What are head-tail sources? How do they provide evidence that double radio sources include jets of fast-moving particles?

Understanding the structure of radio galaxies is crucial for astronomers. These galactic structures emit strong radio waves and often have distinct components such as lobes, jets, and an active nucleus.

In regards to head-tail radio sources, we focus on a distinct class of radio galaxies that feature an asymmetrical shape. The 'head' refers to the brighter region, where an active galactic nucleus (AGN) can be found, while the 'tail' is a trail of radio emission that can extend for vast distances, showcasing a peculiar flow structure. This unique morphology provides insights into how galaxies interact with their surroundings in clusters, and how these interactions can influence their appearances and behaviors. Patterns such as these are not only visually captivating but also rich with data about the galaxies' dynamics.

Relativistic jets are one of the most energetic phenomena observed in the universe, consisting of streams of charged particles being ejected at velocities close to the speed of light from the regions close to black holes or neutron stars.

When we turn our gaze to head-tail radio sources, these jets become visible due to their interaction with the surrounding intracluster medium, causing the jet to bend and form the characteristic 'tail' shape seen in these sources. The behavior and shape of these jets, in fact, provide crucial evidence for understanding the powerful forces at play in active galactic nuclei. The physics powering these jets are still under investigation, and studying them contributes significantly to our knowledge of high-energy astrophysical processes.

The Active Galactic Nucleus, or AGN, is the lively central region of a galaxy that emits massive amounts of energy across the electromagnetic spectrum, from radio waves to gamma rays.

At the heart of an AGN, a supermassive black hole pulls in surrounding matter, which spirals inwards, heats up, and radiates energy. In head-tail radio sources, the AGN serves as the 'engine' powering the relativistic jets that produce the longer 'tail' structures. The intense gravitational and magnetic fields in the vicinity of AGNs enable them to propel particles to high velocities, which is an essential factor in shaping their unusual morphology and giving us a snapshot of the processes occurring near the edge of a black hole.

Galaxies, especially those in clusters, do not exist in empty space; they are immersed in an 'ocean' of thinly distributed gas known as the intracluster medium (ICM).

The ICM is primarily composed of hot, ionized gas, which fills the space between the galaxies in clusters. It is this interaction with the ICM that causes the bending of relativistic jets emitted from the AGN, leading to the formation of the tail seen in head-tail radio sources. The properties of the ICM, such as its density and magnetic field, are key factors in determining how the jets will evolve and appear to us, making the ICM an important element of study in understanding cluster dynamics and galaxy evolution.

Synchrotron radiation is a form of electromagnetic radiation that's emitted when charged particles, like electrons, are accelerated in a curved path or spiral around magnetic fields.

This process is responsible for the radio emissions we detect from head-tail radio sources. When particles traveling in relativistic jets encounter the ICM's magnetic fields, they emit synchrotron radiation, producing the observable radio waves. Key features like the brightness, polarization, and spectrum of the synchrotron radiation give astronomers clues about the speed and composition of the jets, as well as the strength and structure of the magnetic fields within the intracluster medium. This radiation thus provides a means to study the relativistic phenomena in the universe without directly observing the high-speed particles.