Khagol No.133

| 05 | KHAG L | No. 133 - JULY 2024 Just as a fixed set of vehicles does not constitute a traffic jam, the spiral arms are not associated with a specific group of stars or gas clouds. Akin to the traffic jam analog, disc stars and gas clouds enter the spiral arm and slow down for a while by falling into the potential well resulting from the enhanced disc density therein; they eventually climb out of the well and then continue on their orbits around the galactic center. Consequently, we have a moving region of high stellar and gas density involving different parts of the disk at points in time. In both the case of the traffic jam and the galaxy, the wave moves slowly and independently of the overall flow of the medium. Further, the traffic jam can persist long after the procession has left the road; similarly, the spiral structure can persist long after the perturbing source has ceased to exist. Density waves & normal modes of oscillation The essence of the density wave theory is to determine the functional form of the perturbed surface density, which allows the perturbation to be a normal mode of oscillation of the system. For such a mode, the disturbance in the gravitational field assoc i a t ed wi t h t he asymme t r i c distribution of matter equals the perturbation field required to invoke the asymmetric response. Energy exchange between wave and matter Due to the differential rotation of the disk, there occurs an exchange of angular momentum and energy between the disc and the spiral wave. It can be shown that this exchange primarily happens at the so- called Lindblad Resonances, where the frequency at which the star intersects the crests and troughs of the spiral wave potential is either zero (i.e., the star is always in phase with the potential) or equals the oscillation frequency of the star near the circular orbit. In fact, at the Outer Lindblad Resonance (OLR), energy is absorbed, and the wave gets reflected. On the other hand, at the Inner Lindblad Resonance (ILR), energy is emitted, and the wave gets damped. Stationary or Standing Waves in the radial direction If a spiral wave is absorbed at the ILR, then the spiral structure will not be long-lived. However, if the wave gets reflected before it reaches the ILR, a wave pattern quasí- stationary in the radial direction can be established. Some physical features of the disk inhibit the wave from reaching the ILR (the Q-barrier) and thus favor the formation of standing wave patterns, also calledwavemodes. Why are spiral structures mostly two- armed? Higher-order resonances are dynamically less significant, also because they either lie outside the galaxies or very close to the galactic center. For a 3-armed spiral, for example, the resonances are near the co- rotation radius. Interestingly, however, the resonances are roughly separated by the galaxy's radial extent in the case of a 2- armed spiral. These possibly explain the ubiquity of two-armed spirals in the local universe. References: 1. Galaxies&Cosmology: Combes, Boisse, Mazure&Blanchard, Springer 2. Galaxies & Galactic Structure: Elmegreen, PrenticeHall 3. Dynami cs o f Ga l ax i es : Be r t i n , CambridgeUniversity Press 4. Physics of Astrophysics II: Shu, University ScienceBooks 5. Physics of Gravitating Systems II: Fridman & Polyachenko, Springer- Verlag. Dr. Arunima Banerjee is a theoretical astrophysicist specialising in the dynamics of galaxies and the application ofmachine learning in galactic astronomy. She is currently an associate professor and the associate chair at the Department of Physics at the Indian Institute of Science Education and Research Tirupati and a Visiting Associate of IUCAA, Pune. She is also an Editorial Board Member of the Journal of Astronomy & Astrophysics of the Indian Academy of Sciences and the Chairperson of the Working Group for Gender Equity of the Astronomical Society of India. Music, painting, and reading are her favourite pastimes.