4 Observational results

The available data from the present study are:

1.

Table 9, giving for each object the linear representation of the colour-$\log r$ relations, i.e. the selected inner and outer radii of the fit, the zero point colour with its probable error $\sigma_{\rm C}$ and the slope. The probable errors for the slopes are given above. This table allows easy calculation of the colours at any radius, notably the effective radius $r_{\rm e}$ and others as considered below.

2.

In electronic form only, the tables giving for each galaxy, as a function of the radius r, the V magnitude $\mu _V$ and the colours U-B, B-V, V-R, V-I. These tables are presently available with the U-B indices as observed, and consistent with Table 9. They will be later made available after correction for suspected effects of PSF far wings (see discussion in 3.2.5).

3.

Series of graphs from the above tables, showing the colours as a function of $\log r$ or of $\mu _V$. Examples of these graphs are shown here to illustrate a number of properties of the radius-colour relations.

4.

Table 10 gives the "central'' colours according to several definitions: we have considered the colours integrated inside the area of radius r=3 $^{\prime \prime }$, and the colours calculated for r=1.5 $^{\prime \prime }$ from the linear representations of Table 9. They should be nearly equal if these representations remain valid at small r, which is not always the case: see below for a description of typical deviations.

5.

Table 11 collects colours measured at the outermost range of the available data expressed in V magnitude. This gross limit varies between $\mu_V=23$ (for NGC 4472) and 24.5. It is controlled by the size of the object and the "cleanliness'' of the nearby field.

The SA0 galaxies NGC3115, 3607, 4550 and 5866 have been observed with the E-type sample. The corresponding results are given in the tables, but they have been discarded from the discussion.

Figure 7: Colour-colour diagram of U-B against B-V: the colours are calculated from the linear representations of Table 9 at the effective radius $r_{\rm e}/8$ (dots), at $r_{\rm e}/2$ (circles) and at the outermost range near $\mu _V=24$ (stars) from Table 11. Although the various symbols refer to widely different regions of the galaxies they define a common relation.

4.1 Statistical comparison with previous work

• Assuming that colours result only from population variations, "perfect'' correlations between zero point colours in different pass-bands are expected. We have compared colour-colour correlations for previous surveys and the present one. For B-I against B-V we find from Goudfrooij et al. (1994) a coefficient of correlation $\rho=0.61$ (41 objects) compared to $\rho=0.94$ from Table 9 (37 objects). For U-R against B-R the data of Peletier et al. (1990) (38 objects) give $\rho=0.58$, while the present data leads to $\rho=0.94$. The improvement is less if we compare our results with the discussion in RM00 where the calibration of the available photometry was reconsidered;
• The correlation between colour gradients should also be as good as allowed by errors of measurements. Again we compare the correlation coefficients between gradients from the literature and our results for the same colours. From Goudfrooij et al. (1994), considering the gradient $\Delta_{BI}$ against $\Delta _{BV}$, we find $\rho=0.78$ which is quite good. The result derived from Table 9 is still better, i.e. $\rho=0.85$. The improvement is more pronounced when the $\Delta _{UR}$ against $\Delta _{BR}$ correlation is calculated from Peletier et al. (1990): one obtains $\rho=0.36$ only, instead of $\rho=0.88$ with our data.
• Our colour measurements could be obtained at much lower surface brightness (or larger radii) than in previous work. The present data extend to $23.2 <\mu_V <25.2$, with a median value near $\mu_V=24.5$ in all colours. The tables by Goudfrooij et al. (1994), available from CDS Strasbourg, are mostly limited to $\mu_V=22.75$ in B-V and $\mu_V=22.25$ in V-I (median values), due to the small fields of the frames, notably in the I band (and probably an ad hoc cut off). Their published graphs generally extend to the same radius in both colours. The tables from Peletier et al. (1990), again at the CDS, extend to $\mu_V=23.2$ in B-R and $\mu_V=22.6$ in U-R (mean values) due to a systenatic cut-off at 10% of the sky. The printed tables may be still more severely truncated.
  

  Figure 8: Colour-colour diagram of V-I against U-V: the colours are calculated from the linear representations of Table 9 at the effective radius $r_{\rm e}/8$ (dots), at $r_{\rm e}/2$ (circles) and at the outermost range near $\mu _V=24$ (stars) from Table 11.

  
  

  In summary, our colour data generally extends 1.5 to 2 magnitudes deeper than in previous works, so that "external colours" refering to the level $\mu_V=24.5$ whenever possible, are presented in Table 11 with realistic error estimates.

