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

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34 2 • The star formation history of NGC 5471 Figure 2.10: Final CMD with isochrones divided into five age bins, drawn by their concave hull, and shown with different colours. The right hand side of the figure shows the luminosity function. The line across the bottom shows the completeness limit at the 50% level. −0.5 0.0 0.5 1.0 1.5 m547M−m 675W′ 0 20 40 60 80 100 120 N (stars) 27 27 26 26 25 25 24 24 m 547M 23 23 m547M 22 E(B-V) = 0.2 22 21 21 20 19 4-10 Myr 10-25 Mr 25-50 Myr 50-75 Myr 75-100 Myr 20 19
2.4. Discussion 35 are some traces of the blue-plume at M V = −9, indicating ongoing star formation. A strip of stars following a patch from [0.6,25] to [1.1,22.5] form the red giant branch. Mass in stars In order to quantify the relative importance of the present burst to the star formation history during the previous 100 Myr in NGC 5471, we make a rough estimate of the stellar mass from the CMD. We follow three different approaches. One method consists in assigning a mass to each of the stars in the CMD, corresponding to the closest isochrone, thus counting the stars in the mass range 6-55 M ⊙ and integrating a Salpeter IMF in the mass range 1-100 M ⊙ . The result of this calculation gives ∼ 2 × 10 5 M ⊙ . The second method is as follows: first we assign a mean stellar mass of 6 M ⊙ to the stars in the most populated bin of the luminosity function, m 547M = 23.5. This mass is then scaled up by the ratio of the total m 547M of NGC 5471 to the magnitude of the bin m 547M = 23.5. The resulting mass is used to normalize the IMF at the bin corresponding to 6 M ⊙ . The result of integrating a Salpeter IMF in the range 1-100 M ⊙ thus normalized gives a value of ∼ 5 × 10 5 M ⊙ . A third independent method to estimate the mass is based on the fit of the CMD by means of Hess diagrams. We have used the code STARFISH (Harris and Zaritsky, 2001, 2002) to find a best fit to the CMD by a linear combination of Hess diagrams at different ages. The solution found by the code is only sensitive to stars in two age bins in the range 50-100 Myr, giving an average SFR ∼ 0.01 M ⊙ yr −1 . Integrating this SFR for 50 Myr gives a total mass of ∼ 5 × 10 5 M ⊙ , in the same range of values found by the previous two methods. Although these methods give only an order of magnitude estimate, we can conclude that the integrated mass of stars formed in the past 10-100 Myr is of the order of magnitude or less than the current star formation event, that we have calculated above as ∼ 10 6 M ⊙ . If the star formation proceeds in an isolated gas cloud, we would expect that the rate at which stars form would decrease with time, as less gas is left available to form new stars. This is what is observed in many giant H ii regions with two or more generations of star formation, where the latest event is less massive than the previous. In the case of NGC 5471 the results from the integrated photometry and from the CMD indicate that the current star forming event is more massive than the rate at which stars have formed during the previous 10-100 Myr. This we can understand in the context of the large H i spiral arm in which the region is immersed, as mentioned above. In this respect, Waller et al. (1997) conclude from a morphological study of M101 that NGC 5471 may be the result of tidal interactions of M101 with the nearby galaxies NGC 5477 and NGC 5474 in the time interval 100-1000 Myr ago. Spatial distribution of the star formation history Given the long duration of the star formation in NGC 5471, ∼ 100 Myr, it is natural to question the spatial location of the star formation through time along the extent of the
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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 and 26: 1.1. Overview 5 These massive star
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Page 60 and 61: 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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86 3 • IFS of a GEHR in NGC 6946
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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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