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

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62 3 • IFS of a GEHR in NGC 6946 on the orography of the observatory. The empirical DAR correction is performed by tracing the intensity peak of a reference object in the FOV, like a star or an unresolved point source (i.e. cluster or nucleus of a type I AGN). Thus, the effect of the DAR can be easily measured by fitting a two-dimensional Gaussian function to each narrow-band image of the datacube to the bright point source to determine the centroid position. This can be done by using the imcntr task in IRAF. A medium-order polynomial function (3 to 5 is usually enough) is fitted to the distribution of the centroid coordinates as a function of wavelength, giving the relative shifts in right ascension and declination (that is to say, shifts in x and y pixel coordinates) for each slide. When fitting a polynomial function to the centroid coordinates, it is important to introduce some kind of rejection limits below and above the fit according to the residual sigma, in the same way as when fitting a continuum to the 1D spectrum of a standard star in the flux calibration procedure. This avoids bad fits due to errors in the centroid measurement or common artifacts at the edges of the resampled images. The measured offsets are used to shift each spectral image of the datacube to a common centroid position. The shift is estimated from the difference between the polynomial-fit coordinates and the fixed one. The imshift task in IRAF can be used to perform the relative shifts to each image. It has to be stressed that using empirical correction for DAR is only possible if a point source in the FOV is available. There are several factors which determine the extent of effects for DAR. Among the most important ones are the airmass and the geometrical size of the spaxels. In effect, the smaller the spaxels the more of the flux at different wavelengths end up on separate spaxels. As described before, the fibers on the PPak IFU are circular, of 2. ′′ 68 diameter, and are separated by 3. ′′ 6. On the other hand, DAR effects become important for large airmasses, with values greater than 1.1. The fact that all our observations were obtained at airmasses lower than the nominal value, combined with the large size of the fibers on the PPak IFU, allows us to neglect DAR corrections. To check the last statement, we transformed the blue RSS file to a datacube format, and we measured the centroid coordinates of a star in the FOV. The results are presented in Figure 3.12. The estimated centroid coordinates are plotted as a function of wavelength. The red line represents a polynomial fit to the distribution. As it can be seen, the position does not change significantly along the whole wavelength range, implying that the DAR effect is negligible. This allows us to work with RSS files, which are more friendly in terms of data analysis, and avoids to transform it to datacube format, which would impose an inevitable interpolation of the data in single ditherings (as in the case of the red data).
3.4. Results 63 x-coordinate (pixel) 39.0 38.8 38.6 38.4 38.2 38.0 y-coordinate (pixel) 26.0 25.8 25.6 25.4 25.2 4000 4500 5000 5500 6000 6500 Wavelength (◦A) Figure 3.12: Example of the DAR correction for the blue datacube. The estimated centroid coordinates of a point source in the FOV are plotted as a function of wavelength in x direction (upper panel) and y coordinate (lower panel). The crosses correspond to the measurements in the datacube, where the red line denote a polynomial fit to the distribution. 3.4 Results 3.4.1 Subtraction of the underlying population Underlying stellar populations in starburst galaxies have several effects in the measurement of the emission lines produced by the ionized gas. Firstly, since they contribute to the continuum of the galaxy, the measurement of the equivalent widths of the emission lines is not a trustable estimator of the age of the ionizing stellar population (Terlevich et al., 2004). Secondly, the presence of a conspicuous underlying stellar population depresses the Balmer and Paschen emission lines and do not allow to measure their fluxes with an acceptable accuracy (Diaz, 1988). This can affect all the properties derived from the relative intensities to the flux of these recombination lines, like the reddening or the ionic abundances. In the case of helium recombination lines, this effect is present too, complicating the derivation of the helium abundance (Olive and Skillman, 2001). FIT3D (Sánchez et al., 2006) is a package to fit and deblend emission lines which can handle both RSS images and datacubes. It includes several spectral synthesis routines to fit
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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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Page 60 and 61: 40 3 • IFS of a GEHR in NGC 6946
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112 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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