Introduction

This is part of a study of a sample of nearby Magellanic irregular galaxies with important star formation. The main interest in this class of objects is the large number of Giant H II Regions that each one contains. By studying each galaxy independently, the uncertainties in the galaxy distance determination are then ruled out and, therefore, the GEHRs in each galaxy constitute a homogeneous sample to test the luminosity versus velocity dispersion relationship, proposed as a distance indicator (Melnick 1979).

Observationally, the critical point is the precise determination of the parameters involved in the correlation; the determination of the size of the GEHRs, the measurement of emission line widths, and the luminosity, all require careful work, together with a discerning use of traditional methods. This has been an important fraction of the recent PhD work by Fuentes-Masip (1997) in which a complete catalogue of the H II regions of the galaxy NGC 4449 has been compiled. The case of Magellanic irregulars is particularly difficult because there are large numbers of star-forming entities which may overlap. The study, then, needs good spatial resolution to isolate individual regions, particularly if the bulk of the emission is engulfed by extended, low-intensity, ionized gas emission. This is the case of the galaxy NGC 4449 with an intense diffuse nebular luminosity present all over the optical body of the galaxy (Malumuth et al. 1986; Scowen 1992) and 252 star-forming regions that have been catalogued (Sabbandin & Bianchini 1979). Among them a large number have been classified as giant H II regions (Fuentes-Masip, 1997). The optical angular size of the galaxy is arcmin, defined at the isocontour at erg cm s for H, which implies kpc, assuming a distance of 5 Mpc (1''=25 pc). It presents a moderately high SFR estimated as 1.4-1.5 yr. The extent of the H I is about 10 times the size of the stellar component and the total mass of gas is

© Copyright Astronomical Society of Australia 1997