10 Conclusions

The main results of the present paper, where we analyze the statistics and properties of HII regions in the disc of NGC 7479 are summarized below.

1.

Using a high quality continuum-subtracted H$\alpha$ image of the grand-design spiral NGC 7479, we have catalogued a total of 1009 HII regions. The catalogue includes positions, radii and HH$\alpha$ fluxes of all HII regions. Tables containing all these data are available through CDS or directly from the authors.

2.

The slope of the LF agrees broadly with slopes for other galaxies of comparable morphological types.

3.

We have found a change in slope in the LF of the HII regions of NGC 7479 that occurs at a luminosity slightly higher ($\simeq$0.2 dex) than that found in other galaxies of the same morphological type. Due to the intense star formation in the bar of NGC 7479, we decided to construct separately the LF's for the HII regions of the bar and of the disc, finding that the LF of the disc is in no way different from that found in previous papers for galaxies of the same morphological type. The anomaly in the global LF is thus due to different star formation conditions in the bar. This must be due to the effects of gas dynamical parameters on the stellar IMF, and the physical conditions in the clouds, as we do not find significant differences between the physical properties of the HII regions of the bar and of the disc.

4.

The integrated distribution function of the HII region diameters can be well fitted by a exponential function. The value of the characteristic diameter lies within the range reported previously in the literature for galaxies of similar morphological types.

5.

The characteristic scale size of the HII regions of a galaxy depends on the absolute luminosity of the galaxy; it is larger for more luminous galaxies (Hodge 1987). The result for NGC 7479 agrees with the fit presented by Hodge.

6.

The dependence of log L on the volume ( r³) for the regions in NGC 7479 lies close to a straight line, with a slope near unity. There is a tendency for the relationship to show curvature, corresponding to the onset of density bounding at high luminosities, but the dispersion in luminosity at a given volume is such that using only this plot we cannot draw a clear physical inference on this point.

7.

The densities, filling factors, masses, and ionization indices derived from the luminosities and sizes of a selected set of representative regions, through the range of observed luminosities for NGC 7479, are in agreement with those found in the previous literature on extragalactic HII regions.

8.

The physical properties encountered cover a wide range, but all have moderately low electron densities, always much less than 10 cm^-3, of the order of mean interstellar number densities in galaxies. These values, differ from the values found for the central densities of HII regions (Rozas et al. 1998) by two orders of magnitude, but if we assume gaussian internal density distributions and weighting the densities as shown in Table 2 of Rozas et al. (1998), by volume, using the diameters determined here, we find mean electron densities within 20$\%$ of the values found here.

9.

In the 3D diagram of number-luminosity-size, and in the projection on the luminosity-radius plane, we detect curvature in the high luminosity range, which would be predicted for the transition between ionization-bounded and density-bounded regions; this is only hinted at in the luminosity-volume graph. The characteristic mass scale for the onset of the curvature, or the change of slope shows the limit of the masses of the clouds of neutral gas which give rise to stellar associations (Rozas et al. 1996b; Beckman et al. 1999; Rozas et al. 1998). The luminosities which are associated with these changes in slope coincide in value in all galaxies studied by us, and also in those few galaxies subject to a similar study for which there exist data of equivalent quality: NGC 157, NGC 3631, NGC 6764, and NGC 6951 (Rozas et al. 1996a), M 51 (Rand 1992), NGC 6814 (Knapen et al. 1993), NGC 4321 (Knapen 1998, in preparation). The value of this critical luminosity is $\log {L}= 38.6 \pm 0.15$ erg s^-1, (for an assumed value for H₀ of 75 kms^-1Mpc^-1), and it coincides with the luminosity at which there is a "glitch'', i.e. a jump, and/or a change of slope, in the luminosity functions, as reported in Rozas et al. (1996a) and previous papers (e.g. Kennicutt et al. 1989; Rand 1992; Knapen et al. 1993).

10.

The total flux emitted in HH$\alpha$ by NGC 7479 is $L_{\rm H\alpha}$(total) = (1.3 $\pm$ 0.2)$\$10⁴² erg s^-1, which indicates that NGC 7479 is a high luminosity galaxy in HH$\alpha$. Its luminosity is over three times higher than the total HH$\alpha$ luminosity for the most luminous galaxy of the sample of grand design galaxies cited above (NGC 157, NGC 3631, NGC 6951, NGC 6764), and higher too than the total HH$\alpha$ luminosity of NGC 247 and NGC 7793, studied by Ferguson et al. (1996). This is due principally to the intense star formation in the very strong bar that characterizes this galaxy.

11.

The diffuse flux from the ISM of NGC 7479 has been calculated by three methods to bracket the observational uncertainty inherent in observing any galaxy projected in a plane and due to crowding effects. We have found that the diffuse flux in HH$\alpha$ is a high fraction, between $35\%-60\%$ of $L_{\rm H\alpha}$(total). This result is similar to the ionized gas fraction found by Ferguson et al. (1996) in NGC 247 and NGC 7793 ($30\%-50\%$ of the total HH$\alpha$ emission).

12.

With the assumption that the most luminous HII regions, those with L>10^38.6 erg s^-1, are density bounded, they are likely to be the main ionizing sources of the interstellar medium, since:

(a)

The calculated leak-out of Lyc photons from these regions is enough to ionize the diffuse medium in this galaxy (in fact it exceedes the $L_{\rm H\alpha}{\rm (DIG)}$ by a factor of over 2), even if we take into account only the leak-out of Lyc photons from regions of the disc. This excess is especially important here since a significant fraction of these photons will not be absorbed within the galaxy and must escape into the intergalactic medium.

(b)

There is a clear geometrical correlation between the observed diffuse HH$\alpha$ flux and the positions of the density bounded HII regions.

This result will be followed up with detailed models to include the degree of clumping in the diffuse medium (Zurita et al. 1999) in order to further investigate whether the diffuse HH$\alpha$ is caused by escaping Lyman photons from, especially, the most luminous HII regions.

13.

On the basis of our hypothesis of density bounding the calculated ionizing flux which escapes completely from the galaxy without being trapped in the diffuse component is ${\geq} 1.4$ 10⁵⁴ photons s^-1. If this were generalizable for spirals and irregulars, it would have important consequences for the ionization of the intergalactic medium (see also Zurita et al. 1999).

Acknowledgements

The William Herschel Telescope is operated on the island of La Palma by the Royal Greenwich Observatory in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. We acknowledge Dr. J. Knapen for his help during the observations. This work was partially supported by the Spanish DGICYT (Dirección General de Investigación Científica y Técnica) via Grants PB91-0525, PB94-1107 and PB97-0219. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. C. Heller acknowledges support from DFG grant Fr 325/39-1, 39-2.