Internal variation of electron density in galactic HII regions

A study on the internal variation of the electron density in galactic I-III regions has been conducted on a sample of 15 objects of different sizes an d evolutionary stages. The [S II] lambda 6716/lambda 6731 line ratio was ad opted as electron density indicator. Long slit spectrophotometry of high si gnal-to-noise ratio with spectral dispersion of 0.75 Angstrom pxl(-1) and s patial scale of 0." 90 pxl(-1) were obtained at different slit positions an d orientations. No systematic spatial variation of electron density was det ected in nearly half of the objects studied (S 255, S 257, S 271, S 285, S 301, S 305, NGC 3372 and IC 1275). They are in general the most diffuse and probably evolved objects with low mean densities in the range N-e approxim ate to 20-140 cm(-3). The remaining objects (S 288, S 307; NGC 2579, NGC 35 03, Gum 62, Gum 64a and M 20) with mean densities N-e approximate to 80-360 cm(-3), have shown a statistically significant electron density dependence on position. In most of these cases, the spatial variation of density may be interpreted as a radial gradient with the density decreasing from the ce ntre to the edges. M 20 shows a systematic non-radial variation of electron density with maximum values occurring at its prominent dark lanes. A mean filling factor of the order of phi = 0.1 was found compatible with the data . Based on their density profiles, NGC 2579, Gum 62, Gum 64a and possibly N GC 3503 were indicated as candidates of showing a 'champagne flow'.