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

58 3 • IFS of a GEHR in NGC 6946
the spectrophotometric standard star frames.
All the corrections performed to the images, mentioned in precedent sections, do not
correct for the differences in the shape of the emission lines along the cross-dispersion axis.
This effect is also present in sky-subtracted spectra from long-slit spectroscopy, showing
characteristic residuals in the location of the sky emission lines. This effect is more severe
in fiber-based spectrographs, since the effective dispersion and shape of the emission lines
varies fiber-to-fiber.
As mentioned in Section 3.2, PPak includes additional fibers that probe the sky far enough
from the science FOV to avoid contamination by the astronomical object. One simple method
to perform the sky subtraction is to extract all the 36 sky fibers, create a median sky spectra
and substract it from the science exposures. Nevertheless, this does not take into account
the distortion in the wavelength solution along the cross-dispersion. To increase the accuracy
of the sky substraction, a second method can be applied. The routine create sky ppak
determines the sky spectra corresponding to any science fiber by an interpolation of the
spectra obtained through the sky lines. As the set of Hii regions in our images presents an
inhomogeneous distribution through the field of view of the IFU, we followed another method
to derive a representation of the sky. The routine create sky clip creates a sky spectra by
obtaining the median between a certain number of adjacent spectra, clipping those ones
with a flux over a certain threshold of the standard deviation. The results from the last two
methods are very close from each other in our case, with differences less than 3%. The use of
one or other method depends on the nature of the science observations, so it is advisable to
check both solutions. Both results create a better representation of the sky than the simple
method, with less deep residuals, but still present. These residuals are specially strong in the
red part of the spectrum (8000-11000 Å).
As seen in Section 3.1, the Hii complex is located at the extreme of the NE arm of the
galaxy. As the sky fibers surround the object, some of them might have some diffuse nebular
emission from the end of the arm contaminating the sky spectra. We compared the flux of
the Hα line in representative spectra with and without sky subtraction. The difference in
the flux was negligible 8 . This reliable, free of nebular emission, sky spectra was subtracted
from the science frames.
3.3.8 Mosaics
As explained before, adjacent spectra in a RSS file may not correspond to spatially near
positions, so it is important to reorder the spectra to their original position in the sky. A
position table is required, relating the spectra to their locations in the sky. Then, it is possible
to create a regular gridded data cube by interpolating the data spatially, and reconstruct
the original image of the target at any wavelength. This step is needed if DAR correction is
8 The Hα contribution of the sky spectra is about 10% for the lowest value in the FOV.