We have used the NRAO VLA to observe the Hubble Deep Field for a total of 152 hours at a frequency of 8.5 GHz. The VLA field of view at 8.5 GHz has a diameter of 9.2 arc minutes down to the eight percent power point of the VLA primary beam covering an area of 6 x 10-6 sr. This includes the HDF with nearly full sensitivity as well as all of the surrounding flanking field images with reduced sensitivity. The observations were divided among the A, C, and D configurations of the 27-element array to examine the radio emission from the HDF on angular scales up to about 100 arc seconds with resolutions ranging from 0.1 to 10 arcseconds, corresponding to linear scales of 1 kpc to 1 Mpc for moderate redshifts. We observed with both LHC and RHC polarizations with an effective bandwidth of 200 MHz. The rms noise level using all of the data was 1.8 microjanskys with an effective resolution of about 4 arc seconds.
At 8.5 GHz, we catalogued a total of 28 sources within the 9.2 arc minute diameter field of view stronger than 9.0 µJy (snr = 5). Of these, seven sources are found within the HDF. All seven are identified with galaxies of various types, and five have been detected at 6.7 or 15 microns by ISO (Goldschmidt et al. 1997). We also find weaker radio emission from six other relatively bright galaxies below the 9 µJy completeness level but stronger than 6.5 µJy (3 < snr < 5). Figure 2 shows the radio contours at 8.5 GHz superimposed on the HDF optical image.
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Figure 2. Radio contours superimposed on HDF image. Contour levels are 3.5 and 5 times the rms noise or 6.3 and 9.0 µJy which corresponds to the lower limits of our supplementary and complete lists respectively. |
We also used the VLA for about 50 hours in its A configuration at 1.4 GHz in order to obtain information on the radio spectra of galaxies in the HDF. At 1.4 GHz, the resolution of the VLA is limited to about 2 arc seconds. Complementary observations made with the MERLIN array by Muxlow et al. (in preparation) give greatly improved resolution. The combination of the VLA and MERLIN data will give sensitive radio images of the HDF with an angular resolution of 0.2 arc see. This will allow a detailed comparison of the optical and radio emission.