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

62 3 • IFS of a GEHR in NGC 6946
on the orography of the observatory.
The empirical DAR correction is performed by tracing the intensity peak of a reference
object in the FOV, like a star or an unresolved point source (i.e. cluster or nucleus of a type
I AGN). Thus, the effect of the DAR can be easily measured by fitting a two-dimensional
Gaussian function to each narrow-band image of the datacube to the bright point source
to determine the centroid position. This can be done by using the imcntr task in IRAF.
A medium-order polynomial function (3 to 5 is usually enough) is fitted to the distribution
of the centroid coordinates as a function of wavelength, giving the relative shifts in right
ascension and declination (that is to say, shifts in x and y pixel coordinates) for each slide.
When fitting a polynomial function to the centroid coordinates, it is important to introduce
some kind of rejection limits below and above the fit according to the residual sigma, in
the same way as when fitting a continuum to the 1D spectrum of a standard star in the
flux calibration procedure. This avoids bad fits due to errors in the centroid measurement
or common artifacts at the edges of the resampled images. The measured offsets are used
to shift each spectral image of the datacube to a common centroid position. The shift is
estimated from the difference between the polynomial-fit coordinates and the fixed one. The
imshift task in IRAF can be used to perform the relative shifts to each image. It has to
be stressed that using empirical correction for DAR is only possible if a point source in the
FOV is available.
There are several factors which determine the extent of effects for DAR. Among the
most important ones are the airmass and the geometrical size of the spaxels. In effect, the
smaller the spaxels the more of the flux at different wavelengths end up on separate spaxels.
As described before, the fibers on the PPak IFU are circular, of 2. ′′ 68 diameter, and are
separated by 3. ′′ 6. On the other hand, DAR effects become important for large airmasses,
with values greater than 1.1. The fact that all our observations were obtained at airmasses
lower than the nominal value, combined with the large size of the fibers on the PPak IFU,
allows us to neglect DAR corrections.
To check the last statement, we transformed the blue RSS file to a datacube format, and
we measured the centroid coordinates of a star in the FOV. The results are presented in
Figure 3.12. The estimated centroid coordinates are plotted as a function of wavelength. The
red line represents a polynomial fit to the distribution. As it can be seen, the position does
not change significantly along the whole wavelength range, implying that the DAR effect is
negligible. This allows us to work with RSS files, which are more friendly in terms of data
analysis, and avoids to transform it to datacube format, which would impose an inevitable
interpolation of the data in single ditherings (as in the case of the red data).