The small– and intermediate–scale structure and the fraction of the ISM ionized by non–radiative processes is investigated in a small sample of four local starburst galaxies , imaged with the Hubble Space Telescope Wide Field and Planetary Camera 2 .
The sample comprises three dwarf galaxies , NGC3077 , NGC4214 , and NGC5253 , and one giant spiral , NGC5236 ( M83 ) .
The galaxies span a range in metallicity ( \sim 0.2–2 Z _ { \odot } ) , luminosity ( M _ { B } \sim - 17 – - 20 ) , and environment ( isolated , interacting ) , enabling the investigation of non–radiative ionization processes in a variety of galactic conditions .
For this purpose , the four galaxies were imaged in the lines of H \beta ( 4861 Å ) , [ OIII ] ( 5007 Å ) , H \alpha ( 6563 Å ) , and [ SII ] ( 6717,6731 Å ) .
This is a unique set of data , as very few galaxies ( and only our four starbursts ) have ever been imaged by HST in the relatively faint lines of H \beta and [ SII ] .
The use of the HST has allowed us to trace non–photoionized gas in these galaxies on scales ranging from a few tens of pc to a few hundred pc , and thus provide a full budget for this ionized gas component .
Using the ‘ maximum starburst line ’ of ( ) to discriminate between photoionized and non–photoionized gas , we find that in all four galaxies non–photoionization processes are responsible for a small fraction of the total H \alpha emission , at the level of 3 % –4 % .
Because the non–photoionized gas is associated with low H \alpha 
surface brightness , it occupies between 1/6 and 1/4 of the total imaged area .
The central starbursts yield enough mechanical energy to produce the non–photoioned gas in the four galaxies , via shocks from massive star winds and supernova explosions .
In particular , the starbursts in the three dwarf galaxies deposit a significant fraction , 70 % –100 % , of their mechanical energy into the surrounding interstellar medium ( ISM ) , in order to account for the observed luminosity of the non–photoionized gas .
The morphology of the non–photoionized regions is different in the dwarfs and the giant spiral .
As already established in previous works , non–photoionized gas in dwarfs is mainly associated with extended ‘ shells ’ or filamentary regions , likely areas of supernova–driven expanding gas .
In all three dwarfs star formation has been an ongoing process for the last few \times 10 ^ { 7 } yr to \sim 10 ^ { 8 } yr ; time–extended star formation episodes are a requirement to sustain the observed luminosity of the non–photoionized gas .
In the massive spiral , the non–photoionized gas is concentrated in localized areas surrounded by active star–formation , with no evidence for extended structures on the same ( or smaller ) spatial scales as ( than ) the ‘ shells ’ in the dwarfs .
The two H \alpha cavities in NGC5236 may be evolved regions within the starburst .
This confirms the picture that starbursts remain confined events in massive galaxies , likely due to the deep potential well .