The Nuclei of Arp 220

On 2003 Nov 9, Colin Lonsdale (MIT/Haystack), Phil Diamond, Hannah Thrall (Jodrell Bank), Harding Smith (UCSD) & Carol Lonsdale (IPAC) used λ18-cm VLBI to image the nucleus of the ultra-luminous IR galaxy (ULIRG), Arp 220, with a linear resolution of ~1 pc. The image contained data from the VLBA, GBT, Arecibo, and five other EVN antennas. This image (see figure below) has a background rms of 5.5 μJy/beam, the most sensitive VLBI image yet obtained, and reveals 49 point sources in the field with flux densities between 1200 and ~60 μJy. Comparison with an image from 12 months earlier reveals at least 4 new sources. These are believed to be new radio supernovae (RSN), and if these represent all SN exploding during this interval, an estimate for the SN rate in Arp 220 of 4 ± 2 per yr results. The implied star formation rate is sufficient to power the entire observed FIR luminosity of the ULIRG. The two nuclei of Arp 220 show striking similarities in their radio properties, though the W nucleus is more compact, and appears to be ~3 times more luminous than the E nucleus. (For a full description of this work, see Lonsdale et al., astro-ph/0604570.)




Figure: λ18-cm images of the E and W nuclei of Arp 220. Angular resolution is 5.9 x 2.7 mas at -21 deg. The inset shows the relative locations of the two images, with crosses marking the relative locations of the peaks of the diffuse continuum emission in the two nuclei. This diffuse emission (20 times stronger than the sum of the point-source flux densities) is fully resolved out by the VLBI array. (Click on the image to see a larger version.)