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52 3 • IFS of a GEHR in NGC 6946 3.3.6 Flux-calibration Flux calibration converts the count rate measured by the detector into the flux density collected by the telescope. A loss of flux is unavoidable while the light passes through all the optical elements in the telescope and the instruments, so that an instrumental sensitivity function is needed. The Earths’s atmosphere, depending on airmass A and wavelength λ, also absorbs part of the flux emitted from an object. The sensitivity function is obtained by observing a bright star with a well-known flux density 4 and comparing it with the measured count rate I λ , that is to say, a measure of the ratio between the counts per second and the flux per second: I λ r λ = f λ · t exp · 10 0.4·A V ·κ λ where A V is the airmass and κ λ is the extinction. For Calar Alto Observatory, the extinction has three main contributions (Sánchez et al., 2007a): the Rayleigh scattering at the atmospheric atoms and molecules, the extinction due to aerosol particles (mostly dust), and the extinction due to Ozone. Nevertheless, the last one has a marginal effect in the total extinction at any wavelength. Moreover, the Rayleigh scattering is almost constant along the year. Therefore, considering that the extinction in the V-band is due to a fix contribution or Rayleigh scattering and a variable contribution due to Aerosol extinction, the derived total extinction expression is: ( ) λ −4 ( ) λ −0.8 κ λ ∼ 0.0935 + (0.8 ∗ κ V − 0.0935) 5450 5450 Therefore, it is only necessary to know the V-Band extinction for each night. This quantity is measured by the Calar Alto Extinction monitor (CAVEX) each night. The system is fully automatic, opening half an hour after the beginning of the astronomical night and closing an hour before its end, having a record of almost all nights. The extinction for a particular night can be obtained in CAVEX’s Historic Page 5 . A star is a point source on the sky, but the image of the star is broadened due to the turbulence in the atmosphere and/or diffraction of the telescope optics. Fiber-fed spectrographs suffer light losses when the fibers are smaller than the seeing-disc and the calibration stars are not completely well centered in a single fiber. IFUs based on pure fiber-bundles like the PPak mode of PMAS (or INTEGRAL), have a filling-factor of 60%, which imposses flux losses, preventing a reliable absolute spectrophotometry. Nevertheless, this can be overcome 4 Absolute flux calibrated tables of spectrophotometric standard stars can be found in the following web pages: ftp://ftp.stsci.edu/cdbs/current calspec/ http://www.caha.es/pedraz/SSS/sss.html http://www.eso.org/sci/observing/tools/standards/spectra/ 5 http://www.caha.es/CAVEX/HISTORIC/hcavex.php
3.3. Data Reduction 53 by performing a dithering of the standard star in order to cover the entire field-of-view (see section 3.3.8). The total count rate is measured by summing up all the fibers covering the standard star only. First of all, it is necessary to have a reduce frame (following all the steps described above) and a final mosaic of a standard observed during the night, and extract a 1D spectrum. This can be done by using the E3D Visualization Tool 6 . This frame has to be free of cosmic ray hits. In the case of the spectroscopic standard stars, we had only one science frame per pointing, so before performing the flux extraction it was necessary to clean for cosmic rays. We applied the code L.A. COSMIC written by van Dokkum (2001), using a Laplacian edge detection method. Two special versions of the code have been published, one for imaging data and another one for spectroscopic data 7 , both written in CL IRAF programming language. Special care has to be taken because the spectroscopic version was designed for long-slit spectroscopic data. The algorithm tends to falsely detect cosmic rays in parts of the science frames with prominent emission lines. Thus, the parameters controlling the detection process are chosen in such a way that the number of false detected cosmic rays is reduced to minimum while the number of correct detected cosmic rays is maximized. The best set of parameters had been found by eye varying the parameters with respect to the default values until an acceptable result was achieved. Another task which we found to work well with this kind of data is the lineclean IRAF routine. We applied both methods to the standard star images and chose the best result for each case. Once the frames are cleaned from cosmic rays, the 1D spectrum can be obtained with E3D. First, the sky must be subtracted. This is accomplished by selecting spaxels in areas clean of source. It is a good procedure to select the most external ring of spaxels of the frame. Then, the median sky spectrum is subtracted to the frame. Normally, in PPak data, only the most intense fiber is extracted, assuming that the star is well centered in a single fiber and that the size of the fiber is bigger than the seeing-disc. Nevertheless, these assumptions may not be right. Moreover, the PPak mode does not cover the entire FOV, having a filling factor of 60%, which imposses flux losses. The most extended approach to solve the problem is to determine a relative spectrophotometry, and recalibrate later using additional information, like broad-band photometry. Another approach is to perform a dithering of the standard star, as in the case of the science frames, covering all the FOV. Once the mosaic is sky-subtracted, spaxels with flux of the star must be selected. If using E3D to extract the flux, it is required to keep the number of selected fibers, n fib , as E3D averages the spectra of them. To recover the total counts of the star, the 1D spectrum should be multiplied by n fib , or include this in a factor parameter, which is the number of fibers divided by the exposure time. Finally, 6 The development of the E3D was a primary goal of the European Commission’s Euro3D Research Training Network. 7 http://www.astro.yale.edu/dokkum/lacosmic/
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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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Page 32 and 33: 12 2 • The star formation history
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102 3 • IFS of a GEHR in NGC 6946
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104 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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