could be interpreted as jets that are directed from the highly inclined disk/torus, whose major axis lies at P.A. of , which is close to the galaxy's optical axis of . Alternatively, C2(E) could be an unobscured core and its lack of emission at 18 cm results from synchrotron self-absorption (e.g., Jones & Wehrle 1997). We will describe our investigations of these possibilities together with results of high-frequency observations with the VLBA, in a subsequent paper (Hagiwara et al. in preparation). Another possible explanation of the weakness of component C2(E) in the 18 cm map could be time variation of the flux density between the two observing epochs (almost one year) and, due to the relative faintness of 18 cm flux density, the spectrum of C2(E) apparently shows a flat or positive spectral index (see Table 2). If this was the case, the real spectrum at the C2(E) might be as steep as other components.

On balance, we suggest that C1(C), the most intense continuum peak, might be the position of the `true' radio nucleus. C1(SE), C2(C), C2(W), and C2(E) could be a continuous jet ejected from the nucleus, while C1(NE) is the counter jet extending toward north-east. The bent structure of the jet close to C2(E) could be explained by deflection by gas clouds and the collimation axis of the jet being forced to change (e.g., NGC 1068: Gallimore et al. 1996b).

4.2. Properties of the OH absorption

Table 3 lists the single-Gaussian fitted parameters derived at C1(C) from the observed spectra in Fig. 3. The optical depth in the OH line at 1667 MHz, derived with the VLA in A-configuration was 0.065 0.006 (Gardner & Whiteoak 1986), while that derived from our VLBA data is 0.080 0.012 at the Gaussian peak. The optical depth obtained by the smaller VLBA synthesized beam (HPBW = 0.012") is consistent with that observed with the VLA (HPBW = 4") within the errors. As mentioned in the previous section, we found little significant absorption at other positions in the continuum emission image, leaving the possibility that the foreground absorbing gas is spatially confined. The coincidence of these optical depth values within the errors does not contradict the above description. The velocity width of the absorption line in the 1667 MHz transition obtained with the VLBA (28.4 0.5 km s^-1) is narrower by 30% than that observed with the VLA (39.4 km s^-1). This is attributed to the 35% missing flux density of the background continuum emission (which is resolved on parsec scales in our VLBA image). The extended radio continuum emission imaged by the VLA-A array, which is resolved in the 18 cm VLBA continuum image, seems to contribute several weaker OH velocity components which we cannot see in the VLBA profiles. It is interesting that compared with the HI absorption line widths from pc scale circumnuclear gas in Seyfert nuclei such as NGC 4151 (Mundell et al. 1995; Dickey 1986), the OH absorption of NGC 5793 is much narrower.

Assuming the gas to be optically thin (The optical depth value of 0.08 supports the fact that the OH absorbing gas is not optically thick.), the OH column density at 1667 MHz derived from the optical depth is , where is the OH excitation temperature which typically ranges from 5-10 K (Elitzur 1992). Adopting the velocity integrated intensity of the absorption line in Table 3 the column density is 8.4 cm^-2 using = 10 K, which is in the range of the OH column densities found in molecular disks of nearby galaxies which have dusty nuclear regions (e.g., Baan & Haschick 1984, Baan et al. 1992).

4.3. Distribution and kinematics of the OH absorbing gas

The most intriguing result derived from our observations is that an OH velocity gradient symmetrically spanning the systemic velocity in the central region was detected. We determined a value of 8.7 km s^-1 pc^-1 for the velocity gradient across the continuum components of C1(C). Since the intensity distribution of the OH absorption (Fig. 4) (although it is difficult to make a map of OH optical depth because of the weakness of the continuum except at C1(C)) is well correlated with that of the background continuum emission (Fig. 1), the column density of OH does not vary across the continuum, suggesting that OH gas is distributed over a wider area than the continuum. Because the OH absorption is not seen in the outer region due to a lack of background continuum emission, it might be difficult to determine if the OH velocity field around C1(C) is caused by molecular gas associated with the nucleus. Nevertheless, in this section we examine the kinematical properties of the OH gas, making use of the results of H₂O maser and CO molecular gas observations.

