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

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48 3 • IFS of a GEHR in NGC 6946 500 coadding method simple gaussian 400 300 counts 200 100 0 3500 4000 4500 5000 5500 6000 6500 7000 Angstroms (◦A) Figure 3.5: Comparison of the coadding (blue continuous line) and simple gaussian (green dot-dashed line) method for the aperture extraction reduction step. then used as a reference for all other images. In many cases the science frames do not have enough signal-to-noise through all the fibers for an accurate determination of the tracing. Due to this reason, continuum illuminated exposures are used. The final result is stored as a 2D image with the X-axis as the original dispersion axis and the Y-axis the traced spectrum number. In each row is recorded the location of the peak centroid in the original frame. To ensure a continuous behaviour, it is advisable to fit a polynomial function to the traced result (order 4/5 is enough). 3.3.3 Spectra extraction The next reduction step is to extract the flux corresponding to the different spectra at each pixel along the dispersion axis. This procedure is called aperture extraction. This is usually performed by coadding the flux within a certain aperture around the “trace” of the spectra in the raw data. In PPak data, the aperture extraction normally is done coadding the flux within an aperture of 5 pixels. Nevertheless, sometimes the aperture extraction is not the optimal method to recover the flux corresponding to each spectrum due to the cross-talk problem. Figure 3.5 shows a comparison of the coadding and simple gaussian method for the aperture extraction reduction step in the case of the fiber 167 of one single
3.3. Data Reduction 49 Figure 3.6: Left panel: Example of a RSS of a calibration lamp showing the distortion along the cross-dispersion axis. Right panel: Example of shifts produced by a bad second order correction for distortion. pointing of the standard star BD+28 ◦ 4211. The cross-talk problem can affect fibers with low counts adjacent to intense fibers. This is usually seen as a sinusoidal behaviour in the continuum of the spectra, producing spurious structure which could yield to wrong results in the interpretation of the data 3 . To avoid the cross-talk problem, sometimes it is enough to reduce the coadding width, but this may produce a substantial loss of flux if the aperture is too small. This is strongly dependent on the kind of data, so it is necessary to check if the simple aperture extraction does a good job and, otherwise, use the simple gaussian extraction (which is more time-consuming). In the 2D image resulting from the aperture extraction, the X-axis corresponds to the original dispersion axis, while the Y-axis corresponds to the ordering of the spectra along the pseudo-slit. This is the so-called row-stacked spectra representation (RSS). Each single spectrum corresponds to a particular fiber and the RSS is just a frame, where all the spectra are sorted in the rows of the frame one after another. This representation is very useful because all the information can be stored in a single image, preserving the two-dimensional structure of the data as detected by the CCD plane. However, an extra file is needed with additional information of the exact location (projected position) of each fiber in the sky (the “position table”). Due to the not homogeneous light dispersion along the cross-dispersion axis, spectrographs present the so-called C distortion, so that the distortion is larger in the edges of the slit than in the center. In IFS, additional distortions are added to the intrinsic curvature for grating spectrographs owing to the placing of the fibers in the pseudo-slit. The left panel in Figure 3.6 shows a RSS where the curvature and distortions are seen. These distortions must 3 As the case when fitting synthetic stellar populations to the continuum in order to substract the underlying population.
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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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Page 21 and 22: Chapter 1 Introduction 1.1 Overview
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98 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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