Contents of: VI/111/./abstract/LMETCALF_ARCS.abs

The following document lists the file abstract/LMETCALF_ARCS.abs from catalogue VI/111.
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SCIENTIFIC ABSTRACT
We propose deep imaging with ISOCAM of fields containing gravitationally lensed
galaxies (giant arcs) with redshifts up to 2.2. In this way we take advantage
of the vast ``gravitational telescopes'' constituted by distant galaxy clusters
to study the infrared flux from the lensed galaxies themselves and to probe
these fields to high redshift for other, IR luminous, lensed objects.

These observations will:
a) - generate infrared images of known giant arcs.
b) - provide the infrared flux of the giant arcs and hence extend our knowledge
     of the properties of field galaxies at high redshift.
c) - search for infrared arcs that would be produced by a primeval population of
     infrared luminous galaxies.
d) - will be complementary to ISOCAM proposals to make deep integrations on 
     galaxy clusters.

OBSERVATION SUMMARY

Given the R magnitudes for these arcs, and assuming normal galaxy colours, we
can calculate their flux in the M band (4.8 microns). This is, for the arcs
to be studied here, in the range 3 to 12 microJanskys per square arcsecond.

We will use CAM in micro--scanning mode (5x5 microscan, 1.3 pixel step size) in
the following configurations, for all targets:

- filter LW1 (4 to 5 microns), on chip integration time (tint) 20 seconds, pixel
  f.o.v. 3 arcseconds, exposure time 3600 seconds and
- filter LW6 (7 to 8.5 microns), on chip integration time (tint) 20 seconds, 
  pixel f.o.v. 3 arcseconds, exposure time 3600 seconds.

These configurations will allow us to reach S/N ratios of 3 and 11.5 respectiv-
ely, in the two configurations described above, for arcs having fluxes of 8 
microJanskys per square arcsecond. Therfore, while this is a difficult measure-
ment, it is nonetheless well within the bounds of possibility that the arcs 
can be detected with CAM given its sensitivity.

Quite apart from the sensitivity of CAM, another potentially serious constraint
is the possibility of confusion of the IR arc images with foreground galaxies in
the lensing cluster. The diameter of the ISO diffraction spot is roughly, in
arcseconds, 0.84 times the wavelength. This implies diffraction spot sizes of
3.8 and 6.5 arcsedonds, in the filters chosen here. We have therfore chosen the
3 arcsecond per pixel p.f.o.v. with the LW1 filter and the 3 arsecond p.f.o.v.
with the LW6 filter. These are a compromise between sampling of the PSF for the 
wavelenghts employed and achievable S/N and should avoid serious confusion with 
nearby sources for the arcs studied here except for the case of A370, where 
careful modelling of the foreground cluster will be necessary in order to 
compensate, during data reduction, for source confusion.

Observing between 4 and 5 microns reduces flat-fielding problems inherent at 
longer wavelengths where the background is higher. The choice of the LW1 filter
and the 3 arcsec per pixel field of view maximises the resolution and the 
sampling of the PSF for useful S/N, vital if the confusion limit is to be 
avoided. The combination of the 3 arcsecond p.f.o.v. and the LW6 filter allows 
us to take advantage of any growth in the IR flux going further into the IR (if 
the lensed galaxies are AGNs) and provides greater sensitivity due to the larger
 
pixel size. 

For the 3" p.f.o.v, the Zodiacal background flux is much greater than the source
 
flux. Then detector responsive transients which can occur when a source moves 
over the array, should not be too severe, since the array sees essentially 
uniform illumination, even during microscanning, after the initial transient at 
source acquisition. The transient behaviour of the detector should be better for
 
the higher pixel signal encountered with the higher background seen in the LW6 
filter.

Micro-scanning reduces the impact of residual flat-fielding noise and enhances 
the possibilities for later application of super-resolution techniques.

© Université de Strasbourg/CNRS

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