Energy estimates for structures around the galaxy NGC 4438 change markedly with scale, according to a new preprint analysis. In the two compact bubbles near the centre, bulk motion accounts for the largest share of the estimated energy. In the much larger outflow, thermal gas and magnetic fields provide comparable support.
The study combines archival Chandra X-ray observations with Hα images and Very Large Array radio continuum data to examine the bubbles and outflow across several spatial scales. The Chandra dataset has a total effective exposure of 124.93 kiloseconds.
A mismatch between the radio and X-ray signals
The central region contains two oppositely directed sub-kiloparsec bubbles within a larger outflow. The northern bubble has a radio-bright cap ahead of its shell, a feature identified by comparing the radio and X-ray views.
To interpret the X-rays, the researchers fitted a model that combines emission from hot plasma with a power-law synchrotron component. Joint radio and X-ray modeling found that one electron population could not account for both signals: extending the radio component to higher energies overpredicted the hard X-ray component by about eight orders of magnitude. The analysis therefore favored separate synchrotron-emitting components, although it did not uniquely identify how either electron population was accelerated.
The two signals also show contrasting patterns within the bubbles. The radio spectrum is flattest at the shell, while the X-ray power-law is harder in the cavity. Per unit area, the X-ray power-law normalization in the cavity is three to four times that of the shell and cap. The authors associate the radio-emitting GeV electrons mainly with shocks at expanding shells and say the hard X-ray-emitting TeV electrons require a separate local acceleration process.
Motion dominates the compact bubbles
For the northwest bubble, the reported lower limit on total energy is about 3.1 × 10^54 ergs, with a characteristic power of about 3.9 × 10^41 ergs per second. Bulk kinetic energy remains the dominant part of that estimate, alongside thermal, radiative, magnetic and cosmic-ray contributions.
The southeast bubble appears even more dominated by kinetic energy under the study’s assumptions. Its estimated bulk kinetic energy is about 1.0 × 10^55 ergs, while the lower limit on total energy exceeds about 1.05 × 10^55 ergs and the lower limit on power exceeds about 1.33 × 10^42 ergs per second. A complete radiative budget was not feasible, so the total is a lower limit.
The resolved budgets were built from measured shell, cavity and cap areas combined with simple geometric assumptions about volume. The resulting absolute energy and power values are sensitive to the assumed geometry, shock speed and dynamical time.
Two shortcuts give very different answers
The study finds that a familiar cavity estimate known as 4pV gives lower powers for these compact bubbles than the spatially resolved budgets. The difference is about a factor of two for the northwest bubble and about a factor of five for the southeast bubble.
Applying an empirical radio-jet relation to the nuclear radio luminosity produced the opposite result. It implied a jet power of about 5 × 10^44 ergs per second, more than three orders of magnitude above the resolved estimates. The comparison indicates that a relation calibrated for larger structures may not transfer directly to young, sub-kiloparsec bubbles.
The larger structure points to a longer energy history
At galaxy scale, the outflow’s internal pressure was estimated at about 0.0068 keV per cubic centimetre, compared with about 0.0043 keV per cubic centimetre in the surrounding medium. That gives an internal-to-ambient pressure ratio of about 1.58, with thermal and magnetic pressure contributing comparable support.
An analytic bubble-evolution estimate put the energy injected into the large outflow at about 3.5 × 10^55 ergs. It gave the structure a current age of about 5.9 million years and a mean required power of about 1.9 × 10^41 ergs per second. These are conditional estimates because they depend on an approximate evolution model and an extrapolation of pressure.
The authors regard an active galactic nucleus, or AGN, with activity occurring for about half the time over several million years as a plausible explanation for the required input. They consider nuclear star formation alone unlikely to account for the energetics. The analysis describes one galaxy and supports spatial associations and conditional energy estimates, rather than establishing a causal rule for AGN outflows generally.
The manuscript is a preprint identified as arXiv:2608.25552v1 and dated 26 August 2026. The supplied record does not report a journal or DOI. Its acknowledgements report support from Chinese national, space-program, science-ministry and Jiangsu talent-team funding programmes.
Paper data and sources
Original title: Energy Partition in AGN-driven Bubbles of NGC 4438: From Nuclear Bubbles to a Galaxy-scale Outflow
Authors: Luan Luan, Jiang-Tao Li, Jianghui Xu et al.
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-26
DOI: Not available
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