Published online by Cambridge University Press: 05 September 2013
Abstract
Interstellar medium-related secular evolution of galaxies occurs through continuing infall of gas and removal of gas. Infall can happen through accretion of gas from the intergalactic medium, through interactions and mergers.
Removal of gas can happen through galactic winds, interactions and ram-pressure stripping. In these lectures I will review observational evidence that galaxies accrete and lose gas, quantify the effects and discuss some of the physics involved.
Introductory remarks
When preparing these lectures for a school on secular galaxy evolution I wondered what role the interstellar medium (ISM) plays in secular evolution. On the website of John Kormendy I found a slide which nicely summarises what secular evolution is. The slide, which is shown in John Kormendy's chapter (this volume) is a slightly updated version from the one that appeared in the Annual Review article on secular evolution by Kormendy & Kennicutt (2004). It shows the different processes that drive galaxy evolution. On top are the fast processes, protogalactic collapse and mergers, at the bottom are the slow processes. The slow processes can be subdivided in internal processes, evolution driven by bars, oval distortions and spiral structure and external processes, where evolution is driven by prolonged gas infall, minor mergers, ram-pressure stripping and galaxy harassment. Any slow process, including environmental effects, can be considered part of secular evolution. Internal secular evolution affects the ISM through rearrangement of the gas, most importantly bar-induced inflow.
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