4 Results  

The global photometric parameters are summarized in Table 3, whose entries are as follows.

Column 1: UGC number or, if missing, CGCG number;

Column 2: Other common names;

Column 3 and 4: Equatorial J2000 coordinates in standard units (hh mm ss.s, dd mm ss);

Column 5: Revised Hubble morphological type from the RC3;

Column 6: Total B magnitude or, if missing, photographic magnitude reduced to the $B_{\rm T}$ system, from the RC3;

Column 7: Isophotal optical size, major (D₂₅) and minor axes in arcmin at 25 B-mag arcsec^-2, from the RC3;

Column 8: Heliocentric systemic velocity in km s^-1 as listed in NED, or, if missing, from private archives;

Column 9: "Total'' H magnitude ($H_{\rm T}$) within a circle of aperture D₂₅;

Column 10: Isophotal H magnitude (H_21.5) within the elliptical isophote at 21.5 H-mag arcsec^-2;

Column 11: Major axis (D_21.5) in arcmin of the elliptical isophote at 21.5 H-mag arcsec^-2;

Column 12: Effective diameter ($D_{\rm e}$) in arcmin; this is the major axis of the elliptical isophote containing half of the flux corresponding to $H_{\rm T}$;

Column 13: Concentration index C₃₁, defined as the ratio between the major axes of the ellipses enclosing 75% and 25% of the flux corresponding to H_21.5;

Column 14: Ellipticity $\epsilon=1-b/a$ of the outer elliptical isophotes;

Column 15: Position angle (PA) of the outer elliptical isophotes, computed Eastward from North;

Column 16: Telescope of observation: TIRGO (T), NOT (N), and VATT (V);

Column 17: Estimate (FWHM) of the seeing disk of observation in arcsec, see Sect. 3.4.

Column 18: Notes from the catalogues.

4.1 Magnitudes  

Two magnitudes are here reported for the imaged galaxies: a total value $H_{\rm T}$ and an isophotal one H_21.5.

The total magnitude $H_{\rm T}$ measures the flux contained within a circular aperture the size of the optical diameter D₂₅. For our images this is always an extrapolated value and is computed with a procedure similar to the one outlined in Gavazzi & Boselli (1996) and Gavazzi et al. (1996a), although the values here are not corrected for extinction and redshift. We estimate the average $1\sigma$ accuracy of $H_{\rm T}$ to be $\sim$0.15 mag, half the error being contributed by noise and calibration and half by uncertainty in the extrapolation.

The isophotal magnitude H_21.5 is derived by integrating the surface brightness radial profile from the center out to the elliptical isophote at 21.5 H-mag arcsec^-2. In some cases, due to insufficient field of view, or to a particularly noisy background, or to strong asymmetries, we were not able to fit elliptical contours down to such brightness levels. In these cases we provide an estimate of H_21.5 obtained by (exponential) extrapolation of the outer profile; extrapolated values are enclosed in parenthesis and constitute roughly 10% of the total. In practice, the extrapolation was performed by fitting a weighted linear regression to three outermost points of the surface-magnitude radial profile. In the few cases where such regression was deemed not satisfactory, the procedure was repeated with the 6 outermost points of the profile. We estimate the average $1\sigma$ accuracy of H_21.5 to be $\sim$0.08 mag in case of interpolation and twice as much for the extrapolated values. Again, such values are only corrected for atmospheric extinction.

  

Figure 5: The difference between the isophotal magnitude H21.5 and the total magnitude $H_{\rm T}$ vs. H21.5 (left panel), and vs. the average surface magnitude $\langle\mu_{21.5}\rangle$ (right panel). In both panels open symbols are used when the radial brightness profile extends to levels fainter than 21.5 (interpolation); solid symbols are used otherwise (extrapolation). In the left panel the dotted lines are the loci of constant $\rho$, the ratio of the isophotal radius D21.5/2 to the disk exponential folding length $r_{\rm d}$.In the right panel the dotted lines are loci of constant $\mu(0)$, the face-on disk surface brightness whose value in H-mag arcsec-2 is reported on top of each line

