Faint Submillimeter Galaxies behind the Massive Lensing Cluster A2390
  Current studies on Submillimeter Galaxies (SMGs) mostly focus on bright sources with 850 micron flux greater than 2 mJy, and the results have shown that they are likely high redshift mergers with z > 2 and could be a dominant population on star formation in the early Universe. However, bright SMGs only contributes 20-30% of the 850 micron extragalactic background light (EBL), meaning the bulk of the cosmic star formation still hidden by dust and our current understanding is biased. We have started a program to study an unbiased sample of highly-amplified and intrinsically faint SCUBA detected SMGs in the field of massive lensing clusters. Here we report the newly obtained SMA observations at 850 micron on one of our sample source, A2390-5, behind the massive lensing cluster A2390. We successfully detect the source with a flux of 3.95 mJy. Surprisingly, it does not have any counterpart in any other wavelengths even though there are tentative candidates, which implies a very dusty and high-z nature. With less than 1" positional accuracy and the adoption of z = 5, we obtain the amplification factor of 12 using current lensing model, which makes A2390-5 a faint SMG with a de-lensed flux of 0.33 mJy. Together with our previous detection on another faint SMG, both of them have no counterpart in other wavelengths and their properties are very different than previously thought from the single-dish data. We emphasize the importance of direct submillimeter high-resolution studies on faint SMGs, which could be the dominant population of the high-z star formation. 
Chian-Chou Chen and Lennox L. Cowie (Institute for Astronomy,   University of Hawaii)
astro-ph.CO