Astron. Astrophys. 354, 823-835 (2000)

Astron. Astrophys. 354, 823-835 (2000)

8. Conclusions

The main results of the present paper, where we use measured H [FORMULA] flux to analyze the ionized gas of NGC 3359 are summarized below.

Using a high quality continuum-subtracted H image of the grand-design spiral NGC 3359, we have catalogued a total of 547 HII regions. The catalogue include positions, radii and H fluxes of all HII regions. Tables containing all these data are available through CDS or directly from the authors.
The slope of the LF agrees broadly with slopes for other galaxies of comparable morphological types.
We have found a change in slope in the LF of the HII regions of NGC 3359 that occurs at a luminosity close to that found in other galaxies of the same morphological type.
The integrated distribution function of the diameters can be well fitted by a exponential function for the galaxy analyzed. The value of the characteristic diameter lies within the range reported previously in the literature for galaxies of similar morphological types.
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 3359 agrees with the fit presented by Hodge.
The largest fraction of ionized gas is found along the arms and in a set of high brightness star formation region on the bar. There is also a significant component of diffuse emission throughout the entire disc. The scale length in H is 2.3 kpc.
The U-I map defines the detailed locations of SF. In this colour map we can see elongated segments of SF together "hot spots" which compose each segment, divided by the dust, because the I-K image shows most clearly a spiral delineated by the strong dust lanes running along the arms.
The analysis of the K image confirms its symmetry; this indicates that the overall morphology observed in the NIR is dominated by a global density wave. This is not so clear in the H image: the SF is occurring in a more stochastic way.
The densities, filling factors, masses, and ionization indices derived from the luminosities and sizes of a selected set of representative HII regions, through the range of observed luminosities for NGC 3359, are in agreement with those found in the previous literature on extragalactic HII regions.
The electron densities encountered are moderately low, with values always less than 10 cm^-3, of the order of mean interstellar number densities in galaxies.
The diffuse flux from the ISM of NGC 3359 has been calculated by three methods to avoid the observational problems inherent in observing any galaxy projected in a plane and due to crowding effects. We have found that the diffuse flux in H is a high fraction, between 20%-30% of (total).
With the assumption that the most luminous HII regions, those with erg s^-1, are density bounded, they are likely to be the main ionizing sources of the interstellar medium, since the calculated leak-out of Lyc photons from these HII regions is enough to ionize the diffuse medium in this galaxy. 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.

Online publication: February 25, 2000
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