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

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60 3 • IFS of a GEHR in NGC 6946 1e−15 1.0 0.8 [SIII] 9532 ◦A−1 ) s−2 Flux (erg cm−2 0.6 0.4 [SIII] 9069 0.2 Absorption correction factor 0.0 2.2 2.0 1.8 1.6 1.4 1.2 1.0 8800 9000 9200 9400 9600 9800 Wavelength (◦A) Figure 3.11: Correction for atmospheric water-vapour absorption bands. Upper panel: Spectrum from one intense fiber of the red RSS file of the object. Dashed (blue) line represents the corrected final spectrum and solid (green) line the uncorrected one. Sulphur lines [Siii] λλ 9069,9532 Å are labeled. Bottom panel: Absorption correction factor applied to the RSS file (see text for an explanation on how to obtain the correction factor). [Siii] λλ 9069,9532 Å. First, the fitted continuum to the standard star counts-spectrum is divided by the original 1D spectrum with the absorption features. This creates a normalized correction spectrum, with values equal to unity when both continua are the same, and a value greater that 1 which indicates the absorption factor. The correction spectrum is set to one at all wavelengths except for the range in which the correction is going to be applied. Then, the final flux calibrated mosaic of the science target has to be multiplied by the correction spectrum. This correction should be applied after the flux calibration. To build the correction spectrum, the standard star closer in time and position in the sky to the science exposures is chosen, in order to ensure that the conditions of the atmosphere were similar 11 . Figure 3.11 shows an example of atmospheric absorption correction for a particular fiber of the RSS file of the object. In the lower panel, the absorption correction factor applied to all spectra in the red RSS is plotted, obtained as explained above. In the upper level, the solid (green) line represents the uncorrected spectrum for a particular intense fiber, while the dashed (blue) line is the spectrum after multiplying by the correction factor. The 11 The atmospheric absorption features change in shape and intensity along the night.
3.3. Data Reduction 61 strong absorption is clearly seen particularly at 9300 Å with corrections around a factor of 2. Taking into account that the theoretical ratio between the sulphur lines is I(9532Å) ≃ 2.44 · I(9069Å), it is immediately obvious that the intensity of the [Siii] 9532Å line is heavily cut up by atmospheric absorptions bands, since both sulphur lines are of the same intensity before correction. After applying the correction factor both lines are pushed to higher intensities. Nevertheless, the total intensity of the 9532 Å line is not recovered, since the ratio between them is still well below the theoretical one. This ratio is not accomplished at any fiber, meaning that the absorption in the range from 9300 Å is stronger. Thus, we must rely on the 9069Å line when deriving temperatures or using empirical calibrators, applying the theoretical ratio to obtain the intensity of the 9532 Å line. Taking into account the low correction factors obtained near the location of the 9069 Å line, we can assume that the intensity recovered is good to an estimated accuracy of about 7 per cent. 3.3.10 Correction for differential atmospheric refraction The refraction of light by the Earth’s atmosphere is wavelength-dependent. This effect is called differential atmospheric refraction (DAR), and results in the image of an object to appear at different positions in the telescope focal plane depending on the wavelength of observation. The amplitude of the shift due to DAR is a function of zenith distance, causing that the red and blue ends of an object spectrum will appear at different spatial positions. The effect is important when fiber-to-fiber intensities of different wavelengths are combined, but negligible if spatially integrated intensities are used. IFUs have the advantage over traditional long-slit spectroscopy of determining and correcting for DAR (Arribas et al., 1999), once the data is totally reduced and converted to datacube format. Any DAR correction requires to resample the data spatially, which produces a loss of the initial spatial configuration of the spaxels, changing from one spectrum (wavelength versus flux) per spaxel (spatial coordinate) to a 2D flux image at each wavelength point. There are several ways of correcting for DAR. One approach is to apply a theoretical correction based on the Filippenko (1982) formulae, in which an image offset vector for each wavelength bin is used 12 . Each image, at every wavelength, is thereafter shifted with respect to a pre-defined reference wavelength, using a (fractional) bi-linear interpolation procedure. Doing this the intensity in each spaxel becomes a function of four spaxels - introducing an unavoidable smoothing. Nevertheless, in many cases the empirical correction works better than the theoretical one. The reason is that theory assumes that the layers of the atmosphere with equal refraction index are flat and perpendicular to the zenith of the telescope. However, it is known that this is only a first order approximation, since these layers depend strongly 12 Parallactic angle (orientation of the IFU with respect to the downward direction), and physical conditions at the time of the observations like humidity, pressure, and temperature are needed.
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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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Page 32 and 33: 12 2 • The star formation history
Page 60 and 61: 40 3 • IFS of a GEHR in NGC 6946
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110 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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