As for the relation between the two types of magnitudes, we find $\langle H_{21.5} - H_{\rm T} \rangle = 0.20$, exactly what was found for the sample in Gavazzi et al. (1996a). The relation between the two H magntudes is illustrated in Fig. 5, where their difference is plotted vs. the magnitude itself and vs. the average H surface magnitude. Given the already quoted accuracies for the two magnitudes, the distribution appears to consist of a normal range, for $H_{21.5}-H_{\rm T} <$ 0.3 mag, and by a deviant tail for the higher values. Such large differences are partly due to the inclusion of some faint spurious objects, such as foreground stars superimposed on the outer disk. In general, the points for the extrapolated values, that is when also H_21.5 had to be estimated by extrapolation of the brightness profile, are distributed similarly to the others, which implies that, as expected, most of the variance is contributed by $H_{\rm T}$. A significant correlation is evident between $(H_{21.5}-H_{\rm T})$ and H_21.5 itself; upon inclusion of all points, the slope is $0.050\pm 0.014$. A similar and tighter trend is detected in the dependence on $\langle\mu_{21.5}\rangle$,the average surface magnitude within the isophotal elliptical contour at 21.5 H-mag arcsec^-2 (right panel). Such correlations are likely due to the narrow range of apparent diameters of the sample, see Sect. 2.1. This selection criterion causes the faintest galaxies to be often those with fainter surface brightness (and lower inclination) and, consequently, with smaller isophotal size and fainter isophotal magnitude.

We conclude that the accuracy in estimating our H magnitudes, especially the total magnitudes $H_{\rm T}$, degrades for the faintest galaxies of the sample, and that an appreciable part of the error is systematic in the sense of yielding too bright $H_{\rm T}$ values for fainter H_21.5 and/or $\langle\mu_{21.5}\rangle$. Such a trend spells a word of caution for the use of heavily extrapolated magnitudes in, say, distance measurements such as the Tully-Fisher relation.

Since the difference $H_{21.5}-H_{\rm T}$ is ideally determined only by the outer disk, we note that, for an exponential disk with folding length $r_{\rm d}$, such difference depends only on $\rho=(D_{21.5}/2)/r_{\rm d}$, the ratio between the isophotal radius and the folding one:

$$H_{21.5}-H_{\rm T} = 2.5 \, \log_{10}\left[\frac{1}{1-{\rm e}^{-\rho}\,(1+\rho)}\right]\,\, ,$$

if $H_{\rm T}$ is identified with the total, asymptotic magnitude of the disk. Also, if $\mu(0)$ is the disk central surface magnitude:

$$\left[\,\langle \mu_{21.5} \rangle - \mu(0)\,\right]-(H_{21.5}-H_{\rm T})=5\,\log_{10}\left(\frac{\rho}{\sqrt{2}}\right) \cdot$$

In the left panel of Fig. 5, the dashed lines are curves of constant $\rho$, while in the right panel they are curves of constant $\mu(0)$. The average (outer) disk appears to have an isophotal radius $D_{21.5}/2 \simeq 3.5\, r_{\rm d}$, with a central brightness $\mu(0) \simeq 17.5$ H-mag arcsec^-2.

4.2 Diameters

The D_21.5 diameter, reported in Col. 7 of Table 3, is the major axis of the elliptical isophote at 21.5 H-mag arcsec^-2. This is provided, in arcmin, for all the objects observed in the H band. As for the isophotal magnitudes, in some cases D_21.5 had to be measured by (exponential) extrapolation of the outer profile; extrapolated values are enclosed in parenthesis. The average $1\sigma$ accuracy of D_21.5 is usually about 5'' and three times worse in case of extrapolation.

