6 Nuclear and disk spectral characteristics

The distribution of the H$\alpha$ line emission along the slit was traced for each 2D spectrum. The local peaks in the distribution were taken as the centers of HII regions, that were defined to comprise all the spatial sections within the FWHM around that peak. The resulting 1D spectra were measured as explained before, producing the data collected in Table 5. In the following the region comprising the center of the galaxy is referred as the Nuclear Region. Typical sizes for the Nuclear Regions range between 0.5 and 2 kpc. The spectra are not flux-calibrated. However, since the lines used in the analysis are very close in wavelength, the count ratios are a good measure of the flux ratios.

Given the spectral coverage of the data, the standard diagnostic tools to classify the spectra (Baldwin et al. 1981; Veilleux & Osterbrock 1987) cannot be used. But, as shown by the early work by Keel (1984), the [NII]/H$\alpha$ line ratio can be used as a rough, first order estimator to classify the spectral nuclear types (SNT) into Active Galactic Nuclei (regardless of the kind of activity, since into the AGN category we have included both Seyfert and LINER-like nuclei) and HII like objects. Accumulated evidence has shown that this line ratio is very sensitive to the presence of any kind of activity, thus allowing for an easy spectral classification of the nuclei. The existence of absorption under the Balmer line could however induce the misclassification of some objects, and special care has to be taken.

Apart from a spectroscopic classification of the nuclear spectra, we have also attempted to give an estimation of the metallicity of the disk and its possible gradient, taking the [NII]/H$\alpha$ line ratio as an estimator. For low metallicity objects both nitrogen and oxygen are of primary origin and their abundances correlate well (Masegosa et al. 1994). For higher metallicities, a fraction of the measured Nitrogen is of secondary origin, which modifies the previous relation, even if it is still monotonic and, therefore, useful to probe Z. van Zee et al. (1998) have found that, for $12 + \log~ ($O/H) < 9.1, a relation does exist of the form $12 + \log~ ($O/H $) = 1.02 \log~ ($[NII]/H $\alpha) + 9.36$. The use of the [NII]/H$\alpha$ line ratio to estimate Z has the advantage of being insensitive to reddening. But, as Stasinska & Sodré (2001) have pointed out, that calibration relation is adequate only for HII regions, and important errors could be produced when the integrated spectra of spiral galaxies, or a complex ISM with shocked gas is being analyzed. Based on the [NII]/H$\alpha$ ratio Denicoló et al. (2002) obtained an improved calibration of the oxygen abundances. They clearly showed the power of this estimator when analyzing large survey data to rank their metallicities, even if the uncertainties on individual objects can reach up to 0.6 dex, mainly due to O/N abundance ratio and ionization degree variations. Here we only consider it to study global trends of Z from the spectra of HII regions or HII-like nuclei in the collected sample of spiral galaxies.

6.1 Nuclear spectra

Nuclear H$\alpha$ emission has been detected in 91 out of 98 galaxies in the sample. For the other 13, the stored data were corrupted. The nuclear spectra are presented in Fig. 21. Three of the galaxies without H$\alpha$ emission (namely, NGC 3976, NGC 5641, and NGC 2424) show [NII] emission, suggesting that shock ionization would be important in these nuclei. They were classified as LINERs in previous studies (Carrillo et al. 1999), and they belong to the class of interacting systems. For the remaining 4 galaxies (NGC 2344, NGC 2545, NGC 3835 and NGC 5147) only very faint or even absent emission has been detected partly due to the poor S/N ratio of the spectra. In any case the emission cannot be strong. They do not show any other peculiarities and can be considered as normal spiral galaxies (Jansen et al. 2000).

Given the purpose of the work and the rather low S/N ratio in many of the spectra we have not applied any correction for absorption. To cope with the problem of the presence of absorption under H$\alpha$, we visually inspected all the spectra, identifying the cases where it was conspicuous. All those galaxies were classified as SNT=3, i.e., nuclei in which the Balmer absorption is so strong that the measured [NII]/H$\alpha$ ratio is not reliable to classify it. For the remaining nuclei, without any appreciable absorption under H$\alpha$, those with spectral characteristics of HII regions were classified as SNT=1, and those with line ratios similar to active galaxies as SNT=2. Indeed this is a rather crude classification but, as we will see later, some conclusions on the nuclei of spiral galaxies and their relation to some global properties can be drawn.

The distribution of the [NII]/H$\alpha$ nuclear values shows that for most of the galaxies it is lower than 1 (see Fig. 14). The data are presented in Table 5. We notice that all the galaxies with SNT=3 have EW(H $\alpha)\leq 10$, what produces an artificially high ratio if no correction is applied to cope with the underlying absorption and are consequently excluded hereafter from the discussion.

Figure 14: Distribution of nuclear [NII]/H$\alpha$ for the whole sample. NGC 7217 has not been included due to the large measured ratio (see text).

