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

74 3 • IFS of a GEHR in NGC 6946
Density
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Figure 3.18: Map of the electron density, n e , calculated from the ratio of [Sii] λλ 6717,6731 Å lines.
The scale is in units of cm −3 .
3.4.5 Electron density
The electron density of the ionized gas, and other physical conditions (such as temperatures
when available), have been derived from the emission line data using the same
procedures as in Pérez-Montero and Díaz (2003), based on the five-level statistical equilibrium
atom approximation in the task temden, of the software package IRAF (De Robertis
et al., 1987; Shaw and Dufour, 1995). See Appendix B for a detailed description of the relations
of the physical conditions of the gas and ionic abundances. We have taken as sources
of error the uncertainties associated with the measurement of the emission-line fluxes and
the reddening correction, and we have propagated them through our calculations.
Figure 3.18 shows a plot of the spatial variation of electron density, n e , as derived from the
ratio of the [Sii] λλ 6717,6731 lines (representative of the low-excitation zone of the ionized
gas), assuming a temperature of 8000K. The map shows an overall low density through all
the region, with most of the values below 100 cm −3 , and presenting a few denser knots.
One of them shows the extended region of knot A, while two denser zones surround knot C.
At any rate, the errors involved in some of the other regions are compatible with a general
constant density, well below the critical value for collisional de-excitation. This low nominal
value is consistent with other values from the literature (Ferguson et al., 1998). In terms of
the [Sii] λλ 6717/6731 ratio, the field average ratio is 1.41 ± 0.12 (1 σ error).