The Supernova Remnants in Arp 220

The merging galaxy Arp 220 is the prototypical luminous infrared galaxy. It exhibits extreme far-infrared luminosity (~10^{12}L_sun), and a wide variety of molecular lines originating in an extremely dense central condensation of dusty gas.  It is also the prototypical OH megamaser galaxy.  The unique status of Arp 220 arises because at a distance of only 76 Mpc, it is the nearest example of a truly powerful IR galaxy, and the associated phenomena can be studied in greater detail than in any other such galaxy.  It is the focus of considerable attention in the search for unambiguous evidence of starburst or AGN activity, in order to shed light on the ultimate origin of the quasar-like bolometric luminosity, and on the possibility of an evolutionary sequence triggered by galaxy merger events.

In 1996, one of the then largest VLBI experiments was conducted on Arp 220, using the compact OH maser peak as a convenient phase reference. This permitted continuum imaging of the galaxy with high sensitivity, revealing the existence of roughly a dozen unresolved sub-mJy point sources scattered across the nuclear region of the galaxy. This was interpreted as spectacular evidence of an ongoing burst of intense star formation traced by radio supernovae (RSN).  Both the OH maser results (Lonsdale et al., 1998, ApJL, 493, L13) and the continuum results (Smith et al., 1998 ApJL, 493, L17) have significant implications for the field.  However, these studies were severely sensitivity-limited, allowing only modest spectral resolution on the masers, and probing only the peak of the RSN luminosity function. Nearly all the RSN were found in the westernmost of the two nuclei in Arp 220, only 2 very faint candidates being seen in the eastern nucleus.

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 Arp 220, about 10 of which lie in the eastern nucleus.  The brightest source in the field is slightly below 1 mJy.

This preliminary image constitutes dramatic evidence that intense star formation is occurring in both nuclei, and not just the western one. It is of particular importance to deduce the supernova rate in luminous IR galaxies as a measure of the intensity and luminosity of star formation activity.  By probing the luminosity function of these RSN more deeply with the new observations, and combining the data from several epochs to develop accurate light curves, a refined estimate of the RSN rate will be possible.  Further data processing will reveal many  more details about the maser emission.  For example, weak maser amplification along lines of sight to the 30 RSN candidates will constitute a valuable probe of detailed properties in regions inaccessible to other techniques because of opacity and lack of resolution.  Future VLBI observations with Arecibo may have the sensitivity to observe these phenomena in other IR galaxies.