Judged from the [NII]/H$\alpha$ ratio, we find 11 AGN candidate objects (about 10%) in our sample. Seven of them were already observed by Ho et al. (1997), who classified them as 6 LINERs and 1 Seyfert. For the remaining 4 galaxies we find that 2 of them are Seyfert 1 based on the width of the H$\alpha$ line. For the other two the information we have is not enough to classify them as Seyfert 2 or LINER. The largest line ratio is found for N7217, with [NII]/H $\alpha = 8.6$. It is a known LINER (Filippenko & Sargent 1985) frequently quoted to illustrate the signature of strong shocks (see the models by Dopita & Sutherland 1995).

Regarding the HII-like nuclei, we find that they define a rather narrow distribution of the [NII]/H$\alpha$ ratio (Fig. 15). Only two galaxies (not shown in the figure) depart from the general trend, N5172 with [NII]/H $\alpha = 1.92$, and N5678 with [NII]/H $\alpha = 1.16$. For N5172, our data are of very poor S/N ratio and the [NII]/H$\alpha$ ratio we obtained is very uncertain. And N5678 is a composite LINER/HII galaxy after Filho et al. (2000). Excluding those two objects, the remaining galaxies present a range of values of the line ratio, corresponding to values typical of irregular galaxies and disk HII regions (see Vila-Costas & Edmunds 1993 and McCall et al. 1985). For all the data classified as SNT=1, (with the quoted exceptions), included the latest spirals (t > 6), the median value is 0.38, with a dispersion of 0.07. Excluding late type spirals it amounts to 0.39 with the same dispersion.

Figure 15: Distribution of nuclear [NII]/H$\alpha$ for the galaxies with nuclear spectral type SNT = 1.

In spite of the rough character of the estimator we use here, some correlations are already hinted. There is a relation between the metallicity of the HII-like nuclei and the morphological type, the early type spirals having more metallic nuclei than the later spirals (Fig. 16, and Table 6). This result is consistent with the suggestion by Oey & Kennicutt (1993) that early type spirals are more metal rich than later types. That results rests on the difference between Sa/Sab galaxies in one side, and the later types in the other, since we don't find any difference between Sb/Sbc and Sc/Scd objects. Indeed, the later than Scd types are still of lower Z and look like a different family. The results reported here are in agreement with the work by Zaritsky et al. (1994) based on a completely different data set, in the sense that a tendency does exist for the metallicity to decrease when moving along the Hubble sequence. We have investigated if such a trend could be due to a systematically stronger H$\alpha$ absorption in early types. The absence of any appreciable trend between the H$\alpha$ EW and the morphological type t argues against that explanation, and leaves the relation as genuine.

Finally, the analysis of a possible Z enhancement produced by the presence of instabilities like bars or by the interaction with nearby neighbors has produced negative results. The range and median value of Z does not seem to be altered when those aspects are taken into account.

Figure 16: [NII]/H$\alpha$ ratios as a function of the morphological type.

 

 

Table 6: [NII]/H$\alpha$ ratios for the three spectral nuclear types, and for the different morphological types.

	[NII]/H$\alpha$
SNT = 1	0.38
SNT = 2	0.98
SNT = 3	0.76
t = 1, 2	0.46
t = 3, 4	0.38
t = 5, 6	0.38
t > 6	0.27

We have shown in Paper II that the relations are better defined if the type is replaced by a more quantitative parameter such as the B/D ratio, or the inner gradient G, with which it is tightly correlated. Here too, we find a good correlation between Z and the gradient G (see Fig. 17), statistically significant at 99.99% confidence level (R = 0.63, for 38 objects). This adds to the previous finding about the quality of the parameter G to characterize the global properties of spiral galaxies.

The other two global properties that appear to be related to the metallicity are the absolute magnitude, M_B and the maximum rotation velocity. We find that Z increases with both, the central velocity and the luminosity, i.e, massive galaxies are more metal rich. This agrees with the results reported by Zaritsky et al. (1994) and by Dutil & Roy (1999). Again no difference is found between isolated and interacting galaxies. These results would suggest that the instabilities produced by gravitational interaction, even if they can drive gas to the center (Barnes & Hernquist 1991; Mihos & Hernquist 1996), do not have major effects in the central region for mild interaction as the ones reported in this work.

Figure 17: Nuclear [NII]/H$\alpha$ ratios as a function of the gradient of the solid-body region of the rotation curve, G (G is in logarithmic scale). Interacting galaxies are marked with solid circles.

