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The Abundance Properties of Nearby Late-Type Galaxies.I. The Data

We investigate the oxygen and nitrogen abundance distributions across the optical disks of 130 nearby late-type galaxies using around 3740 published spectra of HII regions. We use these data in order to provide homogeneous abundance determinations for all objects in the sample, including HII regions in which not all of the usual diagnostic lines were measured. Examining the relation between N and O abundances in these galaxies we find that the abundances in their centres and at their isophotal R_25 disk radii follow the same relation. The variation in N/H at a given O/H is around 0.3 dex. We suggest that the observed spread in N/H may be partly caused by the time delay between N and O enrichment and the different star formation histories in galaxies of different morphological types and dimensions. We study the correlations between the abundance properties (central O and N abundances, radial O and N gradients) of a galaxy and its morphological type and dimension.


Introduction
The chemical composition of a galaxy is one of its most fundamental characteristics.
Here we focus on disk galaxies. Because the chemical enrichment depends on various physical processes, such as the star formation history and the mass exchange between the galaxy and its environment, progress in our understanding of galaxy formation and evolution processes depends to a large extent on improving our knowledge of the detailed chemical properties of galaxies, such as the radial distribution of element abundances across galactic disks. Establishing the macroscopic properties of spiral and irregular galaxies that likely govern the distribution of heavy elements across their disks is very important in understanding the (chemical) evolution of galaxies.
Accurate abundance determinations in a sample of galaxies are mandatory for such investigations. The classical T e method, often referred to as the direct method, is generally considered to provide the most reliable oxygen and nitrogen abundances in H ii regions.
When this method cannot be used (due to the lack of measurements of the required weak auroral lines in the spectra of H ii regions) then combinations of the strong nebular line intensities in spectra of H ii regions can be used as indicators of their oxygen abundances, as was first suggested by Pagel et al. (1979) and Alloin et al. (1979). This approach is usually referred to as the "strong-line method" and has been widely adopted. The establishment of calibrations (i.e., of relations between metallicity-sensitive emission-line combinations and metallicity) was the subject of numerous studies (Dopita & Evans 1986;McGaugh 1991;Pilyugin 2000Pilyugin , 2001Kewley & Dopita 2002;Pettini & Pagel 2004;Tremonti et al. 2004;Liang et al. 2006;Stasińska 2006;Thuan et al. 2010, among many others).
A calibration is defined not only by the adopted indicators but also by the objects that serve as calibrating data points. A sample of H ii regions with abundances derived through the T e method is used to construct an empirical calibration. A set of photoionization models is used to construct a theoretical (model) calibration. Even if the same indicator is used in the empirical calibration and in the theoretical calibration and if the same spectral measurements in a given H ii region are used, those calibrations can produce significantly different abundance estimations. Metallicities derived using theoretical calibrations tend to be systematically higher (up to ∼0.7 dex) than those derived using the empirical calibrations (see reviews by Kewley & Ellison 2008;López-Sánchez & Esteban 2010;López-Sánchez et al. 2012). Therefore, oxygen abundances in extragalactic H ii regions obtained in different studies using different calibrations can be significantly different.
Spectroscopic measurements of H ii regions within and beyond optical radii of galaxies were carried out in many works (see list of references below). In these studies, usually the H ii regions in one or several galaxies are measured and the radial distributions of the element abundances across the disks of those galaxies are estimated. Since often different methods for abundance determinations are used in different works, the resulting abundances from these studies are not homogeneous and cannot be directly compared to each other. Therefore, the abundances in a sample of galaxies can be analyzed only after those abundances are homogenized, i.e., all the abundances are redetermined in a uniform way. This is the first step in our present investigation. It should be noted that there have been several attempts to use uniform abundances for the determination of radial abundance gradients in a sample of galaxies; e.g., Vila-Costas & Edmunds (1992) for a sample of 30 galaxies, Zaritsky et al. (1994) for 39 galaxies, van Zee et al. (1998) for 11 galaxies, Pilyugin et al. (2004) for 54 galaxies, and Moustakas et al. (2010) for 21 galaxies. However, those samples contain a relatively small number of galaxies (whereas our present sample includes 130 galaxies).
