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

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46 3 • IFS of a GEHR in NGC 6946 development existent tools. Reducing data of IFUs is slightly different from the reduction of pure imaging or spectroscopic data, because IFS is a combination of both and may also depend on the characteristics of the particular IFU. The steps needed for the PMAS data reduction can be summarized in the following sequential list: • Pre-reduction • Identification of the position of the spectra on the detector along the dispersion axis • Extraction of each individual spectrum • Distortion correction of the extracted spectra and dispersion correction (wavelength calibration) • Fiber-to-fiber transmission correction • Flux-calibration (including cosmic-ray detection and removal in standard stars) • Sky-substraction • Mosaic/dither reconstruction • Other corrections (DAR and/or atmospheric absorption) Data reduction was performed using R3D (Sánchez, 2006), E3D (Sánchez, 2004), PyRAF (a command language for running IRAF 2 tasks based on the Python scripting language), and a self-made Python module (called pyR3D) which contains a Python wrapper for R3D, extra tools (as DAR or atmospheric absorption correction) and other routines. This module helps to diminish the number of steps needed for the reduction, working with a final small script which contains the whole reduction process with all the setting values. Moreover, it automatically adds to the header of each processed image all the parameters needed for other routines. This also helps to have a brief information about the main processes that a image has passed through. 3.3.1 Pre-reduction The pre-reduction steps consist in all the corrections applied to the CCD data that are shared with the reduction of any other CCD-based data and can be easily done by pyRAF. This comprises the creation of a master bias (created from a combination of several bias frames) and the subtraction of this image from all raw images. Then it follows the 2 The Image Reduction and Analysis Facility is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, In. (AURA) under cooperative agreement with the National Science Foundation (NSF).
3.3. Data Reduction 47 FlatFielding correction done by the application of the master CCD flat (usually provided by the Calar Alto Observatory) to all the raw images. Next step consists on the combination of different exposures on the same target and same pointing. This also performs the cosmic ray rejection. In the case of the standard stars, we had only one exposure per pointing, so it was necessary to clean for cosmic rays using other method. This is briefly explained when describing the flux calibration procedure. 3.3.2 Identification of the position of the spectra The raw data from a fiber-feed spectrographs consist in a collection of spectra, stored as a 2D frame, aligned along the dispersion axis. For each wavelength, each spectrum is also spread along the perpendicular (“crossdispersion” or “spatial”) axis. As seen in Figure 3.4, spectra are separated by a certain width, following a characteristic profile which may be considered Gaussian. When the spectra are tightly packed, as in PPak mode, contamination occurs among neighbors. This is indicates the dispersion axis. Spectra are separated 5 Figure 3.4: Section of a PPak raw data. The arrow the so-called cross-talk. It is important ∼ pixels across the cross-dispersion axis, following a to take into account, not only in raw pseudo-Gaussian of FWHM ∼ 3 pixels, contaminating data but also in final processed formats, the adjacent spectra. that adjacent spectra at the CCD may originate from distant locations in the sky plane. Special care has to be taken to compensate effects of instrument flexure, optical distortions, etc, which cause that the spectra are not perfectly aligned along the dispersion axis. This effect is corrected by calculating the shifts between the calibration frame and the corresponding object frames. Therefore, it is necessary to find the location of the projection of each spectrum at each wavelength along the CCD in order to extract its corresponding flux. The very beginning IFS data reduction step is to identify where the spectra lie on the CCD. This is done by using continuum illuminated exposures at every location where the telescope is pointing. An ARC exposure (obtained at the same location) is also needed, since flexures may also affect the wavelength calibration. The location of the spectra (apertures) are identified by extracting a slice perpendicular to the dispersion direction, finding where the peaks are. Each aperture is traced along the dispersion direction usually by fitting a Gaussian function. It is important that the trace is continuous. The aperture mapping is
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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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96 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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