Infall of gas in galaxies and triggered star formation

While the importance of merging, accretion, and infall processesin determining galactic evolution is well established boththeoretically and observationally, details on how such processesare taking place nowadays even in our own Galaxy are stillrelatively poorly known, especially due to large remaininguncertainties on the location and origin of high velocity clouds.In this paper we focus on the possible role that galacticoutflows and gas infall may have on directly triggering starformation in the halo and in galactic disks. While compellingevidence has been accumulating in recent years suggesting thatsome level of star formation directly triggered by outflows isvery likely to exist in the halo of some galaxies, the evidencefor star formation dynamically triggered by infall is far moreelusive due to confusion with other, more efficient large-scalestar forming mechanisms operating in the galactic disk. Despite ofincreasingly realistic simulations of the gas circulation betwenthe gas and the halo and of high velocity cloud impacts ongalactic disks, the efficiency of star formation directlytriggered by such impacts remains an open question.