Rd Ekers

21-CM ABSORPTION-LINE IN NGC1275

DE YOUNG, Roberts and Saslaw1 detected H I absorption in NGC1275 with a velocity of 8,120 km s−1. This is ∼ 3,000 km s−1 larger than the systemic velocity of NGC1275 and is the same as that of the enigmatic system of gas filaments extending 40″ from the nucleus2,3. The observations of De Young et al. were made with a single dish with insufficient angular resolution to determine, which of the various radio continuum sources were being absorbed. They argue that the absorption is probably occurring in the components < 0.2″ rather than in the 5′ halo component, but they were unaware of an intermediate scale 30″ component4. Here we present observations made with sufficient angular resolution to decide unambiguously between these possibilities. Our data can also be used to place a limit on the H I mass of possible foreground objects which could be causing the absorption.

VLA Observations of Sgr A

The radio source Sgr A has been mapped with the Very Large Array (VLA) at 6 and 20 cm with an angular resolution of 5″ × 8″ arc. In agreement with the earlier “WORST” map, the non-thermal source Sgr A East shows a shell structure, while the thermal source Sgr A West shows a spiral-like morphology. We suggest that Sgr A East is a supernova remnant (SNR) near the galactic centre. Its surface brightness is the third largest in our galaxy after Cas A and the Crab Nebula. The diameter is 9 pc and the source fits the surface-brightness diameter relationship of Clark and Caswell (1976) if a distance of 10 kpc is assumed.

The Nature, Location and Environment of SgrA East

We have observed SgrA at 332 MHz (92 cm) with a resolution of 12 arcsec (0.6 pc) using the four configurations of the VLA. These results illustrate the dramatic and almost unique variation of radio spectral index within the central 3–4 arcmin of the galactic center. SgrA East is a non-thermal shell source that could be a supernova remnant or a very low-luminosity example of a radio component associated with the active nucleus of a spiral galaxy. The most dramatic aspect of the new 332 MHz observations is the appearance of the the SgrA West spiral features in absorption against SgrA East. Based on these results, SgrA East is situated behind SgrA West, the center of the galaxy. The halo is in front of or surrounds the former sources. The HII regions to the east of SgrA East (1 = -0°.02, b = -0°.07) are probably associated with the 50 km/s molecular cloud. The 7 arcmin halo (20 pc) has a non-thermal spectrum with turn-over below 1 GHz.

Source-noise in radio synthesis images

Open Access. Radio interferometry: Theory, techniques, and applications; Proceedings of the 131st IAU Colloquium, Socorro, NM, Oct. 8-12, 1990.