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Hubble Deep Field - South
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Hubble Deep Field - South

May 16, 2005
This photo uses a near-IR frame obtained with the Hubble Space Telescope NICMOS instrument instead of the VLT I-frame. The HST image has nearly the same total exposure time as the VLT images. Their combination is meaningful since the VLT and NICMOS images reach similar depths and show more or less the same faint objects. This is the result of several effects compensating each other: while more distant galaxies are redder and therefore better visible at the infrared waveband of the NICMOS image and this image has a better angular resolution than those from the VLT, the collecting area of the UT1 mirror is over 11 times larger than that of the HST.

It is interesting to note that all objects in the NICMOS image are also visible in the VLT images, with the exception of the very red object just left of the face-on spiral. The bright red object near the bottom has not before been detected in optical images (to the limit of R ~ 26 mag), but is clearly present in all the VLT Test Camera coadded images, with the exception of the U-band image. Both of these very red objects are possibly extremely distant, elliptical galaxies [2].

The additional information that can be obtained from the combination of the VLT and the infrared NICMOS images has an immediate bearing on the future work with the VLT. When the infrared, multi-mode ISAAC instrument enters into operation in early 1999, it will be able to obtain spectra of such objects and, in general, to deliver very deep infrared images. Thus, the combination of visual (from FORS) and infrared (from ISAAC) images and spectra promises to become an extremely powerful tool that will allow the detection of very red and therefore exceedingly distant galaxies. Moreover, it is obvious that this sky field is not very crowded - much longer exposure times will thus be possible without encountering serious problems of overlapping objects at the "confusion limit".

Technical information: Photo 35b/98 is based on 16 U-frames (~370 nm; total exposure time 17800 seconds; mean seeing 0.71 arcsec) and 15 B-frames (~430 nm; 10200 seconds; 0.71 arcsec) were added and combined with 8 R frames (~600 nm; 7200 seconds; 0.49 arcsec) as well as a HST/NICMOS H-band frame(a H-band HST/NICMOS image from the ST-ECF public archive) (~1600 nm; 7040 seconds; 0.2 arcsec) to make this colour composite. Individual frames were flat-fielded and cleaned for cosmics before combination. The field shown measures 1.0 x 1.0 arcmin. North is up; East is to the left.


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