So far, little attention has been paid to the radial distributions of nitrogen abundances in the disks of galaxies, despite the fact that this provides several advantages for the study of the chemical evolution of galaxies. Indeed, since at 12+log(O/H) 8.3, secondary nitrogen becomes dominant and the nitrogen abundance increases at a faster rate than the oxygen abundance (e.g., Henry et al. 2000), the change in nitrogen abundances with galactocentric distance should then show a larger amplitude in comparison to oxygen abundances and, as a consequence, should be easier to measure. Furthermore, there is a time delay in nitrogen production as compared to oxygen production (Maeder 1992;van den Hoek & Groenewegen 1997;Pagel 1997). Thus the comparison between the radial distributions of oxygen and nitrogen abundances in the disks of galaxies can shed additional light on the chemical evolution of galaxies. Therefore we consider here not only the radial distributions of oxygen abundances but also those of nitrogen abundances.
Our paper is organized in the following way. We describe the method used for the oxygen and nitrogen abundance determinations in the H ii regions of our galaxy sample in Section 2. We describe the observational data that were used to determine the abundances in the H ii regions in Section 3. We discuss the abundance properties in the disks of nearby galaxies (within the optical isophotal radii) in Section 4. We summarize our results in Section 5.
Throughout this paper, we will use the following standard notations for the line intensities: With these definitions, the excitation parameter P can be expressed as: P = R 3 /(R 2 +R 3 ).

Modification of the C method
A new method (called the "C method") for oxygen and nitrogen abundance determinations has recently been suggested . The idea of the C method is a very simple. We have compared a several combinations of the strong-line intensities in the spectrum of a given H ii region with those in the spectra of a sample of reference H ii regions with known abundances in order to find the counterpart for the H ii region under study. It is assumed that the oxygen and nitrogen abundances in the studied H ii region are the same as in its counterpart. A counterpart can be selected by comparison of four combinations of strong-line intensities: P = R 3 /(R 2 + R 3 ) (excitation parameter), logR 3 , log(N 2 /R 2 ), and log(S 2 /R 2 ).
However, there are recent measurements of spectra of many H ii regions where the intensities of [O ii]λ3727+λ3729 or [S ii]λ6717, [S ii]λ6731) lines are not available (e.g., in Sánchez et al. (2012) or in the Sloan Digital Sky Survey (SDSS); see York et al. (2000)). It has been argued that the oxygen and nitrogen abundances in H ii regions can be estimated even if the [S ii]λ6717+λ6731 emission line is not measured (Pilyugin et al. 2010) or if the [O ii]λ3727+λ3729 emission line is not available (Pilyugin & Mattsson 2011). The C method can be adapted to such cases (Pilyugin et al. 2013). To find the counterpart for the H ii region under study, one does not need to compare the four combinations of strong-line intensities, but can instead also use only three combinations: 1) logR 3 , P , and log(N 2 /R 2 ), or 2) logR 3 , logN 2 , and log(N 2 /S 2 ). When this first set of combinations of strong-line intensities is used to find the counterpart then the resulting oxygen and nitrogen abundances will be referred to as ( This sample of reference H ii regions contains 250 objects and will in the following be used for abundance determinations. This sample will be referred to as E2013 sample (etalon sample 2013) below.

Modification of the P method
Furthermore, only blue spectra were observed for H ii regions in a number of galaxies (e.g., Oey & Kennicutt 1993;Zaritsky et al. 1994;Werk et al. 2011 the transition zone and the lower branch. These boundaries are somewhat arbitrary, but were chosen so as to give the best calibrations with the existing data. Two distinct relations between the oxygen abundance and the strong oxygen line intensities will be established in the following, one for the upper branch (the high-metallicity calibration) and one for the lower branch (the low-metallicity calibration).
The relation between the oxygen abundance Z P ≡ 12 + log (O/H) P and R 3 and P can be fitted by a polynomial of the form (Pilyugin 2001;Pilyugin & Thuan 2005) where we have used the notation Z ≡ 12 + log (O/H) for brevity. To take into account the dependence on the excitation parameter P , the coefficients of Eq. (1) are chosen to have the form The coefficients a 0 , a 1 , a 2 , b 0 , b 1 , and b 2 can then be determined by looking for the best fit to our sample of H ii regions. We wish to derive a set of coefficients in Eq. (1) The obtained lower-branch P calibration (for 12 +log(O/H) 8.0) is Hence, here we use the empirical metallicity scale defined by the H ii regions with abundances derived through the direct method (T e method).

