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

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6 1 • Introduction Figure 1.2: Comparison of morphology of a giant Hii region and a BCD: giant Hii region NGC 604 (in M33) obtained with the HST/ACS (left; credits: NASA, AURA/STScI, D. Garnett, J. Hester, and J. Westphal) and Hα image of the BCD SBS-0335-052E observed with HST/ACS (right; program 10575. all BCDs are Hii galaxies, as only in a fraction of BCDs are the Hii regions dominant (Cairós et al., 2001). From the point of view of the emission line classification, Star Forming Galaxies are a subset of Emission Line Galaxies, identified as having strong and narrow nebular emission lines and low chemical abundances (contrary to active galaxies, which have higher metallicities and broadlines), some of them being the least evolved galaxies known and candidates for young galaxies just beginning to form stars. Several taxonomies have been suggested for this group. Melnick et al. (1985) proposed a subclassification in Starburst Nuclei, Clumpy Irregulars, and Hii Galaxies. This classification is by increasing order of the luminosity ratio of the Star Formation to that of the host galaxy. According to this taxonomy, Hii galaxies are gas-rich dwarf galaxies experiencing a violent star formation period which dominates the optical spectrum of the host galaxy. They have one of the highest intensity levels of star forming activity. In general, Hii galaxies have a central region which contains one or more star forming knots, with a diameter of several hundred parsecs with high surface brightness, and a low luminosity underlying galaxy (M V ≥ -17). The activity of the star formation episodes cannot be sustained continuously for long periods of time, since the central region cannot have enough gas to fuel that processes for longer than 10 9 years and to match the gas content and metallicity with theoretical considerations (Thuan et al., 2004).
1.1. Overview 7 Figure 1.3: Color-magnitude (CMD) diagram V vs. V-I of IZw18 compared with Padova tracks with Z = 0.0004 from Aloisi et al. (1999), Figure 9 of that work. The stellar mass of each track is given in M ⊙ . The faintest clump of red objects in the CMD can be populated by old stars, extending the look-back time of the galaxy up to several Gyr. Regarding morphology, these galaxies form a heterogeneous class. Figure 1.2 shows the color composition of NGC 604 (left panel, Hα in red color), a prototypical giant Hii region in M33 and an Hα image of the BCD SBS-0335-052E. As it is clearly seen, both present a remarkable similar morphology, with filamentary structure and multiple well structured shells. See also Figure 10 of Tenorio-Tagle et al. (2006). Other important characteristic of Hii galaxies is their low metallicity (Z ⊙ /50 ≤ Z ≤ Z ⊙ /3; Kunth and Sargent, 1983). The galaxy with the lowest metallicity in the Local Universe is IZw18, with Z ≈ Z ⊙ /50, first reported by Searle and Sargent (1972) together with IIZw40. The fact that Hii galaxies are metal-poor objects and very blue seems to point that they are young objects. Nevertheless, there are evidences which indicate the presence of populations older than the ones in the starburst. This is seen in the behaviour of the surface brightness profile which is exponential in the external zones, or in the the color index, which turns redder in the V-R and V-I. In the particular case of IZw18, it was considered as the best candidate for a truly young galaxy. Early studies of the stellar population of IZw18 did not reveal any old population (Hunter and Thronson, 1995). This contradicted some models which predict that during a starburst, the heavy elements produced by the massive stars are ejected with high
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
Page 16 and 17: vi LIST OF FIGURES 3.12 Example of
Page 19 and 20: List of Tables 1.1 Integrated prope
Page 21 and 22: Chapter 1 Introduction 1.1 Overview
Page 23 and 24: 1.1. Overview 3 Figure 1.1: Sample
Page 25: 1.1. Overview 5 These massive star
Page 29: 1.2. Aims and structure of this wor
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Page 60 and 61: 40 3 • IFS of a GEHR in NGC 6946
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56 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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