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

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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
1.1. Overview 3 Figure 1.1: Sample of different scale and morphology Hii regions: Orion Nebula obtained with the HST/ACS (upper left; credits: NASA, ESA, M. Robberto and HST Orion Treasury Project Team); giant Hii region NGC 3603 observed with HST/WFPC2 (upper right; credits: W. Brandner, E.K. Grebel, Y.-H. Chu and NASA); 30 Doradus in the Large Magellanic Cloud (LMC) taken with HST/WFPC2 (lower left; credits: NASA, N. Walborn, J. Maíz-Appellániz and R. Barbá); radio and near-infrared composite of W49A (lower right; credits: ESO). giant complexes with linear dimensions of the order of 10 2 - 10 3 pc, varied morphology, and an inhomogeneous distribution of gas (Shields, 1990). The luminosities and densities of the largest Hii regions in different galaxies vary widely and independently, although the objects in any one galaxy are often similar. Figure 1.1 shows a sample of Hii regions of different scale and morphology and Table 1.1 some general integrated properties of well known Hii regions. The reasons for studying giant Hii regions in the Local Universe are numerous. Because they are sites of intense star formation, they are used to study large groups of massive young stars, such as OB associations and star clusters. Due to their proximity, it is possible to perform detailed studies of their individual stars, but also their relative larger distance as compared to the regions in our Galaxy, allows to obtain integrated properties. Very impor-
Page 1: Facultad de Ciencias Departamento d
Page 5: A Abigaíl, mi ut A mi familia, por
Page 8 and 9: La gente del grupo de Astrofísica
Page 11 and 12: Contents List of Figures List of Ta
Page 13: CONTENTS iii B •Physical conditio
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52 3 • IFS of a GEHR in NGC 6946
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β[H 84 3 • IFS of a GEHR in NGC
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106 4 • Long-slit spectrophotomet
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Hδ 116 4 • Long-slit spectrophot
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Table 4.4 continued SDSS J1657 Knot
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150 Conclusions and future work has
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152 Conclusions and future work oth
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154 Conclusions and future work The
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156 Conclusiones Las abundancias to
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158 Conclusiones consistentes con l
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160 Appendix A where c=0.434C. This
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162 Appendix B where R S2 = I(6717
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164 Appendix B Sulphur As in the ca
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166 Appendix B Argon For argon, the
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Appendix C Empirical calibrators In
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C • Empirical calibrators 171 One
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Appendix D Stellar photometry resul
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D • Stellar photometry results of
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190 REFERENCES Bertelli, G., Bressa
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192 REFERENCES Girardi, L. & Bertel
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194 REFERENCES Kunth, D. & Sargent,
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196 REFERENCES Pérez-Montero, E.,
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198 REFERENCES Terlevich, R., Melni
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