The data
We have carried out a fairly comprehensive compilation of published spectra of H ii regions in late-type galaxies. Only those galaxies where radial abundance gradients can be estimated were taken into consideration.

The general properties of our sample of galaxies
Our final list includes 130 galaxies. We have taken the de-reddened line intensities as reported by the authors. In some papers only the measured fluxes are reported. In these cases, the measured emission-line fluxes were corrected for interstellar reddening using the theoretical Hα to Hβ ratio (i.e., the standard value of Hα/Hβ = 2.86) and the analytical approximation to the Whitford interstellar reddening law from Izotov et al. (1994).
The spectroscopic data so assembled form the basis of the present study. Our total list contains 3904 spectra including 162 spectra of H ii regions beyond the isophotal radius R 25 .
Here we will examine the radial oxygen and nitrogen abundance distributions within the isophotal radius in every galaxy. The radial oxygen and nitrogen abundances beyond the isphotal radius will be discussed elsewere. The deprojected radii of the H ii regions were computed using their coordinates (or offsets from the nucleus) as reported in the original papers, as were the position angle and inclination listed in Table 1. In some publications, the positions of the observed H ii regions (or their identifications in catalogues) were not reported, but the deprojected radii were listed instead. In these cases these deprojected radii were used (after correction for the galaxy distance adopted here, if necessary). The fractional radii (normalized to the optical isophotal radius R 25 ) were obtained with isophotal radii from Table 1.
The radial oxygen abundance distribution within the isophotal radius in every galaxy was fitted by the following equation: where 12 + log(O/H) R 0 is the oxygen abundance at R 0 = 0, i.e., the extrapolated central oxygen abundance, C O/H , is the slope of the oxygen abundance gradient expressed in terms of dex R −1 25 , and R/R 25 is the fractional radius (the galactocentric distance normalized to the disk's isophotal radius R 25 ). We also determined the oxygen abundance gradient expressed in terms of dex kpc −1 . If there were data points with large deviations, in excess of 0.2 dex, those points were rejected, and were not used in the determination of the final relation.
The derived parameters of the oxygen abundance distributions are presented in Table 2.
The name of the galaxy is listed in column 1. The extrapolated central 12 + log(O/H) R 0 oxygen abundance and the gradient (the coefficient C O/H in Eq. (5)) expressed in terms of dex R −1 25 are listed in columns 2 and 3. The oxygen abundance gradient expressed in terms of dex kpc −1 is listed in column 4. The scatter of oxygen abundances around the general radial oxygen abundance trend is reported in column 5.
As in the case of the oxygen abundance, the radial nitrogen abundance distribution in every galaxy was fitted by the following equation: Again, if there were data points with large deviations, in excess of 0.3 dex, those points were rejected, and were not used in the determination of the final relation. These different rejection criteria for oxygen abundances, 0.2 dex, and nitrogen abundances, 0.3 dex, are used for the following reason. There is no one-to-one correspondence between nitrogen and oxygen abundances, but instead there is a scatter in N/H at a given O/H. Therefore one can expect that the natural scatter in N/H at a given galactocentric distance can be larger than that in O/H assuming similar uncertainties in both abundance determinations. The derived parameters of the nitrogen abundance distributions are presented in Table 2. The extrapolated central 12 + log(N/H) R 0 nitrogen abundance and the gradient (the coefficient C N/H in Eq. (6)) expressed in terms of dex R −1 25 are listed in columns 6 and 7. The nitrogen abundance gradient expressed in terms of dex kpc −1 is listed in column 8. The scatter of nitrogen abundances around the general radial nitrogen abundance trend is reported in column 9.
A list of references for the emission line flux measurements in the H ii regions is given in Table 3.