  

Figure 6: The NIR isophotal diameter D21.5 vs. the optical size D25 from the RC3. In the left panel only D21.5 values derived by interpolation are reported and the sample objects have been grouped into three classes according to the (optical) axial ratio b/a, and plotted with differerent symbols. In the right panel extrapolated and interpolated D21.5 values are shown with different symbols. The dashed lines are for the case D21.5=D25 and the dotted ones for D21.5 = 0.89 D25, the average slope for the sample

Figure 6 illustrates the comparison between our D_21.5 and the B-band D₂₅ from the RC3. In the left panel only D_21.5 values obtained by interpolation are reported and the data set has been divided into three bins of (optical) axial ratio b/a; the bin boundaries are those which result in bins with equal number of objects. Within the uncertainties, the relation D_21.5 vs. D₂₅ does not deviate from linearity. As a whole we find $D_{21.5} = 0.89\, D_{25}$, which implies that the 21.5 H-mag arcsec^-2 is not as deep as the standard 25.0 B-mag arcsec^-2; in other words, the (B-H) colour of the outer galaxy regions is bluer, on average, than 3.5 (see also de Jong 1996). As for the comparison of the different inclination bins, we find no significant difference or trend: $D_{21.5} = 0.88\, D_{25}$ for b/a < 0.6, $D_{21.5} = 0.85\, D_{25}$ for 0.6 < b/a < 0.8, and $D_{21.5} = 0.91\, D_{25}$ for b/a > 0.8. If any, the effects of internal extinction are not noticeable on this relation, which confirms the overall transparency of the outer disk. A last comment regards the rather large scatter about the average regression with $1\sigma \simeq 30\%$. This is true, and approximately constant, over the whole range of apparent size and does not depend on particularly deviant cases; as shown in the right panel of Fig. 6, it actually remains the same upon exclusion of the extrapolated D_21.5 values. The scatter can be attributed to the different methods of measurement: an objective estimate from the elliptically averaged profile in the case of D_21.5, and inspection of the 2D image for D₂₅. Especially in the case of late spirals, inspection of B-band plate material is strongly affected by spiral structure and by the presence of outer H II complexes. Further discussion of this issue will be found in Sect. 4.4.

The ratio between the isophotal optical and NIR diameters is found to be a weak function of the galaxy colour. This is illustrated in the upper panel of Fig. 7 where the ratio is plotted against the total B-H index, $m_{B}-H_{\rm T}$. The solid curve represents an exponential disk with a central B-H=3.5 mag and different scale lengths in the two bandpasses; for this special value of the central colour (3.5=25-21.5) the ratio D_21.5/D₂₅ is equal to the scale lengths ratio, and the total colour only depends on this ratio. Due to the well-known colour-magnitude relation (Tully et al. 1982), the dependence on colour also implies a certain dependence on the absolute luminosity; the colour-magnitude relation for our sample is shown in the lower panel of Fig. 7. The indicative absolute magnitudes are computed assuming a redshift distance with H₀ = 100 km s^-1 Mpc^-1 and upon reduction of the velocity to the Local Group centroid according to RC3 (no infall correction); assuming a solar absolute magnitude of 3.39 H-mag, as in Gavazzi et al. (1996b), the mean value of about -22.5 mag is equivalent to $2.3 \ 10^{10}$ $L_{\hbox{$\odot$}}$.No clear correlation was instead found between D_21.5/D₂₅ and the apparent parameters (magnitude, size, inclination) so that our D_21.5 estimates appears to be generally free of measurement biases.

  

Figure 7: The ratio between the isophotal H-band diameter D21.5 and the B-band D25 vs. the total B-H colour $m_{\rm B}-H_{\rm T}$(upper panel), and the colour-magnitude relation for the present sample (lower panel). Only the galaxies with D21.5 measured by interpolation are reported. The line shown for comparison illustrates the behaviour of an exponential disk with fixed central surface magnitudes (see text)

The effective diameter $D_{\rm e}$ reported in Col. 12 of Table 3 is the major axis in arcmin of the fitted elliptical isophote containing half the flux corresponding to the total magnitude $H_{\rm T}$. The average $D_{\rm e}$ uncertainty, not including the error on $H_{\rm T}$ is about 2% but can be worse, up to 10%, in case of peculiarly disturbed morphologies.

