Footprints of gas accretion in low redshift massive galaxies

Galaxies need to accrete gas to sustain their star formation.Even the most extreme HI-rich galaxies exhibit regular morphologies, suggesting that most of the gas accretion is happening in a slow and gentle way.The correlation between the color gradients and gas fraction in star-forming disk galaxies supports the picture of cosmological gas accretion (as a necessary supplement to fountain mechanism). The brightest cluster galaxies (BCG) are as old as galaxies of the same stellar masses, suggesting that the location of centre of dark matter halo does not guarantee gas accretion. We find an HI-rich environment around HI-rich galaxies. This result holds for both the HI in surrounding satellites and the HI in low-mass systems which can not be directly detected but cumulates to significant signals. It supports a picture in which the HI-rich central galaxies accrete gas from an extended gas reservoir present in their environment.