4.2 Description of isophotal colour profiles

Most of the profiles relating colour to $\log r$, or equivalently to the surface brightness $\mu _V$, are regular, meaning that they deviate very little from a straight line in the range of abscissae relevant to the present data, roughly r >2 $^{\prime \prime }$ and $\mu_V < 24.5$. In this case, the "central colours'' integrated within r < 3 $^{\prime \prime }$, and the colours calculated from the linear representation at the average radius of r=1.5, differ very little. Central colour according to these two definitions are given in Table 10: compare Cols. 5 and 6 for U-V, and 3 with 7 for V-R. Figure 2 gives an example of a set of regular colour profiles for NGC 4473. Of course the linear colour-$\log r$ relation is only approximate and breaks down at small r for all galaxies observed at high resolution. Carollo et al. (1997) obtained V-i maps of a number of E-galaxies from HST frames, disclosing minute colour structures in some cases. In RM99, the CFHT resolution proved sufficient for the detection and classification of B-R central "red peaks''. These are smoothed out, at least partly, with the OHP seeing.

Non regular profiles have been observed in the following cases:

1.

When an important dust pattern occurs near the center of an object, it produces a central red hump in the colour profile. This is the case for galaxies with the value of 3 for the dust pattern importance index (DPII) introduced in RM99, such as NGC 2768, 4125 and 4374, but also for 5813 and 5831. The dust ring of NGC 3607, type SA0, produces noticeable bumps in its colour profiles. Figure 3 gives an example of the colour profiles of such a centrally dusty galaxy, i.e. NGC 4125. In such cases, the integrated colours within r=3 $^{\prime \prime }$ are redder than the extrapolated colours at r=1.5: this "extra reddening'' is smaller in U-B than in other colours. The consideration of the central reddening in V-R, a colour nearly insensitive to age-metallicity variations but sensitive to dust, gives a possibility to correct other colours for dust effects. This has been done, in some applications, for galaxies of DPII 3.

2.

A few objects show a central red hump in some of the colour profiles, specially U-B and also eventually B-V. This is the case of NGC 3377, 3379, 3610(?) and 4494. Figure 4 illustrates the case of NGC 3377. For such objects the "extra reddening'' defined above is near zero in V-R and V-I. It is therefore permitted to attribute it to a metallicity increase rather than to dust.

3.

On the contrary, the central colours U-B, B-V may be bluer than the extrapolation of the linear portion of the profile. This occurs for objects with larger than average colour gradients. The best such case is NGC 4636; others are 4283, 4478 and the SA0 4550. An "extra blueing'' in U-V is then apparent from the comparison of indices in Cols. 5-6 of Table 10. This is also marginally the case for such giant galaxies as NGC 4406, 4472, 4649: according to RM99 the colour profile of such objects tend to flatten out near the center.

4.

NGC 4486 is remarkable in showing a central "blue deep'', the U-B colour beeing 0.51 at r=0 as compared to 0.72 near r= 6.5 $^{\prime \prime }$. This central feature is probably somehow related to the famous non-thermal jet of this galaxy. The jet is of course very conspicuous in U-B, with a peak colour near -0.1. Needless to say, both the central "blue deep'' and the jet are affected by seeing (and our attempts to correct its effects).

4.3 Correlations of interest

4.3.1 Correlations between colour gradients

The correlations between the various colour gradients have already been noted as useful tools to evaluate the probable errors in gradients. These correlations are displayed in Figs. 5 and 6.

The coefficient of correlation between $\Delta _{UB}$ and $\Delta _{BV}$ is 0.75; that between $\Delta _{VI}$ and $\Delta _{BV}$ is only 0.40. For $\Delta _{VR}$ it falls to 0.20, because the errors are of the same order of magnitude as the V-R gradients. Imposing regression lines running through the origin, the relative slopes are $<\Delta_{UB}/\Delta_{BV}=2.2>$, $<\Delta_{VI}/\Delta_{BV}=0.8>$ and $<\Delta_{VR}/\Delta_{BV}=0.25>$. These relative slopes are in good agreement with the results in RM00, and its conclusion, i.e. the negligible influence of dust upon colour gradient, is confirmed.

The distribution of colour gradients for E-galaxies may be of interest. The following parameters are found: $<\Delta_{UB}>\ =-0.152$ with $\sigma=0.044$; $<\Delta_{BV}>\ =-0.070$ with $\sigma\ =0.021$; $<\Delta_{VR}>\ =-0.018$ with $\sigma=0.012$; $<\Delta_{VI}>\ =-0.054$ with $\sigma=0.023$. The dispersions are not much larger than the errors estimated above. The distributions are asymmetric: there are 4 objects with $\Delta _{BV}$ larger than $+1.8\sigma$ above the mean, but none at less than the same deviation. These galaxies, with $\Delta _{BV}$ clearly steeper than average, by about twice the estimated probable error of measurement, are NGC 4283, 4478, 4564 and 4636, seemingly a random collection.

