4 Discussion

Diagnostic mapping of the flux ratios [OIII]/H$\alpha$ and [SII]/H$\alpha$ have allowed us to spatially resolve the fast, high-density LISs within the low-density photo-ionized environment of NGC 5189 (described in Section 3). The regions of fast LISs is shown in Figure 3. There are two main LISs in the inner region of NGC5189 within $50\hbox{$^{\prime\prime}$}\times 50\hbox{$^{\prime\prime}$}$ ($0.13$pc $\times 0.13$pc at $D=546$pc) from the central star: the larger LIS expands toward the northeast of the nebula ( ${\rm PA} \approx 60^\circ$), while the smaller LIS expands toward the southwest ( ${\rm PA} \approx 240^\circ$).

We notice that these inner LISs in NGC 5189 are also bright in both H$\alpha$ and [OIII], in addition to [SII] (see Figure 1), contrary to the typical definition of LISs (Gonçalves et al., 2001; Gonçalves et al., 2009; Corradi et al., 1996, and references therein). These inner LISs are brighter in the [SII]/H$\alpha$ flux ratio, and fainter in the [OIII]/H$\alpha$ flux ratio. However, these LISs are not fainter than the surrounding main nebula in the [SII] or [OIII] emission absolute fluxes.

The kinematic data of a long-slit passing through the central region covering the low-ionization envelopes indicate that they have maximum projected velocities up to 35-45 kms$^{-1}$ (the slit b in Sabin et al., 2012). According to these projected velocities, the inner LISs in NGC 5189 are not moving significantly faster than other regions, so their features may not be typical of the so-called fast, low-ionization emission regions (FLIERs) seen in around 50% of PNe (e.g. Balick et al., 1993; Balick et al., 1994; Perinotto et al., 2004; Hajian et al., 1997; Danehkar et al., 2016; Balick et al., 1998). Typically FLIERs appear point-symmetric and their low-ionization outflows move supersonically with respect to the main nebula (Balick et al., 1993; Balick et al., 1994). Nevertheless, fast bipolar outflows detected in some PNe move faster than the main nebula, but their excitation characteristics may not correspond to LISs (e.g. Danehkar, 2015; Guerrero et al., 2008; Trammell & Goodrich, 1996; Corradi et al., 1997; Miranda et al., 2012; Guerrero & Manchado, 1998; Fang et al., 2015).

From Figure 2, the characteristics of these LISs within the nebula are typical of the shock-ionization, so their unprojected expansion velocity should be higher than the surrounding high-excitation material. These low-ionization envelopes expanding along an apparently symmetric axis may be caused by the past powerful outburst from the progenitor post-AGB star, plowing into the previously ejected material. While these envelopes are ionized by UV radiation from the hot central star ( $T_{\rm eff}=165$ kK; Keller et al., 2014), their propagation through and interaction with the previously expelled matter makes the shocked wind regions that produce additional thermal energy for ionization (see e.g. Freeman et al., 2014; Guerrero et al., 2013; Dopita et al., 2017). Studies of WFC3 images showed that flux ratio such as [OIII]/H$\alpha$ could be enhanced by bow-shock features (Guerrero et al., 2013). Moreover, X-ray Chandra imaging observations suggested the presence of wind-shock-heated bubbles within PNe (Freeman et al., 2014). Shock-ionization modeling demonstrates how a shock propagating at $\sim 40$ km/s into the pre-existing material can heat up them (Dopita et al., 2017), while there is also evidence for the shock-excitation of LISs in some PNe (Ali & Dopita, 2017). Therefore, shock-ionization, in addition to photo-ionization, provides thermal energy that contributes to a deviation from the photoionization pattern in the diagnostic diagrams (on one side of the photon-shock dividing line in Fig. 2).

As Figure 4 shows, the low-ionization envelopes contain several filaments and knots that are bright in [SII] compared to H$\alpha$ and [OIII]. Numerical simulations of radiative shock models reveal that radiative shock can form knots and filaments in a non-accelerated medium such as PNe (Walder & Folini, 1998a; Walder & Folini, 1998b). It is possible that the knots seen in the low-ionization envelopes of NGC 5189 provide the seeds for cometary-like knots such as those seen in the Helix nebula (Matsuura et al., 2009; O'Dell et al., 2004; Meaburn et al., 2013). The kinematics and composition of these early structures can provide valuable constrains on the origin and evolution of knotty structure in PNe (Redman et al., 2003).

Currently, it is not fully understood how fast LISs and bipolar outflows are formed in PNe. It has been proposed that rotating stellar winds and strong toroidal magnetic fields generate equatorial density outflows (e.g., García-Segura & López, 2000; García-Segura et al., 1999; García-Segura, 1997; Frank & Blackman, 2004). Alternatively, axisymmetric superwind mass-loss could result from a common-envelope phase for a binary system consisting of a white dwarf or a low-mass companion (e.g., Nordhaus & Blackman, 2006; Soker & Livio, 1994; Soker, 2006; Nordhaus et al., 2007). Previously, Miszalski et al. (2009) associated complex morphologies, such as those seen in NGC 5189, with post common-envelope nebulae. Recently, the periodic variability of the central star of NGC 5189 was discovered and found to be related to binarity with a 4-day orbital period (Manick et al., 2015). Additionally, Bear & Soker (2017) listed NGC 5189 among PNe with potential triple progenitors based on its complex morphology. The low-ionization envelopes of NGC 5189 could therefore be the result of a binary or triple stellar evolutionary path.

We note that as seen in Figure 2 (bottom), NGC5189 seems to demonstrate patterns of Seyfert-like activity (see e.g. Kewley et al., 2001; Kewley et al., 2006). Interestingly, recent studies of early-type galaxies indicate that excitation classifications based on the BPT diagrams for a considerable fraction of them can be attributed to post-asymptotic giant branch (post-AGB) nuclei of planetary nebulae (Annibali et al., 2010; Sarzi et al., 2010). The ionization contribution from post-AGB central stars of planetary nebulae can therefore be partially responsible for LINER-like and Seyfert-like line ratios in galaxies. Although the diagnostics observed in this very high-resolution view of NGC5189 are typically Seyfert-like, the central stars of planetary nebulae, which are responsible for ionizing the nebulae, are typically orders of magnitude weaker than even the lowest luminosity AGN associated with LINERs (see e.g. Ho, 2008). We therefore expect the local attenuation of the post-AGB Lyman continuum, e.g. by dust or geometric effects, to become very important on larger scales. A more complete and extended system should therefore be consistent with LINER-like ratios in galaxies, similarly to what is seen in attenuated AGN emission (e.g. Singh et al., 2013).