An immense 65,000-light-year radio bubble surrounds this nearby galaxy

Located approximately 61.6 million light-years away, NGC 4217 is a well-known spiral galaxy among astronomers. However, a new observation campaign has uncovered a gigantic structure never seen before in its halo. A radio bubble, of exceptional size, extends more than 65,000 light-years from the galactic disk.


These new observations were made possible thanks to the Jansky Very Large Array (JVLA) and the LOFAR network, cutting-edge radio telescopes. By combining data at different frequencies, the team led by Volker Heesen, from the University of Hamburg, was able to map this bubble with unprecedented precision. The researchers discovered that the radio emission was particularly intense along the walls of the bubble, while its center showed a slight depression.

This "bright edge" structure is quite unusual. The northeastern part of the bubble stands out, with clues suggesting the presence of a shell on that side. The bubble is also impressive in height: 19,200 light-years at 144 MHz and 9,400 light-years at 3 GHz, measurements far beyond the typical sizes of radio bubbles in other galaxies.

One of the most surprising discoveries concerns the speed of the galactic wind around the bubble. It increases from 186 to 373 miles per second (300 to 600 km/s), nearly reaching NGC 4217's escape velocity. This galactic wind could therefore play a key role in the formation and evolution of this structure.

Scientists estimate that supernovae at the core of the galaxy are responsible for this bubble, releasing enough energy to inflate it over a span of 35,000 years. However, a significant portion of this energy would be dissipated in the form of radiation, thus limiting the bubble's expansion.

With a magnetic field intensity measured at 11 microgauss, this bubble appears to have very different properties compared to other identified radio bubbles. Its size and strength place it among the most imposing phenomena observed to date in the field of galactic halos.

This discovery offers new insight into the dynamics of spiral galaxies. It helps to better understand the interactions between stellar explosions, galactic winds, and magnetic fields that shape these gigantic structures.

What is a radio bubble in a galaxy?

A radio bubble is a bubble-shaped structure detected in the halos of some galaxies, emitting radio waves. It is often created by explosions of massive stars, such as supernovae.

It forms when large amounts of energy are released into the surrounding medium. This energy, in the form of a galactic wind, pushes matter outward, creating a cavity surrounded by a bright shell of ionized gas.

The bubble can extend tens of thousands of light-years. Its size and properties depend on the amount of energy released and interactions with galactic magnetic fields.