Khagol No.133

| 04 | KHAG L | No. 133 - JULY 2024 phenomenon from the vantage points of two observers: first on an airplane and the second on the service road. The first observer reports a region of high traffic density around the procession and sluggishly moving with it. Interestingly, the second observer notes that the jam never consists of the same vehicles for long periods of time; they are promptly replaced by other vehicles coming from behind. The traffic jam is, therefore, an example of a density wave, and so are the galactic spiral arms. Of Galactic Spiral Arms and Traffic Jams According to the Tuning Fork diagram, Hubble's morphological classification of galaxies is that galaxies are of two types: elliptical and disc galaxies. In the ellipticals, the stars are random motion- supported against gravitational collapse; in the discs, they are rotationally- supported. The spiral arms constitute the most spectacular features of disc galaxies and distinguish them from other astronomical objects. Observational studies indicate that the spiral arms rotate as a rigid pattern with a constant angular speed called the pattern speed. Spiral galaxies are ubiquitous, and more than 60 % of disc galaxies in the nearby universe are spirals. Spiral arms and galactic secular evolution: The quadrupole moment inherent in the gravitational potential of non- axisymmetric disc dynamical features, such as the spiral arms, results in torques that induce angular momentum transfer between the disc and the dark matter halo. Spirals also trigger gas inflow from the outer galaxy, which feeds the active galactic nuclei. Furthermore, being the overdense regions in the galactic disc, they favor the compression of gas clouds and therefore are the primary sites of galactic star formation. Therefore, the spiral arms play a fundamental role in the secular evolution of galaxies. The winding dilemma: Until the middle of the last millennium, the mechanism of the formation and sustenance of spiral structures in the galactic discs was an enigma. It was well-known that galaxies rotated between 20 to 100 times since the formation of their host galaxies. This meant that if the spiral arms were material arms, i.e., composed of the same stars and gas clouds, they would have wound up 20 to 100 times during this period. Such arms are called material arms, and the problem of wrapping them is known as the winding dilemma. However, observed galaxies do not have spiral armswrapped upmore than once or twice in the azimuthal direction. This implies that either spiral arms are short-lived or they are notmaterial arms Traffic jams, galactic spiral arms, and density waves: Let us try to understand how galactic spiral arms form from the following scenario: A procession slowly moving down the highway and triggering a traffic jam by creating a bottleneck. On approaching the procession, vehicles first slow down momentarily while passing by the procession, then slowly pick up speed, and finally continue on their way at their normal speed. Now let us study this TheWhirlpool Galaxy (Credit: NASA/ESA and TheHubbleHeritage TeamSTScI/AURA)