Remark: an attempt to sort the E-galaxies by flattening as measured in MM94, and to look for some relation to the gradients, lead to negative results. Similarly no significant difference was found between diE and other galaxies.

The SA0 NGC 4550 has quite exceptional gradients in all colours, and an admixture of dust and relatively young stars could be invoked to explain its properties. This object also has very remarkable kinematics, as first described by Rubin et al. (1992); a model has been proposed by Rix et al. (1992). The few other S0s in the present sample are similar to Es with regard to their colour gradients.

Figure 9: Correlation between the near center U-V colour in abscissae, and the Mg2 index from Faber et al. (1989), in ordinates.

Figure 10: Correlation between the U-V colour and the Mg2 index at the effective radius. This index, and the $Mg_{\rm b}$ one merged in the data, are taken from Kobayashi & Arimoto (1999).

4.3.2 Colour-colour diagrams

Many colour-colour diagrams can be built from the present data. The indices used may be calculated from the linear representations in Table 9 at the effective radius $r_{\rm e}$, at a near center position $r_{\rm e}/8$, and at an intermediate position $r_{\rm e}/2$. The system of $r_{\rm e}$ here used is an average of estimates in MM94, Prugniel & Héraudeau (1998), and the RC3. It is satisfactory that the various indices $(B-V)_{r_{\rm e}}$, $(B-V)_{r_{\rm e}/2}$, $(B-V)_{r_{\rm e}/8}$, define a common diagram with the corresponding U-B. This may suggest that a common physical variable controls the variations inside an object, and the object to object changes, of the two colours. Such graphs readily show larger than average calibrations errors: for instance, the U-B colour of NGC 3610 is clearly too red for its B-V.

We have also traced colour-colour diagrams for the "central'' colours, (Table 10), i.e. integrated in the radius r < 3 $^{\prime \prime }$. They are similar to those traced with the interpolated colours, but with larger dispersions: this is not surprising since the central colours suffer from larger errors (see Sect. 3.2.7).

Finally, one can trace colour-colour diagrams with the "outermost colours'' collected in Table 11. They are in fair agreement with the diagrams derived from Table 9, and extend these towards the blue. We show in Fig. 7 a composite colour-colour diagram U-B against B-V, using the colours ar $r_{\rm e}/8$, $r_{\rm e}/2$ and the outermost range. Similarly Fig. 8 displays the diagram of V-I against U-V.

4.3.3 The $\mathsfsl{U-V}$ colour as a metallicity index for ellipticals?

Burstein et al. (1988) showed a correlation between the central Mg₂ index and a global B-V colour measured in a large aperture (see also Bender et al. 1993). This type of correlation is reconsidered here using the U-V colour which is much more sensitive to metallicity than B-V, and taking advantage of recent estimates of the Mg₂ index far from center.

Two correlations between the Mg₂ index and U-V are considered in Figs. 9 and 10. The first shows the relation between the two quantities near the galaxy center: the $Mg_{\rm 2C}$ index is taken from the tabulation by Faber et al. (1989). The $(U-V)_{\rm C}$ index is the integrated colour in a circle of radius r=3 $^{\prime \prime }$. The value for three galaxies with important central dust patterns have been corrected by reference to the central bump in (V-R), a colour sensitive to dust but less so to metallicity changes. The coefficient of correlation reaches 0.825. Taking $(U-V)_{\rm C}$ as x and $Mg_{2\rm C}$ as y we find the regression $y=0.221\pm.027x-0.061\pm.003$.

The second, i.e. Fig. 10, displays the correlation between the U-V colour and the Mg₂ index at the effective radius $r_{\rm e}$. $Mg_{2 r{\rm e}}$ has been taken from Kobayashi & Arimoto (1999) (KA99). To increase the number of data points the $Mg_{\rm b}$ gradients were introduced, using the linear relation $Mg_{\rm b}\,=15\,Mg_2$ derived from the distribution of the values of both indices in the tables of KA99. The coefficient of correlation still reaches 0.72. Again, with the colour in x and the $Mg_{2 r{\rm e}}$ in y we find $y=0.283\pm.069x-0.133\pm.007$. The difference to the above correlation for central indices is barely significant. The quality of these correlations proves that both indices are essentially controlled by the same physical variables, and leaves little room for the effects of diffuse dust upon the colours of E-galaxies.