In the case of (O/H) P abundances, there is the following problem. The relationship between oxygen abundance and strong oxygen line intensities, the P calibration, is double-valued with two distinct parts usually known as the "lower" and "upper" branches of the R 23 -O/H relationship. The expression for the oxygen abundance determination in high metallicity H ii regions, Eq. (3), is valid only for H ii regions that belong to the upper branch, with 12+log(O/H) 8.3. Thus, one has to know a priori on which of the two branches the H ii region lies. We can overcome this problem in the way suggested in Pilyugin et al. (2004). It has been known for a long time (Searle 1971;Smith 1975) that disks of spiral galaxies show radial oxygen abundance gradients, in the sense that the oxygen abundance is higher in the central part of the disk and decreases with galactocentric distance. We thus start from the H ii regions in the central part of disks and move outward until the radius R * where the oxygen abundance decreases to 12+log(O/H) ∼ 8.3. It should be noted that it is difficult to establish the exact value of R * due to the scatter in oxygen abundance values at any fixed radius. An unjustified use of Eq. (3) in the determination of the oxygen abundance in low-metallicity H ii regions beyond R * would result in overestimated oxygen abundances, and would produce a false turnover in the slope of the abundance gradients (Pilyugin 2003). Therefore, H ii regions with galactocentric distances larger than R * , i.e., those with 12+log(O/H) 8.3 were rejected.
The derived radial distributions of the oxygen and nitrogen abundances in 130 galaxies are presented in Figure 1 The values of the gradients in a number of galaxies (e.g., in NGC 12, the first galaxy in our list) are rather small and are comparable (or even lower) to the uncertainties of gradients. It can be assumed that there is no abundance gradient in such a galaxy, and its abundance can be specified by the mean of the central abundance and the abundance at the isophotal R 25 radius of the galaxy.
Thus, the radial oxygen and nitrogen abundance distributions across the optical disk in every galaxy are individually fitted by a single relation. It looks like a rather good approximation for the majority of galaxies. However, a small change in slope in the abundance distribution cannot be excluded in the disks of several galaxies (e.g., NGC 925, NGC 3184, NGC 5457). It is interesting to note the following. Pohlen & Trujillo (2006) studied the surface brightness profiles of a sample of late-type (Sb to Sdm) spiral galaxies using imaging data from the SDSS survey. They found that the surface brightness profiles can be divided into three classes. A small fraction of galaxies (around 10 %) belong to type I, which comprises those galaxies that have a normal (standard) exponential disk down to the noise limit. The surface brightness distribution of the rest of the galaxies is better described as a broken exponential. About 60 % of the galaxies belong to type II, which means that they show a down-bending profile with steeper outer part. About 30 % of the galaxies belong to type III, implying that they show an up-bending profile with shallower outer part. Thus, the abundance distribution profile does not seem to follow strictly the slope of the surface brightness profile. It should be noted, however, that the number of measured H ii regions in many galaxies is too small to allow one to detect a small bend in the radial abundance distribution.

Notes on individual galaxies
NGC 1512. The five H ii regions that are located in the "bridge" between the interacting galaxies NGC 1512 and NGC 1510  were excluded from our analysis. Dicaire et al. (2008) found that there is a significant discrepancy between the kinematically derived inclination of NGC 1512 and its photometric value (i phot = 65 • and i kin = 35 • ). They noted that the explanation is quite clear: the photometric parameter is mainly representative of the bar which contributes a large part of the light resulting in more edge-on values. However, the outer isophotes are much more face-on. Therefore, the kinematic value of the inclination is adopted here.
NGC 3227. NGC 3227 is a nearby Seyfert galaxy that is interacting with its gas-poor dwarf elliptical companion NGC 3226. Mundell et al. (1995) discovered a cloud of H i close to, but physically and kinematically distinct from, the galactic disk of NGC 3227. They suggested that this cloud (J1023+1952) might be a dwarf galaxy that is either preexisting and being accreted by NGC 3227, or a newly created tidal dwarf galaxy (Mundell et al. 1995(Mundell et al. , 2004. However, the oxygen abundance of the cloud (H ii regions 24 and 25 from Werk et al. (2011) and of the H ii region measured by Lisenfeld et al. (2008)) follows the general trend of the radial abundance distribution in the disk of NGC 3227. The oxygen abundances of the H ii region in the cloud estimated in different ways using the emission line measurements by Lisenfeld et al. (2008)  NGC 3359. The NGC 3359 is a giant, very strongly barred spiral galaxy.