4.3 Concentration indices

The concentration index C₃₁, Col. 13 of Table 3, is defined as the ratio between the major axes of the elliptical isophotes enclosing 75% and 25% of the flux corresponding to H_21.5 (Gavazzi et al. 1990). We have explored the possibility of biases in the measurement of C₃₁, by checking for correlations of the index with apparent magnitude, apparent size, and ellipticity, and found no evidence of any. As noted by Gavazzi et al. (1996a), C₃₁ depends on the galaxy absolute luminosity and, to a lesser degree, on morphological type; such relations, for the present sample, are illustrated in Fig. 8. As was the case also in Gavazzi et al. (1996a), the highest concentration indices are found in the most luminous Sb spirals. It also appears from Fig. 8 that the relation between $\log C_{31}$ and NIR absolute magnitude is not simply linear but definitely concave or L shaped. That is to say that, not only the mean value, but also the variance of the concentration index increases with luminosity. In Fig. 8 we indicate the value of C₃₁ of a pure exponential disk: the upper value, $\log C_{31}\simeq 0.45$ is found when the 25% and 75% fractions refer to the total disk luminosity; the lower one, $\log C_{31}\simeq 0.41$, when the fractions refer to the luminosity within 3.5 exponential scale lengths, that is the average value corresponding to H_21.5 (see Sect. 4.1). Not surprisingly, there is a definite tendency of the lowest luminosities and latest morphologies to conform to a pure disk but some low concentration indices are present also among the luminous, early-type objects. A more detailed study of the characteristics of the bulge and disk components of these objects is deferred to forthcoming papers.

  

Figure 8: Log10 of the NIR light-concentration index C31 vs. the absolute H-band magnitude (left panel), and vs. T, the index of stage along the Hubble sequence from the RC3 (right panel). Solid symbols are used for galaxies hosting active nuclei (Seyferts, LINERS, and starbursts). The horizontal lines identify the C31 values of pure exponential disks (see text)

Ten of the sample galaxies, that is $\sim$6% of the total, are reported to host active nuclei, either Seyferts or LINERS or starbursts; they are marked with solid symbols in Fig. 8. While in the present sample they are quite luminous, there is no particular tendency to high C₃₁ values; also the distribution among the morphological types is rather uniform but for the avoidance at $T\geq 6$.We find no difference, for C₃₁, between the different classes of activity.

  

Figure 9: $\langle\mu_{21.5}\rangle$, the average NIR surface brightness within the isophote at 21.5 H-mag arcsec-2 vs. the apparent H magnitude (left panel) and vs. the isophotal D21.5 diameter (right panel). As in Fig. 6, different symbols refer to different ranges of optical axial ratios

Figure 9 is a scatter diagram of the average surface brightness within the 21.5 H-mag arcsec^-2 isophote versus H_21.5 and vs. D_21.5. While there is a definite correlation between $\langle\mu_{21.5}\rangle$and H_21.5, in the sense that faint surface brightnesses are preferentially observed in the faintest sample objects, it disappears almost completely between $\langle\mu_{21.5}\rangle$and D_21.5. The correlation is therefore determined by the limited range of the selected diameters (see Sect. 2.1) rather than by intrinsic properties. As for the isophotal diameters, the trend of $\langle\mu_{21.5}\rangle$with inclination is not significant.

4.4 Ellipticities and Position Angles  

The results of the elliptical-isophote fitting have been used to compute estimates of ellipticity $\epsilon$ and position angle PA of the outer regions. Our $\epsilon$ and PA values are computed directly from the output of the IRAF-STSDAS routine "ellipse'' and are the weighted average of the three outermost points where the ellipticity is still evaluated with a precision better than 0.1. The position angles are not reported for galaxies nearly face-on and particularly uncertain cases are flagged with a " : ''. We estimate the $1\sigma$ uncertainty of the ellipticity to be $\sim$0.05, in well-behaved cases.

