Seeing Galaxy Formation in a Cold Light


Prior to the advent of the SCUBA array all of the detections of high redshift 850µ sources were made with single element bolometers and were targeted to a small number of previously known and generally rather exotic objects. With the advent of SCUBA and its greater sensitivity, targeted observations have become possible on more common objects selected at other wavelengths, such as Cimmati et al.'s detection of the ultrared galaxy Hu-Ridgway 10.
However the first major surveys to determine the typical 850µ populations only became possible with SCUBA, and the first work to show that field populations of luminous 850µ sources were present was that of Smail, Ivison and Blain ( Smail et al. 1997, Blain et al. 1998 and Smail et al. 1998) who used the trick of observing towards high redshift rich clusters where gravitational lensing by the cluster brightens the background sources and makes them easier to detect. The most recent report on this work by Smail et al. gives 17 sources detected above typical 3 sigma levels of 5-6 mJy in 36 square arcminutes of sky.
The lensing technique has many advantages in yielding sources which are brighter both in the submillimeter and optical, and which are very well suited for follow-up observations, but it does have the disadvantages that the actual magnification --- and so the flux of the observed object --- is relatively uncertain, and that there may be contamination of the background source counts by objects lying within the cluster.
The new deep field surveys avoid this issue, but at the expense of much longer exposure times and of identifying sources which may be harder to do followup studies on. However, the current blank field counts are indeed slightly lower than those inferred from the lensing technique, suggesting that there is indeed contamination or selection bias in the cluster-determined counts. This is shown in the figure below.
The green box show Smail et al.'s counts with the best estimate of the magnification factor. Within the possible magnification uncertainties, it may slide along the green line with 1 sigma uncertainties given by the dashed green lines. (The lensing leaves the average sky brightness invariant.) The counts from the deep field surveys are shown by the solid and open boxes with 1 sigma error bars, and the purple-shaded region shows the 95% confidence range at fainter magnitudes from a fluctuation anlaysis on the SSA13 and Lockman fields. The red line shows Guiderdoni et al.'s model E, which is known to match the 850µ sky background, and the dashed red line shows this model renormalised to match the blank field counts. The Smail et al. counts are about a factor of two higher than the blank field counts when the best estimate of the magnification is used. Recent work on optical counterparts of the lensed sample ( Smail et al. 1998) also suggests that about a third of these galaxies are cluster or foreground, consistent with the excess in the counts. When this correction is made, the two sets of counts are broadly consistent within the still substantial small number uncertainties.