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

3.3. Data Reduction 59
compulsory. If it is not the case, this step can be avoided and work with the RSS file and its
corresponding position table.
In the case of PMAS in the PPak mode it is necessary to interpolate at each wavelength
the intensity at each spatial location in order to create a data cube in an artificial squared
grid pattern. This can be done by using E3D or its complementary routines. Nevertheless,
when performing dithering exposures to fill the holes between adjacent fibers to get a filling
factor of 1, the procedure is slightly different and interpolation can be avoided. First, all the
dithering exposures must be reassembled in a single RSS file. In the case of covering all the
FOV, the exposures have to have certain overlap and the offsets between adjacent exposures
had to be selected in a way that the overlap is complete between spectra. We used the routine
Mosaic rss overlap of R3D to build a final RSS for each night which covered the entire
FOV. As mentioned before, it is possible to construct the datacube without interpolating the
data. This is accomplished by means of the regularization scheme, in which all the spectra
result from the combination of measured values. A grid of a certain pixel scale, smaller than
that of the original fibers 9 is constructed over the FOV, and for each pixel the spectrum that
enters in it is averaged, obtaining a final datacube.
In the second night, when getting the exposures in the 7000-10100 Å range (GROT =
-72), the de-rotator and guiding stopped at the second pointing of the dithering exposures 10 ,
loosing the pointing for the second and the third dithering exposures. This prevents us
to perform a mosaic covering all the FOV. We tried to recover the positions of the last
dithering, but no good results where achieved in the last mosaic, so we ended with only the
first dithering for the red part of the spectrum. Nevertheless, this is not important if only
ratios are involved and absolute flux from emission lines are not needed. In section 3.4.8, we
will see that this problem will not affect our results when dealing with integrated properties.
The mosaic in the blue range was built without any problems. The final RSS blue mosaic
contained 993 spectra (3 × 331), while the red RSS frame 331, since it is only the pointing
for the first dithering exposure.
3.3.9 Correction for atmospheric absorption
In some cases, the absorption of the atmosphere is so strong that it can depress some
emission lines, altering fluxes and ratios needed to obtain the physical properties of the
target. This is specially important in the red range of the spectrum, namely from 8000 Å.
We have followed the procedure described by Diaz et al. (1985) to remove the atmospheric
water-vapour absorption bands in order to determine the strengths of the sulphur lines
9 A pixel size of 1/3 of the original spaxel size is recommended. This value should also be used when
interpolating to create datacubes when there are no dithering exposures or building maps for emission lines.
In this case, a nearest neighbour interpolation is also recommended.
10 It has to be taken into account that the dithering method in PPak is experimental, so the telescope does
not record in the header any information of the offsets. These values have to be kept in the log of observations
in order to build the mosaic.