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

50 3 • IFS of a GEHR in NGC 6946
be corrected fiber-to-fiber before finding a common wavelength solution, so a 2D modeling of
the distortion map with an analytical function cannot be done. Using an ARC exposure, the
peak intensity of single emission line is traced along the cross-dispersion axis, and shifted to
a common reference, by a linear shift. This is a first order distortion correction. In order to
carry out a second order correction, a few emission lines are selected. Their peak intensity
is traced and a polynomial distortion correction is performed to recenter all the lines to a
common reference. Due to the fact that PMAS is a well behaved instrument in terms of
distortion, only a low order polynomial function is needed. In this step it is important to
select well separated bright lines and distributed along the dispersion axis (it is not necessary
to select a large number; in fact, it is better to have only a few). Otherwise, the algorithm
may mistake two lines or the fit may not be correct, producing some anomalous shifts in
the trace. This wrong solution is clearly seen in right panel of Figure 3.6, where a detailed
portion of a RSS shows shifts along the cross-dispersion axis. Our set of red ARC exposures
had only a couple of intense lines, and they were only placed at the very beginning of the
left side of the CCD, producing shifts in the second order correction along the last quarter
of the frame. To correct for the shifts, it is enough to perform a smoothing operation in the
solution. In particular, this can be controlled by the BOX parameter in the mdist cor sp
R3D routine.
Once a satisfactory correction (first and second order) is achieved, the solution is finally
applied to the science exposures.
3.3.4 Wavelength solution (dispersion correction)
The wavelength solution is found by identifying the wavelengths of the arc emission
lines, using an interactive routine. The solution of the ARC spectra is transformed to a
linear wavelength system by a one dimensional spline interpolation. The accuracy of this
solution depends on the order of the polynomial selected, the number of lines identified,
and the coverage of emission lines along the CCD (the more homogeneously distributed, the
better) which, in turn, depends on the instrument. For PMAS in PPAK mode, it is enough
with 9 emission lines and a polynomial function of order 4 (accuracy of ∼ 0.15 Å). For the
blue spectrum, the ARC exposures with a HgCdHe had enough strong emission lines in order
to find a good dispersion solution. Unfortunately, the same calibration lamp was used for
the red spectra, having only a few intense lines in the left side of the wavelength range, as
mentioned in secion 3.3.3. For this reason, sky lines were used in order to perform a dispersion
correction. This is achieved by using the science frames themselves, which contains strong
emission sky lines. In the red and near-infrared spectral regions, the OH emission bands of
the natural night-sky spectrum dominate. Osterbrock and Martel (1992) tabulated accurate
wavelengths of the individual lines in these bands. 12 to 15 lines homogeneously distributed
along the full wavelength range were selected, achieving an accuracy of 0.5 Å. Figure 3.7