It turns out that such a blind, although objective, procedure is often inaccurate. Indeed, an inspection of the radial profiles of $\epsilon$ and PA in Fig. 4 shows that they are often determined by the geometry of the spiral pattern, which often dominates even in the NIR, rather than by the effective orientation of the disk. This is true in particular for late spirals seen nearly face on and, obviously, for the more disturbed, peculiar morphologies. In addition our images have a restricted field of view and are somewhat shallower than the plates from which the optical values were estimated and therefore our estimate of the outer disk can be, in some cases, rather uncertain. As a consequence, the values we derive sometimes deviate considerably from those reported in the catalogues, which are also shown for comparison in Fig. 4. A direct comparison of our ellipticities, $\epsilon_{\rm H}$, and those from the RC3, $\epsilon_{\rm B}$, is shown in Fig. 10. As expected the scatter is rather large and not appreciably influenced by the most uncertain values; we count 25 galaxies out of 174 for which $\vert\epsilon_{\rm H} - \epsilon_{\rm B}\vert \gt 0.2$.It is also quite clear that most discrepant values are found for low-inclination objects, where $\epsilon_{\rm H}$ tends to be definitely larger than $\epsilon_{\rm B}$.

  

Figure 10: The NIR ellipticity of the outer isophotes $\epsilon_{\rm H}$vs. $\epsilon_{\rm B}$, the ellipticity listed in optical catalogues. Solid points refer to particularly uncertain $\epsilon_{\rm H}$ values. The dashed line is for the case $\epsilon_{\rm H} = \epsilon_{\rm B}$;the dotted lines enclose the region $\vert\epsilon_{\rm H} - \epsilon_{\rm B}\vert < 0.2$

4.5 Non-axisymmetric structures  

The fact that the elliptical-isophote fitting might occasionally be misled by non-axisymmetric structure is bound to bear an influence on the parameters derived by the brightness profile. For example, a strong bar on a very tenuous disk will induce artificially high values of the ellipticity; the result of such narrow isophotes will be larger isophotal diameters and consequently incorrect estimates of H_21.5 and C₃₁. To check the impact of such an effect we have selected the galaxies for which our estimates of $\epsilon$ and PA differ most from the catalogued values and have repeated the fitting procedure by imposing on the outer regions the $\epsilon$ values from the RC3. In most cases, as expected, such optical values favour rounder images with a consequent shrinking of D_21.5. The effect, in any case, is not dramatic and confined to less than 10%, although it reaches over 30% in the two most deviant cases: UGC 1471 and 1626. The effect on H_21.5 is in the sense of making it brighter at lower ellipticity, but rarely exceeds the errors associated with noise and calibration. Also the influence on C₃₁ is appreciable only in the worst cases and even here within 10%; lower ellipticities tend to yield lower concentration indices.

  

Figure 11: Elliptically averaged radial profile of surface magnitude $\mu$, ellipticity $\epsilon$, and position angle PA for the barred Sb UGC 12039. The influence of the bar on the isophotal fitting between 5 and 20 arcsec is clearly illustrated in all three profiles. The solid line in the top panel represents the surface magnitude profile obtained by imposing at all radii a fixed $\epsilon = 0.29$ and a fixed ${\rm PA} = 0^\circ$, the average values of the outer disk

In a certain number of cases, the isophotal fitting, although strongly influenced by non-axisymmetric structures in the inner regions, is able to recover the actual $\epsilon$ and PA of the outer disk. These cases ($\sim$20) are characterized by sudden and strong jumps in their $\epsilon$ and PA profiles. A good example is UGC 12039, which has both a strong bar and strong spiral arms. As shown in Fig. 11, the radial profiles of $\mu$, $\epsilon$, and PA all show an obvious jump at $\sim$20 arcsec, that is right after the fading of the bar. In the same figure we also show the $\mu$profile obtained by imposing a fixed $\epsilon$ and PA value, 0.29 and $0^\circ$ respectively, which are the average values for the outer regions. The influence of the bar on the derived surface brightness profile is clearly depicted, with the strongest deviations reaching $\sim$0.5 mag. However, in these cases, the global photometric parameters (magnitudes, diameters, indices) estimated in the two ways are virtually identical, with differences well within the limits of the quoted accuracy. For uniformity with the rest of the sample, and given the small bearing on the global parameters, we prefer to retain also in case of strong asymmetries the fitting procedure with free $\epsilon$ and PA. In turn this yields the observed $\epsilon$ and PA profiles, together with higher order azimuthal Fourier components of the luminosity distribution (Carter 1978), which will be used in a forthcoming paper of this series to investigate the properties of bars and non-axisymmetric structures in general.