4 Optical photometry  

Now we present the results of our B and R bands CCD photometry. Details of the available data and the reduction procedures are given in Appendix C. Our CCD photometry is slightly fainter ($\sim 0.28$ mag) than that of BC77 who claimed a corrected B-magnitude inside a 114'' circular aperture of 12.61 mag, while it is 0.02 brighter than the value of 13.01 mag reported by de Vaucouleurs & de Vaucouleurs (1972). By excluding the contribution of the two nearby galaxies NGC 126 and NGC 127 and integrating the total luminosity up to 169'' we get $B=12.87\pm0.1$ mag (in good agreement with the value of $12.77\pm0.13$ of de Vaucouleurs et al. 1991) and $R=11.23\pm0.1$ mag. We obtained (with our choice of H₀) an absolute magnitude $M_B \sim -20.93$. Most of the results of our photometric analysis, together with a number of literature data, are summarized in Table 1.

The isophotes of the galaxy are nearly round in the center and become progressively influenced by the presence of the disk after few arcsec. The simmetry of the figure is good and there are no indications of subcomponents and/or substructures. A first set of profiles was extracted (in each photometric band) along the major, minor and intermediate axes of NGC 128. We averaged 10 profiles taken in a cone of 1 degree aperture.

The B-band folded light profiles of NGC 128 are plotted in Fig. 10. The asymmetry along the major axis starts approximately at 20'' and it is clearly seen between 50'' and 90''. On the other hand, along the minor and intermediate axes the light profiles are more symmetric and become progressively asymmetric because of the presence of the nearby galaxies NGC 126 and NGC 127.

  

Figure 10: Upper panel: The intermediate axes profiles of NGC 128. Lower panel: Observed major and minor axes profiles of NGC 128. The 1$\sigma$ error is $\sim$0.02 mag arcsec-2 at $\mu_B = 18$ and increases up to $\sim$0.25 mag arcsec-2 at $\mu_B = 26$. The dashed lines represent the profiles obtained by BC77

The profiles given by BC77 are shown in the figure with the dashed lines. A small systematic offset seems to exist between the two data sets, but on average the agreement is very good, taking into account that they used photographic data.

A second set of profiles were extracted (for the R-band image) along cuts perpendicular and parallel to the major axis in the East and South directions respectively. Note the shift of the peak of surface brightness toward West (Fig. 11 upper panel). From the bottom panel of that figure we have a marginal indication for the presence of a bar. Following Wakamatsu & Hamabe (1984) (who presented the evidence for the presence of a bar in the edge-on galaxy NGC 4762) we tentatively recognized in the light profiles (along the South direction which is less affected by dust) the typical bump usually attributed to the existence of a bar. This is indicated by the fact that the bump becomes more and more insignificant at larger z. Differently from NGC 4762 this behaviour is not followed by a plateau and a cut-off characteristics of the presence of a lens. So, our suspect for the presence of a bar rests only on the marginal evidence given in Fig. 11.

  

Figure 11: Upper panel: The light profiles of NGC 128 along cuts perpendicular to the major axis (South direction). West is on the right side. Bottom panel: Profiles parallel to the major axis (East direction). South is on the right side

The B-R folded color profiles of the major axis are shown in Fig. 12 (together with the B-J, B-H, and B-K colors). They have been preferred on a 2D color map in order to increase the S/N ratio.

We notice a clear reddening toward the center ($\sim$0.15 mag) starting approximately at $r\sim$10'' and we estimate an average color $\langle B-R \rangle = 1.68\pm0.03$. Such reddening is not symmetric along the various direction of profile extraction, a fact that further support the existence of a disk-like structure around the center.

  

Figure 12: The B-R, B-J, B-H, and B-K color profiles of NGC 128. From left to right: the major axis, the minor axis, and the intermediate axes

The color profiles become bluer where the arm of NGC 127 encounters the disk of NGC 128, the maximum shift between the two sides reaching 0.06 mag. The B-R color increases of $\sim$0.20 mag in the inner 15 kpc. Along the minor and intermediate axes the B-R color gradient is even larger, reaching 0.3 mag in the direction of the X-structure.