Magnetism is a fundamental force with special properties.
• Most baryonic matter is ionized. Magnetic fields are easy to generate.
• Magnetic monopoles do not exist or are extremely rare. Magnetic fields are hard to destroy.
• Magnetic fields need illumination. Magnetic fields are difficult to observe.
The scarceness of data leaves many questions on cosmic magnetic fields.
• When and how were the first fields generated?
• Did significant fields exist before galaxies formed?
• How and how fast were the fields amplified?
• How did fields affect the evolution of stars, planets, galaxies and galaxy clusters?
• How strongly is intergalactic space magnetized?
Magnetic fields are often ignored in astrophysics, in particular in models of galaxies and the interstellar medium (ISM), although they are a major agent in the ISM and in galaxy halos, and important for the structure and evolution of galaxies.
• Magnetic fields contribute significantly to the total pressure which balances the gas disk of galaxies against gravitation (Fletcher & Shukurov, 2001).
• Magnetic turbulence distributes energy from supernova explosions within the ISM (Subramanian, 1998).
• Magnetic reconnection is a possible heating source for the ISM and halo gas (Birk et al., 1998).
• Magnetic fields increase angular momentum transport and hence the gas inflow rate in barred galaxies (Beck et al., 2005; Kim & Stone, 2012).
• Magnetic fields affect the dynamics of the turbulent ISM (de Avillez & Breitschwerdt, 2005) and the gas flows in spiral arms (Gómez & Cox, 2002).
• Magnetic fields make the gaseous spiral arms more patchy and drive gas outflows into the halo (Pakmor & Springel, 2013).
• The shock strength in spiral density waves is decreased and structure formation is reduced in the presence of strong fields (Dobbs & Price, 2008; Fletcher et al., 2011).
• Magnetic fields stabilize gas clouds and reduce the star-formation efficiency to the observed low values (Vázquez-Semadeni et al., 2005; Price & Bate, 2008).
• Magnetic fields are essential for the onset of star formation as they enable the removal of angular momentum from the protostellar cloud via ambipolar diffusion (Heitsch et al., 2004).