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Appendix A: Influence of the dust lane

Appendix A provides additional evidence to that discussed in Sect. 4 regarding the potential influence of the dust lane on our view towards NGC 3178’s nucleus.

For this reason we present Fig. A.1, where the 2MASS K_S band image of NGC 3718 is shown. What can be noted in Fig. A.1 is that the 2MASS K_S band contours (in red) appear to be almost entirely unaffected by the presence of the dust lane. They appear to be significantly rounder and better defined, especially when compared to the SDSS r band contours (in green), which are severely deformed by the presence of foreground dust in the region. Additionally, our Subaru K_S band contours (in yellow) appear to be more of a smaller scale inward extrapolation of the 2MASS K_S band contours (as expected), rather than of the visible light, heavily, distorted contours. Moreover, the innermost ~4 arcsec of our Subaru K_S band are located ~1.5 arcsec from the region where the contour deformation in the optical becomes catastrophic, which naturally coincides approximately with the projected beginning of the dust lane (white dashed box).

We consider it safe, therefore, to treat the central region of NGC 3718 in the NIR, as being unaffected by the presence of the dust lane when we attempt to interpret our NIR color maps.

	Fig. A.1 2MASS KS band image of NGC 3718. Over-plotted can be seen 1) 2MASS KS band contours (red); 2) SDSS r band (~0.6 μm) contours (green); 3) Subaru KS band contours (yellow), and 4) distance (blue dashed line length ~3.5 arcsec) of the optical and NIR peak flux positions to the approximate beginning of the dust lane (outlined with the white dashed box).
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Appendix B: The z-J bands alignment

Appendix B focuses on the validity of the coordinates calibration method used on the Subaru data, described in Sect. 3.5.

For this reason we present Fig. B.1, where the SDSS z band image of NGC 3718 is displayed. In this image, the black dashed ellipse has a major axis of ~4 arcsec, while the blue dashed line indicates that this is, approximately, the radial distance where the SDSS z band contours begin to deviate from being symmetric with respect to the peak flux positions. What can be seen is that the SDSS z band contours are well defined, relatively round, and, more importantly, symmetric within this region. Especially when compared to the SDSS r band contours (green contours in Fig. A.1), they seem to be almost entirely unaffected by the presence of dust within the region of interest and we consider them to be suitable for use,

for an accurate estimation of the photocenter of NGC 3718 in this band. Moreover, our J band contours appear to be similar to the SDSS z band contours, i.e. in terms of similar roundness. The z band ellipticities range between ~0.13−0.15, while in J band they range between ~0.09−0.12 in the radial interval ~0.5−1.5 arcsec, indicating that, at least to a large extent, both bands appear to be similarly unaffected by the presence of foreground dust.

We consider, therefore, that the aforementioned symmetry of the SDSS z band contours in the central ~4 arcsec of NGC 3718, as well as their similarity to the J band contours, safely allow us to apply the centering method described in Sect. 3.1. The derived photocenters are used to align the SDSS z band with our Subaru J, H, and K_S bands in order to astrometrically calibrate them.

	Fig. B.1 SDSS z band image of NGC 3718. Over-plotted can be seen: 1) the SDSS z band contours (yellow) and 2) the Subaru J band contours (red). The black dashed ellipse and the blue dashed line show the symmetry of the SDSS z band contours within the central ~4 arcsec of NGC 3718.
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Appendix C: The e-Merlin radio map position

	Fig. C.1 Same as Fig. 13, but with the radio map position dictated by the old phase reference source position. This is the equivalent position to the one reported by Krips et al. (2007), namely RA: 11:32:34.8534 ± 0.0005, Dec: +53:04:04.523 ± 0.004.
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© ESO, 2015