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

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50 3 • IFS of a GEHR in NGC 6946 be corrected fiber-to-fiber before finding a common wavelength solution, so a 2D modeling of the distortion map with an analytical function cannot be done. Using an ARC exposure, the peak intensity of single emission line is traced along the cross-dispersion axis, and shifted to a common reference, by a linear shift. This is a first order distortion correction. In order to carry out a second order correction, a few emission lines are selected. Their peak intensity is traced and a polynomial distortion correction is performed to recenter all the lines to a common reference. Due to the fact that PMAS is a well behaved instrument in terms of distortion, only a low order polynomial function is needed. In this step it is important to select well separated bright lines and distributed along the dispersion axis (it is not necessary to select a large number; in fact, it is better to have only a few). Otherwise, the algorithm may mistake two lines or the fit may not be correct, producing some anomalous shifts in the trace. This wrong solution is clearly seen in right panel of Figure 3.6, where a detailed portion of a RSS shows shifts along the cross-dispersion axis. Our set of red ARC exposures had only a couple of intense lines, and they were only placed at the very beginning of the left side of the CCD, producing shifts in the second order correction along the last quarter of the frame. To correct for the shifts, it is enough to perform a smoothing operation in the solution. In particular, this can be controlled by the BOX parameter in the mdist cor sp R3D routine. Once a satisfactory correction (first and second order) is achieved, the solution is finally applied to the science exposures. 3.3.4 Wavelength solution (dispersion correction) The wavelength solution is found by identifying the wavelengths of the arc emission lines, using an interactive routine. The solution of the ARC spectra is transformed to a linear wavelength system by a one dimensional spline interpolation. The accuracy of this solution depends on the order of the polynomial selected, the number of lines identified, and the coverage of emission lines along the CCD (the more homogeneously distributed, the better) which, in turn, depends on the instrument. For PMAS in PPAK mode, it is enough with 9 emission lines and a polynomial function of order 4 (accuracy of ∼ 0.15 Å). For the blue spectrum, the ARC exposures with a HgCdHe had enough strong emission lines in order to find a good dispersion solution. Unfortunately, the same calibration lamp was used for the red spectra, having only a few intense lines in the left side of the wavelength range, as mentioned in secion 3.3.3. For this reason, sky lines were used in order to perform a dispersion correction. This is achieved by using the science frames themselves, which contains strong emission sky lines. In the red and near-infrared spectral regions, the OH emission bands of the natural night-sky spectrum dominate. Osterbrock and Martel (1992) tabulated accurate wavelengths of the individual lines in these bands. 12 to 15 lines homogeneously distributed along the full wavelength range were selected, achieving an accuracy of 0.5 Å. Figure 3.7
3.3. Data Reduction 51 1600 1400 OH (9-4) OH (6-2) 8344.59 OH (7-3) 8827.07 1200 7913.71 Count Rate 1000 800 OH (8-3) 7276.41 7523.99 7750.65 8430.17 8919.61 OH (8-4) 9375.97 600 8504.84 400 9001.33 9476.87 9567.36 200 0 OH (4-0) OH (5-1) 7500 8000 8500 9000 9500 10000 Wavelength (◦A) Figure 3.7: Example of sky lines used in the red spectra to find the wavelength solution. The most important transition bands are also labeled. shows an example of sky spectra; some of the lines used in the wavelength calibration are also labeled. 3.3.5 Fiber-to-fiber response correction Not all the fibers of the IFU have the same physical properties. In particular, the transmission can vary significantly from fiber to fiber. In addition, the path of the light through the spectrograph optics has an impact on the sensitivity. To correct for this, an exposure of a continuum, well-illuminated, and flat source is required, like a sky flat. The fiber-flat exposures taken during twilight were used to normalize the response of each fiber over the entire wavelength range, assuming that the light input to each fiber is uniform. The sky flat exposure has to be reduced, tracing the location of each specrum, extracting, correcting the distortions, and applying a wavelength solution, following the same steps as mentioned above. Then, a median spectrum is obtained using all the spectra in the frame. Finally, each spectrum is divided by this median one, obtaining a fiber-flat file. To correct for the differential transmission fiber-to-fiber, the science files must be divided by the fiber-flat one.
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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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100 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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