A spatially resolved study of ionized regions in galaxies at different ...

2 1 • Introduction
The origin of the term “starburst” dates back to the early observations of dust-obscured
star-forming regions in the centers of nearby galaxies at the end of the seventies and beginning
of the eighties, but the basic concept extends back much further (e.g., Hodge, 1969a; Searle
et al., 1973).
The level of intensity of a starburst is highly variable. According to Terlevich (1997), in
a starburst galaxy the energy output of the starburst (L SB ) is much bigger than the one
coming from the rest of the galaxy (L G ), a galaxy with L SB ∼ L G is a galaxy with starbursts,
and in a normal galaxy L SB ≪ L G . This classification shows the variety of environments of
the bursts. It is clear that the visibility of the burst depends not only on its intensity but
also on its environment.
Terlevich (1997) also proposed a division in phases of the starburst. The first one, the
nebular phase, is characterized by the presence of strong emission lines form gas photoionized
by young massive stars, with an age of less than 10 Myr. The early continuum phase, from
10 to 100 Myr, with some Balmer lines in absorption and others in emission. Finally, the
late continuum phase, with only some weak emission lines. In this work we will pay special
attention to the first phase, of which Hii galaxies are typical examples.
Spectroscopically, Hii galaxies are essentially identical to the giant Hii regions found in
nearby irregular and late-type galaxies. The correlation among structural parameters (Hβ
luminosity, velocity dispersion, line widths) and between these parameters and chemical
composition (Terlevich and Melnick, 1981) favours the interpretation of Hii galaxies as giant
Hii regions in distant dwarf irregular galaxies similar to the ones found nearby (Melnick
et al., 1985). However, there are also Hii galaxies with masses and luminosities in the range
typical for elliptical galaxies, suggesting that they may be young galaxies.
Therefore, studies of Hii galaxies and giant Hii regions are of relevance to understand Hii
regions and processes of formation and evolution of massive stars, as well as the evolution
of galaxies.
1.1.1 Giant Extragalactic Hii regions
Giant Extragalactic Hii regions (GEHRs) are the most spectacular star-forming regions
in normal galaxies. The class of giant Hii regions consists of a very heterogeneous group of
objects in terms of size, brightness and metallicity. For example, it is intriguing that the
spiral galaxy M101, which is quite similar to the Milky Way, has Hii regions that are so
much larger and brighter than the largest one in our Galaxy (Giannakopoulou-Creighton
et al., 1999).
GEHRs are large, very bright Hii regions that can be observed in the discs of both latetype
spiral (Sc) and irregular galaxies. The large amount of ionized gas (10 4 - 10 6 M ⊙ ) is
surrounded by massive molecular clouds. Clusters of OB stars, generating ionizing photons at
a rate of 10 51 - 10 52 s −1 , ionize the surrounding low density gas (N e ≈ 10 - 100 cm −3 ), creating