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

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150 Conclusions and future work has to be a large reservoir of gas to maintain this star formation rate, something that is confirmed by the huge Hi concentration in which the region is located. From the spatial distribution of the stars, we can conclude that the star formation in NGC 5471 has proceeded in a general spatio-temporal sequence from the halo innwards to the core, something also seen in 30 Doradus (Walborn and Blades, 1997). The current event of ionizing clusters is contained within a large bubble, which is likely to have been produced by the stars that formed ∼ 20 Myr ago. This leads to the general question of if a low density “seed” generation of evolved intermediate mass stars is in fact a prerequisite to trigger a starburst event (Brandner, 2002). More data of similar kind are obviously needed in order to have an answer. The total oxygen abundances of the four resolved knots analysed spectroscopically (Skillman, 1985; Kennicutt et al., 2003) show a similar value with only one of them showing a slightly lower oxygen content. No more detailed studies for this region have been yet done. The recently developed technique of Integral Field Spectroscopy offers the opportunity to perform a spatially resolved study of the physical conditions of the ionized gas of GEHR exploring at the same time the properties of their ionizing stellar populations. This has been done in this work for the giant Hii region complex in NGC 6946 using PPak attached to the 3.5m telescope of the CAHA observatory. The configuration was chosen to cover the whole spectrum from 3600 up to 10000 Å, allowing the measurement of the [Siii] lines in the near infrared. This is the first time to our knowledge that data of this kind have been obtained for such a wide spectral range. From the created maps, we have selected four main knots, labelled from A to D, to perform a detailed integrated spectroscopic analysis of these structures and of the whole PPak field-of-view. For all the knots the density has been found to be very similar and below 100 cm −3 . The electron temperature of [Oiii] was measured in knots A, B, C and the integrated PPak-field, and was found to be around 8000 K. The temperatures of [Oii] and [Siii] were also measured in the four cases. The abundances derived from the ”direct method” are typical of high metallicity disk Hii regions, and very uniform among all knots, with a mean value of 8.65. This is comparable to what has been found by other authors for regions at similar galactocentric distance (Larsen et al., 2006). The S/O and N/O ratios are very similar in all structures. Therefore, a remarkable abundance uniformity is found despite the different excitations found throughout the nebula that can be appreciated in the [Oiii] λ 5007/Hβ, [Nii] λ 6584/Hα and, [Sii] λλ 6717,6731/Hα maps. This uniformity is also supported by the behaviour of the S 23 parameter which is approximately constant despite the wide range of excitation observed. This confirms the results found in the classical studies by Skillman (1985), Diaz et al. (1987), Rosa and Mathis (1987), and more recently in spatially resolved studies of Hii galaxies (Kehrig et al., 2008; Lagos et al., 2009). On the other hand, there are stellar populations old enough as to produce supernovae since at least one is reported to exist in the region (Mayall, 1948). This implies either a very fast and effective
Conclusions and future work 151 mixing with the surroundings or that, as suggested by Stasińska et al. (2007), the new metals processed and injected by the current star formation episode are not observed and reside in the hot gas phase. The metals from previous events, on the contrary, would be well mixed and homogeneously distributed through the whole extent of the region. The Hα luminosity of the emission knots amounts to about 50% of the total PPak field of view. Knot A is the brightest of them and provides 75% of the combined flux from the knots and 38% of the total Hα flux. This knot has the highest excitation as characterised by the [Oii]/[Oiii] ratio and dominates the excitation deduced for the PPak field integrated spectrum. On the other hand the measured Hβ equivalent width for this entire region is about 40 Å, much lower than the ones measured for the individual knots that range from 60 to 100 Å. That is, the integrated spectra shows high excitation and a relatively evolved age which does not correspond to the individual knot evolutionary stages. Wolf-Rayet features have been detected in knot A and C, and a weak contribution by these stars has been also found in knot