 

Table 9: Linear representation of colour against $\log r$. Successive columns: NGC No.; Type; $r_{\rm i}$ inner radius of calculation; $r_{\rm o}$ outer radius of calculation; r₀ radius of colour evaluation (in log and ''); U-B at r₀ and estimated standard error; $\Delta _{UB}$ radial gradient; B-V at r₀ and estimated standard error; $\Delta _{BV}$ radial gradient; V-R at r₀ and estimated standard error; $\Delta _{VR}$ radial gradient; V-I at r₀ and estimated standard error; $\Delta _{VI}$ radial gradient; du Dust visibility index (Michard 1999) Notes: 2974: V-R and V-I not measurable; 3193: V-I not measurable.

NGC	Type	$r_{\rm i}$	$r_{\rm o}$	r0	U-B	$\Delta _{UB}$	B-V	$\Delta _{BV}$	V-R	$\Delta _{VR}$	V-I	$\Delta _{VI}$	du
2768	diE	10	80	1.467	$0.521\pm.03$	-0.093	$0.892\pm.02$	-0.054	$0.536\pm.02$	-0.012	$1.136\pm.02$	-0.081	3
2974	diE	8	80	1.404	$0.429\pm.01$	-0.237	$0.905\pm.01$	-0.080	-	-	-	-	3
3115	SA0	8	100	1.464	$0.501\pm.01$	-0.169	$0.912\pm.01$	-0.069	$0.585\pm.01$	-0.010	$1.222\pm.01$	-0.047	0
3193	unE	6	40	1.169	$0.461\pm.02$	-0.163	$0. 924\pm.01$	-0.086	$0.567\pm.02$	0.009	-	-	0
3377	diE	8	80	1.399	$0.298\pm.02$	-0.144	$0.854\pm.01$	-0.064	$0.510\pm.01$	-0.014	$1.067\pm.01$	-0.080	1-
3377	diE	8	80	1.403	$0.289\pm.02$	-0.170	$0.852\pm.01$	-0.075	$0.505\pm.01$	-0.028	$1.074\pm.01$	-0.105	1-
3379	unE	8	100	1.460	$0.538\pm.02$	-0.111	$0.918\pm.01$	-0.043	$0.585\pm.01$	-0.013	$1.206\pm.01$	-0.037	1-
3605	boE	4	30	1.043	$0.403\pm.03$	-0.175	$0.821\pm.02$	-0.062	$0.517\pm.02$	-0.014	$1.076\pm.02$	-0.038	0
3607	SA0	15	100	1.569	$0.451\pm.02$	-0.111	$0.882\pm.01$	-0.067	$0.537\pm.02$	-0.017	$1.149\pm.02$	-0.046	3
3608	boE	5	50	1.201	$0.415\pm.03$	-0.189	$0.949\pm.01$	-0.064	$0.542\pm.01$	-0.020	$1.175\pm.01$	-0.038	1-
3610	diE	4	50	1.159	$0.448\pm.04$	-0.132	$0.785\pm.01$	-0.075	$0.503\pm.01$	0.003	$1.144\pm.02$	-0.030	0
3613	diE	6	60	1.287	$0.470\pm.03$	-0.174	$0.875\pm.02$	-0.074	$0.533\pm.04$	-0.018	1.$100\pm.04$	-0.050	1-
3640	boE	5	60	1.266	$0.455\pm.03$	-0.120	$0.892\pm.02$	-0.043	$0.544\pm.02$	-0.012	$1.162\pm.03$	-0.045	0
3872	diEp	5	60	1.202	$0.506\pm.03$	-0.127	$0.928\pm.02$	-0.072	$0.571\pm.01$	-0.005	$1.176\pm.02$	-0.078	0
4125	diE	10	80	1.445	$0.497\pm.03$	-0.165	$0.890\pm.02$	-0.083	$0.568\pm.03$	-0.011	$1.183\pm.03$	-0.046	3
4261	boE	5	80	1.320	$0.588\pm.03$	-0.170	$0.948\pm.01$	-0.089	$0.585\pm.01$	-0.014	$1.246\pm.02$	-0.048	1-
4278	diE	10	60	1.361	$0.408\pm.02$	-0.149	$0.845\pm.01$	-0.088	$0.554\pm.03$	-0.031	$1.145\pm.03$	-0.093	?
