Radio studies of the Galactic Centre - II. The arc, threads and related features at 90 cm (330 MHz)

Abstract

We present 90-cm VLA∗ observations of a ∼ 2° × 2° field around the Galactic Centre. The full field was mapped with a resolution of 100 × 64 arcsec2, and smaller regions with resolutions as high as 14 × 9 arcsec2. The resulting images contain most of the unique features associated with the Galactic Centre, which include the Arc and its linear filaments, the arched filaments, the 'threads', the thermal 'spiral' Sgr A West, the non-thermal source Sgr A East and Sgr C. When compared to higher-frequency observations there are, however, significant differences which we mostly attribute to the increasing optical depth of the thermal components. We show that most of the linear filaments of the Arc in the vicinity of G0.16-0.15, although non-thermal, have an intrinsically positive 90/20-cm spectral index (α ∼ + 0.3, where flux density S ∞ να), although at least one of the filaments shows a 90/20-cm spectral index of -0.4. The thermal feature G0.18-0.04 is seen in absorption, showing clearly that it is in front of the Arc. The positive latitude portion of the Arc is embedded in an extended region of ionized gas with an emission measure > 105 pc cm-6. The 'threads' show up as prominently as the Arc at 90 cm. Other isolated linear features in this region, e.g. G359.54 + 0.18, G359.80 + 0.17 and Sgr C., appear similar to the threads. The 90/20-cm spectral index of three of these features is relatively steep (α -0.6) and the filament in Sgr C has α = - 0.55 ± 0.4, whereas the 'thread' passing just north of Sgr A (G359.96 + 0.09) has a flat 90/20-cm spectral index. Hence there appear to be two classes of non-thermal filaments. The first class, the majority of which occur in the Arc, have flat or inverted spectra in which relativistic electrons must be produced, possibly in situ, on time-scales of less than 104 yr. The other class, which includes most of the isolated linear features, have relatively steep spectra and need not be directly associated with sources of relativistic particles. Two features observed at wavelengths longer than 1 m, namely the northern galactic lobe and the low-frequency jet, do not appear as distinct features at 90 cm. We suggest that these emission areas are caused by 'windows' in an otherwise optically thick thermal gas.