The nuclear region of Arp 220 imaged with continuum VLBI at 18 cm. The
angular resolution is ~4 milliarcsec
(~1.5 pc), and the image is 1.6 arcsec across. Two clusters of point
sources are visible, separated by ~1 arcsec,
corresponding to the twin nuclei in Arp 220. These sources are
interpreted as radio supernovae associated with
an intense nuclear starburst. The rms noise level is ~8 uJy/beam.
In Nov 2002, a new VLBI experiment was conducted by Lonsdale
(Haystack), Lonsdale (IPAC), Smith (UCSD) & Diamond (Jodrell) which
included several large European dishes, the VLBA, the phased-VLA, the
GBT, & Arecibo. When combined with almost continuous
recording at a rate of 256 Mbit/s, this experiment is probably the most
sensitive VLBI observation to date. A preliminary continuum
image, shown above, boasts a rms noise level of only 8 uJy/beam, and
this can be reduced by further processing to perhaps as low as 6
uJy/beam. The more than 3-fold improvement in SNR afforded by Arecibo,
the GBT and a wider recording bandwidth has resulted in the detection
of roughly 30 RSN candidates in