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

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/