Besides this we have obtained average radial colour profiles, giving the mean isophotal colour against the mean isophotal radius (see plots in the Appendix).
It has been attempted to generalize the above considerations by determining the FC functions needed to bring the PSF's of a group of images of similar resolutions to a single narrower common PSF, indeed a Gaussian of suitable FWHM. Sometimes the best PSF in the group was used for matching instead of a Gaussian. The results of this exercize are given in Tables 1, 2, 3 under the label of "improved FWHM". It may be noted that we suceeded in matching the PSF's to very similar width's in most cases, and with noticeable improvements of between 20 to 33% of their width.
The eventual effect of deconvolution artefacts was checked by performing the isophotal analysis upon both the original and "improved" frames: the parameters q and e4 provide very sensitive tests. Other tests are feasible by comparing the colours measured from pairs of frames matched by either convolution or deconvolution: if new colour features occur with the "improved" frames, one should be cautious!
- Improper PSF matching leaves important systematic errors in observed colours both near the galaxian center and along the minA of near edge-on disk galaxies;
- PSF asymmetries may also give important errors in minA light and colour profiles, unless they are very similar in both the frames used. Such asymmetries, specially in the outer wings of the PSF, are not readily corrected by our techniques of PSF matching;
- The background will lead to large errors in colours at low light levels: these will come in part from uncertainties in the adopted sky background constants, but also from residual background fluctuations and trends. These are no doubt present after the flat-fielding and other correction techniques, and they were large with one of the used OHP CCD's, at least in the I band. A poor choice of the average background level also results in calibration errors;
- The unsufficient resolution of the frames, even if properly matched, leads to important errors if sharp colour features are present: this is certainly the case near the galaxian center, and for many dust patterns.
Errors in CFHT observations have been discussed in our previous Paper IV, from 8 pairs of duplicate observations of mostly elliptical objects, plus NGC 3115. This study has been repeated with emphasis upon objects containing a thin disk, such as NGC 3115, 3377, 3610. The analysis of duplicate frames was conducted in such a way as to derive errors upon the quantities to be considered in the present study.
A number of duplicate observations also occur in our collection of OHP frames. Because our sample mostly contains very flat galaxies, with very large transverse light gradients, the seeing effects are quite large. The experiments with duplicate OHP frames led to the rejection of part of the results from this material, except where it could supplement the higher resolution data without introducing unwanted bias.
Another technique is needed to estimate errors in the TBL data. It should be noted that there are 6 galaxies in common between the TBL and OHP series: the comparison of measurements from these two sources gives indications upon errors in both, and, together with the above noted experiments upon duplicate frames in the OHP material, opens a way to estimate a set of errors for both sources. It should be emphasized however that errors are mostly systematic, leaving no hope to estimate these by straightforward statistics. The two applied techniques, that is pseudo-colours C-C from duplicate frames on the one hand, and comparisons of colours from different sources on the other, are not equally sensitive to the various systematic errors listed above. For instance, near centre colour features are much sharper at 1 arcsec resolution than with the OHP seeing: this introduces large local differences between the colour distributions of the TBL and OHP series.
Probable errors derived by the above techniques are summarized in Tables 5 and 6, separately for the three sources of material and various quantities of interest. In Tables 5 are given the errors for radial colour profiles. First appear the errors for the separate semi- majA an minA as in Fig. 3 (noted , ). For OHP material is much larger than as expected from poor PSF matching. Then come the errors , for mean colours along the majA and minA respectively: they are smaller mainly because the effects of PSF asymmetries cancel out. Finally is given the estimated mean errors for the isophotal radial colour profiles.
Table 6 contains estimated mean errors for azimuthal colour profiles. Again errors for the tips of the majA and for the arcs in the minA regions are distinguished: such errors are expected to apply to such graphs as in Figs. 1, 2. We also give the error expected for the colour differences between majA and minA such as tabulated in Table 10. The above estimates were mainly obtained for B-V and B-R colours, while errors are probably larger for U-V and V-I. The contrast of E-S0 galaxies against sky background is reduced by roughly 1 mag in U and 0.5 mag in I (greatly varying with zenith distance!). Besides this, the S/N ratio is poor for part of our U OHP frames, and the background unclean for part of the I frames of the same source. Larger mean OHP errors for U-V and V-I were "estimated" and are entered in the relevant plots.
Finally the standard error upon the colour gradients in Tables 7 and 8 has been evaluated to 0.03 from a comparison between the various data series.
Errors in calibrations should be added to the errors analysed above. They are of course closely bound to the availability and accuracy of calibration data. These are abundant for NGC 2549, 3115, 4111, 4350, 7332, 7457, but rather scanty for NGC 3098, 4036, 5308, 5422. The data are best in B-V, and less good in U-V (due to scarcer data and poorer S/N in U), or V-I (due to the use of an ancillary correlation with B-V). It is believed that calibration errors reduce to 0.01 in the best cases and may reach 0.04 in the worse.
Remark: The survey in BP94 includes minA colour profiles for 4 objects of the present sample. These authors obtained their own photometry by observing standard stars (although in rather unsatisfactory conditions). To quote their paper, their "photometric accuracy is estimated to be better than 0.05 mag in R and I and 0.10 mag in B and U". We could compare results for 7 B-R or R-I colours measured at the same place, i.e. s=5 arcsec along the minA. We find for Them-Us a mean of .02 and a of 0.05, well in line with estimated calibrations errors in BP94 and above.
Incidentally we also compared minA gradients (measured in identical ranges). These are in good agreement for NGC 5422 and 7457, but not for 5308 and 7332: in these two objects BP94 gradients (their Table 2) are anomalous, B-R increasing outwards instead of decreasing as usual. This might be an effect of "differential seeing" if the R frame was distinctly sharper than the B one.
. in B-V,
. in V-I,
. in B-R,
. in U-V, .
The coefficients for galactic reddening take into account the approximate passbands used in our observations, that is Johnson's U, B and V, Cousins's R and I. The corrections have been applied only to tabulated results, not to graphical data dealing with individual galaxies.
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