High-resolution millimeter observations of CO (J = 1-0) with the Nobeyama Millimeter Array (NMA) found a rotating molecular gas structure in the central region of the galaxy with a radius of 1 kpc along P.A. = spanning = 3305-3661 km s^-1 (Hagiwara et al. 1997, Hagiwara 1998). In this paper, the OH velocity gradient shows a reversal of the sense of rotation observed in the central region of 10 pc. This suggests that the OH absorbing gas that we detected with the VLBA is kinematically distinct from the outer disk observed in CO. This kind of compact gaseous subsystem which is kinematically independent from the larger scale galactic molecular gas disk has been observed in other galaxies, such as, NGC 4826 (Braun et al. 1992), NGC 253 (Anantharamaiah & Goss 1996), and NGC 3079 (Sawada-Satoh et al. 2000).

In the following we consider that the velocity gradient observed around C1(C) might arise from a rotating molecular disk with a radius (r) centered on the nucleus. In order to explain the H₂O spectrum showing systemic and red- and blue-shifted features in NGC 5793, Hagiwara et al. (1997) proposed a nearly edge-on circumnuclear molecular disk/torus with a radius of r 4 - 20 pc and a rotation velocity of = 245 km s^-1. If we suppose that absorbing OH gas is located in the same molecular disk/torus, the observed velocity gradient () along the major axis of P.A. = 140^o can be expressed as follows;

where l is the projected distance from the source center, i = 73^o is an inclination of the molecular rotating disk, and sin is adopted (Nakai 1995). We adopt the inclination of the OH disk is to be 73^o, that of galaxy's optical disk. For = 245 km s^-1 and = 8.7 km s^-1 pc^-1, r is estimated to be about 28 pc. Using this value for the radius of a molecular disk/torus around C1(C), the mass confined within the disk is 4.2. This estimate is larger by approximately a factor of three than the mass previously derived from water maser profiles (Hagiwara et al. 1997), because they adopted r = 10 pc. We must note, however, that the mass estimate will depend on the assumed inclination angle of the rotating gas disk. The fact that H₂O maser features were preferentially observed in the edge-on molecular disks supports our assumption (e.g., Miyoshi et al. 1995). On the other hand, the gas mass of molecular hydrogen in the outer disk, that is determined from the CO intensity estimated from the CO profile, is 8.0 (Hagiwara 1998). The OH gas mass within a radius of 10 mas or 2.3 pc from the continuum peak corresponds to about 5% of that in the CO disk with a radius of 1 kpc, implying that the density of observed OH gas is significantly greater than that of the outer molecular gas disk.

One of our motivations for this observation was to investigate whether or not the OH absorption in NGC 5793 takes place in the same molecular cloud that contains the H₂O maser. The centroid velocities of the H₂O emission peaks at = 3449 and 3519 5 km s^-1 are close to those of the OH absorption lines observed with the VLA (Gardner & Whiteoak 1986), although only the OH feature at 3449 km s^-1 was detected in our VLBA observation (the 1667 MHz spectrum of Fig. 3). The 3519 km s^-1 absorption feature, which is weaker than that at 3449 km s^-1, was not clearly detected, probably because the extended background continuum necessary to observe it in absorption is resolved with the VLBA. Another OH velocity component was found in the velocity range of the H₂O maser feature at 3449 km s^-1, however there is no clear evidence for a rotating OH molecular disk/torus around the nucleus or a relationship with the proposed water maser disk proposed in Hagiwara et al. (1997). The large value of the OH column density along the line of sight, the high mass density of the observed absorbing cloud and the existence of the velocity gradient around the nuclear region, dynamically independent from the outer galactic disk, demonstrate that the observed OH absorption probably arises from the dense molecular gas in the circumnuclear region of NGC 5793.

© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000
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