Introduction

Sgr A* is a compact synchrotron radio source located at or very close to the dynamical center of the Galaxy. It is believed to be a starved black hole of roughly 2 x 106 M¤ (e.g. Falcke et al. 1993). Its radio spectrum below 100GHz is flat or slightly inverted and varies on time scales of a few months. In the short mm and submm range the spectrum rises more steeply (e.g. Zylka et al. 1992 - ZML92). No positive detections of Sgr A* at FIR and MIR wavelengths have been reported, but upper limits in the 8-18 micron range (Gezari et al. 1994) indicate that the spectrum must drop by more than an order of magnitude between the submm and the MIR regime.

The exact point of this turnover has been a matter of some debate in recent years. ZML92 claimed a 350 micron detection of SgrA* (with a large beam) of 18 ±9Jy, while Dent et al. (1993) clearly showed with the higher resolution of JCMT that this flux must be arising from the circum-nuclear disk (CND) surrounding SgrA*, and they placed an upper limit to its 450-micron flux of 1.5Jy. Both ZML92 and Dent et al. (1993) detected SgrA* at 800 microns, but with fluxes indicating that the source is varying on timescales less than a year.

Results

We obtained long-term status on JCMT (M/94A/U19) to answer the two questions: At what point in the submm regime does the spectrum turn over? What is the nature of the variability of the submm spectrum? To answer the first question we attempted to map SgrA* at 600 microns, in addition to 800 and 450 microns. To answer the second question we will be returning to JCMT in each of the next two semesters to repeat the observations. The results from the first run, outlined here, will be reported in Zylka et al. (1994, A&A in prep).

During the excellent weather conditions of Feb 27 to March 1 1994, with 230-GHz opacity below 0.03 throughout, we made raster on-the-fly maps, using the single channel bolometer UKT14, of the Sgr A region. Figure 1 shows our results in the form of isophotal contour maps of the region at 800, 600 and 450 microns. The axes are marked in arcsec offset from the position of SgrA*. The maps shown represent a combination of 5 maps at 800 microns, each with 1 second per point integration, and 7 and 9 maps respectively at 600 and 450 microns, each with 2 seconds per point integration.

The bright source seen at the centre of the 800-micron map is SgrA*, and the source can also be seen in each of the other 2 maps. These maps represent the first detections of SgrA* at 600 and 450 microns. However, the maps all show the structure of the surrounding region consistent with that seen in previous 800 & 1100 micron maps (eg: Dent et al. 1993). The CND extends over the central 12pc of the Galaxy. At a galactocentric radius of roughly 1pc the dust and hydrogen column densities drop to low values and form the Central Cavity. Our submm images show that the bottom of this cavity is rather flat, in agreement with Dent et al. (1993), and that Sgr A* sits in the centre. By fitting gaussians to SgrA* at each wavelength, we obtain fluxes of 3.5 ±0.5 Jy, 4.0±1.2 Jy and 3.0 ±1.0 Jy at 800, 600 and 450 microns respectively. Hence we observe a flat spectrum throughout the submm regime.

Our 800-micron detection of SgrA* is just consistent to within the combined errors with the values of Dent et al. (1993) and ZML92. However the full range of observed 800-micron flux densities for SgrA* is 3.1 - 5.6 Jy. These measurements have been made with different telescopes, and varying calibration problems. Therefore we intend to repeat the 800-micron measurements on a six-monthly timescale, using the same telescope, with the same calibration, to check whether this variation is real, or simply an artefact of different calibrations. Our detection at 450 microns is higher than the upper limit of 1.5Jy found for SgrA* by Dent et al. (1993). This appears to indicate that the source is variable at 450 microns on timescales of order 2 years, which we will also attempt to verify.

Conclusions

Our new data have resolved some of the existing paradoxes associated with the spectrum of SgrA*, by ruling out any dust model for the emission mechanism. In addition, our new detections at 450 and 600 microns will enable us to model the synchrotron emission mechanism which our data appear to support. Due to the variable nature of this mechanism we plan to continue to monitor this source at JCMT over the next 2 semesters to more tightly constrain the model.

References:

Dent et al. (1993) ApJ 410, 650

Falcke et al. (1993) A&A 270, 102

Gezari et al. (1994) in: "The Nuclei of Normal Galaxies" eds Genzel & Harris Zylka et al. (1992) A&A 261, 119 - ZML92

Zylka et al. (1994) A&A in press

D.Ward-Thompson, ROE

R.Zylka & P.G.Mezger, Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D53121 Bonn