Energy Partition in AGN-driven Bubbles of NGC 4438: From Nuclear Bubbles to a Galaxy-scale Outflow

Deep within the centers of many galaxies, supermassive black holes act as cosmic engines. As they consume surrounding gas, they do not just swallow it; they often launch powerful jets of material at nearly the speed of light. These jets push against the gas that fills the space between stars, creating massive bubbles and driving winds that can heat entire galaxies. This process, known as feedback, is crucial because it regulates how galaxies grow and how stars form. However, scientists have long struggled to understand exactly how the energy from these jets is spent. Does it mostly push gas outward as motion? Does it heat the gas until it glows? Or does it get stored in invisible magnetic fields and high-speed particles? To answer this, researchers must look at these bubbles at different sizes, from the tiny, fresh bubbles right next to the black hole to the enormous, ancient structures that stretch across the whole galaxy.

A team of astronomers turned their attention to a disturbed spiral galaxy called NGC 4438, located about 14.4 million light-years away in the Virgo Cluster. This galaxy is a rare natural laboratory because it displays two distinct stages of this jet activity simultaneously. Close to the center, there are two small, compact bubbles, each about 200 parsecs across, which represent the jet's current, early-stage interaction with the dense gas nearby. Further out, stretching roughly 10,000 parsecs from the center, lies a massive, lopsided outflow that shows the long-term result of the jet pushing against the galaxy's environment. By studying both the small, young bubbles and the large, mature outflow in the same system, the researchers could trace how the jet's energy changes as it travels outward. They combined deep X-ray observations from the Chandra Space Telescope with high-resolution radio images from the Very Large Array and optical images of glowing gas to build a complete picture of the energy involved.

The researchers found that the energy in these bubbles is not distributed evenly, and it changes dramatically depending on where you look. In the small, nuclear bubbles near the center, the dominant form of energy is the bulk kinetic energy of the gas itself—the sheer force of the gas being pushed outward at high speed. This indicates that these bubbles are still in a violent, early phase of expansion. However, the team also discovered that the particles creating the light in these bubbles are not all the same. The radio waves, which come from electrons moving at high speeds, and the hard X-rays, which come from even faster electrons, appear to be produced by two different groups of particles. The radio-emitting particles seem to be created at the edge of the bubble where a shock wave is currently pushing through the gas. In contrast, the particles producing the hard X-rays are concentrated in the center of the bubble and likely originate from a different, more intense acceleration process closer to the black hole itself. This suggests that the jet is not just a simple stream of particles but a complex system with multiple ways of speeding up matter.

As the jet energy travels from these small nuclear bubbles out to the giant, galaxy-scale outflow, the balance of energy shifts. The massive outflow is no longer dominated by the raw motion of the gas. Instead, the energy has been converted into heat and magnetic pressure. The researchers found that the magnetic fields in the large outflow are strong enough to help support the structure, acting alongside the hot gas to keep the bubble inflated. This transition shows that as the jet pushes further into the galaxy, it slows down and its energy is redistributed, heating the surrounding gas and strengthening the magnetic fields that thread through the space. The study also revealed that the small nuclear bubbles are expanding into different environments on either side of the galaxy. The northern bubble is pushing into denser gas, causing it to slow down and heat up more quickly, while the southern bubble is moving through thinner gas, allowing it to retain more of its speed and kinetic energy.

A significant part of the study involved testing how well standard methods for measuring jet power work on these different scales. The researchers found that the common techniques used for large, mature bubbles in other galaxies fail when applied to these small, young nuclear bubbles. Methods that rely on measuring the pressure of the gas inside the bubble significantly underestimate the total energy because they miss the huge amount of kinetic energy still present in the fast-moving gas. Conversely, a popular formula that estimates jet power based on radio brightness, which works well for large bubbles, drastically overestimates the power of these small nuclear bubbles. This discrepancy highlights that the physics of how jets lose energy depends heavily on their size and age. The small bubbles are still in a chaotic, shock-dominated phase, while the large outflow has settled into a more stable state where the energy is shared between heat and magnetic fields.

Ultimately, the study confirms that even a relatively quiet active galactic nucleus, like the one in NGC 4438, can drive powerful, multi-phase outflows that reshape their host galaxy. The energy required to create the massive 10,000-parsec outflow is consistent with the jet activity seen in the small nuclear bubbles, provided the jet has been active for several million years with a duty cycle of about 50 percent. This means the jet does not need to be firing constantly at full power; it can turn on and off or vary in intensity, yet still build up the enormous structures observed. The work demonstrates that to truly understand how black holes influence their galaxies, astronomers must look at the entire life cycle of these jets, from the tiny, energetic bubbles near the source to the vast, cooling clouds they leave behind.