Martin & Roy (1995) estimated the radial gradient using the [O iii]/Hβ and [N ii]/[O iii]
indicators (one-dimensional calibrations). They found the radial abundance gradient break near the corotation radius. Zahid & Bresolin (2011) carried out new measurements and determined abundances through the N2 and O3N2 calibrations following Pettini & Pagel (2004). They concluded that, with a high degree of confidence, a model with a break fits the data significantly better than one without a break. The C-based abundances show no gradient across the entire galaxy. The T e -based abundances in three H ii regions in NGC 3359 are in agreement with no gradient.
NGC 3718. This galaxy is so peculiar that it is difficult to categorize it morphologically (Tully et al. 1996).
NGC 4559. The H ii regions in NGC 4559 with galactocentric radii larger than 0.4R 25 measured by Zaritsky et al. (1994) were excluded since they are in the transition zone.
NGC 4625. This is a Magellanic spiral with R 25 ∼ 3 kpc and an extended faint disk reaching four times the optical R 25 radius of the galaxy in the ultraviolet. NGC 7518. This is a hydrogen-deficient galaxy in the Pegasus cluster. Following Robertson et al. (2012), oxygen lines from "blue" spectra (360 nm -560 nm) have been used in the abundance determinations.
NGC 7529. This galaxy of the Pegasus cluster has a normal H i content. There is no determination of the position angle, but the inclination is small. Again following Robertson et al. (2012), oxygen lines from "blue" spectra have been used in the abundance determinations.
NGC 7591. This galaxy of the Pegasus cluster has a normal H i content. As before, following Robertson et al. (2012), oxygen lines from "blue" spectra have been used in the abundance determinations. Our data show a well-defined sequence in the O/H -N/H plot. This sequence exhibits the well-known turnover in the sense that the slope at low metallicity is shallower than that at high metallicity. The commonly accepted explanation of this change of slope is that nitrogen can be interpreted as having both primary and secondary components. Nitrogen production is primary at low metallicity, but for 12 +log(O/H) 8.3, secondary nitrogen becomes prominent, and nitrogen increases at a faster rate than oxygen (Henry et al. 2000).
was found using both central abundances and abundances at the R 25 radii of the galaxies.
This relation is presented in Figure 2 by the dotted line. attributed to the errors in the abundance determinations but part of it seems to be true abundance scatter. Two major mechanisms have been proposed for generating a spread.
One mechanism invokes a time delay between ejections of the freshly manufactured oxygen and nitrogen into the interstellar medium by a given stellar generation. The N/O ratios may be an indicator of the age of a galactic system, indicating the time that has passed since the bulk of star formation activity (Edmunds & Pagel 1978). Thus, the N/O ratio in a galaxy would then depend on its star formation history. The second mechanism for causing N/H variations at a given O/H is a variation in the efficiency of enriched galactic winds (Pilyugin 1993) or/and in the inflow of gas into the galaxy (Henry et al. 2000). It is believed that galactic winds do not play a significant role in the chemical evolution of large spiral galaxies (Tremonti et al. 2004; It has been known for a long time that galaxies of different morphological types have different star formation histories, i.e., spiral galaxies with early morphological types have a larger fraction of old stars (Sandage 1986). One can then expect that N/H at a given O/H may depend on the morphological type of galaxy expressed in terms of T type ). On the other hand, the star formation history of a galaxy also strongly depends on its mass (or luminosity) as is epitomized in the galaxy downsizing effect, where the star formation acitvity shifts from high-mass galaxies at early cosmic times to lower-mass galaxies at later epochs (Cowie et al. 1996). Again, one can expect then that the N/H ratio at a given O/H will depend on the galaxy mass (Pilyugin & Thuan 2011).
There is a relatively tight linear correlation between the absolute magnitudes and the logarithms of the linear diameters of nearby galaxies (van den Bergh 2008). Therefore, the linear diameter of a galaxy can be used as indicator of its luminosity (and mass). Thus, if the spread in N/H at a given O/H is caused by the time delay between nitrogen and oxygen enrichment and the different star formation histories in different galaxies then the N/H at a given O/H should correlate with morphological T type or/and with linear radius of the galaxy. Figure 3 shows the residual of Eqs. (7),(8) as a function of the morphological T type and linear radius of the galaxy R 25 . Figure 3 indicates that there may exist some correlation between the N/H at a given O/H and morphological type or/and linear radius of the galaxy.
