What can be learned from the shape of galaxies about the complex connections between different materials in the universe
Our cosmic address is quite complex, we are on a planet belonging to the solar system, which itself surrounds the center of the Milky Way galaxy, which is part of a cluster of galaxies (Virgo cluster) and so on. The one who is in the background and provides the framework for all these structures is the dark matter which many believe is found in the halo surrounding each of the galaxies.
Dr. Jonathan Stern, from the School of Physics and Astronomy at Tel Aviv University, studies the complex relationships between different materials in the universe, and the interactions between galaxies and their cosmic environment, while trying to better understand the physical processes that cause galaxies to develop and change their shape over time.
"Galaxies are not isolated entities," he says. "They make contact with their environment, absorb matter from the intergalactic medium and also emit matter (galactic winds). All galaxies are surrounded by a halo of dark matter: a region that surrounds a galaxy and contains a large amount of dark matter, a substance that does not emit light but affects the galaxy through gravity. This halo affects galactic dynamics and the interaction between galaxies and their environment. This halo has an effect on galactic dynamics. For example, if the diameter of the Milky Way galaxy is about 100 light years, the halo of dark matter extends by about 2 million light years (20 times). The gas that is inside the dark matter halo is called the 'galactic surrounding medium', and it forms a sort of spherical 'pool' of material within which the galaxy grows and develops.
The gas found in the dark matter halo around galaxies is a key component in understanding the structure and evolution of galaxies. Its influence is evident at all levels of the galactic structure, from the distant galactic environment to the black hole at its center. It affects the dynamics of stars and gas, and the processes that take place inside the galaxy. The ongoing study of the intergalactic medium and the deeper understanding of its properties and effects may provide important insights into the universe and its cosmological processes.
A process in which gas from the intergalactic medium is drawn into the galaxy is called "galactic absorption", and it is what enables the creation of new stars in the galaxy. This absorption can occur continuously or in the form of sporadic and concentrated events, depending on the galactic conditions and the environment of the galaxy. Various lines of evidence show that galaxy growth by accretion is more dominant than growth by galaxy mergers, at least in the last 10 billion years.
The way in which gas is attached to the galaxy, and hence the rate of star formation, depends on the cooling and heating processes in the surrounding galactic medium: physical processes that affect the temperature of the gas around the galaxy. Cooling occurs when a gas emits radiation and therefore loses energy and cools. Heating can also occur as a result of the compression of the gas during the absorption process, and also as a result of stellar activity or supernova events, which add energy to the surrounding galactic medium.
The temperature of the surrounding galactic medium, which results from the various heating and cooling processes - determines its critical property, which is a term familiar to all of us - the speed of sound. The speed of sound is the speed at which sound waves travel through a certain material. According to Dr. Stern, if the speed of sound in the galactic medium is low, then the absorption process will be "supersonic" and can lead to the creation of shock waves and disturbances in the gas, which will cause the absorption process to be uneven. On the other hand, if the speed of sound is high, then the movement will be "subsonic", the gas will flow more continuously, and the adsorption rate will be uniform.
Dr. Stern is investigating, with the help of a grant from the National Science Foundation, the possibility that subsonic adsorption creates galaxies in the shape of a thin disk, like the Milky Way, while supersonic adsorption creates "messy" galaxies, that is, without a clear geometric shape, a common form in small galaxies.
Dr. Stern: "The reason why many galaxies, especially disk galaxies like the Milky Way, appear flat and not spherical, even though dark matter and 'normal' matter are present in all directions, is related to the process of galaxy formation and its dynamics over time. In the initial phase of halo formation, matter (gas and dark matter) begins to clump together due to gravitational forces. The normal matter then loses energy to radiation, and therefore flows into the center of the halo. During this adsorption process, the gas retains its angular momentum, causing it to spin faster and faster as it moves closer to the center. At some point the rotation speed and centrifugal forces are so strong that the gas turns into a disk. In subsonic flow, only after the prolapse to the disc is complete, stars begin to form, so they are arranged in a thin disc. On the other hand, in a supersonic flow, stars are formed even before prostrate, so their geometric shape is much less ordered."
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