B, leading to a derived total number of WR stars of 125, 22 and 5, respectively and provide O/WR number ratios which are consistent with SB99 models for an age of about 4 Myr for the metallicity of the region. Knot D, with no WR features, shows weak Hα emission, low excitation, and the lowest Hβ equivalent width value all of which points to a more evolved state. Once the assumption of uniform abundance has been confirmed true for GEHR from IFS observations, spectrophotometric observations of two Hii galaxies, IIZw71 and J1657, were carried out to study the physical properties of the individual bursts of star formation. The star formation processes in Hii galaxies are found to occur in low density environments. In all the knots of J1657 the electron densities have been found to be lower than 30 cm −3 , well below the critical density for collisional de-excitation. For IIZw71, two densities were measured: n([Oii]) and n([Sii]), the latter one being slightly higher than the former. Although by factors between 5 and 10 larger than for J1657, the densities found are still below the critical value for collisional de-excitation. We extracted information on the four brightest knots of IIZw71 in Hα that we labelled from A to D and of the three knots of J1657, labelled from A to C. In this latter galaxy, we have measured four line temperatures: T e ([Oiii]), T e ([Siii]), T e ([Oii]), and T e ([Sii]), reaching high accuracies. For IIZw71, the electron temperature of [Oiii] was measured in knots B and C, with an accuracy of 14%. The temperature of the main knot of J1657 has been found to be about 2000 K lower than the other two knots, while the [Oiii] temperature of knot C of IIZw71 is 1000 K higher than B. The temperatures related to the high ionization zone are similar for both galaxies, with a mean value of 13000 K. The temperature measurements for both galaxies allowed the direct derivation of ionic abundances of oxygen, sulphur, nitrogen, neon and argon. The total abundances of these species are in the same range of metallicities measured in Hii galaxies, between 7.78 and 7.99. Knot A of J1657 presents a slightly larger abundance by 0.1 dex with respect to the
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Facultad de Ciencias Departamento d
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A Abigaíl, mi ut A mi familia, por
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La gente del grupo de Astrofísica
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Contents List of Figures List of Ta
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CONTENTS iii B •Physical conditio
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vi LIST OF FIGURES 3.12 Example of
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List of Tables 1.1 Integrated prope
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Chapter 1 Introduction 1.1 Overview
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1.1. Overview 3 Figure 1.1: Sample
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1.1. Overview 5 These massive star
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1.1. Overview 7 Figure 1.3: Color-m
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1.2. Aims and structure of this wor
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12 2 • The star formation history
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40 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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Page 120 and 121: 100 3 • IFS of a GEHR in NGC 6946
Page 126 and 127: 106 4 • Long-slit spectrophotomet
Page 136 and 137: Hδ 116 4 • Long-slit spectrophot
Page 144 and 145: Table 4.4 continued SDSS J1657 Knot
Page 172 and 173: 152 Conclusions and future work oth
Page 174 and 175: 154 Conclusions and future work The
Page 176 and 177: 156 Conclusiones Las abundancias to
Page 178 and 179: 158 Conclusiones consistentes con l
Page 180 and 181: 160 Appendix A where c=0.434C. This
Page 182 and 183: 162 Appendix B where R S2 = I(6717
Page 184 and 185: 164 Appendix B Sulphur As in the ca
Page 186 and 187: 166 Appendix B Argon For argon, the
Page 189 and 190: Appendix C Empirical calibrators In
Page 191 and 192: C • Empirical calibrators 171 One
Page 193 and 194: Appendix D Stellar photometry resul
Page 195 and 196: D • Stellar photometry results of
Page 207: D • Stellar photometry results of
Page 210 and 211: 190 REFERENCES Bertelli, G., Bressa
Page 212 and 213: 192 REFERENCES Girardi, L. & Bertel
Page 214 and 215: 194 REFERENCES Kunth, D. & Sargent,
Page 216 and 217: 196 REFERENCES Pérez-Montero, E.,
Page 218 and 219: 198 REFERENCES Terlevich, R., Melni
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