( ( If ( ( the ( intergalactic ( gas density ) ) ) ( is ( very low ) ) ) ) ( , ( ( the shell ) ( ( will ( have ( ( ( a ( larger ( on-average radius ) ) ) and ) ( a ( ( more conical ) form ) ) ) ) ) . ) ) ) )
(ROOT (S (SBAR (IN If) (S (NP (DT the) (JJ intergalactic) (NN gas) (NN density)) (VP (VBZ is) (ADJP (RB very) (JJ low))))) (, ,) (NP (DT the) (NN shell)) (VP (MD will) (VP (VB have) (NP (NP (DT a) (JJR larger) (JJ on-average) (NNS radius)) (CC and) (NP (DT a) (ADJP (RBR more) (JJ conical)) (NN form))))) (. .)))
If the intergalactic gas density is very low, the shell will have a larger on-average radius and a more conical form.
(ROOT (S (NP (NNS Gases)) (VP (VBP have) (NP (DT the) (JJS lowest) (NN density))) (. .)))