1 Introduction

Since the work of Hodge and Kennicutt in the 1970's and 1980's (e.g. Hodge 1976, 1987; Kennicutt 1984, 1989; Kennicutt et al. 1989) it has become clear that the set of HII regions in a spiral, or irregular galaxy, is worth studying as a population, i.e. for its statistical properties as well as for the physical properties of the individual regions. As well as being of key importance for the energy balance of the interstellar medium as a whole, the HII region population reflects, in a manner readily observable, the parameters of the massive exciting stars within the regions, and is therefore of interest in the continuing study of the star formation rate (SFR) and the initial mass function (IMF) at the massive end of the stellar mass range (Kennicutt 1992). The number of complete studies of this type since the beginning of the CCD era is restricted. We ourselves have contributed a series of studies of the statistical properties of HII regions in NGC 4321 (Cepa & Beckman 1990), NGC 3992 (Cepa & Beckman 1989), NGC 6814 (Knapen et al. 1993), and of NGC 157, NGC 3631, NGC 6764 and NGC 6951 (Rozas et al. 1996; Rozas et al. 1996). In these papers we concentrated mainly on the HII region luminosity functions (LF's) in HH$\alpha$ and their global properties, as well as the disc-wide geometrical distribution of emission, although in Rozas et al. (1996b), Beckman et al. (1999), as well as in Rozas et al. (1998) we derived the internal physical parameters: emission measure, electron density, and total mass of representative samples of, especially, the most luminous regions. Other studies based on statistical properties of HII regions in spiral galaxies are found in Rand (1992) and Knapen (1998).

Here we offer the most complete study to date of the HII regions in a single galaxy: NGC 7479, in which all the techniques developed in the previous series of papers have been brought to bear. NGC 7479 is a barred spiral, of type SBc (from de Vaucouleurs et al. 1991; hereafter RC3), with a particularly well defined and well developed bar, which virtually dominates the luminosity of the galaxy, emitting almost 50% of its energy in any representative optical band (Blackman 1983). Its arm structure is asymmetric and this asymmetry has been reasonably ascribed to the after-effects of a recent merger with a low mass companion (Mihos & Hernquist 1994; Laine 1997; Laine & Heller, in preparation). We list its basic parameters in Table 1, from RC3, except for the position angle and the inclination of the galaxy, taken from Laine & Gottesman (1998). This study is not designed to differentiate between galaxies which have recently suffered a merger and those which have not. The global properties of the HII regions in NGC 7479 do in fact show some interesting differences from the series of galaxies cited in the preceding paragraph, and we show that this is due to the major star formation zones along the bar, which may have been affected by the merger. We will itemize the differences and discuss them in detail in the text. However many of the basic parameters of the population of HII regions do not differ greatly from those in other galaxies. In particular the LF, at luminosities above the completeness limit, approximates a power law with exponent not far from 2, as found by Kennicutt et al. (1989) for a set of 30 galaxies of different morphological types, using pre-CCD data. However, the studies by Rozas et al. (1996a,b), Knapen et al. (1993), cited above, based on CCD data of excellent S/N and angular resolution, revealed a local peak at log $L_{\rm Str}$ = 38.6 (erg s^-1) (which we have termed the Strömgren luminosity) in the LF of each of the galaxies treated, accompanied by a change in slope, which we have attributed to the transition from ionization bounding at lower luminosities to density bounding at higher luminosities (Rozas et al. 1998; Beckman et al. 1999). An initial objective of the present study was to see if such a "glitch'' exists in the LF of NGC 7479. Further objectives were to measure the basic physical properties of the HII regions, to see whether the properties of those along the bar differ measurably from these in the disc.

 

Table 1: NGC 7479: Basic parameters

  

Figure 1: Grey scale representation of the HH$\alpha$ continuum-subtracted image of NGC 7479

In Sect. 2 of the paper we describe the observations and their reduction, and the preparation of the catalogue of HII regions: HH$\alpha$ fluxes, positions, and diameters. This section is rather less routine than is customary, since in all our previous work we used interactive region by region integration to obtain the fluxes of the many hundreds of HII regions in a given galaxy, but here we have used for the first time a semi-automatic method, which saves a major fraction of the time, and will be needed for multi-galaxy statistical studies. In Sect. 3 we derive the LF's. In Sects. 4 and 5 we produce diameter and flux distributions, and in Sect. 6 we show the luminosity volume relations. The derivation of the physical parameters of individual regions is described in Sect. 7, with their resulting values. Number-luminosity-diameter relation is described in Sect. 8. In Sect. 9 we compare the measured diffuse HH$\alpha$ flux in the disc of NGC 7479 with the computed ionizing flux escaping from the density bounded HII regions, testing, both in terms of energy balance and geometry, the hypothesis that this escaping flux produces the diffuse HH$\alpha$; and in Sect. 10 we draw our statistical and physical conclusions.