Internal variation of electron density in galactic HII regions

A study on the internal variation of the electron density in galactic H II regions has been conducted on a sample of 15 objects of different sizes and evolutionary stages. The [SII] λ 6716/λ6731 line ratio was adopted as electron density indicator. Long slit spectrophotometry of high signal-to-noise ratio with spectral dispersion of 0.75 Å pxl^-1 and spatial scale of (0″″. 90 pxl^-1 were obtained at different slit positions and orientations. No systematic spatial variation of electron density was detected 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 ≈ 20-140 cm^-3. The remaining objects (S 288, S 307, NGC 2579, NGC 3503, Gum 62, Gum 64a and M 20) with mean densities N_e ≈ 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 centre 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 (φ = 0.1 was found compatible with the data. Based on their density profiles, NGC 2579, Gum 62, Gum 64a and possibly NGC 3503 were indicated as candidates of showing a 'champagne flow'.