5 Summary and Conclusions

In this work, we used deep HST WFC3 imaging of NGC5189 in [SII], [OIII], and H$ \alpha $ to map low- and high-excitation regions within the inner ( $ 120\hbox {$^{\prime \prime }$}\times 90\hbox {$^{\prime \prime }$}$ or 0.32 pc $ \times $ 0.24 pc) region of the nebula centered on its [WO] central star. The HST images (see Figure 1) illustrate that NGC5189 contains multiple filamentary structures and several knots distributed inside the nebula. The inner region close to the central star includes filamentary loops, which are bright in the H$ \alpha $, [OIII], and [SII] emission lines.

We employed diagnostic diagrams consisting of [OIII]/H$ \alpha $ and [SII]/H$ \alpha $ ratios to distinguish between fast, dense LISs (low-ionization structures) and low-density photoionized medium. For the excitation diagnostic ([OIII]/H$ \alpha $ vs. [SII]/H$ \alpha $ ratios), the dividing line between the shock- and photo-ionized regions was adopted based on shock model simulations by Raga et al. (2008), which demonstrates the shock-ionization effects of fast, dense material passing through a low-density environment. The adopted photon-shock dividing line of $ 1.89 \log($[SII]/H$ \alpha $ $ )+2.0 = \log($[OIII]/H$ \alpha $$ )$ is parallel with the Seyfert-LINER classification line (Kewley et al., 2006) distinguishing between Seyfert-type and LINER-type activities in galaxies. We have used this approach to map fast LISs within the NGC5189 nebula. We identified two low-ionization envelopes in the inner $ 50\hbox{$^{\prime\prime}$}\times 50\hbox{$^{\prime\prime}$}$ ($ 0.13$pc $ \times 0.13$pc) region of NGC5189 from the central star: one of them is a large envelope expanding toward the northeast ( $ {\rm PA} \approx 60^\circ$), whereas its counterpart is a smaller envelope expanding toward the southwest of the nebula ( $ {\rm PA} \approx 240^\circ$).

Although the hot central star of NGC5189 provides UV radiation to ionize these low-ionization envelopes, propagation of dense LISs through the previously ejected materiel create wind-shock-heated features that could add a deviation from the photoionization pattern (see Fig. 2). We also noticed that the HST diagnostic view of NGC 5189 corresponds to Seyfert-like activity in the BPT diagram (see Fig. 2 bottom). Nebulae locally ionized by post-AGB stars can considerably contribute to LINER-like and Seyfert-like patterns in the BPT diagram on large scales of galaxies (see e.g. Annibali et al., 2010; Sarzi et al., 2010).

Currently, only one long-slit spectrum of the central region is available, so 3-D kinematic structures of the LIS envelopes cannot properly be determined. Further high resolution kinematic observations, such as long-slit high-resolution spectroscopy at several positions and orientations in the $ 120\hbox {$^{\prime \prime }$}\times 90\hbox {$^{\prime \prime }$}$ region will present further constraints to determine their 3-D morphological and kinematic characteristics. We also note that accurate reddening correction of HST images for the interstellar extinction can be done if H$ \beta $ $ \lambda $4861 and radio maps are available. Further high-resolution deep HST multiwavelength emission-line imaging and multiple long-slit spectroscopy will certainly provide crucial details about morpho-kinematic structures of inner, low-ionization envelopes in this planetary nebula.

Ashkbiz Danehkar