6.2 Extranuclear HII regions

The extranuclear HII regions detected in all our 2D spectra were measured and used to characterize the metallicity of the disks. As discussed before, our data does not allow us to conclude on any individual galaxy, but can be used to look for general trends when the population of the disks is considered as a whole. To be able to combine data from different galaxies, we have normalized to R₂₅(as given in the RC3) the galactocentric distances of the HII regions. In principle the choice of one or another radius to normalize could have some effect on the results of Z gradients (Zaritsky et al. 1994). Since we are interested in general trends and not in a real quantification of the gradient we consider that the choice of the normalization is of minor importance and decided to use the isophotal radius, accessible for most of the galaxies studied. From the HII regions measured we have only selected all the data with H$\alpha$equivalent width larger than 10 Å. In that way we select the better S/N data, and avoid including regions with important Balmer absorption, that could induce inconsistencies in the estimation of the metallicity. The number of regions we consider here is 392, in 98 galaxies. In the following we report the results obtained when the general metallicity trends in the disk are analyzed in relation to the morphology of the galaxy, the Nuclear Spectral Type and the effects produced by interaction. Different authors have claimed (see Vila Costas & Edmunds 1992 for a review and Zaritsky et al. 1994) that a Zradial gradient does exist in disk like galaxies. In Fig. 18 the metallicity estimator [NII]/H$\alpha$ is presented for the different morphological types. A slight tendency seems to be present for the gradient to be steeper in later types, whereas it is about zero for Sa and Sb spirals. This agrees with the claim by Oey & Kennicutt (1993) of a larger global metallicity and almost flat gradients in early type spirals.

Figure 18: Disk [NII]/H$\alpha$ ratios as a function of the distance to the center (normalized to R25).

We have calculated from the disk data the expected central [NII]/H$\alpha$ values, using the formal fitting to the data. The values we find in that way ranges from 0.41 for the earlier types, to 0.28 for the latest types. They compare very well with what we have found just measuring the line ratio of the nuclear regions, 0.44 for the early types, and 0.27 for the latest types. That consistency adds confidence to the reality of the trends we have found, and to the way of estimating the metallicity form the line ratio.

The H$\alpha$ Balmer line has been extensively used to measure the ratio of the current to the average past Star Formation Rate in Galaxies (see Kennicutt 1983; Kennicutt et al. 1994; Stasinska & Sodré 2001). Kennicutt (1994) found smooth progression in the Star Formation History with the Hubble type, with a ratio of current to past SFR increasing from 0.01-0.1 for Sa type to 0.5-2 for a typical Sc disk. The data reported here are only barely consistent with such a claim. In Fig. 19, where the H$\alpha$ equivalent width is plotted versus the radial distance, the only effect is a larger dispersion on later types than in earlier spirals towards larger EW in the later types but a clear separation between different morphologies is not obvious. It has to be noticed that Kennicutt data refer to the integrated EW whereas here we are trying to get the trend based on the distribution of HII regions crossed by the slit through the disk of the galaxies. We cannot extract a definitive conclusion from our data and, therefore, we cannot say that our data are in contradiction with Kennicutt's study, even if such a conclusion is hinted at by our results.

Figure 19: Disk equivalent widths of H$\alpha$ (in Å) as a function of the morphological type, t. EWs are plotted in logarithmic scale.

Let's now compare isolated and interacting systems. Regarding the metallicity, it appears that interacting galaxies tend to show a larger [NII]/H$\alpha$ ratio in all the mapped regions (see Fig. 20). The median value of [NII]/H$\alpha$ for the disk of isolated normal galaxies is 0.27, in contrast with a median value of 0.35 for the interacting systems.

Figure 20: Metallicity estimator for the HII regions in the disks of isolated and interacting spirals.

It is usually accepted that the interaction process drives gas to the central regions producing an enhancement in the star formation events. Consequently, a larger Z in the bulges of those galaxies would be predicted, and it's found in our data and other studies. The point here is that we have also found a higher Z along the disks of the same galaxies. It seems then that the interaction affects the whole galaxy, producing star formation in all the disk, depending on the conditions (see for example, Márquez & Moles 1994).

However, no difference in the H$\alpha$ EW has been found between interacting and isolated galaxies, as if the global star formation rate now was essentially the same in both families. To understand this result we have to take into account that only mildly interacting systems are included in our sample, for which the effects of the interaction are expected to be much less important than in stronger interactions. In that sense, we notice that our results are compatible with those found by Kennicutt et al. (1987) for a large fraction of galaxies in their complete pairs sample. Combining both results, higher Z and normal present star formation rate, it seems that the enrichment is only produced as a secular, accumulative effect along the galaxy life, without marked episodes, in those mildly interacting systems. This result is consistent with those by Bergvall et al. (2001), who find reddest disks in interacting galaxies. Nevertheless, we have already noticed that the morphological types of interacting galaxies tend to be earlier than for isolated ones, so the reported higher metallicities could be reflecting the difference in metallicities between early and late type galaxies. Larger samples of isolated galaxies would be needed to further analyze the metallicities of early types spirals as compared to those of interacting spirals with the same morphologies.

The situation for AGNs is somewhat similar, since active spirals are known to mainly reside in early type spirals (see for instance Moles et al. 1995). Given the limited sample we are considering (11 AGNs, 7 of them belonging to interacting systems) eventual differences in metallicity cannot be addressed.