To verify this, we found a relation between N/H and O/H for spiral galaxies (with T < 7.5) where T and logR 25 are "secondary parameters", N/H = f (O/H,T ,logR 25 ). It should be noted that T and logR 25 are not a perfectly independent parameters since there is some correlation between them. The relation obtained for central abundances is The values of the coefficients in the terms containing T and logR 25 are similar. However, the T value is a more important second parameter than the logR 25 value since the variation in T values (from 1 to 7.5) is much larger than the variation of logR 25 values (from ∼0.6 to ∼1.5). Therefore, the variation in N/H due to variation in the morphological type is larger than that due to variation in the linear radii of galaxies. For example, the Sab galaxies (T = 2) have nitrogen abundances larger than on average 0.1 dex (i.e., by around 30%) than Sd In this case the logR 25 value is the more important second parameter than the T value.
Although the morphological type is the more important second parameter in the relation for central abundances while logR 25 is the more important second parameter in the relation for abundances at the optical R 25 radii of the galaxies, it is difficult to draw a solid conclusion whether this is physically meaningful since there is a correlation between T and logR 25 .
That the relation between N/H and O/H depends on the additional parameter(s) T and/or logR 25 suggests that the scatter in N/H at a given O/H can be caused, at least partly, by the time delay between nitrogen and oxygen enrichment and the different star formation histories in different galaxies.

Abundances and gradients as a function of morphological type and galaxy radius
The upper left panel of Figure 4 shows the central oxygen (O/H) R 0 abundance in a galaxy as a function of its morphological T type (data from Table 1 and Table 2).
The (O/H) R 0 -T diagram shows that there is a trend in central oxygen abundance with morphological type for galaxies later than Sc (T 5) such that the central oxygen abundances are lower in galaxies of later types. This trend disappears for early-type spiral galaxies (T 5).
The upper right panel of Figure 4 shows the central oxygen abundance in a galaxy as a function of its isophotal radius R 25 . Since there is a relatively tight linear correlation between the absolute magnitudes and the logarithms of the linear diameters of nearby galaxies (van den Bergh 2008), this diagram can be considered as some kind of analog of the standard "luminosity -metallicity" diagram. The (O/H) R 0 -R 25 diagram shows that there is a weak correlation between the central oxygen abundance and optical radius for small galaxies (R 25 10 kpc) in the sense that the smaller galaxies have on average lower oxygen abundances. This correlation disappears for large galaxies (R 25 10 kpc), i.e., the most oxygen-rich galaxies of different radii have similar central oxygen abundances. It has been advocated that the constant maximum value of the observed central oxygen abundance in the most oxygen-rich galaxies suggests that the observed oxygen abundance in the centers of those galaxies represents the maximum attainable value of the gas-phase oxygen abundance (Pilyugin et al. 2007). The upper-row panels of Figure 4 show that the observed central oxygen abundance in the most oxygen-rich galaxies in our sample is 12 The dashed lines in the upper row of panels of Figure 4 show this value of oxygen abundance, which seems to correspond to the maximum attainable value of the gas-phase oxygen abundance in galaxies. The observed central oxygen abundance in the most oxygen-rich galaxies from our sample is a factor ∼2 higher than the gas-phase oxygen abundance in the solar neighbourhood, 12 + log(O/H) ∼ 8.5 (e.g., Pilyugin et al. 2006). The maximum attainable value of the oxygen abundance in galaxies obtained here is in agreement with the value from Pilyugin et al. (2007). Because some fraction of the oxygen (about 0.1 dex) is expected to be locked in dust grains (e.g., Esteban et al. 1998), the maximum value of the true (gas + dust) oxygen abundances in H ii regions of spiral galaxies is 12 + log(O/H) R 0 ∼ 8.95.
The lower left panel of Figure 4 shows the central nitrogen (N/H) R 0 abundance in a galaxy disk as a function of its morphological T type. The comparison of the panels in the left column of Figure 4 shows that the changes in the central oxygen and nitrogen -25abundances with morphological type of a galaxy are rather similar. The lower right panel of Figure 4 shows the central nitrogen abundance in a galaxy disk as a function of its optical isophotal radius R 25 . Again, the comparison between the right column panels of Figure 4 indicates that the general behaviour of the central nitrogen abundances as a function of optical radius is similar to that for oxygen.
