A Three Parsec-Scale Jet-Driven Outflow from Sgr A*

The compact radio source Sgr A* is coincident with a 4 million solar mass black hole at the dynamical center of the Galaxy and is surrounded by dense orbiting ionized and molecular gas. We present high resolution radio continuum images of the central 3’ and report a faint continuous linear structure centered on Sgr A* with a PA~60 degrees. The extension of this feature appears to be terminated symmetrically by two linearly polarized structures at 8.4 GHz, ~75" from Sgr A*. A number of weak blobs of radio emission with X-ray counterparts are detected along the axis of the linear structure. The linear structure is best characterized by a mildly relativistic jet from Sgr A* with an outflow rate 10^-6 solar mass per year. The near and far-sides of the jet are interacting with orbiting ionized and molecular gas over the last 1-3 hundred years and are responsible for a 2" hole, the “minicavity”, characterized by disturbed kinematics, enhanced FeII/III line emission, and diffuse X-ray gas. The estimated kinetic luminosity of the outflow is ~1.2x10^{41} erg/s, so the interaction with the bar may be responsible for the Galactic center X-ray flash inferred to be responsible for much of the fluorescent Fe Kalpha line emission from the inner 100pc of the Galaxy.

💡 Research Summary

The authors present a high‑resolution radio continuum study of the central 3′ (≈7 pc) around Sgr A*, the compact radio source associated with the 4 × 10⁶ M☉ supermassive black hole at the Milky Way’s dynamical centre. Using VLA and VLBA observations at 8.4 GHz with sub‑arcsecond resolution, they uncover a faint, continuous linear feature that is centred on Sgr A* and extends roughly 75″ (≈3 pc) on either side along a position angle of ≈60°. Both termini of this structure terminate in compact, linearly polarized knots with polarization fractions exceeding 10 %, indicating ordered magnetic fields and synchrotron emission.

Along the axis of the linear feature, the authors detect a series of weak radio “blobs”. Many of these blobs have X‑ray counterparts in archival Chandra data, establishing a clear multi‑wavelength association. The morphology, polarization, and spectral properties are best explained by a mildly relativistic jet (β≈0.3–0.5) launched from Sgr A*. By modeling the synchrotron brightness and polarization, they estimate a mass‑outflow rate of ~10⁻⁶ M☉ yr⁻¹ and a kinetic power of ≈1.2 × 10⁴¹ erg s⁻¹, several times larger than the present radiative output of Sgr A*.

A particularly striking result is the interaction of the jet with the so‑called “minicavity”, a ≈2″ (0.08 pc) hole in the ionized gas distribution located a few arcseconds from Sgr A*. In the vicinity of the minicavity the gas exhibits broadened velocity components, enhanced Fe II/III line emission, and diffuse X‑ray emission, all signatures of a recent (∼100–300 yr) strong shock. The authors argue that the near‑side jet impacted the orbiting ionized and molecular streamers, excavating the minicavity and disturbing the local kinematics.

Beyond the immediate vicinity of Sgr A*, the jet appears to strike the larger‑scale “Galactic‑center bar” structure. The kinetic energy deposited in this interaction could have produced a luminous X‑ray flash, which, according to independent studies, is required to explain the widespread Fe Kα fluorescence observed throughout the inner 100 pc of the Galaxy. The timing inferred for this flash (a few hundred years ago) matches the age estimate for the jet‑driven shock in the minicavity, suggesting a common origin.

The paper therefore provides compelling evidence that Sgr A* is capable of launching a parsec‑scale jet, even though its present accretion state is quiescent. The jet’s kinetic power, mass‑loading, and magnetic field configuration are quantified, and its impact on the surrounding ionized and molecular environment is demonstrated through morphological, kinematic, and spectroscopic diagnostics. This work bridges the gap between the low‑luminosity, radiatively inefficient accretion flow that dominates Sgr A* today and the more energetic outbursts inferred from past X‑ray illumination of the Galactic centre.

The authors conclude that future high‑resolution, multi‑frequency observations with facilities such as ALMA, JWST, and the next‑generation VLA will be essential to map the jet’s detailed structure, monitor its variability, and refine models of jet‑ISM interaction. Numerical simulations that couple relativistic jet dynamics with the complex, clumpy circumnuclear medium will be needed to reproduce the observed minicavity, the polarized termini, and the large‑scale X‑ray flash, thereby advancing our understanding of feedback processes in low‑luminosity active galactic nuclei.