4283	unE	5	30	1.080	$0.410\pm.02$	-0.256	$0.870\pm.01$	-0.117	$0.533\pm.02$	-0.040	$1.147\pm.02$	-0.078	0
4365	boE	5	120	1.430	$0.552\pm.02$	-0.120	$0.939\pm.01$	-0.058	$0.588\pm.01$	-0.011	$1.226\pm.02$	-0.057	0
4374	unE	8	80	1.394	$0.497\pm.03$	-0.107	$0.915\pm.01$	-0.044	$0.565\pm.02$	-0.020	$1.189\pm.02$	-0.025	3
4387	boE	5	30	1.102	$0.431\pm.02$	-0.106	$0.883\pm.01$	-0.052	$0.565\pm.01$	-0.028	$1.170\pm.02$	-0.036	0
4406	boE	5	80	1.312	$0.463\pm.02$	-0.121	$0.961\pm.01$	-0.078	$0.561\pm.01$	-0.018	$1.204\pm.01$	-0.054	0
4472	unE	5	150	1.510	$0.590\pm.02$	-0.133	$0.963\pm.01$	-0.040	$0.592\pm.01$	-0.005	1.257$\pm.01$	-0.010	0
4473	diE	5	80	1.310	$0.423\pm.03$	-0.169	$0.881\pm.01$	-0.063	$0.598\pm.01$	-0.021	$1.225\pm.01$	-0.065	0
4478	boE	6	50	1.286	$0.300\pm.02$	-0.234	$0.806\pm.01$	-0.103	$0.525\pm.01$	-0.023	$1.129\pm.01$	-0.051	0
4486	unE	10	120	1.559	$0.566\pm.02$	-0.192	$0.921\pm.01$	-0.063	$0.601\pm.01$	-0.019	$1.235\pm.02$	-0.068	0
4494	unE	5	80	1.340	$0.452\pm.03$	-0.114	$0.862\pm.01$	-0.050	$0.518\pm.02$	-0.016	$1.137\pm.02$	-0.019	?
4550	SA0	5	40	1.199	$0.260\pm.02$	-0.252	$0.826\pm.01$	-0.196	$0.511\pm.01$	-0.108	$1.119\pm.02$	-0.185	0
4551	boE	5	40	1.184	$0.475\pm.02$	-0.113	$0.880\pm.01$	-0.040	$0.525\pm.02$	-0.014	1.151$\pm.02$	-0.027	0
4552	unE	5	100	1.366	$0.489\pm.01$	-0.171	$0.943\pm.01$	-0.080	$0.546\pm.01$	-0.035	$1.191\pm.02$	-0.074	0
4564	diE	5	50	1.233	$0.369\pm.02$	-0.247	$0.884\pm.01$	-0.111	$0.545\pm.01$	-0.024	$1.124\pm.02$	-0.083	1-
4621	diE	5	80	1.327	$0.522\pm.02$	-0.182	$0.919\pm.01$	-0.080	$0.584\pm.01$	-0.004	$1.216\pm.02$	-0.033	0
4636	unE	7	100	1.416	$0.495\pm.02$	-0.169	$0.906\pm.02$	-0.115	$0.562\pm.02$	-0.034	$1.185\pm.03$	-0.105	?
4649	unE	5	150	1.515	$0.592\pm.01$	-0.121	$0.982\pm.01$	-0.053	$0.571\pm.01$	-0.036	$1.231\pm.02$	-0.048	0
5322	boE	5	80	1.313	$0.445\pm.03$	-0.146	$0.843\pm.02$	-0.067	$0.530\pm.02$	-0.001	$1.091\pm.03$	-0.028	0
5576	boEp	5	60	1.216	$0.382\pm.02$	-0.147	$0.821\pm.01$	-0.076	$0.521\pm.01$	-0.003	$1.126\pm.01$	-0.040	0
5813	unE	5	60	1.229	$0.477\pm.01$	-0.076	$0.945\pm.01$	-0.045	$0.589\pm.01$	-0.022	$1.244\pm.02$	-0.069	1
5831	diE	4	50	1.175	$0.423\pm.03$	-0.124	$0.876\pm.02$	-0.078	$0.515\pm.02$	-0.040	$1.168\pm.02$	-0.075	?
5846	unE	5	100	1.407	$0.594\pm.02$	-0.129	$0.955\pm.02$	-0.051	$0.586\pm.02$	-0.016	$1.245\pm.02$	-0.035	0
5866	SA0	15	110	1.612	$0.367\pm.02$	-0.190	$0.819\pm.01$	-0.100	$0.537\pm.01$	-0.048	$1.095\pm.02$	-0.042	3+
5982	boE	8	100	1.447	$0.431\pm.02$	-0.117	$0.868\pm.02$	-0.062	$0.522\pm.02$	-0.043	$1.145\pm.02$	-0.080	?