| Description |
scientific abstract: to link the survey of distant clusters to the virgo sample, we will study the mid-ir emission of a sample of clusters in the hydra region. these clusters have been studied in the x ray range with rosat, and in the optical and radio ranges from ground, by m. pierre and co-workers. present results indicate that the correlation amongst the three wavelengths appear stronger for the higher redshift members of the sample, indicating a luminosity selection effect or, possibly, an evolution in cluster properties with redshift. observation summary: we intend to observe these clusters in the isocam filters used in the deep survey and the distant cluster proposal, lw2 and lw3, and with phot c100 at 90 microns. for clusters at z virgul 0.1, we will use aot1 in the microscanning mode, with a pfov of 6, to cover a field of 10.x10.. the objective is to detect a galaxy 10 times fainter than the starburst galaxy ngc6240 at z = 0.1 (0.5 mjy at 7 microns and 1 mjy at 14 microns), with a s/n ratio of 4. we intend to perform raster scans of 8x8 positions, displacing every time the array by 16 pixels. the axis of the raster will be aligned to the spacecraft axis. the time required is 2500 seconds per filter, including all overheads. redshifting the same starburst galaxy at z=0.2, intensities will be 0.1 and 0.2 mjy at 7 and 14 microns respectively. at z=0.3 they will be reduced by a factor of two (i.e. 0.05 and 0.1 mjy). for clusters at zvirgul0.2-0.3, we shall use the following microscanning mode: 4x4 positions, displacement of 16 pixels. the area most covered will be close to 5.x5.; the total observed area will be 2.8 times larger. according to the isocam software simulator, we should obtain with lw2 in 2500 sec a s/n of 3.7 for a 0.1 mjy source; with lw3 barely >1 for a 0.2 mjy source. with flux densities two times fainter and same exposure times, the simulations show that the s/n will be decreased to virgul2 for lw2 and 0.6 for lw3. however, for such long exposures, we think that it will be possible to use more sophisticated data processing procedures than that provided by the simulator and improve the present predictions. an alternative would be to restrain our observations to a 3.x3. field and use the beam switching mode (cam03). then, taking the last-mentioned observing figures the simulator predicts a s/n of 3 for lw2 and 1.2 for lw3. all clusters will be observed with phot in the camera mode using the chopper with a beam throw of 180. we shall use c100, with the 90 micron filter. we shall perform rasters of 6x6 positions for z = 0.1, of 3x3 for z = 0.2, and 2x2 for z =0.3 displacing the array by steps of 1.5. in both directions. thus, we will cover a field of 10.5.x10.5., 6.x6., 4.5.x4.5. with individual integration times will of 2 min, 3 min, 5 min respectively. our aim is to be able to observe sources of a few mjy with a s/n greater than 3. at the mjy level - or earlier - we expect to reach the confusion level, but what we want to measure is the integrated light of the cluster. |
