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2.2. Molecular Gas

What is seen in the molecular phase is just the contrary: apparently large H2 concentrations pile up at the galaxy nuclei in interacting systems. Up to 50% of the dynamical mass could be under the form of molecular hydrogen in merging systems (Scoville et al. 1991), even when the the CO / H2 conversion ratio is corrected to a minimum (Bryant & Scoville 1999, Solomon et al 1997).

In summary, the observations suggest that the HI gas is dragged outwards, while the H2 gas is driven inwards, to be consumed in star formation. In fact these two tracers (HI and CO) shed light on two aspects of the same gas component. In normal spiral galaxies, there exists a sharp cut-off in the HI line distribution (21cm) (van Gorkom et al 1991, Corbelli & Salpeter 1993); it is interpreted as a ionizing front, due to photoionisation from the extragalactic background. HII is the third gas phase to take into account.

While star formation is enhanced in interacting galaxies, this is not true in groups and clusters. This is mainly due to the stripping of the outer gas acting as reservoirs of fuel for star formation. The HI deficiency in compact groups is in average 3 (Williams & Rood 1987, Williams et al 1991, Huchtmeier 1997, Oosterloo & Iovino 1997), and there is no significant CO deficiency or enhancement (Boselli et al 1996, Leon et al 1998, Verdes-Montenegro et al. 1998). The missing gas is heated by repeated interactions, and joins the coronal hot phase. ROSAT survey of 22 HCG (Ponman et al 1996) have revealed that 75% possess a diffuse hot gas. There appears to be a correlation between X-ray and spiral galaxy fraction (Pildis et al. 1995).