2 mm GISMO Observations of the Galactic Center. II. A Nonthermal Filament in the Radio Arc and Compact Sources

TL;DR

This study uses 2 mm GISMO observations to analyze the Galactic Center, detecting a nonthermal filament in the Radio Arc at the shortest wavelength, revealing new details about emission mechanisms and source characteristics.

Contribution

First detection of a nonthermal filament at 2 mm wavelength in the Galactic Center, providing new insights into its spectral properties and relation to surrounding regions.

Findings

The nonthermal filament is detected at 2 mm, the shortest wavelength for such detection.

The spectral index steepens from -0.2 to -0.7 with distance from the Sickle H II region.

Several unresolved sources are identified, including potential H II regions and a molecular cloud.

Abstract

We have used the Goddard IRAM 2-Millimeter Observer (GISMO) with the 30 m IRAM telescope to carry out a 2 mm survey of the Galaxy's central molecular zone (CMZ). These observations detect thermal emission from cold ISM dust, thermal free-free emission from ionized gas, and nonthermal synchrotron emission from relatively flat-spectrum sources. Archival data sets spanning $3.6 \mu$m to 90 cm are used to distinguish different emission mechanisms. After the thermal emission of dust is modeled and subtracted, the remaining 2 mm emission is dominated by free-free emission, with the exception of the brightest nonthermal filament (NTF) that runs though the middle of the bundle of filaments known as the Radio Arc. This is the shortest wavelength at which any NTF has been detected. The GISMO observations clearly trace this NTF over a length of ~0.2$^\circ$, with a mean 2 mm spectral index which is steeper than at longer wavelengths. The 2 mm to 6 cm (or 20 cm) spectral index steepens from $\alpha \approx -0.2$ to $-0.7$ as a function distance from the Sickle H II region, suggesting that this region is directly related to the NTF. A number of unresolved (at $21''$) 2 mm sources are found nearby. One appears to be thermal dust emission from a molecular cloud that is associated with an enigmatic radio point source whose connection to the Radio Arc is still debated. The morphology and colors at shorter IR wavelengths indicate other 2 mm unresolved sources are likely to be compact H II regions.