2.7. Deep submm-wave surveys
Images of the redshift z = 0.25 cluster of galaxies Abell 1835 in both the optical and submm wavebands were compared in Fig. 1. This image provides a realistic impression of the appearance of deep optical and submm images of the sky. Note that the only relatively low-redshift or cluster member galaxy that contributes any submm-wave flux is the central cD galaxy: the other cluster galaxies are either quiescent, neither forming stars nor heating dust, or are insufficiently luminous to be detectable at 850 µm using SCUBA. Background galaxies at much greater redshift, which have faint optical counterparts as compared with the cluster member galaxies, dominate the image. This is a direct visual demonstration of the strong bias towards the detection of distant galaxies in submm-wave surveys that was illustrated in Fig. 4. These background galaxies are magnified by a factor of order 2-3 due to the gravitational lensing potential of the foreground cluster over the full extent of the image. The effects of gravitational lensing can be determined using accurate models of the cluster potential, that are constrained with the help of data from HST images and spectroscopic redshifts for multiply-imaged optically-selected galaxies. The uncertainty in the results is comparable to the uncertainty in the calibration of the submm images.
As shown in Fig. 1, existing deep submm images are much less visually stimulating than deep optical images, because their angular resolution is not sufficient to image the internal structure in distant galaxies. The limited resolution also imposes a confusion limit to the depth for submm surveys, at which the noise level is dominated not by atmospheric or instrumental noise but by the telescope resolution blurring together signals from faint unresolved galaxies. It takes about 50 h of integration using SCUBA to reach the practical confusion limit in a single field. Confusion is discussed in more detail in Section 3.1.
There are a variety of published results from deep submm galaxy surveys. Surveys aim to detect high-redshift galaxies exploiting the powerful K-correction effect in the submm waveband. Over 500 arcmin2 of blank sky has been surveyed using SCUBA by several groups (Barger et al., 1998; Hughes et al., 1998; Barger et al., 1999a; Eales et al., 1999, 2000; Borys et al., 2002; Fox et al., 2002; Scott et al., 2002; Webb et al., 2002a). These range from an extremely deep survey in the area of the Hubble Deep Field-North (HDF-N; Williams et al. 1996) by Hughes et al. (1998) searching for the faintest detectable populations of submm galaxies, to wider-field shallower surveys to detect brighter sources that might be easier to follow-up and could be used to trace large-scale structure (Borys et al. 2002; Scott et al. 2002). About 30 5-arcmin2 lensed cluster fields have been imaged using SCUBA (Smail et al., 1997, 2002; Chapman et al., 2002a; Cowie et al., 2002; Kraiberg Knudsen et al., 2001; van der Werf and Kraiberg Knudsen, 2001), to various RMS depths between 0.5 and 8 mJy. 8 By exploiting the magnification effect of gravitational lensing, which extends over fields several arcminutes across, due to rich clusters of galaxies at moderate redshifts, the population of distant galaxies in the source plane behind the lensing cluster can be probed to greater depths than is possible in a blank field (Blain, 1998).
The detection rate of galaxies using SCUBA based on published papers appears to have declined over time since 1998. In significant part this is due to the absence of the sustained excellent observing conditions on Mauna Kea that were experienced during the El Nino winter of 1997-1998, just after SCUBA was commissioned.
A wide variety of complementary, and sometimes overlapping surveys have been made using MAMBO at the IRAM 30-m telescope (Bertoldi et al., 2000, 2001; Carilli et al., 2001), during several winters. These fields include the cluster Abell 2125 and the ESO-NTT Deep Field (Arnouts et al., 1999).
A compilation of the results from all the SCUBA and MAMBO surveys is presented in Figs. 9 and 10. A full summary of deep projects that have been undertaken or are underway can be found in Ivison (2001). In addition, larger shallower submm surveys of the Galaxy (Pierce-Price et al., 2000), 9 and perhaps the CMB images obtained using the BOOMERANG balloon-borne experiment (Masi et al., 2001), can be used to search for brighter submm-wave galaxies.
8 1 Jy = 10-26 W m-2 Hz-1 Back.
9 A JCMT project (Vicki Barnard et al.) is currently searching for candidate high-redshift galaxies detected in wide-field SCUBA images of star-forming regions in the Milky Way (Barnard et al., 2002). Back.