Gas accretion onto galaxies
There is much debate on the importance of gas accretion in disc galaxies to feed the star formation activity in nearby galaxies. Several pieces of evidence show that this should be the case, one for instance is the observation that the gas available in discs today is capable of sustaining star formation at the current rate only for 1-2 Gyr. This paradox is referred to as the gas consumption problem. For this reason astronomers look since decades for signs of gas accretion from the environment surrounding galaxies.

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A possibility often considered is that gas accretion comes from gas-rich satellite galaxies that, after an interaction with a larger galaxy, either merge with the latter of lose their gas to be eventually collected by the main galaxy. These phenomena are certainly at place, as we can see from the Figure above that shows the HI distribution and kinematics of a couple of galaxies undergoing a minor merger. However it is not clear whether the contribution of satellites to gas accretion can be important enough to solve the gas consumption dilemma. Our recent estimate shows that this contribution is, in fact, negligible in the Local Universe (Di Teodoro & Fraternali 2014).