5 Narrow band photometry  

As before the details of the data reduction are given in Appendix D. NGC 128 is already known to possess a counter-rotating gas component (Pagan 1994; Emsellem & Arsenault 1997). In Fig. 8 together with the unfolded rotation curve of the galaxy we plotted the behaviour of the gas component resulting from the long slit data. It appears a clear counter-rotating gas component which extends up to $r\ \raise-2.truept\hbox{\rlap{\hbox{$\sim$}}\raise5.truept \hbox{$\gt$}\ }8''$ (2.2 kpc) around the nucleus.

The contours of the H$\alpha$ and [NII] emission lines of NGC 128 and NGC 127 are shown as solid lines in Fig. 13. In the same figure we overlaid the contours of the B-band image of the galaxy (dashed lines) and a grid to help the evaluation of the size of the emission region.

  

Figure 13: H$\alpha$ and [NII] emission contours. The solid line gives the emission coming from NGC 128 and NGC 127. The dashed line shows the contours of the B-band image of the galaxies. North is on the left, West is up

Notice the presence of an inner disk-like component with the major axis tilted toward NGC 127. The disk is approximately at $\sim$ 40$^\circ$ from the equatorial plane of NGC 128 and it is extended to $\sim$10'' from the center in both directions. The position angle of the gas component decreases toward the galactic plane to $\sim$25$^\circ$ (in good agreement with the value quoted by Emsellem & Arsenault 1997).

The total H$\alpha$ flux (computed in Appendix) was used to calculate the total mass of the gas disk. Following Osterbrock (1974), Kim (1989) derived the mass from the relation:

$$M_{\rm HII} = 2.8\ 10^2 \cdot \left(\frac{D}{10}\right)^2 \... ...$})}{10^{-14}}\right) \cdot \left(\frac{10^3}{n_{\rm e}}\right)$$ (3)

where $M_{\rm HII}$ is expressed in solar masses, D is the distance of the galaxy in Mpc, and $n_{\rm e}$ is the electron number density in cm^-3, and F(H$\alpha$) is given in erg s^-1 cm². Our final value is $M_{\rm HII} = (2.7\pm1.3) \ 10^4\ M_{\hbox{$\odot$}}$, which rests on our value of H₀, an uncertainty of 0.4 Mpc for the distance of the galaxy, on the measured flux error, and an assumed electronic density of $(10\pm 3)\ 10^2~\rm e^- cm^{-3}$.This density was estimated following Kim (1989) from the ratio of the two lines $\lambda$ 6717/6731 ($\sim$0.75 a value which is compatible with such electronic density) between the [SII] emission lines detected in the spectra.

We also computed the gas mass using CIGALE observations. By integrating the total $\mbox{H$\alpha$}\ + [\rm NII]$ flux within 25'' and assuming $n_{\rm e} = 500\ \rm e^- cm^{-3}$we get $M_{\rm HII} = 7\ 10^4\ M_{\hbox{$\odot$}}$. Although the flux is integrated here on a larger area, we believe it is likely an overestimated value, due to the poor photometric calibration of the CIGALE data.