According to the relation between nitrogen and oxygen abundances given in Eq. (7), the value of 12 + log(N/H) R 0 ∼ 8.42 corresponds to the maximum attainable value of the gas-phase oxygen abundance in galaxies with 12 + log(O/H) R 0 ∼ 8.85. This value of nitrogen abundance is indicated in the lower row of panels of Figure 4 by the dashed lines.
One can see that the value of 12 + log(N/H) R 0 ∼ 8.42 can be adopted as the maximum value of the observed central nitrogen abundance in the most nitrogen-rich galaxies at the present-day epoch. However, in contrast to the case of oxygen, it is not necessary that this value corresponds to the the maximum attainable value of the gas-phase oxygen abundance in galaxies because of the time-delay between nitrogen and oxygen enrichment of the interstellar medium.
The panels in the left column of Figure   of a galaxy (right panels). Inspection of the left column panels of Figure 5 shows that the values of the abundance gradients in units of dex kpc −1 do not correlate with the morphological type of a galaxy. According to Zaritsky et al. (1994), the lack of a correlation between gradients in units of dex kpc −1 and the macroscopic properties of late-type galaxies may suggest that the relationship between these parameters is more complex than a simple correlation. Indeed, Vila-Costas & Edmunds (1992) have concluded that a correlation for non-barred galaxies is seen. The panels in the right column of Figure 5 show that shallow gradients can be found both in small and large galaxies while steep gradients are seen only in small galaxies in the sense that the smaller a galaxy the steeper its gradient.

Summary
We We found that the morphological type, T , is a more important "second parameter" in the relation for central abundances, while the logR 25 is a more important second parameter in the relation for abundances at the R 25 radii of our galaxies. Since there is a correlation between T and logR 25 it is as yet unclear whether this difference is meaningful. The fact that the relation between N/H and O/H depends on additional parameter(s), namely T and/or logR 25 suggests that the scatter in N/H at a given O/H can be caused, at least partly, by the time delay between nitrogen and oxygen enrichment and the different star formation histories in different galaxies. The best fit to N/H as a function of O/H is close to a linear relation at low metallicity (12 + log(O/H) 8.0).
The central oxygen abundances (O/H) R 0 show a trend along the Hubble sequence of galaxies of late morphological types (T 5) such that the oxygen abundances are lower in galaxies of later types. This trend disappears for early morphological types. The central oxygen abundance also correlates with optical galaxy radius for small galaxies, R 25 10 kpc, being lower in galaxies of smaller radii. The trend disappears for galaxies with large radii. The maximum gas-phase oxygen abundance in large (10 kpc R 25 30 kpc) galaxies (or in galaxies of early (1 T 5) morphological types) is constant, 12 + log(O/H) ∼ 8.85. This implies that the observed central oxygen abundance of the most oxygen-rich galaxies in our sample is a factor of ∼2 higher than the gas-phase oxygen abundance in the solar neighbourhood. The central nitrogen abundances (N/H) R 0 show a similar behaviour.
The observed central nitrogen abundance in the most nitrogen-rich galaxies of our sample is 12 + log(O/H) ∼ 8.42.
The radial O and N abundance gradients (in units of dex kpc −1 ) within the optical radius do not show any significant correlation with the morphological type and optical radius. However, the spread in the gradients increases with decreasing galaxy radius in the sense that shallow gradients are seen both in small and large galaxies while steep gradients occur only in a small galaxies. The smaller a galaxy the steeper is the gradient that it may show.
The abundance data set presented in this paper serves as the foundation for other investigations we are carrying out. In a forthcoming paper (Pilyugin et al. 2014), we examine relations between the radial abundance distribution across the disk and the disk surface brightness profiles in the optical B and infrared K bands for a sample of nearby We acknowledge the usage of the HyperLeda database (http://leda.univ-lyon1.fr). Thim, F., Hoessel, J. G., Saha, A., et al. 2004, AJ, 127, 2322 Thuan, T. X., Pilyugin, L. S., & Zinchenko, I. A. 2010, ApJ, 712, 1029Tikhonov, N. A., & Galazoutdinova, O. A. 2002 Torres-Peimbert, S., Peimbert, M., & Fierro, J. 1989, ApJ, 345, 186 Tremonti    Table 3. List of references for the emission line flux measurements in the extragalactic H ii regions.