Nonthermal Pressures: Key to Energy Balance and Structure Formation Near Sgr A* in the Milky Way

TL;DR

This study investigates the energy balance and structure formation near Sgr A* by analyzing thermal and nonthermal processes, revealing the dominance of nonthermal pressure and magnetic stabilization of gas clouds in the Galactic Center.

Contribution

It provides detailed measurements of thermal and nonthermal pressures, magnetic fields, and gas dynamics near Sgr A*, highlighting the role of nonthermal pressures in energy balance and structure formation.

Findings

Nonthermal pressure from turbulence nearly balances magnetic and cosmic ray pressures.

Magnetic fields can stabilize gas clouds against gravitational collapse.

The medium is a low-beta, supersonic plasma with a high Alfvén Mach number.

Abstract

The circumnuclear region of the Galactic Center offers a unique laboratory to study energy balance and structure formation around Sgr A$\star$. This work investigates thermal and nonthermal processes within 7 pc distance from Sgr A$\star$. Using MeerKAT 1.3 GHz radio continuum data and ALMA H40 radio recombination line emission from the ACES survey, we separate free-free and synchrotron components at $\sim$0.2 pc resolution. With a thermal fraction of $\simeq$13%, the 1.3 GHz emission shows tight correlations with the Herschel PACS infrared data. The correlation between the equipartition magnetic field and molecular gas traced by JCMT $^{12}$CO (J=3$\rightarrow$2) observations reveals a balance between the magnetic field, cosmic rays, and molecular gas pressures south of the circumnuclear disk on $\sim$0.7 pc scales. Unlike the magnetic field and ionized gas, the molecular gas density declines in the cavity (R$\leq$2 pc) toward the center, likely due to feedback from Sgr A$\star$. We find that nonthermal pressure from turbulent gas nearly balances magnetic and cosmic ray pressures and exceeds thermal pressure by two orders of magnitude. The medium surrounding Sgr A$\star$ is filled by a low-$\beta$ (thermal-to-magnetic energy), supersonic plasma, with an Alfv\'en Mach number $\simeq$ 4 (assuming equipartition). Analysis of the mass-to-magnetic flux ratio suggests that the circumnuclear region is mostly subcritical and, therefore, the magnetic field can help stabilize gas clouds against gravitational collapse.