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

68 3 • IFS of a GEHR in NGC 6946
Creating maps
Maps are data files stored in 2D fits files, reflecting the original arrangement of the spaxels
in the sky. In the case of non regularly gridded IFU or RSS files, it is necessary to interpolate
the data, as mentioned in previous sections. We have used interpol routine from E3D with
a nearest neighbour interpolation, a grid parameter 13 of 10 −12 and a pixel size of 1 ′′ , close to
the recommended 1/3 of the original spaxel size. All the maps shown in next sections, unless
mentioned, have a North down and East left orientation, and a 1 ′′ /pixel scale. A distance of
5.9 Mpc to NGC 6946 is assumed, so 1 ′′ in the sky corresponds to 28.6 pc.
We have masked all values in the maps with relative errors ≥ 25%. This is applied to
any quantity: flux, ratio or combination of lines used in empirical parameters.
3.4.3 Hα maps and morphology
From Figure 3.14 it is clear that the morphology of the complex of Hii regions is well
recovered from the IFU data. The upper panel is an Hα flux map created from the RSS
blue data as explained in section 3.4.2. The masked values follow the criteria explained in
that section, as well as the orientation (north down, east left). The lower panel shows an Hα
(not continuum-subtracted) image taken at the Kitt Peak National Observatory (KPNO)
2.1-meter telescope (same source as Figure 3.3) showing the PPak field and with the same
orientation as the map of the upper panel.
Hodge and Kennicutt (1983) catalogued more than 10000 Hii regions in 125 galaxies.
This HK catalog contains 540 identified Hii regions in NGC 6946. We have labelled all the
important knots following the HK identification. As it can be seen in the Hα map, only the
most intense knots are clearly defined, and for this reason we have labelled them with the
first four alphabetical letters. Interestingly, knot B is not identified in Hodge and Kennicutt
(1983). Since they have enough signal-to-noise, we will present the main measured properties
of these four knots when discussing the integrated properties in section 3.4.8. It has to be
remarked that the lower panel image has not been continuum-subtracted, so some knots are
seen more intense since their continuum sources are included in the image, but their Hα
emission are not so strong. This is the case of knots HK83-013, HK83-014, HK83-009 and
HK83-006.
Figure 3.15 shows the continuum emission near Hα (top) and Hβ (bottom) maps together
with the corresponding emission line intensity isocontours overplotted, while Figure
3.16 shows the continuum emission map near 8500 Å together with the Hα isocontours overplotted
as reference. The red continuum comes from the first pointing. Maps of the continua
are representative of the stellar emission, free of contamination from the gaseous emission
lines. From the Hα and Hβ isocontours it can be seen that both emission structures are
13 The grid parameter represents the resolution of the triangulation.