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

TL;DR

The paper investigates energy balance and structure formation in the Galactic Center within 7 pc of Sgr A* by separating thermal free-free and nonthermal synchrotron emission using H40α ALMA data and MeerKAT 1.3 GHz radio observations. It maps thermal and nonthermal components at ~0.2 pc resolution, correlates radio with infrared tracers, and derives an equipartition magnetic field from the nonthermal emission. The results show nonthermal and turbulent pressures dominating over thermal pressure, a low-$\beta$ plasma with $M_A \sim 4$, and a mostly subcritical mass-to-magnetic flux ratio, implying magnetic fields help stabilize gas clouds against collapse while SMBH feedback reshapes the inner ISM. These findings highlight the important role of magnetic fields and nonthermal processes in the energy balance and structure formation near Sgr A*, with implications for star formation in extreme Galactic-center environments and guidance for future high-resolution observations.

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-$β$ (thermal-to-magnetic energy), supersonic plasma, with an Alfvén 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.

Figures (11)

• Figure 1: The circumnuclear region (R$<$7 pc) mapped by different observations: Left, from top to bottom- MeerKAT RC emission at 1.3 GHz, ALMA H40$\alpha$ RRL integrated intensity at 99.0 GHz, and Herschel FIR emissions at 70 $\mu$m. Right, from top to bottom- JCMT $^{12}$CO (J=3$\rightarrow$2) integrated intensity at 345.8 GHz, MSX MIR emission at 21.3 $\mu$m, and Herschel FIR emissions at 160 $\mu$m. Circles in the lower right corners indicate the resolutions or beam sizes (see Table \ref{['table:1']}). Stars indicate the position of Sgr A$^{\star}$. The northern and southern parts of this region are indicated in the RC map. The names of the components within the mini-spiral structure are labeled on the RRL map.
• Figure 2: Maps of the thermal free-free emission (top), nonthermal synchrotron emission (middle), and thermal fraction (bottom) at 1.3 GHz in the circumnuclear region. The beam size of $4\hbox{$^{\prime\prime}$} \times 4\hbox{$^{\prime\prime}$}$ is shown in the lower right corner of the first map. Contours in all maps show the thermal intensity at levels of 0.05, 0.1 Jy beam$^{-1}$. Stars indicate the position of Sgr A$^{\star}$.
• Figure 3: Radial profiles of the RC emission (mean intensities in rings of 4$^{\prime\prime}$ width vs radial distance to Sgr A$^{\star}$) and its thermal and nonthermal components at 1.3 GHz. Also shown is the radial profile of the thermal fraction ( magenta). Error bars represent the standard deviation divided by the square root of the sample size per ring.
• Figure 4: Relationships between the nonthermal ($\rm S^{nt}$, top) and thermal ($\rm S^{th}$, bottom) intensities at 1.3 GHz and the 160, 70, and 21 $\mu$m IR emissions in the circumnuclear region. Solid black lines indicate the OLS fits, and dotted lines represent the 1:1 line. Orange and blue data points represent the northern and southern parts. Pink and purple lines show the OLS fits to the north and south, respectively. The green arrow referred to the vertical distribution.
• Figure 5: Equipartition magnetic field strength mapped in the circumnuclear region (R$<7$ pc) with contours of the 160 $\mu$m emission tracing a relatively cool ISM. Colorbar represents the strength in units of $\mu$G. The beam size of $18\hbox{$^{\prime\prime}$} \times 18\hbox{$^{\prime\prime}$}$ is indicated in the lower right corner, and the star represents the position of Sgr A$^{\star}$. Contour levels are 40, 46, and 52 Jy pixel$^{-1}$, respectively.