Intergalactic hydrogen gas
The top image shows the interacting galaxy pair NGC6221/15 (Koribalski & Dickey 2004, MNRAS 348, 1255). The distribution of neutral hydrogen gas (within a narrow velocity range) as obtained with the ATCA is shown in blue, overlaid onto optical images. Notice the extended HI envelopes of the two spirals compared to their stellar distributions and the magnificent HI bridge between them. The low-surface brightness galaxy named Dwarf 3 (magnified in the inset) may have formed out of the tidal debris. Another website gives more information.
The middle image shows the peculiar galaxy IC 2554 (Koribalski, Gordon, Jones 2003, MNRAS). The overall distribution of neutral hydrogen gas as obtained with the ATCA is shown in blue, overlaid onto optical and infrared images. Notice the extended one-sided HI plume emerging from the eastern side of IC 2554 in the direction of the massive elliptical galaxy NGC3136B. While it is likely that the plume was created through tidal interactions between IC 2554 and NGC 3136B, another possibility is that IC2554 resulted from a merger. Another website gives more information.
The bottom image shows the peculiar galaxy NGC3263 (English, Koribalski, Freeman 2003, IAU Symposium 217). The overall distribution of neutral hydrogen gas is shown in red, overlaid onto optical images. The enormous HI cloud to the west of NGC3263/2 extends about 200 kpc in the N-S direction. It is a striking example of neutral intergalactic gas without stars, apart from one clump which is associated with a dwarf galaxy. The galaxy NGC3263 lies within the prominent NGC3256 galaxy group.
ATCA 21cm HI image by B. Koribalski, J. Dickey, S. Gordon, K. Jones, J. English, F. Freeman. Optical images from 2nd-generation DSS. Infrared image from 2MASS.